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Lake Oswego Stormwater Management Manual - March 2016 1► EP* "If . . a' & Oswe . o o • . r . ,. .: ti NO Fo'r ,,_ w 0 5ri-a,,,,, ,t. . ............_ . i id! - ,i— - _ _ ,.fin * - ' rT - rCr , 7 - . - il -- - 7"+-� :- .�.f195 n !� _?-__"ram,: t ' lab * ;#,1 A N.R 40.°4141IIIPPIP -- -). a A r - r _ Illitel 0,01111.44,4 ,. - T' ` 1 t/ i - f as ic-,„ ,-- , 0 „.„_ V _that _ ,,, OEGO . March 2016 Lake Oswego Stormwater Management Manual �, Prepared for C� cf City of Lake Oswego 4 - rei 380AAvenue f—� PO Box 369 V 0 Lake Oswego, OR 97034 OR E G O (503) 675-3999 Prepared by Herrera Environmental Consultants 24 NW 2nd Avenue, Suite 204 Portland, OR 97209 H E R R E RA Telephone (503) 228-4301 The Lake Oswego Stormwater Management Manual is a living policy document that the City will periodically modify as necessary. Modifications may be made to meet regulatory requirements or infrastructure management needs, or to improve the effectiveness of the stormwater program. The City has the right to modify the standards and may establish supplemental requirements under a separate publication or policy that becomes an addendum to this Stormwater Management Manual. The City may also update additional requirements under a separate document. Any modifications to the Lake Oswego Stormwater Management Manual will be included in the latest online version of this document and updated in print when updates are made. Acknowledgements The City wishes to acknowledge the assistance of citizen volunteers and staff on the stormwater manual Project Advisory Committee: Alex Hurley, PLS, and Jonathan Morse, PE, AKS Engineering & Forestry, LLC Brian Wegener, Tualatin River Watershed Council and Tualatin Riverkeeper Corrina Chase, formerly Tryon Creek Watershed Council E. Jay Murphy, LO Sustainability Advisory Board Jeff Ward, Lake Oswego Corporation Justin Wood, formerly Government & Builder Relations Manager, Home Builders Association of Metro Portland Stephanie Wagner, Oswego Lake Watershed Council Bruce D. Goldson, PE, Theta LLC William Tierney (2012-2013) and Skip O'Neill (2015), City Council liaisons David Donaldson (2012-2013) and Megan Phalen (2015), Assistant City Managers Rob Amsberry, City Stormwater Engineering Erica Rooney, PE, City Engineer Guy Graham, PE, formerly City Public Works Director Evan Boone, Deputy City Attorney Jim Bateman, City Public Works Operations Jenny Slepian, Sustainability and Management Fellow Bonnie Hirshberger, City Citizen Information Specialist Hamid Pishvaie, City Assistant Planning Director David Gilbey, formerly City Water Quality Coordinator Mary Larkin, Herrera Environmental Consultants (consultant team project manager) Eryn Deeming Kehe, JLA, Inc. (consultant, committee facilitator) Kate Forester, Herrera Environmental Consultants (stormwater graphic and manual design lead) Carol Slaughterbeck, Herrera Environmental Consultants, Herrera principal in charge Additional City staff members contributed to this manual: Todd Knepper, PE, Engineering Program Supervisor, provided a review of the draft manual; Pam Peterson, Public Works Beautification Coordinator, provided a review of planting design guidance and species selection. Finally, City staff, project advisory committee members, and the consultant team have benefitted from over a decade of interaction with our regional colleagues in stormwater management. We hope this manual integrates the collective knowledge and insights gained by this experience. --Anne MacDonald, CEG City Stormwater Quality Coordinator (June 2013-present) Cover photo: Reflections on Oswego Lake, Francie Manning, 2014 Lake Oswego Photo Contest awardee Table of Contents Letter From the City i Notice of Manual Adoption ii Ordinance 2695 iii Chapter 0.0 Abbreviations vii 0.1 Abbreviations and Acronyms vii Chapter 1 Introduction 1 1.1 Manual Purpose 1 1.2 Organization of This Manual 4 1.3 Why Stormwater Matters 5 1.3.1 Water Quantity Impacts from Development on Receiving Waters and Conveyance System 6 1.3.2 Water Quality Impacts from Development on Receiving Waters 9 1.3.3 TMDLApprovedAreas/303(d)-Listed Receiving Waters 9 1.4 General Principles of Stormwater Management 11 1.4.1 Structural Stormwater Best Management Practices 12 1.4.2 Non-Structural Stormwater Best Management Practices 12 1.4.3 Conveyance 13 1.5 How Local Conditions Affect Stormwater Management 13 1.6 Updates 14 Chapter 2 Project Planning, Permits, and Stormwater Management Requirements 16 2.1 Project Process 16 2.2 Activities That Trigger Stormwater or Permit Requirements 19 2.3 General Requirements For All Residents, Property Owners, and Business Owners 19 2.3.1 Prevent Illicit Discharges 19 2.3.2 Maintain Existing Drainage Patterns 20 2.4 Project Classifications and Development Types 20 2.4.1 Small Projects 21 2.4.2 Large Projects 21 2.4.3 Development Types 21 Lake Oswego Stormwater Management Manual TOC— 1 2.5 Overview of Minimum Requirements — Design Requirements 21 2.5.1 Perform Site Assessment and Feasibility Analysis 21 2.5.2 Onsite Stormwater Management 22 2.5.3 Design for Water Quality 22 2.5.4 Design for Flow Control 26 2.5.5 System Designed by Licensed Professional 26 2.5.6 Underground Injection Control Review with Oregon Department of Environmental Quality 27 2.6 Overview of Minimum Requirements — Permits 27 2.6.1 City of Lake Oswego Erosion Control Permit 27 2.6.2 NPDES 1200-C Permit from Oregon Department of Environmental Quality 28 2.6.3 Street Opening Permit 28 2.6.4 Other Permits and Project Review 28 2.7 Overview of Minimum Requirements — Submittals 29 2.7.1 Drainage Report 29 2.7.2 Recorded Operations and Maintenance Plan and Deed Restriction 30 2.7.3 Landslide Hazard and Erosion Risk Areas Report 31 2.8 Project Exemptions and Waiver and Payment/Fee In Lieu Information 31 2.8.1 Exempt Projects 31 2.8.2 Adjustments 31 2.8.3 Exceptions 31 2.8.4 Exceptions and Payment/Fee In-Lieu Program Information 32 2.9 City Review of Development Permits 32 2.9.1 Engineering Department 32 2.9.2 Planning Department 33 2.9.3 Building Department 33 2.9.4 Public Works — Operations Division 33 2.10 Review Process for Different Types of Projects 33 2.10.1 New or Remodeled Single-Family Residence 33 2.10.2 Land Divisions, Commercial, Industrial, Institutional and Multifamily Development & Redevelopment Projects 33 2.10.3 Lake Oswego Review Process for Various Project Phases: Design, Construction, and Operations and Maintenance 34 Chapter 3 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 40 3.1 LID Principle #1 — Understand the Site 40 3.1.1 Site Assessment 40 TOC—2 Lake Oswego Stormwater Management Manual 3.1.2 Site Assessment Components 42 3.2 LID Principle #2 — Reduce Runoff through Design 45 3.3 LID Principle #3 — Reduce Pollutants Carried by Runoff 46 3.4 LID Principle #4 — Capture and Treat Runoff 48 3.4.1 BMP Selection for Small Projects 50 3.4.2 BMP Selection for Large Projects 50 3.4.3 Infiltration BMP Considerations 51 3.4.4 Use and Approval of Proprietary Stormwater Best Management Practices 52 3.4.5 Other Considerations in BMP Selection 53 Chapter 4 Stormwater Facility Design Guidelines 56 4.1 Performance Standards 56 4.1.1 BMPs for Pretreatment 56 4.1.2 Onsite Stormwater Management 56 4.1.3 Water Quality BMPs 58 4.1.4 Flow Control BMPs 58 4.1.5 Impervious Area Reduction BMPs 59 4.2 Planting Design 59 4.2.1 Why Plants Matter in Stormwater Design 59 4.2.2 Vegetation Diversity and Its Effect on Water Quality 60 4.2.3 Plant Selection Process: Steps and Considerations 61 4.3 BMP Design Methods and Computations 66 4.4 Soils 66 4.5 Pollution/Flow Control Manhole Design 68 4.6 BMP Design Guidelines 68 4.6.1 Rain Gardens 70 4.6.2 Planters 78 4.6.3 Infiltration Trench 86 4.6.4 Drywell 92 4.6.5 Green Roofs 98 4.6.6 Pervious Pavement 102 4.6.7 Rainwater Harvesting 106 4.6.8 Filter Strips 112 4.6.9 Swales 118 4.6.10 Sand Filters 124 4.6.11 Constructed Wetland 130 Lake Oswego Stormwater Management Manual TOC—3 4.6.12 Ponds 136 4.6.13 Detention Pipes and Vaults 142 4.6.14 Sheet Flow Dispersion 148 4.6.15 Tree Retention 150 4.6.16 Post-Construction Soil Quality and Depth 154 Chapter 5 Conveyance and Detention Design Standards 156 5.1 General Provisions - Conveyance 156 5.2 Extension of Public Conveyance Systems 156 5.3 Conveyance Easements 157 5.3.1 General 157 5.3.2 Standard Conveyance Easement Width 157 5.3.3 Reduced Conveyance Easement Widths 157 5.3.4 Encroachments 158 5.4 Flow Determination for Surface Water Conveyance 158 5.4.1 Land Use Assumptions for Flow Determination 158 5.4.2 Computational Methods for Runoff Calculations 159 5.5 Surface Water Conveyance Design Considerations 159 5.5.1 Design for Full Build-Out 160 5.5.2 Stormwater Conveyance Design Criteria 160 5.5.3 Materials for Piped Conveyance 160 5.5.4 Materials For Open Channels 161 5.5.5 Upstream Impacts 161 5.5.6 Downstream Impacts 162 5.5.7 Cross Lot Drainage 163 5.5.8 Dissipation of Runoff Discharge 163 5.5.9 Separation 163 5.5.10 Alignment 163 5.6 Other Requirements for Public Conveyance Systems 163 5.6.1 Survey 163 5.6.2 Railroad Crossings 163 5.7 Surfacewater/Stormwater Laterals 164 5.7.1 General Provisions 164 5.7.2 Planning Considerations 164 5.8 Conveyance and Detention Design Standards 164 5.8.1 Purpose 164 5.8.2 Downstream Analysis 164 TOC—4 Lake Oswego Stormwater Management Manual 5.8.3 Flood Management Areas Defined 167 5.8.4 Flood Management Design Criteria 167 Chapter 6 Construction Phasing, Stormwater Pollution Prevention, Erosion, and Temporary Sediment Control 170 6.1 Legal Requirements 170 6.1.1 City Code 170 6.1.2 MS4 Permit 170 6.1.3 1200-C Permit 170 6.1.4 General Considerations 171 6.2 Construction Practices 171 6.2.1 Good Construction Practices 171 6.2.2 Poor Construction Practices 172 6.3 Scheduling, Phasing, and Timing of Work 172 6.3.1 Scheduling 172 6.3.2 Phasing 173 6.3.3 Wet Weather Season Erosion and Sediment Control Requirements 173 6.4 Implementing Procedures 175 6.4.1 Communication 175 6.4.2 Prohibition of Certain Activities 175 6.4.3 Managerial Practices 175 6.5 Consideration of Construction Impacts during Planning, Design, and Construction 176 6.5.1 Planning Considerations 176 6.5.2 Design and Construction Considerations 176 6.5.3 Recommended Construction Practices 177 6.5.4 Selecting Construction Best Management Practices 177 6.5.5 Maintenance and Inspection Procedures 180 6.5.6 Post Construction 181 6.6 Documentation 182 6.6.1 Erosion and Sediment Control Drawings 182 6.6.2 Erosion Control Plan 183 6.6.3 Inspection Log 183 Chapter 7 Stormwater BMP Maintenance 184 7.1 Legal Requirements 184 7.1.1 Maintenance Responsibilities 184 Lake Oswego Stormwater Management Manual TOC—5 7.1.2 Inspection 185 7.1.3 Maintenance Access 185 7.1.4 Routine Maintenance Activities 186 7.1.5 Transfer of Property Ownership 191 7.2 Failure to Provide Adequate Maintenance 191 7.3 Submittals and Reporting 192 7.3.1 Operations and Maintenance Agreement/Plan 192 7.3.2 Records of Maintenance Activities 192 7.4 Maintenance Checklists 192 Chapter 8 References 194 Chapter 9 Definitions 198 Appendix A Submittals A A-1 Site Assessment and Feasibility Analysis Checklist A-1.0 A-2 Drainage Report A-2.0 A-3 Impervious Surface Area Reduction Table A-3.0 A-4 Integrated Pest Management Plan Example A-4.0 A-5 Sample Operations and Maintenance Plan A-5.0 Appendix B Infiltration Testing Guidance B B-1 Infiltration Testing Report B-1.0 B-2 City of Seattle Modified Procedure for Conducting Pilot Infiltration Test B-2.0 Appendix C Reading the Soil C Appendix D Biofiltration Soil Mix Specification D Appendix E Rational Method Information E Appendix F Santa Barbara Urban Hydrograph (SBUH) Method Input Data F Appendix G Right Tree Right Place G Appendix H Approved Plant Lists and Seed Mixes H H-1 City of Portland BES Stormwater Manual (2008) Plant List H-1.0 TOC—6 Lake Oswego Stormwater Management Manual H-2 City of Portland BES Stormwater Management Manual (2004) Seed Specifications H-2.0 Appendix I Erosion Prevention and Sediment Control Plan Information I-1 Erosion Prevention and Sediment Control General Notes I-1.0 1-2 Erosion Prevention and Sediment Control Application 1-2.0 1-3 Erosion Prevention and Sediment Control Permit Attachment 1-3.0 Appendix J Source Control J Appendix K Maintenance Checklists K Lake Oswego Stormwater Management Manual TOC—7 Letter From the City "Lake Oswego is a community rich with art, culture, historical resources, quality schools, vibrant business districts, and an abundance of activities and events for all ages." So begins the "New to Lake Oswego" portion of the City's website. Our city began due to local geology; iron-rich sediments between basalt flows attracted the first industry—and the first concentrated settlement. The community has evolved into one that appreciates the area's natural beauty and ready access to recreational and educational opportunities, and our community is committed to being the best it can be through active civic engagement. When Lake Oswego was developing, industrial pollution was the greatest threat to water quality in the Willamette River. Now, the greatest threat to water quality comes from everyday pollutants in stormwater: oil and other fluids that drip on roads, copper from brake dust, pesticides and fertilizers applied to our lawns and garden beds, nutrients and mercury from soil erosion. Just as we all contribute in some way to stormwater pollutants, we all must be responsible for improving water quality through our individual actions, including stormwater management. Our efforts to manage stormwater in Lake Oswego are complicated by several conditions. Most new development is in-fill, occurring on relatively small sites with nearby neighbors, so space for stormwater facilities is limited. In addition, soils throughout the city are generally fine-grained (like clay) and slow to drain water. The area's beautiful hills bring challenges of steep slopes and erosion. Protecting neighboring properties and water quality can be difficult and requires creativity. The Lake Oswego Stormwater Management Manual is intended to guide the selection, design, and operation of stormwater and drainage facilities by technical users, including City staff and designers in the development community. The manual: • Reviews regulatory requirements and design standards (Chapter 2); • Provides guidance for characterizing development sites (Chapter 3); • Outlines multiple possible design approaches for managing water quality and drainage (Chapters 4 and 5); • Describes expectations for construction activities and related site controls necessary to protect water quality (Chapter 6); • Reviews facility maintenance needs (Chapter 7); and • Provides the City's additional requirements for submittals, resources for stormwater design, and guidance for continuing stormwater management via source control (appendices). As communicated in the manual, stormwater management is increasingly site-specific. The process begins with an assessment of site characteristics and selection of appropriate management practices and features, moves through design and construction, and continues through operation and maintenance. The intent established during design must be carried through construction and continued with routine maintenance by the property owner. The Engineering Department welcomes your involvement in managing stormwater for the Lake Oswego community. Sincerely, Erica Rooney, PE City Engineer Lake Oswego Stormwater Management Manual Ejfjs� ENGINEERING DEPARTMENT 1,C.3 .iieur 0 \`°REGOt'' March 16, 2016 Dr. Scott Lazenby City Manager City of Lake Oswego 380 A Avenue Lake Oswego, Oregon 97034 Dear Dr. Lazenby: On February 16,2016 the Lake Oswego City Council unanimously passed Ordinance 2695, amending several City code provisions related to stormwater. One of these provisions, contained in Lake Oswego Code 38.25.110,requires the City Manager to, "administer this Code,and may furnish additional policy,criteria and information including specifications and procedures for the proper implementation of the requirements of this Code.This information includes the Lake Oswego Stormwater Management Manual." City staff,with assistance from our consultant,has completed the Manual and is ready to fully implement the guidance it contains for public and private development projects. It is my recommendation that you adopt. Please indicate your concurrence with this recommendation by signing below. Sincerely, fae.74_ Erica Rooney, PE City Engineer I,Scott Lazenby,City Manager,concur with this recommendation. �'" y 3I4(//‘ Signature Date' 503.635.0270 380 A Avenue P❑ BOX 369 Lake Oswego,OR 97034 www.ci.oswega.or.us ii Lake Oswego Stormwater Management Manual Effective Date: March 17, 2016 ORDINANCE NO.2695 AN ORDINANCE OF THE CITY COUNCIL OF THE CITY OF LAKE OSWEGO AMENDING LOC SECTIONS 38.06.2.c.i,38.24.5,50.06.006,50.60.004,50,07,003,50.07.004,50.10.003.2 AND ADDING ARTICLE 38.25, REGARDING STORMWATER MANAGEMENT;AND ADOPTING FINDINGS(LU 15-0050). WHEREAS, the regulation of stormwater runoff reduces threats to public health and safety and the environment by decreasing discharge of pollutants to receiving waters; and WHEREAS, creeks, rivers and streams within the jurisdiction of Lake Oswego experience bank erosion and contribute sediment and sediment-bound pollutants to downstream waters; and WHEREAS, inadequate surface and subsurface drainage planning and practice can lead to erosion and property damage and risk to life;and is therefore in the public interest;and WHEREAS, regulation of stormwater runoff is required by the Municipal Separate Storm Sewer System(MS4)permit issued to the City by the Oregon Department of Environmental Quality(DEQ) under the National Pollutant Discharge Elimination System (NPDES) program;and WHEREAS, stormwater is locally discharged to the subsurface through Underground Injection Control facilities (dry wells); pollutants in this discharge, if present, have the potential to contaminate groundwater without appropriate regulation through the Safe Drinking Water Act and OAR Division 44; and WHEREAS, surface waters within and adjacent to the City are subject to Total Maximum Daily Loads(TMDLs)that require the City to further reduce sediment discharges; and WHEREAS,other regulations of stormwater may be applied on a site-specific basis to development activities within waters of the state where compliance with the City's stormwater requirements may be presumed to fulfill ail or part of the permit requirements of the regulating agencies;and WHEREAS, illicit and non-stormwater discharges to the storm drain system can contribute a wide variety of pollutants to waterways, and the control of these discharges is necessary to protect public health and safety, and water quality; and WHEREAS, the City Council has previously found that absent effective maintenance, operation, regulation, and control, existing surface water drainage conditions in all drainage basins within the City constitute a potential hazard to the health, safety,and property of the citizens of the City (LOC 38.24.505)and for effective facility management to occur,the facility must also be protected from excess sediment; Lake Oswego Stormwater Management Manual iii WHEREAS, notice of the public hearing for consideration of this Ordinance was duly given in the manner required by law; and WHEREAS,the Planning Commission has recommended that LU 15-0050 be approved by the City Council;and WHEREAS, a public hearing on LU 15-0050 was held before the City Council of the City of Lake Oswego on February 2, 2016. The City of Lake Oswego ordains as follows: Section 1. The City Council hereby adopts the Findings and Conclusions (LU 15-0050) attached as Attachment C. Section 2. LOC Chapter 50,Articles 50.06.004,50.06.006, 50.07.003,50.007.004, and 50.10.003 are hereby amended as shown in Attachment A (new text shown in bold.liquble-underlined type; deleted text shown in strfi&*,c' . h type). Section 3. LOC Chapter 38,Articles 38.06, 38.24,and 38.26 are hereby amended; and a new Article 38.25 is as added as shown in Attachment B (new text shown in bold,-double-underlined type; deleted text shown in type). Section 4. The City adopts the Lake Oswego Stormwater Management Manual as the primary guidance document for stormwater management. Section 5. Severability, The provisions of this ordinance are severable. If any portion of this ordinance is for any reason held to be invalid, such decision shall not affect the validity of the remaining portions of this ordinance. Enacted at the regular meeting of the City Council of the City of Lake Oswego held on the 16th day of February,2016. AYES: Mayor Studebaker, Buck,Gudrnan,Gustafson,Collins,O'Neill, Manz. NOES:None. ABSTAIN: None. EXCUSED: None. /a V�J Kent Studebaker,a Mayor Dated: LTC i.4A-A.:j I , 2-0ieo Ordinance No. 2695 J Page 2of3 iv Lake Oswego Stormwater Management Manual ATTEST: + Anne-Marie Simpson, City Recorder APP OVER TO RM: David Powell, City Atto ney Ordinance No.2695 Page 3 of 3 Lake Oswego Stormwater Management Manual V vi Lake Oswego Stormwater Management Manual 0 0 0 ° 10 p T@WIlaA.(11©E@o 0.1 Abbreviations and Acronyms AASHTO -American Association of State Highway and Transportation Officials PVC - polyvinyl chloride ASTM -American Society for Testing and ROW - right of way Materials NRCS - Natural Resources Conservation Service BMP - best management practice SBUH — Santa Barbara Urban Hydrograph CFR — Code of Federal Regulations SWPPP— stormwater pollution prevention plan DEQ — Oregon Department of Environmental Quality TAPE —Technology Assessment Protocol — Washington Department of Ecology; GULD - DMA— Designated Management Agency general use level designation ESC — erosion and sediment control TIA—total pervious area FEMA— Federal Emergency Management Agency TMDL—total maximum daily load GIS —geographic information system UIC — underground injection control HDPE - high-density polyethylene US EPA— United States Environmental Protection Agency IPM — integrated pest management USGS — United States Geological Survey LID — low impact development LOC — Lake Oswego Code MS4 - municipal separate stormwater system NPDES - National Pollutant Discharge Elimination System NRCS — Natural Resource Conservation Service OAR— Oregon Administrative Rule ODOT— Oregon Department of Transportation OMP— operations and maintenance plan PIT - pilot infiltration test psi - pounds per square inch Lake Oswego Stormwater Management Manual Section 0.1 vii e- � oaco� on 1.1 Manual Purpose This manual replaces the existing Surface Water Design Workbook (Lake Oswego 2003). It contains guidance and standards to implement the City's stormwater management program and, thereby, to reduce the impacts of stormwater. Included in this manual are the minimum requirements for stormwater facilities and other best management practices (BMPs), which include low impact development (LID) techniques as well as facilities such as rain gardens, green roofs, and pervious pavement. Engineering criteria and design standards are also included. The City is required to update its 2003 workbook to meet requirements in its National Pollution Discharge Elimination System Best management practices, or (NPDES) Municipal Separate Storm Sewer System (MS4) permit, BMPs, can be structural BMPs as well as other legal requirements. In addition to addressing the such as , rain gardens, or constructteded w weetlands.They can mandated regulatory components, the City updated its manual to also be source control measures improve and protect water resources, and provide transparency to prevent pollution described in to City staff and the stormwater community about stormwater Appendix D. Source control also plan review, inspection, and maintenance processes (City of Lake includes non-structural BMPs related o 2010 . This manual is intended to be a dynamic rather than to site housekeeping and landscape Oswego ) y management. a static document. As such, it will be updated periodically to reflect the state of stormwater practice and treatment technology. Table 1.1 summarizes the requirements that apply to projects that exceed the MS4 permitted impervious area threshold for new or redeveloped areas, along with where they are addressed in this manual. MI • Ile . . _ lige .4 '- _ 4- 4k rr �^4 .4. . _ ,1 1. F, a Oswego Lake Lake Oswego Stormwater Management Manual Section 1.1 1 Introduction Table 1.1 NPDES MS4 Permit Requirements Addressed by this Manual. _ = o _ 15 0) 0 E .co - d = = V as a) di as w ❑ N =p a=) It ..' 'v — E d = a=i m LL d a=) -a 2 • ca a� c , N LL 0 0 L � CU L V) _ :r a L r MS4 Permit Requirement a 's 3 ;, = _ Ca RS ti rn 3 L p ; a' a E re > U .1 E r L = _ L a (/) ts Cr) 0 as o cn I3 I Iy I ,t I , 0 I � N E M c a) 11) +' CO +-' 1C N R L O L N 0 L C L N 'a-) ca = L ram) r.+ +a) a7 ...' ram) ram) = t 3 O — g to as co _d to •- to N (C to •— t = t = y t 7 t a) L r OL t 1C O 1a U Q v) C) CD O ❑ C) to uJ 0 2 Implement the development and redevelopment project thresholds applicable to the City: 3,000 square feet of new or redeveloped impervious area. Projects • exceeding this threshold are referred to in this manual as large projects. Onsite stormwater management: Incorporate site- specific management practices that target natural surface or pre-development hydrologic functions, and • • • • optimize onsite retention based on site conditions. Design for water quality/Design for flow control: Reduce site-specific, post-construction runoff volume, duration, and rates of discharges to the MS4 to minimize • • • • • hydrological and water quality impacts from impervious surfaces. Capture and treat 80%of the average annual runoff • volume. Reference design storm (or continuous simulation method)to demonstrate compliance with requirement to • capture and treat 80%of average annual runoff volume. Include applicable LID and green infrastructure or equivalent approaches in the Stormwater Management Manual as well as the practical uses for these • • • • approaches. Include conditions where the implementation of LID, green infrastructure, or equivalent approaches may be • • • impracticable. Include a list of BMPs including descriptions of site- specific design requirements, descriptions of design • • requirements that do not inhibit maintenance, and conditions where the BMP applies. Include pollutant removal efficiency performance goals • that maximize reduction in discharge of pollutants. City must review, approve, and verify proper implementation of post-construction site plans for new • • development and redevelopment projects. 2 Section 1.1 Lake Oswego Stormwater Management Manual Introduction =6. o .0 y co w r v c = ci as _a) CO 0 in =p a) m - 0 = ns E a EE L uCO = > a) m al O N r..r L V O d L MS4 Permit Requirement a ' y 3 _ _ E co L p a s Q 1- E > ca n - Q E o •OL L d .•. L = • g L Q co 73 co 0 . V 0.0 ai v) CO I IC I , Ig It, � I = N E M c er d IC) +' CO +••' a L w 2 +a, N .+ ya., 2 _ 2 3 O .�. a) 0.u) Q>' V Q� 0.•' Q. — 0.C as •a C W co I Al •_ cC N R CS t = t = d t 3 .0 a) t r 2 t to 0 a: Oav) 00 00 0v) w 02 Include offsite stormwater quality management options where site constraints limit onsite • • stormwater management methods. Require erosion and sediment control (ESC)plans, require prevention and control of non-stormwater L. • waste from construction, perform onsite inspections, and document enforcement procedures. Require operations and maintenance plan, and procedures to maintain facility functions. • Identify and develop strategies, priorities, and tools for preventing or reducing hydromodification impacts to water bodies affected by the MS4 • discharges. Address permit conditions related to Total Maximum Daily Load or 303(d)-Listed water bodies within • • • the City In addition to meeting MS4 permit requirements, this manual also responds to findings of the Clean Streams Plan (Otak 2009) and addresses numerous City Council Goals related to water quality improvement, LID, and water resource protection. Goals of this manual are to improve performance, aesthetics, and maintenance of stormwater facilities, and to ensure that stormwater facilities are amenities that perform well over time, protect the City's water resources, and reflect community needs and values. This manual is intended for use by developers, designers of stormwater facilities, City staff, property owners, contractors, or others responsible for designing or implementing development or redevelopment projects in the City. For simplicity, the term "applicant" is used throughout this manual to refer to the developer, designer, or other party proposing development or redevelopment that is subject to the stormwater regulations and requirements described in this manual. Lake Oswego Stormwater Management Manual Section 1.1 3 71Introduction 1.2 Organization of this Manual Chapter 0.0 lists abbreviations and definitions used within this manual. Chapter 1 includes background information on principles of stormwater management, the purpose and organization of this manual, and an overview of water quality and water quantity impacts on Lake Oswego's water bodies and conveyance system. Anyone who is interested in why stormwater management is important to the long-term viability and sustainability of the City, including how this manual addresses legal requirements, should review this chapter. Chapter 2 provides an overview of project planning and the stormwater review process, while helping applicants determine which minimum requirements apply, depending on the project type, development type, area of disturbance, and amount of new or replaced impervious surface. Chapter 2 reviews the various types of permits and project submittals that may be required before and after construction. Applicants (including developers, designers of stormwater facilities, etc.) of proposed development or redevelopment projects should review this guidance. Chapter 3 provides an overview of LID site design concepts and requirements for the site assessment and feasibility analysis. Chapter 3 is intended to help applicants select stormwater facilities approved for use in Lake Oswego, depending on site characteristics and the stormwater management objective (e.g., flow control, onsite stormwater management, or water quality). Chapter 4 describes performance standards and design criteria necessary for stormwater facilities to meet the City's stormwater requirements. Chapter 4 includes performance standards for all facility types (e.g., pretreatment, onsite stormwater management, water quality, and flow control), plant selection and design guidance, methods of analysis, soil analysis and salvaging information, and standards and specifications for approved facilities. Chapter 5 provides requirements for conveyance facilities, including general provisions related to stormwater conveyance facilities, and technical design criteria for storm sewers and hydraulic structures. Chapter 6 provides information on recommended and required construction practices required to ensure that receiving waters are protected and that stormwater BMPs function as designed. Chapter 7 describes BMP inspection and maintenance requirements to ensure effective performance and operation over time. Chapter 8 includes a list of references cited in this manual. 4 Section 1.2 Lake Oswego Stormwater Management Manual Introduction 1.3 Why Stormwater Matters Stormwater is the water that originates from rainfall and snowmelt. In urban areas, precipitation lands on impervious surfaces (that do not allow water to infiltrate into the ground) such as roads, parking lots, and rooftops. When rain or snow lands on an impervious surface, it cannot enter the soil through infiltration, so it flows overland as surface runoff. Pervious surfaces, such as natural areas and lawns, allow water to infiltrate into the soil. All surface runoff in Lake Oswego eventually finds its way to the Willamette River, Tualatin River, or Oswego Lake. As a steward of these resources, the City can play an important role in protecting them and the human users and animal species that rely on them. 7 \ to Portland 9 > q <O r , A 1 i'-- . . an a., ,,, - ,' Oak Creek �i / ; r O z/, l \ -OliS �Z k' C L all CK<k I g F cst Hdls l LOJHS � ., �y Clvb-�P W11 Krvs<wa , ,/ i' A prenue )ur Lady Carter Creek % e\J The Lake wd 6a`I , CIC M __ `' .. Lak y�,_, 0 /v Lake*eve SQ,m9br \ 0c90 ia °tofLake �a a Y�,..e Oswego ; ��<� 4.41 � I Halh�a� OREGON '- Si�u / 0 e j 0 S.th Shore Blvd St j d s € a a'' LEGEND m g �4 \ - o''u� 0s Lakenelye HS / �\ 9 \i School -'�� I Ra �� Road /- �.�� Laker�dye JHS. We.tridg -� i 4S 1 ��Rtver,Strew.., / / Creek ! /' j v R v<r 4rorti N WdlaMctte River ---- Watershed I - \ Code Oswego Lake �- ---\\ �ll Rye 4 Vr -\/ % Watershediiiii Taala{iry I iv . --\ Vl ---'' Tvalatie River i 1 - - Watershed ..-L....."."'" l ' to West Linn ■ For more information,go to:www.ci.oswego.or.us/engineer/environ/surface.htm Each drainage basin is a complex system of topography, vegetation, and hydrology. Water flows through the drainage basins via a network of interconnected ditches, streams, and storm drains and eventually finds its way to the Willamette River and Tualatin River. The surface water management utility includes all natural and man-made facilities used to manage the quantity and quality of surface water, including drainage easements, culverts, storm drains, ditches, catch basins, and other stormwater BMPs. The City of Lake Oswego is different from many jurisdictions in that the surface water utility also includes natural systems such as stream corridors, rivers, ponds, wetlands, and impoundments. The City is responsible for managing and protecting its surface water management system through the implementation of the City's MS4 permit, Total Maximum Daily Load (TMDL) implementation activities, and other natural resource programs. Lake Oswego Stormwater Management Manual Section 1.3 5 Introduction 1.3.1 Water Quantity Impacts from Development on Receiving Waters and Conveyance System The conversion of pervious surfaces to impervious surfaces through urbanization increases the volume of surface runoff, increases the peak flow rate during or after a precipitation event, and reduces the time between the onset of rain and the peak flows leading to potential flooding. Development also decreases groundwater recharge and base flow. Impacts on Conveyance System and City Residents and Development can affect the discharge of stormwater runoff as Property Owners follows: Flooding - • • • Changing the timing at which the Some areas of Lake Oswego have peak flow occurs, usually earlier no storm drainage facilities, and in the storm other parts of the city have pipe • Increasing the peak flow rate systems that are now undersized, VD b or are open drainage ditches. `'% • Increasing durations(the time During periods of heavy rainfall period during which a receiving on areas with a large amount of T r. Awl water experiences elevated flood ` flows) impervious surface, local flooding may occur, along with increased • Increasing the runoff volume associated risks to health, public Flooding in Lake Osw associated with a storm event. safety, and property. • Reducing base flow discharge between storms. Cost and Feasibility Increasing the size of pipes is often infeasible due to conflicts with other utilities and the need to maintain gravity flow to receiving waters. Even where it is feasible, constructing larger pipes is a costly solution, requiring removal and restoration of the overlying pavement or other surface. Moving Problems Downstream Increasing pipe sizes or constructing regional detention facilities may alleviate flooding concerns in the immediate vicinity of these improvements, but these temporary fixes can ultimately exacerbate downstream problems if, for example, regional detention facilities delay the peak flow so that it coincides with peak flows downstream. Local Applicability and Solutions: Stormwater Planning for the Future This manual prioritizes BMPs that promote infiltration of stormwater wherever site conditions allow. This reduces the volume and rate of stormwater discharged from a site, and has the additional benefit of supporting base flows in streams. Minimum requirements have been established to meet the legal requirements under the MS4 permit and to avoid exacerbating the City's flooding problems as new development and redevelopment occur. The City is planning to update its 1992 Surface Water Master Plan to show "built out" conditions, based on current and projected development densities over the next 20 years. Some of the undersized pipes and underserved areas will be addressed as part of this long-term planning effort. The City will be evaluating the need and feasibility of increasing, replacing, and constructing new stormwater management facilities as part of its long-term planning efforts under the stormwater management plan. Because the cost of such improvements must be borne by ratepayers, however, the City is promoting LID practices described in this manual to work with existing infrastructure. 6 Section 1.3 Lake Oswego Stormwater Management Manual Introduction Impacts on Channel Processes and Relevancy to City Residents and Property Owners Streams and other receiving waters are affected in many complex ways as the timing, volume, and peak flows of stormwater change due to development. As watersheds develop, soils are compacted and more surfaces are converted from native vegetation to impervious surfaces. This makes the hydrology become "flashier," with a greater percentage of rainfall converted immediately to surface runoff instead of contributing to evapotranspiration or infiltrating into the soil and recharging groundwater. Pipes and ditches also shorten the time between when rainfall begins and when streams begin to rise. Some streams can experience elevated flow rates for a longer duration. Base flows in streams, which are sustained by groundwater and are critical to fish and other aquatic species during the summer, are commonly reduced. More complex and less intuitive is the relationship between streamflow and sediment in streams. Most of the City's streams are considered alluvial (composed of channel and bank sediment that is mobilized and deposited by channel processes rather than hillslope processes). In an alluvial channel, the channel will respond to changes in flow and sediment supply by adjusting —the channel may become steeper, wider, and/or deeper depending on the condition of the stream banks and streambed. Alternatively, or additionally, higher sediment loads or larger particles may be deposited in the channel (Lane 1955; Leopold and Maddock 1953). The effects on stream channels resulting from changes in streamflow characteristics due to stormwater discharges are referred to as hydromodification. Hydromodification affects residents, property owners, and business managers in the City in the following ways: • Property damage: Properties adjacent to stream channels can experience property damage, loss of trees, and other impacts as stream channels erode or change alignment or geometry due to stormwater impacts. • Flooding: Increased peak flows can result in an increased frequency of bank overtopping. If culverts or bridges are undersized or blocked with debris, properties or roads in the vicinity are more likely to flood during storm events. • Increased cost to ratepayers: As noted in Table 1.1, the City's surface water management utility is legally required to address hydromodification under its MS4 permit. As hydromodification impacts associated with stormwater discharges get worse, the problems in receiving channels get more challenging and costly to fix. • Increased erosion and sedimentation: These can create • substrate conditions that are inhospitable to the incubation and survival of salmonids or the insects upon which they feed. Bank erosion and sediment deposition can smother fish spawning and rearing habitats. • Reduced habitat: Over-bank vegetation, large instream wood, and other important habitat-forming features, which are present in natural channels and are important for stream ecological health, are lost. Photo from ABR, Inc. Cutthroat fry from Springbrook Creek. Local Applicability and Solutions: Prioritizing Infiltration This manual prioritizes LID BMPs that promote infiltration. While this will not undo the hydromodification that has already occurred, reducing the volume of surface runoff by promoting infiltration and reducing impervious surfaces will help reduce further impacts. Lake Oswego Stormwater Management Manual Section 1.3 7 HIntroduction Expected Climate Change Patterns Climate change research has projected trends that will have potential consequences to stormwater runoff, climate patterns, vegetation, and the ability to adapt to these elements. Listed below are the key points of climate change impacts from current research (Oregon Climate Change Research Institute 2010; Dalton et. al. 2013): • An increase in flooding and overall rainfall volume and intensity is predicted, including greater frequency in 25-year storm events. • Annual rainfall is expected to remain similar to the current baseline, but will be more concentrated between mid-October and late April. While the Pacific Northwest is forecasted to see greater fluctuation in temperature and precipitation, hotter, drier summers and warmer winters are predicted. • As precipitation and weather patterns shift, spatial distribution of plant species will change with some plant communities becoming more vulnerable and fragile to these fluctuations. Other vegetation types may thrive, favoring the new conditions and becoming more robust. Communities such as hemlock and cedar forests will become susceptible to the these changes whereas oak savannas are expected to fair better. • Plant selection will become more challenging as availability and resiliency is less predictable. Consequently, inconsistent precipitation and temperature variation may create the need for stormwater facility irrigation in order to support vegetation within those facilities. • Rainfall intensity and volume will test the ability and capacity of facilities to respond to peak runoffs. Adaptations to these storm events may include: Developing new storage and retention structures, and changing sizing factors for pipes and some stormwater facilities. • Lastly, maintenance and restoration of instream flows should encompass riparian protection, increased shade protection, and overall resiliency measures that will adapt to changing climate patterns. Lake Oswego Stormwater Management Manual Section 1.3 8 Introduction 1.3.2 Water Quality Impacts from Development on Beneficial uses include the following for the Willamette River and its Receiving Waters tributaries: As stormwater flows over roads, roofs, and lawns, it collects and (OAR 340-041-0340) carries pollutants that have been deposited on or leached from those • Domestic water supply surfaces, including sediment, excess nutrients, pesticides, metals, • Fishing bacteria, and fuel and oils. Recent watershed health indexing by the City indicates that these pollutants are reaching receiving waters. • Industrial water supply The receiving waters are showing signs of increased urbanization • Boating (e.g. sediment-tolerant aquatic insects and macroinvertebrates, bank Irrigation erosion, bed scour, and water quality degradation). • • Water contact recreation In addition to the health, recreational, aesthetic, and environmental concerns related to water quality, discharge of pollutants from • Livestock watering stormwater can result in violation of state and federal water quality • Aesthetic quality standards established by the Oregon Department of Environmental • Fish and aquatic life Quality (DEQ). These water quality standards have been established to protect the beneficial uses of water bodies. Beneficial uses are • Hydropower legally assigned by basin, and they are intended to establish goals • Wildlife and hunting and requirements for that water body, to make sure its water quality Commercial navigation and is supportive of the most sensitive beneficial use (See box at left). • transportation 1.3.3 TMDL Approved Areas/303(d)-Listed Receiving Waters The federal Clean Water Act requires the State of Oregon to develop a list of impaired or threatened waters within the state. To meet this mandate, DEQ establishes water quality standards and prepares a list of impaired waters, known as the 303(d) list. Waters are added to the 303(d) list if they do not meet the water quality standards set by DEQ. To address water quality issues in 303(d)-listed waters, DEQ must establish TMDLs for identified contaminants. A TMDL is the total amount of a contaminant a water body can accept without violating the water quality standard. Implementation plans are prepared to meet the TMDL goals and are implemented by Designated Management Agencies. The City of Lake Oswego is a designated management agency for reaches of the Tualatin River, Springbrook Creek, Tryon Creek, and the Willamette River within the City's jurisdiction. The City is also the designated management agency for Oswego Lake. TMDL water quality management plans require the City to work toward reduction in concentration of contaminants of concern, so stormwater facilities that discharge to those water bodies should be directed toward reducing the TMDL contaminants (OAR 340-41-0340). As water quality management plans are revised and further developed, the City may require additional treatment to ensure that wasteload allocations approved for each water body are not exceeded. The 303(d) list is also revised biannually, with the potential to identify additional pollutants in receiving waters. The City's MS4 permit requires assessment of how effective BMPs are at removing all pollutants that are known to impair receiving water bodies. Table 1.2 summarizes approved TMDLs and 303(d)-listed contaminants along with affected water bodies. Phosphorus, bacteria, and dissolved oxygen are most frequently associated with stormwater in Lake Oswego. The water bodies in Table 1.2 are within the City of Lake Oswego's jurisdiction and are listed as impaired on the 303(d) list or have an approved TMDL in place. The impaired watersheds account for nearly all of the City of Lake Oswego's area. For that reason, rather than adopt basin-by-basin approaches to targeting pollutants of concern, the City is focusing on identifying stormwater facilities that are effective at targeting these pollutants and applying their use throughout the City. 9 Section 1.3 Lake Oswego Stormwater Management Manual Introductio1 Table 1.2 Impaired Water Quality in Receiving Waters of the City of Lake Oswego. Waterbody v N o O a) > d _1L 2 Y i �' = 2 O O U CD a V C E ' 01 07 O = L c ++ (1) 7 Parameter 3 3 c L o —_ > . 00 Ui co H I- 1- cM 1- M 1- c`M 1- M 1- c.,) 1- M Aquatic Weeds or Algae • • • Biological Biological Criteria • • I • Chlorophyll a • Ammonia • General Dissolved Oxygen • •' •z • • • Chemistry Phosphorus • • • • • Arsenic • Copper • • I • Iron • • I • Metals Lead • • • Mercury • • • • • • Thallium • Zinc • • E.Coli • • Microbial Fecal Coliform • Dioxin • Hexachlorobenzene • Other Organics PAHs • PCBs • Tetrachloroethylene • Aldrin • Chlordane • Pesticides Cyanide • DDT/DDE • Diedrin • • pH • Physical Temperature • • • 1 Year round nonspawning RM 0 to 11.0. 2 Spawning 0 to 13.9 Information from Water Quality Assessment Database(DEQ 2012). Lake Oswego Stormwater Management Manual Section 1.3 10 Introduction Local Applicability and Solutions: Maintaining Facility Function for Long-Term Water Quality Benefits The BMPs included in this manual have been selected based on their effectiveness at removing important pollutants from stormwater. The City is reviewing research funded by the Washington Stormwater Center, the Oregon Department of Transportation (ODOT), the Washington State Department of Ecology, and other agencies to ensure that BMPs and design standards reflect the best performing options. This manual includes maintenance schedules and guidance and source control requirements. Two of the best ways that Lake Oswego residents and property owners can help protect the water quality of the City's receiving waters and keep them safe for recreation, fish, and other beneficial uses are: 1) Maintaining stormwater facilities to make sure that they keep functioning well and 2)Adhering to the source control requirements in Appendix D of this manual, to prevent pollutants from entering the storm system, which eventually discharges to streams or Oswego Lake. 1.4 General Principles of Stormwater Management Ideally, stormwater runoff should be managed at its source by minimizing effective impervious surface area and by using vegetation, amended soils, and infiltration practices to minimize the I amount of stormwater runoff that must be managed off site. Effective 1101 gm source control measures that prevent pollutants from coming into contact with stormwater are also important for protecting receiving tii ,! waters. Once stormwater runoff leaves a site, it is much more costly - }: :=kg and less effective to transport, treat, and control. Low Impact Development (LID): uses site design practices to reduce the amount of stormwater to be managed off site. These practices ar was its are use to prevent include reducing impervious surface area and preserving vegetation contaminated water that results and soil characteristics that promote infiltration and biological from washing our cars from entering treatment processes, among others. LID can also utilize site-scale a storm drain. BMPs that are integrated into the landscape and that receive stormwater from small tributary areas, rather than centralized facilities accepting stormwater from large areas. LID can be highly effective at reducing the impacts of development on streams, lakes, and other receiving waters. The general principles of LID are described in Chapter 3 and include: 1. Understand the site 2. Reduce runoff through design 3. Reduce pollutants carried by runoff 4. Capture and treat runoff 11 Section 1.4 Lake Oswego Stormwater Management Manual Introduction Examples of LID techniques Green roof i '4 'I r..1 -•\--WW; -ter• .11, • Pervious pavement - • cC:.a Cistern to collect and Linear rain gardens Riparian buffer vo reuse roof runoff - - General principles of LID adapted from Center for Watershed Protection's"Managing Stormwater in Your Community:A Guide for Building an Effective Post-Construction Program"(Center for Watershed Protection 2011). See http://www.cwp.org/online-watershed-library/cat_view/65-tools/129-post-construction-guidance-manual-8-tools 1.4.1 Structural Storm water Best Management Practices Structural stormwater BMPs are constructed facilities such as swales, ponds, or rain gardens that are designed to manage stormwater. LID stormwater management techniques capture, filter, store, evaporate, and infiltrate runoff near its source. LID flow control BMPs are typically small facilities, integrated into LID BMPs should receive runoff from the landscape, that receive stormwater from small tributary areas, no more than 6,000 square feet of rather than centralized facilities accepting stormwater from large impervious surface area (see Tables areas. They control flow through infiltration and/or dispersion. LID 4.2-4.4). Larger scale infiltration facilities may be appropriate for flow control BMPs are highly effective at reducing the impacts of some parcels or projects, but the development on streams, lakes, and other receiving waters. City prioritizes small, dispersed stormwater management facilities as Flow control BMPs include detention ponds, tanks, or vaults that the best way to manage runoff and temporarily store stormwater and then release it at rates to match meet City requirements. predevelopment flow rates and/or durations. Flow control BMPs attempt to reduce the flooding and hydromodification impacts of development. 1.4.2 Non-Structural Stormwater Best Management Practices Non-structural BMPs are practices (rather than facilities) designed to prevent pollution. They include erosion and sediment control measures, such as construction phasing, minimizing land disturbance, and stabilizing the site (see Chapter 6). Maintenance practices (Chapter 7) and source control measures described in Appendix D are also examples of important non-structural BMPs that are required to protect Lake Oswego's water bodies. Additional non-structural BMPs include education and outreach, illicit discharge detection and elimination, public involvement, and adaptive management. The City's website (raintoriver.org) provides more information on these ongoing stormwater programs. Lake Oswego Stormwater Management Manual Section 1.4 12 'Introduction 1.4.3 Conveyance Conveyance facilities include inlets, catch basins, manholes, storm drains (pipes), and ditches used to safely convey, or transport, Ditches are typically V-shaped stormwater to a facility or an outfall to a stream or lake. The depressions that transport water. Tconveyance system is designed to convey a certain amount of quicklyu are and designed o convey water are not acceptable for stormwater, referred to as its capacity. water quality treatment, even if vegetated, unless they meet the Chapter 5 provides design information for conveyance facilities swale design criteria. including size, location, structures, and materials. 1.5 How Local Conditions Affect Stormwater Management Lake Oswego has many areas with seasonally high groundwater and shallow bedrock, both of which limit opportunities for LID. Many areas of the City also have steep slopes, which limit stormwater management options. Chapter 3 has site suitability criteria for stormwater BMPs, including tools for identifying areas that are appropriate for LID. This chapter also outlines the analyses that may be required for downstream infrastructure or receiving waters. ""::-,,- ::::%,::- „loop 0.„, . __. :.,......; . . a..- -, _ ---'` r'.�..,.. -.. , , ..,,,,„ Tom' __. _ . _ ,� .� WIMP , . ,.. . _. _ ,. .. ,4„.. . ._„ . _. _.„ _... k ) f _......„:„,,A,. , 1„ .... ..,. _ .... ._ i, ; . . , l . i - x :. ince ans uses in t e ity are most y resi.entia, retro it-type :V 's t at are appropriate at the small site scale are and will be a priority. .'fir The City of Lake Oswego is addressing stormwater runoff at different scales t1 of land use, including integrating stormwater management practices into those different scales to the maximum extent practicable. At the regional scale,this ommercia .evelopment in Lake Oswego tends to include landscaping and community amenities. Well selected means working within the City and neighboring jurisdictions to preserve open • space and critical ecological features, encourage development in areas ripe for redevelopment, and using land efficiently.At the neighborhood scale,this and designed stormwater BMPs can means applying green street principles where practicable, minimizing impervious enhance the landscaping of a commercial surfaces, and making stormwater features public and private amenities. site in addition to providing an important function. 13 Section 1.5 Lake Oswego Stormwater Management Manual Introduction 1.6 Updates This manual was developed between 2013 and 2016. It reflects the current regulatory requirements and state of stormwater science and technology. Stormwater management is a dynamic and emerging science. There has been a surge of research related to stormwater in the past decade, particularly in the following areas: • Compost and amended soil mixes that can improve BMP pollutant removal performance, especially for metals and nutrients like phosphorus (current guidance is in Chapter 4, but as more information becomes available, updates will be provided via www.raintoriver.org.) • BMP sizing and analysis tools to streamline sizing of BMPs. Current BMP selection and sizing guidance is described in Chapters 3 and 4. As the City addresses its hydromodification requirements in the MS4 permit, new sizing tools will likely be developed. Check www.raintoriver.org for current BMP sizing requirements. • Proprietary BMPs that have been tested and approved under the Technology Assessment Protocol — Ecology (TAPE) program, run by the Washington State Department of Ecology, are regularly updated on the TAPE program website: http://www.ecy.wa.gov/programs/wq/stormwater/newtech/index.html. Proprietary BMPs are only allowed for use in Lake Oswego with prior approval from the City, and BMPs must be selected that have been approved for general use under the TAPE program. See Section 3.4.4 for more information. • The City is evaluating fee-in-lieu, incentive, and credit programs, and other ways to add flexibility to its stormwater management program and comply with MS4 permit requirements. These programs are not yet in place. Updates will be provided on the City's website (raintoriver.org) should they be adopted. Lake Oswego Stormwater Management Manual Section 1.6 14 Introduction This page is intentionally left blank. 15 Lake Oswego Stormwater Management Manual pc;m id& EH ror !glib mwgElb7 2 i)Tcic;@[R p EIToro.a, __41imimilii-i_and, gia@E[R J a o 7cgimc;U 2.1 Project Process Requirements for stormwater management planning and permitting vary depending on the size of the project and the type of development proposed. This chapter describes: activities that trigger stormwater and permit requirements, the City's project classification categories and associated minimum project requirements, development types, and permits. Various City departments may be involved in the planning, design, construction, and evaluation of development activities. This chapter also describes departmental responsibilities and provides step-by-step guidance on how to move through the planning and permitting processes. Figure 2.1 City Project Planning, Design, and Permitting Processes. Project Beginning: Design: Inspect and Maintain Conduct Site Assessment& Begin project design Facility Feasibility Analysis - — Planning: Ongoing Assess all permit and design Construction: Operations and requirements Construct Project Maintenance.... a N A 000000r► o fa s 4L a a Consultation with Consultation with Consultation with Engineering Engineering Engineering and Engineering and Engineering Staff Staff Review Staff Planning Staff Planning Staff Inspection PROJECT TIMELINE tn •As-built s drawings • One-year r • Drainage Report inspection ._ •Preliminary Drainage Report . Construction Plan and•Copy of • Annual MS4 �1 • •Draft O&M Plan . O&M Plan O&M plan report -CI3 filed with maintenance vt records deed cStormwater Management Requirements /N /1 0 0 E �,• Erosion Control Permit •a E NPDES 1200-C Permit l- a) ai Ix . Building Permit R C W.• f° Land Use Permit (, aCU Street Opening Permit v 0 =Project Stage When Permit or Event Commonly Occur Figure 2.1 provides an overview of the planning and design process for a project in Lake Oswego. While Figure 2.2 demonstrates a step-by-step approach to planning a project. Lake Oswego Stormwater Management Manual Section 2.1 16 Project Planning, Permits, and Stormwater Requirements Figure 2.2 Project Planning Steps. Planning a Project? Step 1 Review minimum project requirements table and description (Chapter 2) to determine whether the project is small or large, and which requirements apply. Step 2 Review Low Impact Development design practices and complete site assessment and feasibility analysis for the site (Chapter 3). Step 3 Consult with City staff- visit Planning and Engineering departments (Section 2.9). Bring the site assessment and feasibility analysis. Step 4 Plan the project to reduce runoff through design and reduce pollutants carried by runoff(LID principles #2 and #3 in Chapter 3). Step 5 Select onsite, water quality/infiltration, and flow control BMPs that are applicable for the site (Chapter 3). Step 6 Design stormwater management BMPs (Chapter 4) and complete submittals neccessary (Appendix A). 17 Section 2.1 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements Planning a Project? Step 7 Make sure you have all permits and submittals BEFORE hiring a qualified contractor. See Chapter 2 for an overview of permits and Appendix A for submittals. Step 8 Construct your project. If stormwater facilities are professionally designed, have design professional inspect and certify their construction. See Chapter 6 for construction BMPs. Step 9 Request inspection by City staff, complete as-built plans, and pay maintenance and landscape financial securities as appropriate. (Chapter 2) Step 10 Maintain stormwater BMPs (Chapter 7). Employ pollution prevention techniques to prolong the life of stormwater BMPS (Chapter 6). Lake Oswego Stormwater Management Manual Section 2.1 18 Project Planning, Permits, and Stormwater Requirements 2.2 Activities that Trigger Stormwater or Permit Requirements The requirements and guidance provided in this manual apply to a variety of activities. Some of the most common activities include: • Grading or earthwork, drainage, and erosion control activities, whether or not a permit is required • Ground-disturbing activities, whether or not a permit is required • Discharges directly or indirectly to a public storm drainage system • Direct or indirect discharges into receiving waters within or contiguous to city limits • Development • Creation or replacement of impervious surface areas • Repair of large areas of ground-level impervious surface, if there are substantial changes to offsite drainage. • Phased projects that occur over a 3-year period on a single site and that exceed the thresholds in Table 2.1 are considered a single development. They must meet applicable requirements from this manual based on cumulative areas of land disturbance and/or new or replaced impervious surface, as appropriate. 2.3 General Requirements For All Residents, Property Owners, and Business Owners The activities noted above (Section 2.2) include even minor activities • - undertaken by manypeople who work or live in Lake Oswego. Some p p g activities, such as illicit discharge or rerouting stormwater to a neighbor's h -t,;4� property, can cause significant damage even though there may be _ ° no associated increase in impervious surface area. Regardless of classification or development type, every project must prevent illicit • '""� discharge and avoid creating or exacerbating offsite drainage problems. - 2.3. 1 Prevent Illicit Discharges All residents, property owners, and business owners in Lake Oswego must prevent illicit discharges in accordance with the Stormwater Utility Soap leaving a driveway and provision of Lake Oswego Code (LOC) 38.24.006 and must implement running toward storm drain. source control BMPs (Appendix D) to avoid such discharges. Illicit discharges include pollutants resulting from a spill or deliberate dumping. Illicit or prohibited discharges are discharges that: a. Cause or contribute to a violation of the City's MS4 permit, or b. Cause or contribute to a violation of a waste load allocation To voluntarily reduce stormwater contained in a TMDL approved by the US Environmental impacts and help improve the health and quality of Lake Protection Agency (US EPA), or Oswego's water bodies, stay informed by checking www. c. Cause or contribute to a violation of a city, state, or federal law raintoriver.org regularly. or regulation, or 19 Section 2.3 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements d. Cause or contribute to the endangerment of public health, safety, or welfare; the environment; or public or private property. Y 'k'"t�',i Lori►' �!_% r _w...r. - ._._..__ -._014, Y ' . + �. - 4 .2 Commercial car washing causing an n i ici isc arge rom cons ruc ion. illicit discharge. Si and grease illicit disc arge. Illicit discharges such as those shown in these pictures may be subject to fines or other penalties. 2.3.2 Maintain Existing Drainage Patterns Oregon has adopted the civil law doctrine of drainage. Adjoining landowners are entitled to have the normal course of natural drainage maintained. The downhill or downstream owner must accept water that naturally comes to their land from above, so long as the amount and pattern of drainage does not change significantly. Similarly, the downhill landowner may not obstruct the runoff from the upper property if the uphill landowner is properly discharging the water. Because development within Lake Oswego occurs as infill or as redevelopment, protection of adjacent properties from Oregon drainage law,which adverse drainage must be prioritized during design. originates from common law or For the uphill property owner, "properly discharging the water" means case law, has developed without legislative action, and it is embodied keeping the location of the discharge the same, avoiding substantial in court decisions.Therefore,there increases in the acceleration and concentration of stormwater, and are no Oregon Revised Statues to preserving areas where water infiltrates, ponds, and/or evaporates cite pertaining to Oregon drainage rather than flows onto adjacent property. law. For more information on this topic, consult Section 2.2 of The City's updated stormwater code (LOC 38.24) contains additional the current version of the ODOT information on rights and responsibilities related to stormwater Hydraulics Manual (ODOT 2014). management. Enforcement and penalties for violations of stormwater ' requirements are summarized in LOC 38.24.010. 2.4 Project Classifications and Development Types For the purposes of determining stormwater management requirements, Lake Oswego categorizes projects based on effective impervious surface area created or replaced (small and large) and the type of development. To minimize the stormwater management requirements that apply to a particular project, the City of Lake Oswego encourages applicants to reduce impervious surface area. Before determining the project classification, applicants should complete the impervious surface area reduction table in Appendix A. If it is infeasible to manage stormwater at the location of the new or replaced impervious surface that triggers requirements, stormwater from an equivalent amount of existing impervious surface may be managed. Lake Oswego Stormwater Management Manual Section 2.4 20 Project Planning, Permits, and Stormwater Requirements 2.4.1 Small Projects Small projects are defined as new development projects that create greater than or equal to 1,000 square feet and less than 3,000 square feet of potential impervious area. 2.4.2 Large Projects Large projects are defined as new or redevelopment projects that create or replace greater than or equal to 3,000 square feet of potential impervious surface area. 2.4.3 Development Types Additional permit and review requirements depend on the development type. For the purpose of this manual, the types of development discussed include: • Single family residence (new construction or remodeling) • Partitions • New commercial • Multi-family • Subdivision • Other, including landscaping that incorporates impervious hardscape. This manual provides a brief overview of the development process only. It is not intended to provide an in-depth discussion of planning requirements. For information on additional permit and review requirements according to development type, see Section 2.10. 2.5 Overview of Minimum Requirements — Design Requirements Minimum project requirements are summarized in Table 2.1. The table summarizes the design requirements, potential permits, and submittals for small and large projects. Project requirements are organized sequentially according to a typical project process. Each requirement is described in further detail in this chapter. 2.5.1 Perform Site Assessment and Feasibility Analysis A site assessment and feasibility analysis is required for all project classifications. The goal of this analysis is to incorporate stormwater management into the landscape in a way that will preserve onsite drainage, soils, and native vegetation.Another important goal of the site assessment is to identify suitable locations for stormwater facilities before design begins on a project. The applicant will document soils, seasonally high groundwater elevations, existing topography, current hydrologic conditions and natural features, onsite vegetation, land use and zoning, and existing utilities. Completing the site assessment and feasibility analysis involves printing out a site map from LOMap (an interactive map website with geographic information system (GIS) data for these features), conducting a site visit, and gathering additional information as available. Section 3.1 and the drainage report template in Appendix A have more guidance on performing the site assessment and feasibility analysis. 21 Section 2.5 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements 2.5.2 Onsite Stormwater Management As applicants perform the site The goal of onsite stormwater management is to capture, treat, and assessment and feasibility infiltrate stormwater to mimic natural hydrologic conditions. Manyof analysis,they should keep in mind these LID principles: the City's storm drains are undersized. There are also many areas that lack stormwater infrastructure. The City has been addressing 1. Understand the site these problems by requiring onsite stormwater management that 2. Reduce runoff through infiltrates stormwater where feasible through use of rain gardens, design infiltration trenches, or drywells. This requirement complies with the 3. Reduce pollutants carried by MS4 permit requirement to "incorporate site-specific management runoff practices that target natural surface or predevelopment hydrologic functions as much as practicable. The site-specific management 4. Capture and treat runoff practices should optimize onsite retention based on the site These principles are discussed in conditions." greater detail in Chapter 3. See Chapter 4 for design criteria and options to meet onsite stormwater management requirements. Feasibility of infiltration, or Some projects may be able to meet onsite stormwater requirements lack of such feasibility, shall through sheet flow dispersion. See Section 4.6.14 for details. be determined by pre-existing site conditions such as soil permeability,topography, depth 2.5.3 Design for Water Quality to groundwater, or landslide susceptibility.A lack of available The City's MS4 permit requires that City- and development-related surface area on the parcel activities "capture and treat" 80 percent of the average annual runoff. following the placement of LID and onsite stormwater management practices must be given high the desired structure(s)is not priority. grounds alone for determining that infiltration-based stormwater management is not feasible. New development must implement water quality facilities that reduce phosphorus by 65 percent or to the maximum extent practicable. Phosphorus removal efficiency shall be Water quality design standard: calculated as summarized in the worksheet Capture and treat 80 percent of the average annual included with the drainage report template in runoff (1.0 inch in 24 hours) Appendix A. Because LID facilities, such as rain gardens, effectively capture stormwater in addition to treating it, the City prioritizes the use of the following facilities for water quality treatment. • Rain garden • Planter • Sheet flow dispersion Lake Oswego Stormwater Management Manual Section 2.5 22 Project Planning, Permits, and Stormwater Requirements - Table 2.1 Minimum Project Requirements Project Classification Small Project Large Project Minimum Project New development projects that New or redevelopment projects Requirements create that create or replace >1,000 sq. ft. and<3,000 sq. ft. >_3,000 sq. ft. impervious surface impervious area Design Requirements Perform Site Assessment and Feasibility Analysis ✓ ✓ Applicant to perform site assessment and feasibility analysis. Onsite Stormwater Management Onsite stormwater management requirement shall apply. Stormwater facilities shall be sized to infiltrate the 10-year 24-hour storm event, to the maximum extent practicable. Design for Water Quality Stormwater facilities shall be designed to capture and treat 80% ✓ of the average annual offsite runoff volume, based on local rainfall frequency and intensity. Design for Flow Control Peak flow rates of offsite runonff shall be maintained at their ✓ predevelopment levels for the 2-year, 5-year, and 10-year, 24-hour runoff events. 23 Section 2.5 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements Project Classification Small Project Large Project New development projects New or redevelopment Minimum Project Requirements that create projects that create or replace >_1,000 sq. ft. and<3,000 sq. ft. >_3,000 sq. ft. impervious impervious area surface System Designed by Licensed Professional Design professional (engineer or landscape architect)will be required v/* ✓ for facility design and post-construction certification. See Section 2.5.5 for detailed requirements by stormwater facility type and project size. Underground Injection Control (UIC) Review with DEQ Drywell design and construction shall be reviewed by an engineer registered in the State of Oregon. UlCs must either be automatically rule authorized or registered with DEQ. V V UIC permits from DEQ may be required. See OAR-340-44. Drywells shall conform to Oregon Plumbing Code. Permits City of Lake Oswego Erosion Control Permit Erosion control permit shall be required when activity disturbs >_500 sq. ft. OR ✓ ✓ when any part of a disturbed area is within 50 feet of a pond, lake, river, stream corridor, canal, or wetland. May include landscaping projects. *Requirement may be waived by the City. Lake Oswego Stormwater Management Manual Section 2.5 24 Project Planning, Permits, and Stormwater Requirements Project Classification Small Project Large Project New development projects New or redevelopment Minimum Project Requirements that create projects that create or replace >_1,000 sq. ft. and<3,000 sq. ft. >_3,000 sq. ft. impervious impervious area surface NPDES 1200-C Permit 1200-C permit from DEQ shall be required*for projects that disturb 1 acre Consider for all in-water work. or more. Street Opening Permit A street opening permit may be needed (e.g., driveway approaches, work in right-of-way, work in public easements, etc.). Submittals Drainage Report Required See Section 2.7.1 and AppendixA [LOC 38.25.120] for Drainage Report V �/ Requirements. Includes Downstream Analysis. Recorded Operations and Maintenance Plan and Covenant For projects that create or replace >_1,000 sq. ft. of impervious area, operations and maintenance plans shall be prepared by a registered professional (licensed landscape architect or a licensed professional engineer) and recorded as a covenant on the deed V ✓ in the county in which the property/ facility is located prior to Certification of Occupancy. Stormwater facilities shall have a recorded maintenance covenant against the property prior to issuance of a Certificate of Occupancy. See Section 2.7.2. Refer to Chapter 7 for small project maintenance requirements. Landslide Hazard and Erosion Risk Areas Report Required where development is to occur on a potential severe erosion or landslide hazard area. 25 Section 2.5 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements Where sites are not suitable for infiltration, water quality treatment facilities that could be implemented include: • Flow-through rain gardens • Constructed wetland • Filter strip • Flow-through planters • Wet pond • Swale • Sand filter See Chapter 4 for stormwater facility selection guidance, and design criteria and guidelines. 2.5.4 Design for Flow Control Flow control is required for large projects where soils are not suitable for infiltration. Flow control is not required for projects that discharge directly to the Willamette River, the Tualatin River, or Oswego Lake. Flow control design standard: aintain peak flow rates at their predevelopment levels for the 2-year, 5-year, and 10-year, 24-ho runoff events. Section 4.1 has information on design storms, and Chapter 5 describes approved methods for calculations. 2.5.5 System Designed by Licensed Professional .10 Because Lake Oswego is largely built out, only a limited amount of • land is available for surface stormwater facilities such as constructed , titt wetlands and ponds. Since such facilities require dedicated land area -,a..,. - and will be placed within existing neighborhoods, it is essential that they �_,;- •- fit in within the community context, reflect local aesthetics and character, and create a public amenity in addition to meeting water quality and flow control requirements. The City requires stormwater facilities, such as constructed wetlands, ponds, or rain gardens, to be designed by a licensed professional or team of licensed professionals that understand how to incorporate the facility into the community context, ' improve aesthetics, and ensure water quality and flow control function. .- However, rain gardens for small projects do not need to be designed by - a professional unless there is a potential for offsite impacts to adjacent ormwa era in e an.scape properties. helps express local character. The following types of projects, regardless of size, need to be designed and stamped by a licensed professional engineer: Downstream analysis shall • Pervious pavement (pervious concrete, porous asphalt, or demonstrate adequate to pervious pavers) or driving surfaces, including driveways. conveyancewh c of tce distance wheree the ity project site contributes less than 10 percent • Projects adjacent to or within immediate areas that will create of the upstream drainage basin downstream impacts area. Lake Oswego Stormwater Management Manual Section 2.5 26 Project Planning, Permits, and Stormwater Requirements The City requires that designs of the following facilities must be stamped by a design professional: • Drywell - Project Engineer • Proprietary subsurface facility (e.g., Stormchambers) - Project • Infiltration trench - Project Engineer Engineer • Detention pipes and vaults - Project Engineer • Green roofs - Project • Detention ponds - Project Engineer or Landscape Architect Engineer or Landscape Architect For large projects with greater than or equal to 3,000 square feet of contributing impervious surface area, designs for the following facilities must be stamped by a professional engineer or landscape architect: • Rain garden (note that the • Planter • Sand filter maximum contributing area to a single rain garden • Constructed wetland • Swale is 6,000 square feet of • Wet pond impervious surface area). 2.5.6 Underground Injection Control Review with Oregon Department of Environmental Quality Federal regulations require that owners/operators of certain types of stormwater facilities register them with the appropriate agency (DEQ in For confirmation of whether a Oregon). The following facilities, which are included in this manual, are specific facility is considered a likely UlCs that must be registered with DEQ: UIC, refer to OAR 340 44 and DEQ's website: http://www.deq. • Drywell • Proprietary subsurface facility state.or.us/wq/uic/uic.htm. designed for infiltration Owners/operators of such • Infiltration trench systems must complete the UIC General, Industrial and Registration with DEQ includes payment of a fee (the fee amount Commercial Stormwater is based on the risk to groundwater) and completion of a form that at thRegietration Application available e DEQ website: http://www. describes the contributing area and its size, facility design information deq.state.or.us/wq/uic/docs/ (e.g., depth and diameter, design infiltration rate), pretreatment, and forms/GenlndComSW.pdf. site characteristics. Documentation of UIC registration with DEQ must be included with the drainage report prepared for a project that includes a UIC(s). 2.6 Overview of Minimum Requirements — Permits 2.6.1 City of Lake Oswego Erosion Control Permit All projects that disturb soil should take precautions to keep soil from leaving the site, but if projects exceed a certain threshold, they may be subject to submittal and permit requirements. • An erosion control permit and plan are required for land disturbance greater than or equal to 500 square feet OR when any part of the disturbance area is within 50 feet of a pond, lake, river, stream corridor, canal, or wetland, except as noted in the City code. 27 Section 2.6 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements For more information, consult the following sources: • LOC Chapter 52 for specific erosion and sediment submittal requirements, and required content for an erosion control plan. • Erosion Prevention and Sediment Control Planning and Design Manual(Clackamas County et al. 2008) for planning and design information for specific erosion and sediment control BMPs. 2.6.2 NPDES 1200-C Permit from Oregon Department of Environmental Quality A 1200-C permit from DEQ is required for projects with land disturbance of 1 acre or more.An erosion control plan conforming to 1200-C requirements is acceptable for both City and DEQ requirements. These projects are also subject to the City's Erosion Control Permit requirements, but a separate submittal is not required. The City will use the temporary erosion and sediment control documentation from the 1200-C permit to evaluate conformance with Lake Oswego's Erosion Control Permit. Refer to DEQ's website for information on the 1200-C permit: http://www.deq.state.or.us/wq/stormwater/constappl.htm 2.6.3 Street Opening Permit Street opening permits may be needed for project construction. Street opening permits are needed during development or redevelopment of driveway approaches, work in the public right-of-way, work in public easements, or other City defined activities. 2.6.4 Other Permits and Project Review Land Use and Development Review Projects that require land use and development review based on consultation with the City's Planning Department will be reviewed by staff from several departments throughout the application and design, construction, and operations and maintenance stages of project development. Projects with changes to parcel boundaries or commercial development will require land use and development review. Single family development or remodeling projects can also trigger this review when one or more of the following are involved: • Tree removal permit • Floodplain • Zoning overlay • Sensitive lands overlay • Heritage tree • Special street setback Figures 2.3 and 2.4 and Tables 2.2 through 2.4 provide an overview of activities and submittal and review responsibilities for both the applicant and City staff during project development. Section 2.9 provides more information on the roles and responsibilities of different departmental staff related to stormwater management. Applicants should always verify project status initially with the Planning Department. Building Permits Building permits are needed for: • All new construction • Most remodeling projects, particularly when building safety may be affected Lake Oswego Stormwater Management Manual Section 2.6 28 Project Planning, Permits, and Stormwater Requirements • Electrical, mechanical, or plumbing work that is subject to the Oregon Specialty Codes (Oregon Building Codes Division 2012). Construction documents describing the proposed work should be brought to meetings with the City's Engineering Department and should include: • Two plan sets (legible and drawn to scale) including a site plan that shows the following: • Property lines • Lot dimensions • Existing and proposed improvements with distances to property lines indicated • Stormwater BMPs and drainage features with distances to property lines indicated • Location and size of all trees on the property • Building permit application that includes the address of the existing building or, if for new construction, the legal description of the lot • Additional requirements apply for building, electrical, plumbing, and/or mechanical projects. See City website and consult Building Department for more information. 2.7 Overview of Minimum Requirements — Submittals 2.7.1 a Preliminary Drainage Report The Preliminary Drainage Report must document the Site Assessment and Feasibility Analysis, explain how site conditions have influenced selection of stormwater facilities, and present preliminary type, size, and location of proposed stormwater management facilities, along with supporting calculations. The Preliminary Drainage Report should also identify the proposed discharge location. 2.7.1 b Drainage Report A drainage report must be submitted for all small and large projects. The drainage report must document which minimum project requirements from Table 2.1 apply and must demonstrate that the project will meet applicable requirements. Calculations and drawings must be submitted as appendices to the report. The drainage report must address the following topics: • Project owner and location • Other City permits and reviews (for all that apply) • Project classification (small or large) 0 Sensitive lands overlay • Tree removal permit 0 Special street setbacks • Land use planning review conditions of 0 Erosion control/1200-C permits approval • Site map (see Section 3.1.1 for requirements) 29 Section 2.7 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements • Minimum project requirements— summarize which requirements apply and attach calculations and/ or narrative demonstrating how they are met. • Site assessment and feasibility analysis • Downstream analysis • Offsite analysis • Onsite stormwater management • City of Lake Oswego Erosion Control Permit/ NPDES 1200-C permit from DEQ • UIC review with DEQ (if applicable) • Design for water quality • Design for flow control • Recorded operations and maintenance plan and other deed restrictions, for example: • Declaration of covenant for operations and maintenance of surface water maintenance facility (e.g., commercial sites) • Declaration of maintenance and cost-sharing agreement for access, stormwater facility, and other utility easements (e.g., three parcel partitions). • State or Federal resource-related permits (e.g., Department of State Lands/US Army Corps of Engineers) Attach the following: • Infiltration test results • Soil testing results (if applicable) • UIC registration information (if applicable) • Description of approved discharge location • Geotechnical report (if applicable) • FEMA floodplain analysis (if applicable) • Calculations demonstrating sizing in conformance with BMP design guidelines, including impervious area, for each facility. • Application(s) for other resource-related permits (if applicable) Appendix A contains a list of information that should be considered in preparing the drainage report along with minimum submittal requirements. 2.7.2 Recorded Operations and Maintenance Plan and Deed Restriction An operations and maintenance plan (OMP) is required for all stormwater facilities, with the level of detail and requirements scaled to the facility type and project size. The OMP must describe how to properly maintain the facility, the frequency of maintenance required, and the party responsible for maintaining the facility. Chapter 7 provides information on recommended maintenance practices. The checklists in Appendix I are general and do not reflect site- and facility-specific criteria such as specific plants, maintenance access, and unique design features. See Appendix A for an OMP example. In addition to registration with DEQ, owners/operators of facilities that manage stormwater from large projects must file a recorded maintenance covenant against the property in the county in which the property or facility is located. This step needs to be completed before the City will issue a Certificate of Occupancy. Lake Oswego Stormwater Management Manual Section 2.7 30 Project Planning, Permits, and Stormwater Requirements 2.7.3 Landslide Hazard and Erosion Risk Areas Report Where development is to occur on a potential severe erosion or landslide hazard area, Lake Oswego Code requires that a report evaluating soil conditions and potential hazards be submitted to the City. The report shall be prepared by a registered geotechnical engineer or certified engineering geologist and shall contain the following: • Evidence that a field investigation was made to determine the actual hazard • Statements regarding the exact nature and extent of the hazard • Recommendations on site preparation and construction methods to minimize the effects of the hazard • If erosion hazard exists, a specific erosion control plan to be approved by the City Manager, in accordance with LOC Chapter 52, Erosion Control • A description of any hazard area that should not be disturbed by construction activities or post- construction site use (including drainage) • If landslide hazard exists, a statement as to whether or not a proposed development constructed in accordance with the recommended methods is reasonably likely to be safe and to prevent landslide or damage to other property. 2.8 Project Exemptions, Adjustments, Exceptions, Waiver and Payment/ Fee in Lieu Information 2.8.1 Exempt Projects The following developments are exempt from the minimum project requirements in Table 2.1: 1. Developments approved prior to adoption of the this manual 2. Repair or maintenance activities that are not considered to be replacement of impervious surfaces under the MS4 permit. These include repair or maintenance activities on structures or facilities taken to prevent decline, lapse, or cessation in the use of the existing impervious surface as long as no additional hydrologic impact results from the repair or maintenance activity. 2.8.2 Adjustments An "adjustment" consists of use of a non-preferred facility type or strategy that will accomplish the stormwater performance standards. or post-construction site use (including drainage).Adjustments to these requirements may be allowed by the City Engineer. 2.8.3 Exceptions The City may approve exceptions to the requirements of this manual if specifications are met so as to ensure that the proposed exceptions will not increase risks to public health, safety and welfare, or to water quality or quantity, or to public and private property, whether in the vicinity or downstream of the site. If this Management Manual proposes design requirements that may not be technically feasible, the City may also approve an exception to those proposed design requirements. Other exceptions may be appropriate in some cases where requirements can't be met because an emergency exists, such as where there is immediate danger of landslide, damage to public or private property, or failure of a public facility. The City may also require mitigation in the form of offsite stormwater control or other means. 31 Section 2.8 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements When an exception is granted, conditions of approval may be imposed to offset or reduce harm that may be caused by granting the exception, or that would have been prevented if the exception had not been granted. This may include off-site treatment or payment into a fee-in-lieu program as allowed in LOC 38.25.170.3. 2.8.4 Exceptions and Payment/Fee In-Lieu Program Information The City recognizes, as do state and federal regulators, that not all sites have suitable conditions for onsite stormwater management or preferred stormwater facilities that rely on infiltration. Conditions such as a high water table, shallow bedrock, steep slopes, and soils that are poorly drained or have low permeability characterize much of Lake Oswego. The City's MS4 permit requires the City to implement "equivalent pollutant reduction measures" for sites that cannot meet their stormwater management requirements on site. Options identified in the MS4 permit include a payment-in-lieu program, a stormwater quality structural facility mitigation bank, or offsite mitigation within the same subwatershed as the project site. The City does not currently have a payment-in-lieu or offsite mitigation option. The City will be investigating options for providing equivalent pollutant reduction measures over the next several years and will update the stormwater program website, www.raintoriver.org, with any new information. 2.9 City Review of Development Permits The stormwater planning and submittal process will depend on the type of development proposed. Development types discussed in this chapter include single-family residential (this covers both new and redevelopment), land divisions, new commercial, multi-family, subdivision, and other development. Depending on the type of project, consultation with the Engineering, Planning, Building, and Public Works departments may be required. For all development types, the new stormwater code and manual emphasize maximizing onsite stormwater management. In order to move through the planning process efficiently and avoid schedule delays, the applicant must identify requirements for both the individual project classification (e.g. small and large) and specified development type. Several City departments are typically involved with review and permitting during planning, design, construction, and maintenance of new development and development projects that include stormwater management requirements. The following sections provide a brief overview of City departmental responsibilities related to stormwater management. 2.9.1 Engineering Department The Engineering Department provides stormwater review for the Building Division and for all land use applications for partitions, subdivisions, and commercial development. The City Engineer has authority to approve a stormwater system that varies from specific requirements in this manual as allowed by LOC. The Engineering Department is the main point of contact for the following: • Erosion control permit (Lake Oswego and/or 1200-C: applicable to ground disturbing activity greater than or equal to 500 square feet, including some landscaping projects). • Street opening permits: All construction activities in the public right-of-way or within public utility easements require a street opening permit issued by the Engineering Department. For more information and submittal requirements, see https://www.ci.oswego.or.us/publicworks/webforms/ street-opening-permit-application Lake Oswego Stormwater Management Manual Section 2.9 32 Project Planning, Permits, and Stormwater Requirements • Confirming the locations of existing utilities, including stormwater, and driveways • Questions regarding capacity of existing storm drains or drainage ditches • Clarifications on technical specifications related to stormwater facilities or conveyance facilities • Stormwater submittals (e.g., Drainage Report, Operations and Maintenance Plan). 2.9.2 Planning Department At the City Planning counter, applicants can learn what requirements apply to their project site (e.g., zoning, tree protection, and sensitive lands) and confirm whether the project is subject to land use review and development permit requirements. Specific to stormwater, the Planning Department can provide information on the best locations and drainage conditions on a particular property only to protect trees, and to meet setback requirements. 2.9.3 Building Department Building permits are required for most new construction and remodeling projects and for any projects that include electrical, mechanical, or plumbing work that is regulated by Oregon Specialty Codes (Oregon Building Codes Division 2012). Specific to stormwater, the Building Department will review any rainwater harvesting projects (other than rain barrels used strictly for landscape watering) to confirm that they conform to the Plumbing Specialty Code. 2.9.4 Public Works — Operations Division Lake Oswego's Public Works— Operations Division maintains public storm drains, ditches, storm drainage structures (e.g., catch basins and inlets), and non-vegetated stormwater facilities. The Operations Division also maintains public vegetated stormwater facilities, including rain gardens and planters within street rights-of-way. 2.10 Review Process for Different Types of Projects 2.10.1 New or Remodeled Single-Family Residence Figure 2.3 provides an overview for the review process for a new or remodeled single-family residence. When the applicant visits the City Planning counter, the Planning Department will review the proposed project to determine if there are land use and development review requirements. 2.10.2 Land Divisions, Commercial, Industrial, Institutional and Multifamily Development & Redevelopment Projects Figure 2.4 provides a simplified overview of the review process for other projects, such as partitions, new commercial, or multi-family projects. Land use and development review is not required for ministerial developments (less than 200 square feet of new or replaced impervious surface and less than 10 feet in height), unless one of the permits noted in Section 2.6.4 is required. 33 Section 2.10 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements 2.10.3 Lake Oswego Review Process for Various Project Phases: Design, Construction, and Operations and Maintenance Tables 2.2, 2.3, and 2.4 provide a detailed outline of the City review processes for applicants undergoing the design, construction, and operations and maintenance phases of a project. Figure 2.3 Lake Oswego Review Process - New or Remodeled Single-Family Residence Complete Site Assessment and Feasibility Worksheet See Appendix A Engineering Topics / Consultation With Engineering and Planning Topics Planning Staff • Drainage conditions •Zoning • Review minimum requirements in • Development • Discharge location Stormwater Management Manual (Table limitations • Existing utility 2.1) •Setbacks infrastructure (drainage, • Discuss site development and design •Overlay utilities) districts options with Engineering and Planning (neighborhood, • Erosion control (Chapter 4) sensitive lands) •Stormwater Is the property/project subject to one or more •Tree protection/tree performance standards & of the permits, regulations, or requirements removal requirements \ listed this chapter(Section 2.6)? •Streets, sidewalks, and \ right-of-way •Slope stability YES NO • 1 Is a building permit required? Lake Oswego review process See Tables 2.2, 2.3, and 2.4 See Chapter 2.6.4 for information Refer to Site Assessment and Feasibility Worksheet for site constraints and YES NO allowances I 1C ITERATIVE I PROCESS I I I d Confirm all Minimum Requirements from Stormwater Management Manual. For projects Submit construction documents to below threshold areas for stormwater requirements, Engineering staff describing your document new/redeveloped impervious areas and proposed work. existing and proposed drainage for approved point of discharge. Lake Oswego Stormwater Management Manual Section 2.10 34 Project Planning, Permits, and Stormwater Requirements Figure 2.4: Lake Oswego Review Process — Land Division, Commercial, Industrial, Institutional, Multi-Family, or Other Project Complete Site Assessment and Feasibility Worksheet See Appendix A i Engineering Topics Consultation with Engineering and Planning Topics Planning staff •Drainage conditions •Review minimum requirements in Stormwater •Zoning •Discharge location Management Manual (Table 2.1) • Development •Existing utility • Discuss design options with Engineering and limitations infrastructure Planning (Chapter 4) •Setbacks (drainage, public Is the project a Minor or Major Development? •Overlay districts utilities) (Planning Department will Determine (neighborhood, •Erosion control Designation) sensitive lands) •Stormwater OR •Tree protection/ performance standards Is the property/project subject to tree removal &requirements one or more of the permits, •Streets, sidewalks, regulations, and/or requirements and right-of-way listed in this chapter(Section 2.1)? •Slope stability II YES J, 40NO Lake Oswego development \ NO review process Is this project considered a ministerial See Tables 2.2, 2.3, and 2.4 development? Refer to Site Assessment and Feasibility Worksheet for site constraints and allowances / i YES I 1` Is a building or eriosion control permit required? ITERATIVE See Chapter 2.6.4 for information PROCESS \ / YES I NO I i rr Confirm all Minimum Requirements from i Stormwater Management Manual. For projects Submit construction documents to below threshold areas for stormwater requirements, Engineering staff describing your document new/redeveloped impervious areas and proposed work. existing and proposed drainage for approved point \ See Chapter 2.6.4 of discharge. 35 Section 2.10 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements Table 2.2 Lake Oswego Review Process —Application and Design Phase DESIGN PHASE Applicant City Phase 1: Preapplication Conference Schedule preapplication conference with Planning Planning Department will coordinate with other Department for major, minor developments, and department representatives as appropriate. with Engineering Department for ministerial development. Where stormwater management requirements apply (see Table 2.1), a representative from the Come prepared to discuss stormwater management Engineering Department will: including: • Review stormwater management approach • How project will minimize impervious surface area • Convey stormwater standards and discuss • How project will protect and preserve native soils stormwater management options during construction (see Section 3.3) • Discuss site design options that can minimize • How project will meet onsite stormwater stormwater management requirements and management requirements, if applicable (see protect native soils Section 4.1) • Discuss connections to City utilities • Proposed connection to City surface water • Discuss right-of-way and sidewalk extension management system. permits • Plan to meet other minimum requirements that • Discuss operations and maintenance requirements apply (see Table 2.1) •Visit the site, if appropriate Applicant City Phase 2: Project Application Submission The Applicant is responsible for submitting the The Planning Department will assign a land use following: casefile number and prepare a staff report for minor and major developments. • Preliminary drainage report, including proposed BMPs (see Section 2.7 and Appendix A) The Engineering Department will review other • Draft Operations and Maintenance Agrement/Plan applications. (see Section 2.7.2 and Appendix A) Applicant City Phase 3: Land Use Decision If application is Approved: Proceed with final design The Planning Coordinator will notify the Applicant of project. Consult Stormwater Management Manual of whether the application is approved, denied, or for submittal and design requirements related to approved with conditions for minor, major stormwater. See Table 2.3 for Construction Phase developments. Engineering Department staff will Review. notify Applicant on status of other project types. If project is Approved with Conditions: Review Engineering Staff will start stormwater treatment conditions of approval. Contact Engineering facilities (SWTF)file (for internal City use) if Department if any of the conditions of approval appropriate. related to stormwater are unclear. If project is Denied: Consult with Planning and/or Engineer Coordinator for options. Lake Oswego Stormwater Management Manual Section 2.10 36 Project Planning, Permits, and Stormwater Requirements Table 2.3 Lake Oswego Review Process — Construction Phase CONSTRUCTION PHASE _ Applicant City Phase 1: Construction Plan Review Applicant submits construction plans and Engineering Development Coordinator will: drainage report to Engineering. • Review plans and applicable stormwater standards • Submit two plan sets (legible and drawn to scale) for SWTF per staff report conditions that include all plan elements • Issue construction permit for public infrastructure • Schedule pre-construction meeting with contractors and owner Applicant City Phase 2: Construction Applicant pays 120% construction bond Engineering will: (engineers public improvement cost estimate- EPICE) • Ensure facility constructed per plans and sign off on completion • Submit 9% project plan review fee • Bond required to be posted before sign off by City • Length: duration of construction phase • Collect 120% construction bond • Covers: contractor responsibilities • Issued/released by: developer, includes letter of partial acceptance by Engineering Development Coordinator to developer/applicant's insurance company for release of bond Applicant City Phase 3: Post-Construction Applicant submits as-built drawings of facility. Engineering Development Coordinator will: Applicant pays 10% two-year maintenance bond. • Review as-built drawings of stormwater facilities • Length: two years from issuance of Letter of Partial • Engineering and Building Inspectors will Acceptance coordinate final walk-through and punch list for • Covers: public improvements including stormwater compliance with conditions of approval facilities • Issue Letter of Partial Acceptance • Issued/released by: Engineering Department • Release 120% construction bond Applicant pays 5% per year and submits letter from • Collect 10% maintenance bond and 5% landscape landscaper to Planning Department(general bond, as applicable landscaping)or Engineering Department(stormwater facility landscaping).. Engineering Staff will update SWTF file with bond dates and as-built numbers. • Length: two years from issuance of Partial Letter of Acceptance Engineering Staff will issue final (100%) Letter of • Covers: vegetation establishment, compost Acceptance covering and functionality of the facility • Issued/released by: Planning or Engineering Department, as applicable 37 Section 2.10 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements Table 2.4 Lake Oswego Review Process — Operations and Maintenance Phase OPERATIONS AND MAINTENANCE PHASE Applicant City Phase 1: Operations and Maintenance Agreement/Plan Applicant submits final operations and Engineering Staff will: maintenance plan (OMP)to City. • Update SWTF file (add OMP) and submit to Final approved stormwater facility OMP shall be Engineering Development Coordinator recorded with County Clerk by applicant. Water Quality Program Coordinator will: • File SWTF data for future inspections • Enter project information in SWTF database Applicant City Phase 2: One Year After Issuance of Letter of Partial Acceptance Owner submits first-year inspection and Engineering Inspector will: maintenance records in accordance with OMP. • Perform one-year infrastructure inspection and Owner has 30 days to respond/rectify need and prepare a needs and corrections list corrections notice(s). Engineering Development Coordinator will: • Issue facility needs and corrections notice Applicant City Phase 3: Second Year After Issuance of Letter of Partial Acceptance Owner submits second-year inspection and Engineering Development Coordinator will: maintenance records in accordance with OMP. • Issue facility needs and corrections notice Owner has 30 days to respond/rectify need and •Approve corrections corrections notice(s). • Release bond. Applicant City Phase 4: Ongoing Owner submits inspection and maintenance records Water Quality Program Coordinator will: in accordance with OMP. • Schedule random inspections of 10% of facilities to ensure compliance with OMP • Submit annual MS4 report to Oregon DEQ Lake Oswego Stormwater Management Manual Section 2.10 38 Project Planning, Permits, and Stormwater Requirements This page is intentionally left blank. 39 Section 2.9 Lake Oswego Stormwater Management Manual Oh A ' 'eSFEM'gniRo MAW, A HE V , 2aDTLEIMERgT _HE@LI 2g gAran Low impact development is intended to reduce adverse hydrologic and water quality impacts of development on receiving waters and the environment by designing sites to mimic natural processes and replicate predevelopment hydrology. LID is most effective when it is a strategy that is incorporated through all phases of site selection, planning, design, construction, and maintenance. While some LID techniques, such as pervious pavement, can have a higher construction cost than traditional methods, LID methods that reduce runoff or manage stormwater through dispersion or infiltration can save money by reducing downstream stormwater infrastructure costs relative to traditional development methods. Chapter 3 describes LID principles that should guide the process of site analysis and design. It also summarizes the process of completing a site assessment, feasibility analysis, and ultimately selecting stormwater facilities appropriate for the site. 3.1 LID Principle #1 — Understand the Site LID Principles Selecting the right stormwater management technique should start as early as possible in the planning and design #1 Understand the Site process. Before the site design is developed and before any construction begins, the City requires that applicants and/ #2 Reduce Runoff through Design or site operators complete a site assessment and feasibility analysis to evaluate site hydrology, topography, soils, #3 Reduce Pollutants Carried by Runoff vegetation (including vegetation that will be preserved) and any other features that demonstrate how stormwater moves #4 Capture and Treat Runoff through the site prior to development. One goal of this analysis is to incorporate stormwater management into the landscape in a way that will preserve onsite drainage, soils, and native vegetation. Another important goal of the site assessment is to identify suitable locations for stormwater facilities before design begins on a project. 3.1.1 Site Assessment Site Analysis and Feasibility Process Applicants must conduct the site Step 1: Review information needed for all steps of analysis assessment and feasibility analysis prior to consulting with City Building Step 2: Create site maps using LOMaps or Planning departments. Step 3: Conduct site visit Step 4: Gather missing data and information Step 5: Assess feasibility of stormwater management techniques Lake Oswego Stormwater Management Manual Section 3.1 40 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection Site Mapping Site maps are required for the site assessment and feasibility analysis, and for the drainage report. The site map should include the following elements, where applicable, for each drainage facility: • Property lines • Contour lines and critical elevations (City datum) • Lot dimensions • Onsite and offsite drainage patterns (indicate with arrows) and features (including depressions where stormwater appears to infiltrate, downstream conveyance, emergency overflow routes, and flow onto adjoining and mapped parcels) • Project location (Section,Township and Range) • Stormwater infrastructure management subbasin (available on LOMap, Surface Water Map) • Total impervious area (TIA) of project site • Existing and proposed improvements with dimensions and distance from property lines • Proposed stormwater collection, conveyance, treatment, and flow control features • Areas of protected native soils and/or amended soils that have been added for stormwater management function • Sensitive lands delineation • Floodplain delineation • Wetland delineation • Tree protection zones To simplify this process for the applicant, the City has developed an interactive map website called LOMap. LOMap is an online tool that provides GIS data for many of the site characteristics needed for a proper site analysis. Field verification of site conditions is required. Maps can be created with necessary data layers and printed or saved for future reference and sharing. The LOMap mapping tool is online at: http://gis.ci.oswego.or.us/GeoNorth/Flexmap3/Flexmap.html For instructions on how to use LOMap, click the `H' button at the top of the page or go to: http://gis.ci.oswego.or.us/GeoNorth/Flexmap3/assets/docs/help.pdf 41 Section 3.1 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection LOMap GIS Layers • Topographic Contours • Tax lot data • Neighborhood associations • Zoning information • Utility locations (approximate; includes City water, wastewater, and stormwater only; limited private utility data) • Environment layers (e.g., Federal Emergency Management Agency [FEMA] flood zones, limited soil characteristics, and fault lines) • Planning information (e.g., design districts, heritage trees, historic landmarks, permits, special street setbacks, commercial overlays, and sensitive lands) • Fire information • As-built drawings (available through City staff) • Parks • Stormwater management subbasin (Surface Water Map) Note:Map layers, names, and attributes will be updated and changed over time. Visit LOMaps website for latest layer information. Field verification of site features is required. Site Visit After creating maps with available data layers, it is time to conduct a site visit to collect other relevant information necessary to inform design. During the site visit, it is important to investigate the soils, topography, hydrology, vegetation, and any structures and utility locations that may be present. More information on how to investigate each of these site characteristics is provided below. 3.1.2 Site Assessment Components If sandy or gravelly soils are observed, this may be a good site for Soils a drywell or infiltration trench. Sandy or gravelly soils often have infiltration Opportunities for infiltration are constrained by soil conditions. As rates that are too high to provide noted in the Clean Streams Plan (Otak 2009), nearly half of the soils effective treatment, so include a filter strip, swale, or other treatment BMP in Lake Oswego are classified as belonging to Natural Resources prior to infiltration BMP. Note that Conservation Service (NRCS) Hydrologic Soil Group C. Another 30 registration of a BMP with DEQ may percent of the soils are classified as Group D, and the remainder of be required if it is considered a UIC. the soils are categorized as Group B. Soil properties related to hydrologic soil groups affect infiltration potential as follows: • Group B soils have a moderate infiltration rate. They include soils of moderately fine and moderately coarse texture, and are generally most suitable for water quality treatment and capture through infiltration. • Group C soils are typically fine-textured soils (e.g., tills)with slow infiltration rates. These soils may be appropriate for infiltration facilities designed for treatment but are unlikely to have adequate infiltration rates to handle high runoff volumes. Lake Oswego Stormwater Management Manual Section 3.1 42 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection • Group D soils, which include clays, have very slow infiltration rates or a high runoff potential and are not suitable for infiltration. Figure 3.1 is a general soils map of the City of Lake Oswego. Check GIS data available from the City and online soils data from NRCS to evaluate whether soil conditions are likely to be favorable for infiltration. Figure 3.1 Lake Oswego Soils Map 4 rry. 1 rePortland \ )i Milwaukee 7 Z \, I p \ l U yi pi!, _Li: ---, ‘, y's A f, L! LL I,r' / coonr,C ,-_-_A H /�/ Soil Group Tigard Club L_11 l Rd 1 . ileilliwillt4 NI A Alf B -,.- . • -- City Limits 1= ge n -,_ ��- la I use ill ,, r_.„ �oWer ,onesF l - �„V .. Tualatin ,rI sflp N--,_, 7 --1i if --- o -- Hp Ij-� - s�z i West ffm Rivergrova T, r L___ Linn Nyberg s, City Of Lake Oswego I Soil Hydrologic Groups nI ‘) 0 0.25 0.5 0.75 1 1 Riles Verify whether onsite soils match the NRCS and City soil information or if further investigation is needed. Test infiltration rates (Appendix B) and soil quality to assess suitability for infiltration and to pick the best location for LID BMPs. City may require formal surface or subsurface infiltration testing depending upon site conditions and the type of infiltration facility proposed by the applicant. More information related to testing needed to support design of specific facility types is available in Section 4 of this manual. To do a preliminary soil site evaluation, the applicant should dig some small holes in a few representative areas scattered throughout the site to inspect the soils. By moistening a medium-sized handful of soil in their hand and rubbing a small amount between their fingers, the applicant can determine soil texture. 43 Section 3.1 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection Identify whether site soils are predominantly: • Sandy or a sandy loam Can individual sand particles be distinguished between fingers; Does the soil contain more sand than clay or loam? Does the soil appear to drain well? • Silty or silty loam • Does the soil feel smooth and chalky when moistened and rubbed between fingers? • Clay or clay loam Are there areas of protected native • Does the soil feel sticky and hold together when moistened? soils and/or amended soils that have been added for stormwater • Will the soil form a flat "ribbon" of 1 to 2 inches in length if management function?These soils kneaded and threaded through finger tips? might be evident by the presence of healthy native vegetation. See Appendix G for more information on how to read characteristics They would also be loose and unc within site soils. City may require consultation with a geotechnical stor mpacted and infiltrating stormwater. engineer. Topography Several BMPs are not suitable Identify and map breaklines or changes in slope to help characterize near or on steep slopes. Confirm flat, sloping, and steeply sloping topography. To reduce construction safe drainage route to prevent and post-construction impacts, work should be conducted within the concentrateddrainingow stormwater slopes. down steep slopes. existing topography to the maximum extent practicable. The design should be developed in a way that works with existing Ageotechnical engineering analysis grades. and report may be required. • Plan the project to minimize land disturbance such as clearing and grading and cut and fill. • Keep cut and fill slopes as flat as practicable to help preserve soil stability. Hydrology If ponding water is observed on Where is the water draining? This will be easier to confirm on a rainy areas of clay soils(or saturated day, but even in dry conditions, general drainage patterns should be soils during otherwise dry periods), evident. confirm that the area is not a jurisdictional wetland (consult with Planning Department). If Note the following: not, and if a stormwater facility is reLocations of catch basins, inlets, ditches, and other construced,ted d wetland d this area for a • constructed wetland or wet pond. If stormwater facilities an onsite stormwater management or water quality treatment BMP is • Depressions where water likely ponds and/or infiltrates during not needed, consider incorporating storms this wet area into landscaping plans through use of wetland • Offsite areas that slope toward the property and may plantings. contribute offsite drainage • Areas of the site that appear to drain to neighboring properties Lake Oswego Stormwater Management Manual Section 3.1 44 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection Depth to Groundwater Information on depth to groundwater can be found on the NRCS website: http://websoilsurvey.sc.egov.usda.gov/app/WebSoilSurvey.aspx. Information on depth to groundwater can be found from review of well logs held by Oregon Water Resources Department database, accessible here: http://apps.wrd.state.or.us/apps/gw/well_log/Default.aspx Depth to groundwater varies seasonally; assume high groundwater conditions for design purposes. Vegetation Confirm setbacks from sensitive Note location and size (diameter at breast height) of all trees on the areas. Select a vegetated BMP property (Tree code LOC 55.02). with habitat features and plant selection appropriate for the species and habitat present. Identify patches of native vegetation, invasive species and weeds, and dominant vegetation cover types. Existing Structures and Other Infrastructure If the site was previously Identify areas of existing structures, including parking areas, buildings, developed and site history sheds, and any impervious surfaces.Also document any existing is unknown,test soils for utilities, easements, or other site features that could influence feasibility. contamination. Collect Other Data, if Available City staff may have information on previous site development as well as stormwater facilities observed during the site visit. Neighbors may be able to describe past land uses on and surrounding the site, and areas where they have noticed ponded water or water leaving or entering the site. They may also be willing to share observations about infiltration rates on their own site that may be representative of the area. 3.2 LID Principle #2 — Reduce Runoff through Design LID can reduce the adverse ecological impacts associated with In addition to minimizing impervious development. When there is less impervious area due to careful site surface area and reducing runoff planning and design, and native soils are protected or amended to through design, careful site design preserve or promote infiltration, less stormwater enters streams as techniques that work with natural surface runoff. Consequently, hydrologic impacts to channels and other topography, soils, anda vegetationeu p help create places that reduce downstream receiving waters, such as increased peak flows or flow impacts on the environment, are durations, are reduced. public amenities, increase habitat for native species, and beautify Reducing runoff through design means designing a project that is Lake Oswego. appropriate for and tailored to a particular site. It requires knowledge of • the site, including localized depressions, soils with good infiltration capacity, and vegetation. Some key concepts include: • Preserving and enhancing native vegetation to provide a high level of stormwater treatment 45 Section 3.2 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection • Protecting native soils to preserve their infiltrative and treatment capacities • Minimizing impervious surfaces through site design techniques Stormwater BMPs included in this manual (see Chapter 4) are options for further reducing stormwater impacts and stormwater management requirements. The following resources provide demonstrated effective methods for reducing runoff through design. Low Impact Development: Low Impact Development: Technical Guidance Manual for Puget The Lake Oswego Public Works p p g Stormwater Program webpage Sound (December 2012): http://www.psp.wa.gov/downloads/ provides resources and links to LID/20121221_LIDmanual_FINAL_secure.pdf stormwater information relevant to the City. Green Streets: Visit: Innovative Solutions for Stormwater and Stream Crossings (Kloster http://www.ci.oswego.or.us/ et al., 2002). publicworks/stormwater-program Various publications, links, and resources available through the Oregon State University extension stormwater solutions webpage (SCS 1986): http://extension.oregonstate.edu/stormwater/ 3.3 LID Principle #3 — Reduce Pollutants Carried by Runoff The goal of principle #3 is to protect healthy native soils and provide infiltration. Contaminated soil and sediment impair habitat and water quality by transporting pollutants and physically degrading habitat (e.g., clogging spawning habitat with sediment). The practices in this section focus on retaining and protecting native soils.Areas where water is currently infiltrating should be protected. The applicant should flag these areas so construction vehicles do not compact soils. • Soil retention. The duff layer and native topsoil should be retained in an undisturbed state to the maximum extent practicable. In any areas requiring grading/earthwork, remove and stockpile the duff layer and topsoil on site in a designated, controlled area, not adjacent to public resources and critical areas, to be reapplied to other portions of the site where feasible. • Soil quality.All areas subject to clearing and grading that have not been covered by an impervious surface, incorporated into a drainage facility, or engineered as structural fill or slope shall, at project completion, demonstrate that soils have: • Enough organic content to support healthy vegetation; • the correct pH balance (typically from 6.0 to 8.0 and matching the pH of the original undisturbed soil); and • a minimum depth of 8 inches (except where tree roots limit the depth of incorporation of amendments). Additionally, subsurface layers should be scarified at least 4 inches with some incorporation of the upper material to avoid stratified layers, where feasible. Consider the quality of the compost and other materials used for planting beds. Typically, compost should have an organic matter content of 35 to 65 percent and a carbon:nitrogen ratio of 25:1 -35:1. Mulch with a minimum of 2 to 3 inches of organic material. Lake Oswego Stormwater Management Manual Section 3.3 46 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection To attain these results, the applicant must amend soils with stockpiled duff and topsoil and add other amendments, if needed. The City will accept the use of soil mixes that conform to the Portland Bureau of Environmental Services specifications, and will consider appropriately specified and tested alternative mixes (e.g. bioretention soil that meets the Washington Department of Ecology specifications). • Implementation. Consider the following when preserving soil depth and quality. • Do not disturb native vegetation and soil, and protect these areas from compaction during construction. o Amend existing site topsoil or subsoil either at the default recommended rates (listed above) or at custom-calculated rates based on site soil testing results. • Stockpile existing duff and topsoil during grading and earthwork, and replace it prior to planting. Stockpiled topsoil should be amended if needed to meet the organic matter or depth recommendations, either at the default recommended rates (listed above) or at a custom- calculated rate based on site soil testing results. • Import topsoil mix of sufficient organic content and depth to meet the recommendations listed above. • To protect and improve the stormwater benefits provided by soils, including stormwater management and treatment, mulch existing landscape beds with materials such as shredded leaves, arborist wood chips, or compost. Implement techniques like grass-cycling (leaving grass clippings on the lawn when mowing) and avoid use of soluble fertilizers and pesticides, which can damage beneficial soil life. To aid in revegetation with suitable native vegetation, strongly consider inoculating the soil with beneficial fungi (mycorrhizae) suitable for the proposed planting. More than one method may be used on different portions of the same site. Remember, soil that already meets the recommended depth and organic matter quality, and is not compacted, will likely not need to be amended. Visit the following sites to learn more about how to protect, restore, and maintain healthy soils: The Building Soil Manual. Guidelines and Resources For Implementing Soil Quality and Depth BMP T5.13 in WDOE Stormwater Management Manual for Western Washington http://www.buildingsoil.org USDA Soil Biology Primer http://soils.usda.gov/sqi/concepts/soil_biology/biology.html Soils for Salmon http://www.soilsforsalmon.org 47 Section 3.3 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 3.4 LID Principle #4 — Capture and Treat Runoff Lake Oswego is required to prioritize LID stormwater practices and reduce pollutants associated with 303(d)-listed and TMDL water bodies. The City prioritizes implementation of LID by requiring that all small and large projects, as defined in Section 2.4, use and prioritize onsite infiltration to the maximum extent practicable. The applicant should keep runoff dispersed and design the site so that small areas of impervious surface either sheet flow to adjacent vegetation (sheet flow dispersion) or drain to a single bioretention facility (e.g., planter or rain garden) that is incorporated within overall site landscaping. Overflow routes shall be identified for all stormwater drainage from the proposed development, including overflows from BMPs. This information must be shown on the site map provided with the drainage report prepared for the project. Some design practices that can help promote dispersion and avoid concentrating runoff include: • Managing stormwater on site through infiltration to avoid impacts on downstream receiving waters • Amending soils where existing soils lack sufficient organic matter or physical properties to remove pollutants through adsorption, filtration, or other pollutant-removal processes Table 3.1 lists BMPs that are included in this manual and approved for use in Lake Oswego, and indicates the minimum project requirements, key design decision criteria, and project classification associated with each facility type. (Applicable minimum project requirements are summarized in Table 2.1.) Lake Oswego Stormwater Management Manual Section 3.4 48 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection Table 3.1 Stormwater Management Best Practices. Minimum Project Key Design Decision Appropriate Requirements Approval Criteria For c Ina) Q d a) o = o v E Ls c 0 c — L a� 0 D) I- o t o c '2r 0c, co O J Tz 3 al 4- _C "O > > Cr y d a V __ l4 V i d a) < C O Z y 2 2 .. a) . N d 3 3 + ce >+ O ++ .0 co N y � Q E LL C C C) d 0 ce .0 E E a) es 4— y O d _ ++ �. L O a) d O O r 8 8 O O E R j c -0 LI- t •p O C d c C 7. N CO 0 � -Oa V V d a 0) YA i — .0 i c c co a) LA d aL+ +' =0 gm 0 II a) 2 2 R FT m 0. C a) a) `i- C i 13 ) - E m m E as Facility Type co E 0 ❑ ❑ E ❑ o- < ❑ CD E Q co cn cn Rain garden —infiltration 4.6.1 • • • • >_3*** >0.25 >_5 >_10 • • Rain garden —flow through 4.6.1 • • NA >0.25 >_5 NA • • Planter—infiltration 4.6.2 • • • • >_3*** >2 >5 >_10 • • Planter—flow through 4.6.2 • • NA NA >5 NA • • Infiltration trench 4.6.3 • • • • • >_5 >2 >_5 >_10 • • Drywell or proprietary 4.6.4 • • • • • >_10 >2 >_5 >_10 • • infiltration chamber Green roof 4.6.5 • • • NA NA NA NA • • Pervious pavement 4.6.6 • >_4 >2 >_5 >_10 • • Pervious pavement facility 4.6.6 • • >_4 >2 >_5 >_10 • • (receives run-off) Rainwater harvesting 4.6.7 • NA NA NA NA • • Filter strip 4.6.8 • >_3*** >2 >5 >_5 • • Swale 4.6.9 • >_3*** NA >_5 >_10 • • Sand filter 4.6.10 • • • >_3*** NA >_5 • Constructed wetland 4.6.11 • • 0 NA >5 >_10 • Constructed wetland with 4.6.11 . • 0 NA >5 >_10 • detention storage Wet pond 4.6.12 • • • — NA >5 >_10 • Infiltration pond 4.6.12 • • • • • >_3*** >0.25 >_5 >_10 • Detention pond 4.6.12 • • — NA >5 >_10 • Detention pipes and vaults 4.6.13 • • — NA NA NA • Sheet flow dispersion 4.6.14 • >_3 NA >_5 >_10 • • Proprietary treatment BMP 3.4.4 • • • — NA NA NA • • NA= not applicable *And UIC Pretreatment **Setback from structures is measured from edge of excavation for facility. ***Cell bottom may be>1 foot from seasonally high groundwater elevation if contributing drainage area is<3,000 square feet. 49 Section 3.4 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 3.4.1 BMP Selection for Small Projects Bioretention BMPs (e.g., rain gardens and planters) are the preferred BMPs for small sites because they provide treatment and flow control, are not considered UlCs (unless they have an underdrain that discharges to a drywell or other UIC), and, when well-designed, can be a beautiful part of the landscape. Bioretention BMPs with underdrains are suitable in areas only where onsite soils have proven suitable for bioretention and there is no risk of phosphorous export to surface water bodies. Compost-amended soils can export phosphorus. Where rain gardens and planters are not feasible due to site constraints, infiltration trenches, drywells, or proprietary infiltration chambers may be Registration forms for UlCs can be used. Since these technologies are considered UlCs, they will need to found on DEQ's website at: be permitted through DEQ (see Section 2.5.6). For these sites, post- http://www.deq.state.or.us/wq/uic/ construction soil amendment, as described in Section 3.3, is particularly forms.htm important. Submit documentation of registration along with drainage Where an impervious surface is surrounded by lawn or landscaping, report. onsite stormwater management requirements may be met through sheet _ flow dispersion (see Section 4.6.14). 3.4.2 BMP Selection for Large Projects Large projects must meet water quality and flow control requirements, in addition to onsite stormwater management requirements, where applicable. Best management practices to minimize impervious surfaces and overall stormwater management requirements must be considered first. These include: • Installation of green roof • Installation of pervious pavement • Rainwater harvesting • Onsite preservation of vegetation • Soils management to preserve onsite infiltration Stormwater from remaining impervious surfaces should be managed with BMPs that provide both flow control and water quality treatment. BMPs that will meet onsite stormwater management, water quality, and flow control requirements include: • Rain garden • Planter • Combination of an infiltration or flow control BMP from Table 3.1 with a water quality treatment BMP from Table 3.1. If onsite stormwater management requirements apply, onsite stormwater management facilities should be sized to infiltrate 100 percent of the 10-year, 24-hour design storm (3.2 inches in 24 hours). If only water quality and flow control requirements apply, water quality BMPs should be sized to infiltrate the water quality design storm (1 inch in 24 hours.) Lake Oswego Stormwater Management Manual Section 3.4 50 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 3.4.3 Infiltration BMP Considerations Infiltration facilities must meet a minimum infiltration rate standard and have additional requirements for facility setbacks. In addition, if the proposed facility is a UIC, it must meet additional design and permitting requirements. Each of these facility parameters is described in further detail below. Soil Testing Infiltration rate testing is required where infiltration facilities are proposed. Subsurface soil assessment and modified soil infiltration tests shall be conducted by using a pilot infiltration test (PIT). Guidance for conducting a PIT test is included in Appendix B.A correction factor of 2.0 must be applied to the field-tested infiltration rate. Setbacks Stormwater facilities that meet the infiltration minimum project requirements per Table 3.1 are subject to the following horizontal setback requirements: • 10 feet between the top edge of the facility and the edge of the building foundation • 100 feet from contaminated sites • 100 feet from drinking water supply wells or springs • 10 feet from septic systems or drain fields • 10 feet from underground storage tanks • 5 feet from property lines without neighbor agreement • 5 feet between top edge (width) of facility and property lines for flow-through planters • 5 feet from right-of-way • Facilities cannot be located in the right of way or special street setback without approval from the City Engineer. • 7 feet from new utility trenches and 5 feet from other existing utilities. Figure 3.2 Setback Diagram. 5' 10' � I I C J Infiltration Zone 2 0 a %0 51 Section 3.4 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection A geotechnical report is required to determine setbacks for infiltration facilities on slopes >_15 percent or within 200 feet of a steep slope hazard area or landslide hazard area (LOC 50.06.006(2)). Underground Injection Control Devices If an applicant proposes a UIC, the applicant must demonstrate that the UIC is either authorized by rule or has a UIC permit associated with the facility. Use of a UIC to manage drainage from roofs only is automatically rule authorized. Additionally, if the UIC owner/operator cannot obtain DEQ approval, the UIC system must be formally closed and follow DEQ's guidelines for closing the UIC. See http://www.deq.state. or.us/wq/uic/uic.htm for more information. Infiltration BMPs and Contaminated Soils If a potential infiltration facility drains into or is within 100 feet of a stream, lake, pond, wetland, or other open water body, the City may require that soils be tested for toxic pollutants before allowing infiltration- based stormwater management facilities. The City may require additional constituent-specific stormwater treatment in response to receiving water impairment or requirements of applicable TMDLs. Some LID and infiltration BMPs are NOT appropriate for areas with contaminated soils. Restrictions apply to the following BMPs: • Pervious pavement • Planter— infiltration • Sheet flow dispersion • Drywell • Rain garden — infiltration • Infiltration trench Infiltration-based stormwater treatment is rarely compatible with existing septic systems. Applicants may be required to connect to sewer, whether available within 300 feet of an existing line or not, to alleviate conflicts. 3.4.4 Use and Approval of Proprietary Storm water Best Management Practices Proprietary devices are manufactured technologies that remove pollutants through physical, chemical, or biological treatment processes. Lake Oswego allows use of proprietary stormwater facilities where other facilities are not feasible due to site constraints (e.g., steep slopes). City approval is required, and only facilities that have been tested and approved through the Technology Assessment Protocol - Ecology (TAPE) program run by the Washington State Department of Ecology will be permitted. (There is currently no comparable program in Oregon; instead, the City recognizes Washington's program). Under the TAPE program, technologies are field tested to evaluate whether they meet criteria for various levels of treatment (basic, dissolved metals, phosphorus, oil, and pretreatment). • Basic treatment—technologies remove 80 percent or more total suspended solids • Dissolved metals —technologies meet basic treatment goal and remove 30 percent or more dissolved copper and 60 percent or more dissolved zinc for influent concentration ranges • Phosphorus — meets basic treatment goal and removes 50 percent or more of total phosphorus. Lake Oswego Stormwater Management Manual Section 3.4 52 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection Technologies that have been or are being tested under the TAPE program are assigned use level designations as follows: • Pilot Use Level Designation (PULD)— laboratory data suggests that technology may meet the performance goal. • Conditional Use Level Designation (CULD)— both laboratory and field data suggest likelihood of meeting performance goals. • General Use Level Designation (GULD)—technology is approved by the Washington State Department of Ecology. Lake Oswego accepts proprietary technologies that have GULD for basic, dissolved metals, or phosphorus treatment as water quality treatment BMPs, as long as the City has approved use of proprietary BMPs for the project. In determining whether a proprietary device will be allowed for a project, the City will consider both: 1) the effectiveness of the device for removing pollutants to specified levels and 2) the ability of the current and future property owners to undertake the maintenance required to ensure that the device continues to remove pollutants at specified levels. Proprietary technology use level designations are posted on the Washington State Department of Ecology's website at: www.ecy.wa.gov/programs/wq/stormwater/newtech/index.html. 3.4.5 Other Considerations in BMP Selection In addition to the previously discussed feasibility factors, there are a number of other considerations that should be taken into account when selecting or designing BMPs for a particular site. These include, but are not limited to, the following: • Ease of maintenance —All BMPs will require some form of routine inspection and maintenance. Selection of a BMP should consider how easy or difficult the BMP will be to maintain, including frequency, accessibility, and need for specialized training or equipment. You should select or design BMPs to minimize the level of effort and expense associated with these factors. Special attention should be paid to inlet and forebay designs, as these are typically where maintenance needs are most pronounced and where failure is most likely to originate. • Community acceptance— Community acceptance is highly subjective, but a general sense can be gleaned from market surveys, visual aesthetics, and reported nuisance problems and safety concerns. BMPs with low community acceptance can often be improved through better landscaping or more creative design. Note that while underground BMPs can have high levels of community acceptance, this is due primarily to the fact they are "out-of-sight, out-of-mind," which substantially reduces their ease of maintenance. • Ancillary benefits — Beyond stormwater management, many BMPs have ancillary benefits that can have positive environmental and/or community impacts. Many BMPs have the potential to create aquatic and terrestrial habitat for wildlife and waterfowl. Other BMPs contribute to air quality improvements and reduce the urban heat island effect. Still others, like green roofs and trees, can provide energy savings. In most cases, the ancillary benefits are not automatic, and require proper installation, landscaping, and vegetative management to be fully realized. 53 Section 3.4 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection • Climate change adaptability— BMPs are intended to function for multiple decades. With a changing climate, storms are likely to become longer, more intense, and more frequent, while droughts may become more prolonged. Designers should consider future climate change in BMP designs. Approaches could include oversizing the BMP, minimizing bypass during intense storm events, and creating adaptable planting plans, among others. Lake Oswego Stormwater Management Manual Section 3.4 54 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection This page is intentionally left blank. 55 Section 3.4 Lake Oswego Stormwater Management Manual 4 Stormwater FacilityDesign li uidelines This section provides an explanation of the performance standards and design criteria necessary for stormwater facilities to meet Lake Oswego's stormwater requirements. 4.1 Performance Standards The performance standards to be used for BMP sizing are provided in Table 4.1. Table 4.1 Performance Standards for Best Management Practices. BMP Purpose Performance Standard AMI Design Storm* Onsite stormwater management Infiltrate the runoff volume from 3.2 inches (All projects)** the 10-year, 24-hour storm event within 24 hours Water quality Capture and treat 80% of 1.0 inch average annual runoff (Large projects only) Flow control Maintain peak flow rates at their 2 year, 24 hour storm: 2.38 inches predevelopment levels for the (Large projects only) 2-year, 5-year, and 10-year, 24- 5 year, 24-hour storm: 2.85 inches hour runoff events 10 year, 24-hour storm: 3.20 inches *Design Storm data from Clean Streams Plan, (Otak 2009). "*Projects that are able to meet onsite stormwater management requirements will meet flow control requirements without additional measures. If an onsite stormwater facility provides water quality treatment(Table 3.1), additional water quality treatment facilities will not be needed. 4.1.1 BMPs for Pretreatment In Lake Oswego, catch basins Pretreatment is required when using a flow control BMP that does with less than a 18-inch sump are not provide sufficient water quality benefits. The following BMPs are considered conveyance structures, approved with this order of preference for providing pretreatment while catch basins with a sump where required (see Table 3.1): greater than or equal to 18 inches with a snout outlet are considered • Water quality BMPs listed in Table 4.3 pretreatment facilities that remove sediment via settling. • Proprietary BMPs approved under the TAPE program with a general use level designation (GULD) for pretreatment (see Section 3.4.4) • Catch basins with a sump greater than or equal to 18 inches, trapped with outlet (snout). 4.1.2 Onsite Stormwater Management Facilities used for onsite stormwater management must meet minimum design performance goals. Onsite stormwater management BMPs need to infiltrate the runoff volume from the 10-year, 24-hour storm within 24 hours. To reduce cost and size of BMP, consider incorporating green roofs and pervious pavement to reduce the size of impervious area. Lake Oswego Stormwater Management Manual Section 4.1 56 Stormwater Facility Design Guidelines Table 4.2 BMPs That Meet the Onsite Stormwater Management Requirements. Facility Design Performance Design Considerations Goal Rain garden/planter Capture and store the These facilities should be designed to drain the design storm in a surface design storm within 24 hours. Where soil infiltration pond for infiltration rates are not high enough to allow 100 percent through the biofiltration infiltration, additional infiltration BMPs must be soil media. implemented to the maximum extent possible, and an approved discharge location must be identified. Design as distributed facility with no more than 6,000 square feet of contributing impervious surface area. Rain gardens and planters with underdrains are suitable in areas only where onsite soils have proven suitable for bioretention. Amended bioretention mixes will be required if there is a risk of phosphorous export to surface water bodies. Compost-amended soils can export phosphorus. Bioretention soil mixes demonstrated to export phosphorous and other metals are prohibited. Drywell Capture and store the Pretreatment for water quality is required. Where design storm in a drywell soil infiltration rates are not high enough to allow for infiltration through the 100 percent infiltration, infiltration BMPs must be underlying soils. implemented to the maximum extent possible, and an approved discharge location must be identified. This facility is a UIC that must be registered with DEQ. Documentation of facility registration must be provided to the City. Infiltration trench Capture and store the Where soil infiltration rates are not high enough to design storm in a gravel- allow 100 percent infiltration, infiltration BMPs must filled trench for infiltration be implemented to the maximum extent possible, and through the underlying an approved discharge location must be identified. soils. This facility may be a UIC that must be registered with DEQ (unless constructed without an underdrain). Documentation of facility registration must be provided to the City if an underdrain is used. Sheet flow dispersion Provide adequate flow A 10-foot wide vegetated buffer must be provided path and soil quality for up to 20 feet of width of contributing impervious and depth to disperse surface. An additional 5 feet of buffer width must and infiltrate the design be added for each additional 20 feet of width of storm. contributing area or fraction thereof. For example, a 30-foot wide vegetated buffer would be required for a contributing impervious surface that is 50 feet wide. Soils must be protected or restored (BMP 4.6.16 Post-Construction Soil Quality and Depth). See Section 4.6 for design guidelines. 57 Section 4.1 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines 4.1.3 Water Quality BMPs Water quality BMPs must be sized to capture and treat 80 percent of the average annual stormwater runoff. This is equivalent to treating runoff from the first 1.0 inch of an individual storm event. Therefore, design storm volume for volume-based BMPs (constructed wetlands, wet ponds, planters, and rain gardens) can be calculated as the total runoff volume from a storm with 1.0 inch of precipitation. Similarly, design storm discharge for flow-based BMPs (swales, filter strips) can be calculated as the peak discharge from a 1.0-inch, 24-hour storm using the Santa Barbara Urban Hydrograph (SBUH) method and a synthetic NRCS Type 1A hydrograph. Table 4.3 BMPs that Meet Water Quality Treatment Requirements. Facility Design Performance Goal I Design Considerations Rain garden/planter Capture and treat the design Design to drain in 24 hours. storm in a surface pond for movement of water through the Design as distributed facility with no more biofiltration soil media. than 6,000 square feet of contributing impervious surface area. Bioretention facilities with underdrains are suitable in areas only where onsite soils have proven suitable for bioretention and there is not a risk of phosphorous export to surface water bodies. Compost- amended soils can export phosphorus. Flow-through facilities without downstream flow control BMPs shall be sized for the 10-year storm. Swales/filter strips/ Provide treatment through Provide minimum residence time of 9 sheet flow dispersion filtration by vegetation as water is minutes. conveyed through the facility or discharge area. Wet ponds and Capture and treat the runoff constructed wetlands volume associated with the water quality design storm event. Sand filter Capture and treat the peak flow Remove pollutants through contact with associated with the water quality sand media at a hydraulic rate sized to design storm event. ensure drawdown time of 24 hours. 4.1.4 Flow Control BMPs Flow control BMPs are required for large projects, unless the site drains directly to Oswego Lake, the Tualatin River, or the Willamette River. Flow control facilities shall be designed to maintain peak flow rates at their predevelopment levels for the 2-year, 5-year, and 10-year, 24-hour runoff events (see Table 4.1). Infiltration facilities designed for onsite stormwater management can be used to meet some or all of the flow control requirements. Lake Oswego Stormwater Management Manual Section 4.1 58 Stormwater Facility Design Guidelines Table 4.4 Flow Control BMPs. Facility Design Performance Goal Design Considerations Infiltration pond, Infiltrate all or part of stormwater runoff. If *Infiltration trenches may infiltration trench*, there is any surface discharge from facility, be, and drywells are always drywell* flow control standard must be met: maintain considered a UIC that must be peak flow rates at their predevelopment registered with DEQ. levels for the 2-year, 5-year, and 10-year, 24- hour runoff events. Detention pond or Maintain peak flow rates at their Constructed wetland or wet constructed wetland or predevelopment levels for the 2-year, 5-year, pond with storage also provides wet pond with storage and 10-year, 24-hour runoff events. water quality treatment. Detention pipes or Maintain peak flow rates at their vaults predevelopment levels for the 2-year, 5-year, and 10-year, 24-hour runoff events. Rainwater harvesting Capture and store for reuse all or part of stormwater runoff. If there is any surface discharge from facility, flow control standard must be met: Maintain peak flow rates at their predevelopment levels for the 2-year, 5-year, and 10-year, 24-hour runoff events. 4. 1.5 Impervious Area Reduction BMPs Several BMPs in this section are intended to substitute for impervious surface, reducing the area required to be treated by other BMPs. They do not typically provide stormwater treatment or flow control for areas of runon. These are: • Pervious pavement • Green roofs • Tree retention 4.2 Planting Design 4.2.1 Why Plants Matter in Storm water Design Plants play several important roles in the function of stormwater "Picking the Right Plant for the facilities while also contributing to the aesthetic quality of the Right Place" landscape in Lake Oswego. Planting design should respond to In order for plants to thrive and for variable environmental conditions (e.g., soil moisture conditions, a stormwater facility to achieve sun exposure, existing infrastructure, setbacks and sight distances, long-term success, planting pedestrian use and interaction, existing plant communities and design must carefully consider invasive species control, and visual buffering) in conjunction with the placement of each plant within and around a facility.The design surrounding community aesthetics and values. For infiltration should consider the mature height facilities, it is important to choose plants that do not require the use and spread of each plant and of fertilizers or pesticides to thrive. Plants should also be adapted to respond to environmental and local micro-climate conditions and require little to no extra watering cultural site conditions. after the first two years of establishment. In addition to "picking the right plant for the right place" for environmental or cultural reasons, there are several other factors that can affect plant selection and planting design. This section describes some of the most critical factors to consider in planting design and outlines a strategy for ensuring appropriate plant selection. 59 Section 4.2 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines 4.2.2 Vegetation Diversity and Its Effect on Water Quality Diversifying vegetation in stormwater facilities can significantly improve water quality treatment effectiveness by enhancing the ecological function and unit treatment processes occurring in stormwater facilities. Diverse, healthy native or site-adapted vegetation improves water quality by: • Reducing sediment load. The stems and leaf blades of vegetation intercept stormwater and act as physical " .:. filters by capturing sediment and associated pollutant . particles. Emergent plant species are particularly suited to reducing sediment load because of their plant structure and adaptation to water inundation and saturated soils. • Reducing water velocity. Vegetation reduces the velocity • ,; of water moving through a stormwater facility, allowing more "�` time for pollutants to settle to the bottom and minimizing turbulence that may otherwise induce re-suspension of sediments previously deposited in the pond bottom. - an s pay an imp an ro e m I enng sediment and pollutant particles from • Increasing ability of soil to absorb and filter pollutants. stormwater. Root systems increase the potential for water to filter through soil, which increases the adsorption, sorption, and anion exchange interactions that remove pollutants in stormwater. • Absorbing pollutants into plant matter. Plants will absorb some pollutants and heavy metals through their root systems and transfer them into their tissues. Plant species vary in their ability to assimilate pollutants and toxins into their tissues. For example, cattail species have demonstrated the ability to take up a wide variety of pollutants, while bur-reed species have proven to be the most efficient at uptake of lead, zinc, and total petroleum hydrocarbon (Jurries 2003). For stormwater facilities, vegetation is used as a part of the pollution removal process. The physical filtration process results in removal of sediment from runoff, while the biological uptake component can result in removal of metals, phosphorus, nitrogen, and some hydrocarbons from the stormwater. Dense, healthy vegetation will provide good filtration and biological ; w` :ram uptake in a stormwater facility. _ r Indicators of degraded filtration and biological uptake include: • • Low plant density: bare soil and sparse vegetation on facility► ,; bottom and side slopes _ _ � • Unhealthy plant community: poor apparent health of the •,"`t .' : ;. - plant community (e.g. signs of disease, stress, and lack of ". '• vigor) 4. • Monocultures: a planting comprised or dominated by a single •!ma cyan . ac .ernes sominate t is pan species community and are an indicator of poor plant community health. • Dominated by invasive species: invasive species spread rapidly and suppress the establishment, growth, and health of the intended plant community. Lake Oswego Stormwater Management Manual Section 4.2 60 Stormwater Facility Design Guidelines Visual indicators of plant stress or disease might include: • Leaf cupping, curling, or rolling Unseasonal leaf drop • Yellowing, browning, or abnormally discolored leaves Stunted leaf or plant growth • Wilting or abnormal drooping '�y .� ...: - .� S a 'It', � o- 1 : 7 .d" ga i '% ` s `R gk _ 7m ,., , , _. ., ,. „,„:.,,,,, .... . .,,.....,,,..,,,,,/, ,w .,„ .... , . .. .. jj,„..c,,, i. dot _ , ,:..,,,., ,,,.. , i n ,,,,,*.4*, :ervva,f:s, �7 6 �} yV....",,- is cons ruc e• we al as a ea y vane y o naive rees, s ru•s,an• groun•cover species simultaneously providing water quality benefits and a neighborhood amenity. Invasive Species or Noxious Weeds Definition: Non-native species that displace and compete with native or desirable plants and cause economic or environmental harm. Examples of Invasive and/or Noxious Weeds in Oregon: • Knotweed (e.g. Japanese, giant, • Bamboo • Kudzu etc.) • Garlic mustard ▪ Purple loosestrife • Butterfly bush • Teasel • Toadflax(e.g. yellow and • Thistles(e.g Canadian, bull, Italian, dalmation) etc.) ▪ Yellow-flag iris • Hawkweed (several varieties) • Scotch broom • Tansy ragwort • English ivy • Blackberries(Himalayan and • St. Johnswort evergreen) ▪ Clematis(Old man's beard) 4.2.3 Plant Selection Process: Steps and Considerations The following section describes one way to approach planting design for a stormwater facility.A summary of the plant selection process is shown in Figure 4.1. See Appendix H for BMP facility plant lists. 61 Section 4.2 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines Figure 4.1 Plant Selection Process. Information Gathering Step 1 Conduct site assessment and feasibility analysis and collect relevant project information. y Facility Design and Plant Selection Step 2 Select appropriate plants for BMP Q and site location. REVISE PLANT SELECTION Check Availability of Plant Selection with Local Nurseries NO Step 3 Are the selected plants available, or is there enough time to contract with a nursery to grow or acquire the plants needed? / / REVISE I YES PLANT y SELECTION Review Maintenance Requirements of Plant Selection and Placement NO Step 4 Does the plant selection align with long-term maintenance expectations, capabilities, and resources? i YES Update Construction Documents, Plans, and Step 5 Specifications To Guide Installation and Establishment of Vegetation Step 1: Information Gathering Before plant selection can begin, the applicant needs to gather information about existing and proposed site conditions, the surrounding site context, desired future uses of the site, aesthetic requirements, and maintenance considerations that should influence design. Each stormwater facility should be designed in response to the unique characteristics and circumstances of its location. Much of this information can be gathered during the site assessment and feasibility analysis. Lake Oswego Stormwater Management Manual Section 4.2 62 Stormwater Facility Design Guidelines Information gathering should begin by evaluating site characteristics and site circumstances, including but not limited to the following. • Determine hydrologic regime (from local micro-topography and micro-climate); for example, infiltration facilities need plants that tolerate both saturated and dry periods. • Is groundwater present (less drying/moist soils)? • How does water move through and interact with the site? • Evaluate site soils to determine soil composition and design characteristics. • Is a soil test needed to verify quantity of organic matter, identification of potential contamination, or nutrient contents? • If amendments can be used instead of importing new soil, what types of amendments are appropriate for the proposed facility type? • Determine the duration and intensity of sunlight and shade across the site. • Observe the surrounding context. • Determine if the site is near a natural area, a designated sensitive area, within a neighborhood, or within a commercial district • How can the planting design help the facility respond or adapt to the surrounding context? • Assess whether there are potential contaminant sources from surrounding areas. • Are there any potential sources of contaminants that need to be mitigated in the site design (e.g., heavy road traffic, an industrial land use, commercial car wash, etc.)? • Document whether or not the site is adjacent to or near any of Lake Oswego's natural areas. See Parks Plans 2025: Lake Oswego's Parks, Recreation • Is there an opportunity for the site to provide additional and Natural Areas System Plan habitat connectivity or value for a fragmented natural (Lake Oswego 2012a)for more area'? information on the City's parks and natural areas and how they Are there particular species that may be attracted to relate to stormwater and habitat the site (bird populations, amphibians, butterflies, etc.) management goals for Lake p p p Oswego. and that may benefit or be harmed by its location? • Are there reference habitats nearby that provide clues to choosing an appropriate plant selection? • Determine desired future uses. • Is there a community desire or opportunity to incorporate multiple uses for the facility? • For example, can trails, art, or wildlife viewing be incorporated into the design? • Is the site highly visible? • Will the site provide educational opportunities for neighborhood, community, or school groups? 63 Section 4.2 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines • Should the facility blend into the surrounding context or make an artistic statement? • Evaluate the aesthetic requirements and drivers for the site. What personal or contextual ways can plants be organized'? � How can edges and borders be clearly organized around =••• ff landscape facilities to ensure stewardship and notl'� t` marginalization due to perceived "messiness"? k; -44,9N.c in,,4 Step 2: Facility Design and Plant Selection '' 7 - y After gathering enough information about the site being designed, it is time to begin plant selection. As a rule, the applicant should select plants that will provide for year-round water quality and is p an ing •or•er as seen ove a en aesthetic functions. Nativeplants should always be considered first byweeds and does not show clear signs y of care. to meet design goals. (See following description of native versus non-native vegetation to learn more about the advantages of planting native species.) If native species cannot meet the design needs of a site due to specific environmental or cultural goals, horticulturally appropriate species (that is, species that are well adapted to local site conditions and reliably perform well in stormwater facilities) can be used to supplement a native plant selection. All plant lists and species selections should be created based on species available and obtained from local nurseries. The justification for following this protocol is explained below. Native versus Non-Native Vegetation Planting native species should be a priority for stormwater facilities for several reasons.: • Native plants are adapted to local soil, hydrology, and climate conditions. • Native plants compete with invasive species that threaten to overtake facilities and create monocultures. • Native plants are likely to be less susceptible to pests and diseases. Native vegetation also provides the following aesthetic benefits: • Native plant species placed in natural-looking clusters can better blend stormwater facilities into adjacent natural areas, open spaces, and neighborhoods. • Planting a diversity of native plants, including trees, shrubs, and low-growing species, creates a more park-like visual experience for the community. • Native vegetation provides wildlife habitat (food and shelter) and the presence of beneficial organisms (such as pollinators). Lake Oswego Stormwater Management Manual Section 4.2 64 Stormwater Facility Design Guidelines Some non-native, horticultural species are suitable for stormwater Native: facilities. These species are usually chosen when a site is located A species that occurs naturally in a within a highly developed area where an applicant has a specific particular region, ecosystem, and designgoal in mind. Theyare also beneficial when the mature size habitat. of native species is too large to be accommodated in a facility (e.g., Native(other definition): narrow parking swales, flow-through planters alongside buildings, Plant species that occurred naturally etc.) and a specific type of species is desired. For some BMPs, such in a particular region, ecosystem, and as green roofs, there are only a few native species easily found in habitat prior to European contact. nurseries, and it is useful to augment the planting palette with non- Acceptable non-native or native, horticultural species. In these instances, plant lists should horticultural: be reviewed by a landscape architect or horticultural professional Annual or perennial species that is not experienced in native natural systems. persistent or competitive with native vegetation. Step 3: Check Availability of Plant Selection with Local Naturalized species: Nurseries Nonnative species that were introduced by humans to a region, It is important to make sure that the plants selected for a design are ecosystem, and habitat, but have now available at a local nursery. If a plant list contains species that are become a part of natural native plant not typically available or are requested in a large quantity, a nursery communities. can often help find the plants needed if given enough time. Some Undesirable plant species: Plant nurseries can even grow species not normally within their selection if species that are on the regions they are given at least a year of advance notice. If the project timeline noxious weed list and plants that out- is shorter than a year between design and installation, which is often compete and dominate native plant the case, the need to verifyavailabilityofplant species is even more communities. p important. The more a designer can collaborate and communicate with local nurseries prior to plants being delivered to a project, the more likely a facility will end up with the plants intended in the design. Step 4: Review Maintenance Requirements of Plant Selection and Placement During the design development process, it is important to consider the maintenance needs, limitations, and consequences of design decisions. • Review operations and maintenance guidance for your facility Maintenance information and design. annual 0&M checklists can found Assess available maintenance capabilities and resources to in Appendix H of this manual. • maintain the site. • Determine whether additional training or information is needed to properly maintain the facility. • Evaluate the maintenance needs of various planting design alternatives • Will vegetation border walkways, driveways, or other surfaces where pedestrian or vehicle access will be an issue? What is the mature height and spread of each of species? • What are the critical lines of sight that need to be maintained at intersections, along sidewalks, and along bicycle paths for pedestrian and vehicular safety and crime prevention? • Will vegetation border inlet or outlet structures, pervious pavement surfaces, drains, or other infrastructure that could be negatively affected by vegetation debris or root systems? • Does the planting design achieve proper access in and around the stormwater facility for maintenance activities (for example, in and around inlet or outlet structures)? 65 Section 4.2 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines Step 5: Update Construction Documents, Plans, and Specifications After the planting plan and list have been evaluated through all criteria in Steps 1 through 4, update the construction document plans and specifications in order to clearly guide the installation and establishment of vegetation. 4.3 BMP Design Methods and Computations The City's current guidance for sizing and hydraulic analysis of BMPs is to use the Santa Barbara Urban Hydrograph (SBUH) method. The SBUH method is a single-event model that estimates a flow hydrograph for a representative rainfall event. The SBUH method was developed by the Santa Barbara County Flood Control and Water Conservation District. Applicable to urban areas, it converts design storm incremental excess rainfall depths into instantaneous unit discharge hydrographs (Debo and Reese 2003). The SBUH method is appropriate for sizing most BMPs included in this manual. There are proprietary stormwater models available that may be used to conduct SBUH modeling (functionally equivalent algorithms in proprietary programs are also allowed).Alternatively, a spreadsheet may be used. See Appendix F for further information. The City uses the SBUH method to verify sizing of facilities. Precipitation is one of the input parameters for the SBUH method. Lake Oswego periodically updates design methods and computations The City currently uses the NRCS Type 1A, 24-hour rainfall to comply with state and federal distribution resolved into 10-minute time intervals as the standard permit guidelines. Check the design hydrograph. Table 4.5 summarizes precipitation for various website(www.raintoriver.org)to design storms. find current guidance. When sizing flow control facilities, pre-development conditions should be modeled using a CN of 70. Table 4.5 City of Lake Oswego Precipitation. Two other methods are approved by the City for sizing of stormwater facilities: the equations Design Storm/ 24-Hour Rainfall within the BMP Design Guideline for that facility Recurrence Interval Depth or the City of Portland Presumptive Approach (years) (inches) Calculator (PAC). For water quality facilities Water Quality 1.0 sized using the PAC, the facility surface area must be increased by 25 percent. No correction 2-year 2.38 factor is required for onsite stormwater facilities 5-year 2.85 sized using the PAC with flow control criteria. 10-year 3.20 The equations within the BMP sections do not 25 year 3.73 include routing or infiltration, and will therefore 50-year 4.13 result in larger facilities than those sized using 100-year 4.54 the SBUH or PAC. 500-year 5.55 Source: Otak 2009. 4.4 Soils Soil health is fundamental to a functional stormwater facility. Soils play the most important role in treating water quality, successful plant establishment, and the long-term success of a site.A handful of soil can hold millions of microorganisms that are critical to recycling soil nutrients, maintaining soil structure, processing pollutants from runoff, and aiding plants in nutrient and water uptake. Soil should be treated as a community of living organisms. It needs to be protected and handled carefully during construction. Lake Oswego Stormwater Management Manual Section 4.4 66 Stormwater Facility Design Guidelines Consider methods to use and amend onsite soil first. If onsite soil is not NRCS Soil Biology Primer appropriate for the facility design goals, consider a sustainable way to website: salvage soil for use at another local site. If soil or amendments need to http://soils.usda.gov/sqi/concepts/ be brought into a site from an outside source, research where the soil is soil_biology/soil_food_web.html coming from, what materials amendments are made from, and whether or not sustainable practices are being supported. Avoid and minimize soil compaction and disturbance. If subsoils/ The two most common soil testing backfill material needs to be stockpiled during construction, implement labs used to test bioretention soil appropriate erosion control procedures (plastic covering, silt fence, mixes according to specifications etc.). If topsoil needs to be moved and stockpiled during construction, are: place topsoil in shallow linear mounds and sow a temporary cover crop Soil Control Lab(California): over the mounds. Cover crops will stimulate soil activity and protect http://compostlab.com/ topsoils from erosion. The best protection for topsoil during construction is to move it from shallow mounds into its target area as quickly as Soil Test Farm Consultants possible. Leaving topsoil in large piles and covering it with plastic for (Washington): longperiods will result in sterile soil, devoid of beneficial organisms that http://www.soiltestlab.com/ g treat water quality and aid in plant establishment. Salvaging Native Soil • If possible, salvage litter and duff and store separately from the topsoil to prevent loss of any non-weedy seed banks in litter • Do not mix topsoil with subsoil to prevent dilution of beneficial soil microorganism community. • Salvage soil when dry and keep dry for storage (cover with plastic if needed) • Optimum soil mound size is 6 feet wide and 3 feet high, but longer linear piles are preferred to higher piles - Information adapted from"Roadside Revegetation:An Integrated Approach to Establishing Native Plants" FHWA, 2007. Recent research by the Washington State Department of Transportation and the Washington Stormwater Center have found that LID stormwater facilities perform very well in reducing concentrations of total suspended solids, total and dissolved metals, fuel and oils, and many other pollutants. However, preliminary water quality monitoring has found export of phosphorus from LID BMPs (concentrations are higher after treatment). The compost used as soil amendment in LID BMPs, which is critical to plant health, is high in nutrients such as phosphorus. Preliminary monitoring investigations suggest that export of pollutants is especially high in the months following installation. Ongoing research is being conducted by the Washington Stormwater Center and various municipalities on whether construction practices or soil mixes can be modified to reduce export of nutrients from LID facilities. Given the sensitivity of Oswego Lake to phosphorus, the City is following this research and will be updating biofiltration soil mix recommendations and/or BMP design and construction practices based on research findings. 67 Section 4.4 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines 4.5 Pollution/Flow Control Manhole Design Meeting flow control requirements requires both adequate detention storage volume and a control structure that releases stormwater at the design flow rate. Lake Oswego requires that detention facilities include a pollution/flow control manhole. Computer-aided design (CAD) details for the City's pollution/flow control manhole are provided on the City's website. Refer to the BMP design guideline, in Section 4.6.13 for pipes and vaults, provides sizing guidance. 4.6 BMP Design Guidelines The following section includes BMP facility design guidelines for facilities approved for use in Lake Oswego. Section BMP Facility Type 4.6.1 Rain garden 4.6.2 Planter 4.6.3 Infiltration trench 4.6.4 Drywell 4.6.5 Greenroof 4.6.6 Pervious pavement 4.6.7 Rainwater harvesting 4.6.8 Filter strips 4.6.9 Swale 4.6.10 Sand filter 4.6.11 Constructed wetland 4.6.12 Ponds 4.6.13 Detention pipes and vaults 4.6.14 Sheet flow dispersion 4.6.15 Tree Retention 4.6.16 Post-Construction Soil Quality and Depth Lake Oswego Stormwater Management Manual Section 4.6 68 Stormwater Facility Design Guidelines This page is intentionally left blank. 69 Section 4.6 Lake Oswego Stormwater Management Manual L +T 1 'I _. --, I • - �� I �' + ti I J a a • .+ f '-. .. n,i, e ._4iG ' ' . , ''' ar 0 -ens _ . ....•_ ,,....„ ,; .....„...... ..„... _ .... . , • .„. -.. .„...„.. 74F:it: - 4.6.1 Rain Gardens Definition Rain gardens are vegetated depressions that provide stormwater treatment during the capture and infiltration of water runoff through a biofiltration soil medium. Introduction Applicability Table Rain gardens treat stormwater through sedimentation of particles in ponded water, filtration and BMP Parameters phytoremediation through contact with vegetation, Appropriate for and biodegradation and adsorption of pollutants pretreatment V through contact with soil organisms and chemical Appropriate for onsite soil processes. Rain gardens are ideal for residential stormwater management and small commercial sites, within parking lots, and along roadways. They can help fulfill landscaping Provides flow control requirements. (when designed for infiltration) Linear rain gardens may look similar to swales, but Provides water quality they have a flat bottom and pond water to infiltrate treatment it vertically through treatment soils. Swales have a gently sloping bottom and provide treatment Appropriate for residential through filtration by vegetation as water is conveyed through the swale. Rain gardens can have multiple UIC permit required distinct depressions, called cells, that can be linked Design professional hydraulically via overflow structures or berms. required Through thoughtful plant selection and design, a rain garden can become a community and wildlife asset in addition to treating stormwater. Rain gardens can showcase sculpture, serve as a butterfly garden or habitat, or function as a sedge meadow, becoming a local source of pride and reinforcing neighborhood identity. Figure 4.2 Rain Garden Section (Typ). Facility Footprint r I Optional benching above -Design ponding design pondingdepth to depth 9 p S Bottom width ' integrate rain gardens into the landscape \ 1 Freeboard .� • �{ See ' o — construction y , 7 a 4 section for Native soil Overflow ° d soil protection 'A 6 a:p - requirements O s p l °° Biofiltration soil mix • °ii _Mulch Outlet to approved Y T discharge location Infiltration zone Lake Oswego Stormwater Management Manual Section 4.6 70 Rain Gardens Site Requirements , r��, • Cell bottom must be at least 1 foot above seasonal high '• ground water elevation or other layer that limits infiltration ` '' (e.g., bedrock, clay lens) if contributing area is less than 1i "11/4 , l'. _ 1 3,000 square feet of impervious surface. • h„`' • ki f" ,• ;- , ,;;^ • Cell bottom must be at least 3 feet above seasonal high ' ` • ', "' ,' ;. s•f•:;' ground water elevation if contributing area is 3,000 square ,r , ;,,, ,, /� _. feet or greater of impervious surface. 1, i A,y": • Infiltration rate must be at least 0.25 inches per hour for a Rain garden at Glencoe Elementary, Portland, rain garden. Otherwise, the rain garden will pond water for OR. longer than a 24-hour period. If infiltration rate is less than 0.25 inches per hour, facility must have an underdrain. On large projects, additional flow control measures will apply. • Infiltration rain gardens must meet requirements and setbacks listed in Section 3.4.3. Geometry As a rule of thumb, the footprint of a rain garden should be approximately 10 percent of the impervious surface draining to the rain garden if infiltration rates are at least 0.5 inch per hour. This general rule can provide an initial estimate of rain garden size, but final sizing must be determined using the criteria and methods described below. Rain Garden Dimensions • Minimum bottom width: 2 feet • Planted side slopes: no steeper than 3 horizontal:1 vertical (rock or concrete walls may be used for areas that require steeper side slopes) • Minimum freeboard: 2 inches, if contributing area is less than 3,000 square feet; 6 inches for larger contributing areas (up to 6,000 square feet maximum per individual facility without engineered outflow) • Maximum ponding depth: 12 inches . - _ —----- ----_-_ 4 401. , . - .. LAI a - _. .. i .,...- .---,r-ir • ..... ,,,art-,-t,' .. - - ter. -T ^M vo Rain water is directed from this home's rooftop to a rain garden in the front yard. 71 Section 4.6 Lake Oswego Stormwater Management Manual Rain Gardens Design Steps Design Performance Goal Capture and store the design storm in a surface pond for infiltration through the biofiltration soil medium. Step 1: Assess Site Suitability Determine infiltration rate for native soils where the rain garden will be located. Conduct infiltration test. See Appendix B. Step 2: Determine Drainage Basin Characteristics Calculate the drainage area and Natural Resource Conservation Service (NRCS) Curve Number separately for both pervious and impervious surfaces in the basin. See Appendix F for NRCS Curve Number determinations. Parameter Units Value Aimp and Ape drainage area (impervious and pervious) acres CN NRCS curve number (impervious and pervious) unitless Step 3: Calculate Runoff Volume Calculate the storm runoff volume of the onsite design storm or water quality storm (as applicable) separately for impervious and pervious surfaces and then sum the results using the equations: 1,000 * [ - 10] 2 V imp or VpeN= 3,630 *A* ( Pdesign - 0'2 CN ) + 0.8 r 1,000 Pdesign - 10 L CN Parameter Units Values Vimp + VpeN runoff volume (impervious and pervious) cubic feet P design precipitation depth inches water quality: 1.0 design onsite: 3.20 Step 4: Determine Rain Garden Treatment Depth Select the design depths for ponding in the rain garden before overflow and for the soil media. Parameter Units Values rain garden ponding depth feet maximum 12 Dpond Dmed.a rain garden media depth feet minimum 18 Lake Oswego Stormwater Management Manual Section 4.6 72 Rain Gardens Step 5: Develop Preliminary Rain Garden Geometry Select preliminary dimensions of the rain garden including length, width, and side slopes (maximum 3H:1V). Calculate average surface area of the ponding area and the horizontal cross sectional area of the media. Parameter Units Values Apond average ponding surface area square feet square Amedia horizontal cross sectional area of soil media feet Step 6: Check Rain Garden Volume Calculate the design storage volume (Vd) of the rain garden using the equation below. If volume (Vd) is less than the ruoff volume (Vimp + Vpew), adjust the facility geometry (e.g., increase area, ponding depth, media depth) and repeat this step. This equation can be modified if additional subsurface storage layers are included (e.g., gravel sump below and underdrain). Vd = Apond * °pond +Amedia * Dmedia * gmedia,Where nmedia applies to subsurface storage layers. Parameter Units Values ns porosity (soil media) unitless 0.25 119 porosity (gravel layer) unitless 0.40 Step 7: Check Drawdown Time Calculate the storage volume of the rain garden using preliminary geometry. If volume is less than the runoff volume (Vimp + Vper,), adjust the facility geometry (e.g., increase area, ponding depth) and repeat this step. Td = Dpond = 24 hours max Parameter Values Values Td drawdown time hours maximum 24 rain garden ponding depth inches maximum 12 Dpond infiltration rate inches per hour 73 Section 4.6 Lake Oswego Stormwater Management Manual Rain Gardens Materials Biofiltration Soil Mix Rain garden soil media must support long-term plant and soil health and provide treatment to water as it infiltrates. Biofiltration soil shall meet the requirements for topsoil in Appendix K (from the 2013 Portland Stormwater Management Manual). This mixture can be achieved by amending native soils with compost, if soils are sandy loam or coarser. If site has finer grained native soils, a biofiltration soil mix may be needed to meet required infiltration rates (See Section 4.4 for information on amending onsite soils). Component Percent by Volume Sandy loam 60-70% Composted plant-based organic matter 30-40% Fines passing a size 200 sieve are restricted to 5 to 15 percent of the soil volume. Mulch Fine to medium sized hemlock bark or well-aged organic yard debris compost is recommended for rain gardens. It should be placed in the facility only in areas above the design flow depth. Keep mulch material out of the stormwater flow path to avoid clogging inlets or outlets. Mulch must be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It should not be over-applied. In addition to larger cobble flow dissipators (2- to 6-inch round washed river cobble), it is acceptable to use optional mulch in high flow and inundated facilities to provide soil coverage. Use a washed round river rock mulch (3/4 to 1-1/2 inches) to a depth of 2 inches and add compost tea to planters as additional nutrients are needed. Vegetation Objective Establish dense plant growth with a diversity of groundcovers, shrubs, and trees along edges and borders for maximum runoff treatment and weed control. Plant Selection and Diversity Rain gardens often experience moist to saturated soil conditions during the wet, rainy season and dry soil conditions during warm summers. Plant selection should be based on water level tolerances during the rainy season, as well as ability of plants to withstand dry summer conditions. Select a minimum of seven species for each facility. Select species that are suitable for the hydrologic, light, and soil conditions in the proposed rain garden. Rain gardens should be designed so that they do not require mowing. See Appendix H for a list of plants appropriate for rain gardens. Lake Oswego Stormwater Management Manual Section 4.6 74 Rain Gardens Plant Quality Vegetation Type Recommended Size Sedges and rushes 10-inch deep container'; plug2; division; rhizome; or tuber 1 gallon or equivalent; plug; 4-inch pot allowed if the facility is left Grasses and forbs off-line for the first wet season, allowing plants an appropriate establishment period Shrubs 1 gallon; bareroot; or equivalent Deciduous trees 3 gallon container; bareroot or equivalent, minimum caliper should be 1 1/2" at 6 inches above base Evergreen trees 3 gallon container or equivalent; minimum height 6 feet, unless approved by City 110-inch deep container sedges and rushes provide a well-developed root system that more easily adapts to site conditions. 2 Plugs and container stock are more often grown from seed. Consider acquiring plants grown from seed in order to preserve genetic diversity and increase resiliency of vegetation on site. All plants shall meet most recent standards as defined by the American Standard for Nursery Stock. Grouping Plant Species Place smaller plants (1 gallon container or smaller) in odd-numbered clumps (3, 5, 7, etc.) of the same species throughout the planting areas. Avoid planting in rows. Planting Depth All plants should be buried as deep as they were grown in the nursery. Plant bare-rooted plants in a hole wide and deep enough so that roots are not bent, crowded, or exposed to air. Bareroot trees shall be planted so that their root flare is even with the soil surface. Woody Vegetation Shrubs and trees shall be sited to promote long-term health and survival, minimize maintenance, and protect lines of sight. Locate shrubs and trees to allow for maintenance access to the treatment area. Shrubs should be a minimum of 3 feet from sidewalks and roadways. T Rain Garden Side Slope Planting Establish groundcover vegetation that will protect slopes from erosion and provide competition for invasive or weedy vegetation. Plantings along side slopes and tops of banks should maintain vehicular and pedestrian lines of sight near street crossings. 75 Section 4.6 Lake Oswego Stormwater Management Manual Rain Gardens Conveyance and Outlet/Overflow Rain gardens must include overflows that outlet to an approved discharge location. If native soil infiltration rates are less than 0.25 inches per hour, rain gardens must include underdrains that are a minimum of 4" diameter for private facilities and a minimum of 6" for public facilities. Underdrains shall be slotted or perforated polyvinyl chloride pipe (PVC) that conforms to ASTM D 3034, with a pipe stiffness of 46 psi or a minimum of SDR of 35 or approved equal. Installation and testing requirements shall conform to the current Uniform Plumbing Code and Oregon Department Of Transportation (ODOT) Standard Specification 02415.50 for polyvinyl chloride pipe. Slotted perforations (0.064 inches wide x 1.00 inch long, spaced 0.3" on center) are preferred. Construction Mark rain garden boundaries with stakes or flagging prior to construction and avoid any unnecessary soil compaction or disturbance. If soil is compacted, it is critical that it be uncompacted to ensure proper infiltration rates and function of the rain garden. Loosen native soil to a depth of at least 4 inches before placing biofiltration soil mix or amending native soils. This step is key to promoting appropriate rooting depth of plants and infiltration into native subsoils. The rain garden and surrounding areas should be protected from sedimentation that will reduce capacity. Maintenance Proper maintenance is essential for a functioning rain garden. Please refer to the operations and maintenance section (Chapter 7) for more information. Native grasses and sedges do not require cutting or mowing. See maintenance checklists in Appendix I for further maintenance guidance. Herbicides should not be used as a weed control technique within stormwater facilities. Insecticide and fungicide use should also be minimized. See also Appendix A-4 for IPM strategies appropriate for stormwater facilities. Lake Oswego Stormwater Management Manual Section 4.6 76 Rain Gardens This page is intentionally left blank. 77 Lake Oswego Stormwater Management Manual e , .. ` t \ 1 E %. D te.r1PS\ ; / -. , r...' _ % • . . . aii ..�,, ,,,,� „., I _,, ,,i. „. 4.6.2 Planters Definition Planters are vegetated reservoirs with structural walls that treat and, where soils are suitable, infiltrate stormwater through a biofiltration soil medium. Introduction Applicability Table Planters treat stormwater through sedimentation of particles in ponded water; filtration and BMP Parameters phytoremediation through contact with vegetation; Appropriate for and biodegradation and adsorption of pollutants pretreatment V through contact with soil organisms and chemical soil Appropriate for onsite processes. Planters are essentially rain gardens with stormwater management structural walls. The following considerations may help you select the best facility for your site: Provides flow control (when designed for • Planters require less space than rain gardens to infiltration) treat the same contributing area providing they are flat bottomed Provides water quality treatment V • Planters are more costly than typical rain gardens to construct because they have structural walls. Appropriate for residential • Planters typically require a structural engineer to UIC permit required design (because of their structural walls). Planters Design professional may receiving stormwater from downspouts need be required17 energy dissipation at outlet and planters receiving water from the street need a sumped pollution control catch basin at inlets. • Flow-through (lined) planters have no setback requirements relative to building foundations as long as they meet setback requirements of rights-of-way, utilities, and pipelines. They can be constructed adjacent to buildings (for example, to treat roof runoff). • Planters can help fulfill a site's landscaping requirements . • Planter should be designed to consider safety issues (including pedestrian safety). Curbing fencing, or placing planters above grade may be necessary. Figure 4.3 Planter Section (Typ.). Facility footprint See construction section for ` Alternate inlet/building downspout soil protection requirement Overflow Design Walkway Surface inlet ponding / , depth V Mulch Waterproofing/liner •• : ,o ,O , Planter a Biofiltration soil mix 0 0:o a ' i Choker course To approved outlet location FP ` Drain rock .....-s� —Underdrain to run length of planter where required Native soil r . Optional bottom and drain for lined facilities Lake Oswego Stormwater Management Manual Section 4.6 78 Planters Site Requirements • Bottom of planter must be at least 1 foot above "• • seasonal high ground water elevation or other j layer that limits infiltration (e.g., bedrock, clay •- lens) if contributing area is less than 3,000 • square feet of impervious surface. r< • Bottom of planter must be at least 3 feet above ;, , .` }' seasonal high ground water elevation or other ,, impermeable layer if contributing area is 3,000 ;. :/..- • :e square feet or greater of impervious surface. ' • Infiltration rate must be at least 0.25 inch per hour for an infiltration planter. Otherwise, the : . planter will pond water for longer than a 24-hour Stormwater planter using recycled building walls. period. If infiltration rate is less than 0.25 inches per hour, facility must have an underdrain. • Infiltration planters must meet requirements and setbacks listed in Section 3.4.3. • Consider pedestrian safety and access when designing planters. Ensure that planters located next parking areas have clearance for pedestrians exiting vehicles. Geometry Planter Dimensions • Maximum ponding depth: 12-inch • Minimum freeboard: 2 inches if contributing impervious area is less than 5,000 square feet; 6 inches for larger contributing areas • Width: - at least 30 inches for infiltration planter - at least 18 inches for flow-through planter • Depth of biofiltration mix: 18 inches LtP1 : L ., 4 -- ..: qie.mayyy :,g C _- _ S '1144';ifyr ' — .,y 4i ,:p 14, s_Er lry 4, • : „Ili 0, , ,_ ,iive,f.,y,' „ , ,i4i (,..A t. t is .'T ,� . • _ ,,,, ; . Street right-of-way planters. Red osier dogwood providing year-round aesthetic interest. 79 Section 4.6 Lake Oswego Stormwater Management Manual Planters Design Steps Design Performance Goal Capture and store design storm in surface pond for infiltration through biofiltration soil mix. Step 1: Determine Drainage Basin Characteristics Calculate the total drainage area and composite Natural Resource Conservation Service (NRCS) Curve Number for both pervious and impervious surfaces in the basin. See Appendix F for NRCS curve number calculations. Parameter Units Value Amp and Aper drainage area (impervious and pervious) acres CN NRCS curve number (impervious and pervious) unitless Step 2: Calculate Runoff Volume Calculate the storm runoff volume of the onsite design storm or water quality storm (as applicable) separately for impervious and pervious surfaces and then sum the results using the equations: 1,000 * [ - 10] 2 Vimp or Vperv= 3,630 *A*( design - 0'2 CN ) Pdesign + 0.8 L r 1,000 - 10] CN Parameter Units Values Vimp + Vpery runoff volume (impervious and pervious) cubic feet P design precipitation depth inches water quality: 1.0 design onsite: 3.20 Step 3: Determine Planter Treatment Depth Select the design depth for ponding in the planter before overflow. Parameter Units Values Dpond planter ponding depth feet maximum 12 Dmedia planter media depth feet minimum 18 Lake Oswego Stormwater Management Manual Section 4.6 80 Planters Step 4: Calculate Planter Area Determine the surface area of the planter using the equation below. This equation can be modified if additional subsurface storage layers are included (e.g., gravel sump below an underdrain). V. + V imp pery As = Dpond + Dmedia * nmedia,Where nmedia applies to subsurface storage layers. Parameter Units Values AS surface area of planter square feet nmedia porosity (soil media) unitless 0.25 nmedia porosity (gravel layer) unitless 0.40 81 Section 4.6 Lake Oswego Stormwater Management Manual Materials Planters Planter Walls Planter walls shall be made of concrete, unless other materials are approved by the City Engineer. Chemically treated wood that can leach out toxic chemicals and contaminate stormwater must not be used. Waterproof Liners Planters need waterproof liners placed along side walls to prevent potential lateral movement of water. Liners should be 30 mil PVC membrane or equivalent. Drain Rock Drain rock is required below the biofiltration soil mix (i.e., growing medium; see below). For infiltration planters, use 3 -inch to 1'/z-inch washed drain rock. Drain rock shall conform to ODOT Standard Specifications 00430.11 or American Association of State Highway and Transportation Officials (AASHTO) No. 4. Furnish granual drain backfill material of 3 -inch to 1'/2-inch crushed gravel meeting the following gradation requirements: Seive Size Percent Passing (by Weight) 2" 100% 1 1/2" 95- 100% 1 1/4" - 1" - 3/4" 0- 15% 1/2" 0-2% 1/4" - Underdrain If native soil infiltration rates are less than 0.25 inches per hour, planters must include underdrains that are a minimum of 4 inches diameter for private facilities and a minimum of 6 inches for public facilities. Underdrains shall be slotted or perforated polyvinyl chloride pipe (PVC) that conforms to ASTM D 3034, with a pipe stiffness of 46 psi or a minimum SDR of 35 or approved equal. Installation and testing requirements shall conform to the current Uniform Plumbing Code and Oregon Department Of Transportation (ODOT) Standard Specification 02415.50 for polyvinyl chloride pipe. Slotted perforations (0.064 inches wide x 1.00 inch long, spaced 0.3 inches on center) are preferred. When pipes are greater than 10 inches All flow-through facilities shall use 3 -inch to %-inch washed drain rock around underdrain. Lake Oswego Stormwater Management Manual Section 4.6 82 Planters Choker Course Drain rock and biofiltration soil mix must be separated by a 2-inch to 3-inch choker course layer. Granular material should be 3/4-inch to '/z-inch crushed or uncrushed rock or gravel meeting the following gradation requirements: Seive Size Percent Passing (by Weight) 2" 100% 1 1/2" 100% 1 1/4" 100% 1" 100% 3/4" 90- 100% 1/2" 0- 15% 1/4" 0-3% Biofiltration Soil Mix Planter soil media must support long-term plant and soil health and provide treatment to water as it infiltrates. Biofiltration soil shall meet the requirements for topsoil in Appendix K (from the 2013 Portland Stormwater Management Manual). This mixture can be achieved by amending native soils with compost, if soils are sandy loam or coarser. If site has finer grained native soils, a biofiltration soil mix may be needed to meet required infiltration rates (See Section 4.4 for information on amending onsite soils). Component Percent by Volume Sandy loam 60-70% Composted plant-based organic matter 30-40% Fines passing a size 200 sieve are restricted to 5 to 15 percent of the soil volume. Mulch Use washed rock mulch to a depth of 2 inches and add compost tea to planters as additional nutrients are needed. In regularly inundated facilities, apply medium hemlock bark or well-aged organic yard debris compost. It should be placed in the facility only in areas above the designed flow depth. Keep mulch material out of the stormwater flow path to avoid clogging inlets or outlets. Mulch must be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It should not be over-applied. Vegetation Objective Establish dense plant growth with a diversity of groundcovers. Careful consideration should be given to the placement of trees and shrubs. Trees and shrubs shall be kept away from planter edges in areas where they will negatively impact vehicular or pedestrian movement or interfere with traffic line of sight. 83 Section 4.6 Lake Oswego Stormwater Management Manual Planters Plant Selection and Diversity Planters often experience moist to saturated soil conditions during the wet, rainy season and dry soil conditions during warm summers. Plant selection should be based on water level tolerances during the rainy season, as well as the ability of plants to withstand dry summer conditions. Select plants for more extreme (wet and dry) conditions than would be typical in adjacent landscaped areas. Select a minimum of five species for small treatment areas and add diversity for large facilities. Select species that are suitable for the hydrologic, light, and soil conditions in the proposed planter. Planters should be designed so that they do not require mowing. See Appendix E for a plant list appropriate for stormwater planters. Plant Quality Vegetation Type Recommended Size Sedges and rushes 10-inch deep container', plug2, division, rhizome, or tuber 1 gallon or equivalent, plug; 4-inch pot allowed if the facility is left Grasses and forbs off-line for the first wet season, allowing plants an appropriate establishment period Shrubs 1 gallon, bareroot, or equivalent 3 gallon container, bareroot, or equivalent; minimum caliper should Deciduous trees be 1 1/2" at 6 inches above base. Mature size should be suitable for location. Evergreen trees 3 gallon container or equivalent; minimum height 6 feet, unless approved by City. Mature size should be suitable for location. 110-inch deep container sedges and rushes provide a well-developed root system that more easily adapts to site conditions. 2 Plugs and container stock are more often grown from seed. Consider acquiring plants grown from seed in order to preserve genetic diversity and increase resiliency of vegetation on site. All plants shall meet most recent standards as defined by the American Standard for Nursery Stock. Grouping Plant Species Place smaller plants (1 gallon container or smaller) in odd-numbered clumps (3, 5, 7, etc.) of the same species throughout the planting areas. Avoid planting in rows. Planting Depth All plants should be buried as deep as they were grown in the nursery. Plant bare-rooted plants in a hole wide and deep enough so that roots are not bent, crowded, or exposed to air. Bareroot trees shall be planted so that their root flare is even with the soil surface. Lake Oswego Stormwater Management Manual Section 4.6 84 Planters Woody Vegetation Shrubs and trees shall be sited to promote long-term health and survival, minimize maintenance, and protect lines of sight. Locate shrubs and trees to allow for maintenance access to the treatment area. Shrubs should be a minimum of 3 feet from sidewalks and roadways. Trees shall be placed on the margins or tops of slope, unless it is proven that the species selection can thrive in designed hydrologic conditions. City of Portland is currently developing a list of trees acceptable for planting in stormwater facilities. Conveyance and Outlet/Overflow Planters must include overflow outlets that discharge to an approved location. In areas where it is inappropriate to infiltrate (e.g. where infiltration rates are less than 0.25 inches per hour or underlying soils are contaminated), underdrains shall be provided along the length of the planter. Construction Mark planter boundaries with stakes or flagging prior to construction and avoid any unnecessary soil compaction or disturbance. Maintenance Proper maintenance is essential for a functioning stormwater planter. Please refer to the operations and maintenance section (Chapter 7) for more information. Native grasses and sedges should not be cut or mowed. See maintenance checklists (Appendix I) for further maintenance guidance. Herbicides should not be used as a weed control technique within stormwater facilities. Insecticide and fungicide use should also be minimized. See also Appendix A-4 for IPM strategies appropriate for stormwater facilities. 85 Section 4.6 Lake Oswego Stormwater Management Manual 1 r • • Ulm tginrgh 4.6.3 Infiltration Trench Definition An infiltration trench is a linear, gravel-filled trench that distributes stormwater to underlying soils. Introduction I Applicability Table An infiltration trench provides flow control only, so I BMP Parameters pretreatment is required. Since infiltration trenches are classified as underground injection control devices Appropriate for (UICs), unless they are constructed without an pretreatment underdrain, UlCs must be registered with the DEQ. I Appropriate for onsite stormwater management V These guidelines cover infiltration trenches for both Provides flow control small and large projects. Some differences are: (when designed for Observation wells are required for individual infiltration) • infiltration trenches serving large contributing areas Provides water quality (more than 3,000 square feet) I q treatment Appropriate for residential UIC rule authorization required V Geotechnical engineer may be required V Figure 4.4. Infiltration Trench Section (Typ) With Underdrain. Observation well (if required) Underdrain (6" PVC pipe) Finish grade °444i SIN 211e.10C4 - - - - •.- • * Min. 18" Vie- Y v..� * Maintain 6" sp.-41w-.0 + '• •f ti \ washed drain • !•r ` ' ,. , ,. • T+► dr t \ rock below --;vx1. . Sump \— �.4•;•_.r,' r y t;Ng-,4,. _L _A, � �• pipe inlet '1 \-1 AI II \�1 Al I I \-‘ AI I I , I Ail II \-1/ Lake Oswego Stormwater Management Manual Section 4.6 86 Infiltration Trench Site Requirements • Trenches are not approved for slopes greater than 15 percent T z _ • Trenches within 200 feet of a mapped landslide =ram, y hazard area require the review and approval of a - - geotechnical engineer. • Trenches must be located at least 10 feet from • building foundations. • • Trenches must be located at least 5 feet from property lines. : ; • Infiltration trenches are not allowed in the public Infiltration trench also serving as an outdoor gathering space. right-of-way. • Trenches should not be located where they will be subject to vehicular traffic. • Soil surrounding trenches must be native, uncompacted soil. • Bottom of trench must be at least 5 feet above seasonal high groundwater elevation. • Infiltration rate must be at least 0.25 inches per hour unless an underdrain is used. • Infiltration trenches shall be located outside of tree protection zone or at least 10 feet from the base of newly planted trees and large shrubs. • Site must be adequate for required pre-treatment. Expected pollutants may limit allowed pre-treatment options. • Trenches that receive stormwater other than roof runoff must be registered with and rule authorized by DEQ. Geometry Infiltration trenches are rectangular in shape and can be any length. Infiltration Trench Dimensions Impervious Surfaces • Depth of drain rock: 12 inches minimum • Width: 2 to 5 feet • Length: varies t', +.fir +' Vt... `,*r` ' .•' t'M1'' Filter Strips for Pre-Treatment r o 141- , .y N► ! 5'Minimum am,, ' ;l{ Observation well (if required) _ k �._ � ' � ,• • V <t�yy Infiltration Trench 2'-5'Width 87 Section 4.6 Lake Oswego Stormwater Management Manual Infiltration Trench Design Steps The required storage capacity of an infiltration system shall be determined by subtracting the volume of water that can infiltrate from the facility within a 24-hour period from the volume of runoff generated by the contributing basin during a 24-hour, 10-year storm event. Step 1: Site Suitability Confirm that site meets the criteria for infiltration facilities (Chapter 7), perform infiltration test, and confirm that you are willing to register the trench as a UIC with Oregon DEQ (Appendix B). A geotechnical report is required for infiltration facilities on slopes >15% or within 200 ft of a steep slope hazard area or landslide hazard area. Step 2: Determine Drainage Basin Characteristics Calculate the drainage area and Natural Resource Conservation Service (NRCS) Curve Number separately for both pervious and impervious surfaces in the basin. See Appendix E for NRCS Curve Number determinations. Parameter Units Value Aimpand Aper drainage area (impervious and pervious) acres CN NRCS curve number (impervious and pervious) unitless Step 3: Calculate Runoff Volume Calculate the volume of runoff during a 10-year, 24-hour storm separately for impervious and pervious surfaces and then sum the results using the equations below: 1,000 * - 10] 2 Vimp + Vpem= 3,630 *A* ( Pdesign - 0 2 CN ) Pdesign + 0.8 r 1,000 - 10 L CN Parameter Units Values V. + Vpery runoff volume (impervious and pervious) cubic feet Pdesign design precipitation depth inches 3.20 Lake Oswego Stormwater Management Manual Section 4.6 88 Infiltration Trench Step 4: Establish Preliminary Geometry Establish infiltration trench depth, width, and length for preliminary evaluation. Parameter Units Values H trench depth feet W trench width feet L trench length feet Step 5: Calculate Infiltration Volume Calculate the volume that can infiltrate from the infiltration trench to groundwater in a 24-hour period using the trench geometry and the percolation rate determined through the PIT Test. Vi=AdW * Rinf * T * \-1 / 12 Parameter Units Values Rinf infiltration rate Inches per hour AdW surface area of infiltration trench square feet V. infiltration volume cubic feet T time hour 24 Step 6: Calculate Storage Volume Subtract the volume calculated in Step 5 from the runoff volume calculated in Step 3. This is the required storage volume for the infiltration trench. If the required storage volume is not met by the preliminary geometry assumed in Step 4, adjust and repeat Steps 5 and 6 until the criteria are met. 89 Section 4.6 Lake Oswego Stormwater Management Manual Infiltration Trench Materials Upstream Protection Upstream protection: an upstream sediment trap or cover to protect the trench from clogging with debris (e.g., leaves) is required. Drain Rock Drain rock shall be %-inch to 1'/2-inch granular drain backfill material. Drain rock shall conform to ODOT Standard Specifications 00430.11 or American Association of State Highway and Transportation Officials (AASHTO) No. 4. Furnish granual drain backfill material of%-inch to 1' -inch crushed gravel meeting the following gradation requirements: Seive Size Percent Passing (by Weight) 2" 100% 1 1/2" 95- 100% 1 1/4" - 1" - 3/4" 0- 15% 1/2" 0-2% 1/4" - Underdrain An underdrain is required unless infiltration rates of native soils are at least 2 inches per hour. The underdrain shall be perforated, 6-inch-diameter PVC pipe that conforms to ODOT Standard Specification 02410.70. The invert elevation must be at least 12 inches below finished grade. Rock Filter A rock filter, 2 to 3 inches of special filter material per ODOT Standard Specification 02610.10, shall be placed between the drain rock and native soils. Conveyance and Outlet/Overflow An observation well is required for infiltration trenches constructed for large projects. The well allows confirmation of proper drainage. See Chapter 3. Construction The location of the proposed infiltration trench must be clearly marked prior to land disturbance to ensure that the area is protected from construction vehicle traffic. Construction vehicles shall not be allowed within 10 feet of the trench. The bottom of the infiltration trench shall be level. Infiltration rates must be tested after construction to confirm that they are consistent with the design infiltration rates. Lake Oswego Stormwater Management Manual Section 4.6 90 Infiltration Trench Maintenance Protect trench from vehicle traffic to avoid compaction. If ponding is observed in the observation well or at the surface of the trench, clear the underdrain of debris. Avoid planting large shrubs or trees adjacent to trench to prevent damage from root systems. Allow a minimum of ten feet of clearance between the base of large trees and shrubs and the edge of the infiltration trench. See Chapter 7 for additional maintenance guidance. il .. OP i 4.4i , , , , worn— Oa it, I 4.?L. ''' , it . 1' -11111411111.44:- ' 4,- _ r ■ ! , r% A , • 110 1( ,kit Washed gravel used for construction of infiltration trenches. 91 Section 4.6 Lake Oswego Stormwater Management Manual . ! !Ai IX, 1 ` irgri. . . ' 1, • ' ' r ��i 1 emr, ............ 0 . , © T _&... til - , _ rywe... ill__,, ��._ .. 4 ..,�__ . .._ 4.6.4 Drywell Definition A drywell is an underground perforated pipe or chamber surrounded with gravel that collects stormwater runoff and gradually discharges it into underlying soils. Introduction Applicability Table Drywells are "Class V Injection Wells" under the BMP Parameters federal Underground Injection Control Program (OAR Division 44). UlCs, including drywells, must Appropriate for be registered with DEQ and classified as: exempt, pretreatment authorized by rule, or authorized by permit. Appropriate for onsite Drywells do not provide water quality treatment, so stormwater management V water quality treatment is required before stormwater Provides flow control discharges into drywells except rule authorized ODOT (when designed for detail. For drywells used exclusively for residential roof infiltration) runoff from three dwelling units or less, a silt trap is the Provides water quality only pretreatment necessary. Where space is available, treatment rain gardens are preferred to manage residential runoff because they provide both treatment and flow control Appropriate for residential (and are not considered UlCs). UIC rule authorization Designers are urged to review current regulations and required V UIC registration materials from DEQ. Design professional required V Figure 4.5. Drywell Section (Typ). See also City standard detail SD1-07. il T I I I Drywell I inlet pipe I I I Cap with I`Yi1 n - *iiiiiiiiiiiock �� i' oil, �I/,I312"Soil laver 0 0 �i .✓ o Sum p 1,i ,- 8° — Perforated pipe section Y 1 t"*4 `r o. Stone fill material moo: *i±, , Filter fabric liner Building foundation ,� N ',;7`c,, .:;,:i: 8 . ,1 Alt ( )1 10'from building foundation Lake Oswego Stormwater Management Manual Section 4.6 92 Drywell Site Requirements • Drywells must meet the infiltration design criteria - _ ,. described in Chapter 3. `. • ,y • Bottom of drywells must be at least 5 feet above II - -`� ` seasonal groundwater or impermeable layer*. '' �40, ` • The edge of excavation for the drywell must be at - 4 = least 10 feet from building foundations. . • • The edge of excavation for the drywell must be at ` ' ' �-` least 5 feet from property lines. - - " r - • The top of drywell must be lower than the floor 01R`` ' elevation of basements in immediately adjacent . buildings. '` - • Drywells are not allowed on slopes of 25 percent UIC evaluation. or more. • Drywells are not allowed in the public right-of-way. • Soil surrounding drywells must be native, uncompacted soil. • Pretreatment is required unless the drywell is receiving only roof runoff. "Drywells can be constructed with less separation to seasonal high groundwater, but require a water pollution control facility permit from DEQ.This is strongly discouraged. Geometry Drywell Dimensions w • Diameter: 2 feet minimum Atom,. 1 ; i * • • • • • i0 ‘ I Ill i II • # • • • • ° Pi. H ", * A di • • el ali • • i o( 4 ii qi . . al P� a iii 4 ep ' • • • • • I tLi Large drywell. 93 Section 4.6 Lake Oswego Stormwater Management Manual Drywell Design Steps The required storage capacity of a drywell/infiltration system is determined by subtracting the volume of water that can infiltrate out of the facility within a 24-hour period from the runoff volume generated by the contributing basin during a 24-hour, 10-year storm event. Step 1: Site Suitability Confirm that the site meets the criteria for infiltration facilities (Chapter 7), perform an infiltration test, and confirm that you are willing to register the drywell as a UIC with DEQ. A geotechnical report is required for drywells on slopes of 15 percent or more or within 200 feet of a steep slope hazard area or landslide hazard area. Step 2: Determine Drainage Basin Characteristics Calculate the drainage area and Natural Resource Conservation Service (NRCS) Curve Number separately for both pervious and impervious surfaces in the basin. See Appendix E for NRCS Curve Number determinations. Parameter Units Value Aimp and Ape drainage area (impervious and pervious) acres CN NRCS curve number (impervious and pervious) unitless Step 3: Calculate Runoff Volume Calculate the volume of runoff during a 10-year, 24-hour storm separately for impervious and pervious surfaces and then sum the results using the equations below: * - 10] 1,000 [ 2 imp V or VpeN= 3,630 *A* ( Pdesign - 0'2 CN ) Pdesign + 0.8 r 1,000 L - 1 0 CN Parameter Units Values Vimp + Vpen, runoff volume (impervious and pervious) cubic feet Pdesign design precipitation depth inches 3.20 Lake Oswego Stormwater Management Manual Section 4.6 94 Drywell Step 4: Establish Preliminary Geometry and Number of Drywells Establish drywell depth and diameter, and number of units for preliminary evaluation. Parameter Units Values D drywell diameter (inside diameter) feet 2 foot minimum H drywell depth feet N number of drywell units each Step 5: Calculate Infiltration Volume Calculate the volume that can infiltrate from the drywell(s) to groundwater in a 24-hour period using the drywell geometry and the infiltration rate determined through the PIT Test (Appendix B). 1 V.=A R. T * - * N i dW inf 12 Parameter Units Values Rinf infiltration rate inches per hour AdW Horizontal cross square feet sectional area of drywell(s) V. infiltration volume cubic feet T time hours 24 N number of drywell units each Step 6: Calculate Storage Volume Subtract the volume calculated in Step 5 from the runoff volume calculated in Step 3. This is the required storage volume for the drywell(s). If the required storage volume is not met by the preliminary geometry and number of drywells assumed in Step 4, adjust and repeat Steps 5 and 6 until the criteria are met. 95 Section 4.6 Lake Oswego Stormwater Management Manual Drywell Materials Drain Rock Place 12-inch minimum layer of 3/4" to 1-1/2" round rock that conforms to ODOT Standard Specifications 00430.11 orAASHTO No. 4 between pit lining and earth wall, up to the lid. Construction The location of the proposed drywell shall be clearly marked prior to land disturbance to ensure that the area is protected from construction vehicle traffic (construction vehicles shall be restricted within 10 feet of drywell). The bottom of the drywell shall be level. Infiltration rates shall be tested after construction to confirm that they are consistent with the design infiltration rates. Maintenance Proper maintenance and regular inspection is essential for a functioning drywell. Please refer to the operations and maintenance section (Chapter 7)for more information. Lake Oswego Stormwater Management Manual Section 4.6 96 This page is intentionally left blank. 97 Section 4.6 Lake Oswego Stormwater Management Manual Green,. :._.,_ _,....... . .. .... ._ .. ofs-.._..,..-.... .... -, .,.......„.„ ,-.4t.„,: .. ..,_.,:.., .- ,4',.. : '.7.-_-'=. : . , ,,,-- _..-., - - 4.6.5 Green Roofs Definition A green roof is a building roof that is partially or completely covered with vegetation and growing media, atop a waterproof membrane; It is also called an ecoroof or vegetated roof. Introduction Applicability Table A green roof includes the following elements: a thin, layered system of waterproofing, drainage layers, BMP Parameters growing media, and planting to cover impervious roof Appropriate for areas and allow water to be absorbed, detained, and pretreatment V evaporated back into the atmosphere. Appropriate for onsite stormwater management V Green roofs are often employed on sites where other / stormwater management strategies are not feasible Provides flow control V due to lack of available space Provides water quality treatment V Green roofs are mostly "extensive," meaning they are low-maintenance and lightweight, and primarily Appropriate for residential designed for stormwater management, with aesthetics as a secondary goal. Semi-intensive green roofs UIC permit required have somewhat deeper soils or more expansive Structural engineer plantings. Green roofs are not designed for foot required V traffic or recreation. They should be differentiated from accessible "roof gardens" and "terraces," which include deeper soils, paved areas, seating, and furnishings — and that are used more as an amenity than for stormwater management. However, a combination of green roof and "roof garden" or"terrace" may be incorporated on roof tops. Figure 4.6. Green Roof Section (Typ). Approved Gravel plantings mulch Drainage layer with Metal filter fabric edging . �� - �l `Growing i o o 0 o 0 0 o O o smeda 0 0 Oo KI� o 4'emin. o ° o G ' � o O O KIWI f#e4: v 6 Waterproofing / Protection/ i root barrier Building i structure Drain to approved outfall location Lake Oswego Stormwater Management Manual Section 4.6 98 Green Roofs Site Requirements • Flat or slightly sloped roofs on large institutional, commercial, or residential projects. Green roofs a- : 1 3:, '' work on sloped roofs up to a maximum of 4:12 -.•- •:,- roof pitch without additional engineering - and - �_ can be steeper with intermediate structural •, i4e support of soils. • Structural considerations: must be able to carryIlLivi •-- .- additional loads as determined by a structural engineer (15 to 30 pounds per square foot is typical) to support fully saturated conditions. These load recommendations do not include Graphic representation of a green roof. snow load. • Access to roof via crane, lift or other device is recommended to load heavy, and bulky materials up to rooftop surface. Geometry Green roofs can be adapted to fit a wide variety of roofs. Use the design steps and select from the plants and materials described in the following sections to ensure adequate treatment of stormwater. Green Roof Dimensions • Minimum growing media depth: 4 inches • Maximum growing media depth: 8 inches • Minimum roof slope: 1/4 inch per foot • Maximum roof slope: 4:12 roof pitch (greater slopes allowed with engineering/stabilization) • Perimeter edges and access pavers should count for no greater than 10 percent of green roof area • Mulch: rock mulch (non-organic) to avoid wind and water erosion; at drains and edging • Vegetation to cover 100 percent of soil areas • Safety line tie-off points, hand holds, or walking surfaces may be necessary to facilitate maintenance — — • • '• ' yo[r--,,..—• v.tdx- • r ey i Tyr :.. r.. ' v .IK/ — ir+ .-4 1 ' s:. } # "$ igmax. e` ^_ .. , w • ' ` •� 'i w'+ itt 0 t ,-,,: 1 ..tsr: ,,_._ ,„..„40.4-,..,.:,)..:.-,;._-, ,, , , 4 . .bot:„.. ...„,_ 'vi•. a;: i Z:S1. n } +I ■ Sr Flat comme`r'cial/mixed use green roof Sloped residential green roof 99 Section 4.6 Lake Oswego Stormwater Management Manual Green Roofs Design Steps Following is the procedure to confirm that a proposed green roof has adequate capacity to manage the impervious roof surfaces. Step 1: Determine Green Roof Area Determine area (square feet) of roof that will contain vegetated rooftop area. Step 2: Calculate Non-Vegetated Green Roof Area Gravel perimeter edging, pavers, drains and other areas of the green roof can comprise up to 10 percent of the overall square footage of a green roof. Step 3: Calculate Impervious Area Reduction To calculate the total impervious area reduction, sum the total vegetated area with any non-vegetated green roof elements to calculate the total impervious area reduction. The following table provides guidance on the calculation to obtain the quantity to be entered for impervious area reduction: Parameter Value vegetated areas square feet A 0.1 xA non-vegetated areas (pavers, gravel, drains) square feet (maximum) impervious area reduction square feet 1.1 x A (maximum) Materials Root Barrier/ Protection Layer A synthetic, non-biodegradable layer must be placed to protect waterproofing layers and to provide additional protection from roots. Do not use copper or copper hydroxide for a root inhibitor. Drainage Layer/ Filter Fabric A synthetic or mineral layer must be placed over the protection layer to allow for water movement under the growing media. The drainage layer should be 1/2 inch deep with a void space of at least 50 percent, covered by a non-woven (needle-punched) filter fabric to separate the drainage layer from the growing media. Growing Media An engineered growing media mix should be placed over the filter fabric to a depth of at least 4 inches to meet requirements. It should contain no fines, weed seeds, or other materials. Provide documentation of saturated weight (field moisture capacity) that has been tested and documented by a third party. The media mix must consist of: Pumice/Lightweight Aggregate 80-90% Composted plant-based organic matter 10-20% Mineral Mulch A mulch layer of washed gravel or non-decompostable material (no fines) that will not be moved by wind or water movement must be placed on top of the growing media. Lake Oswego Stormwater Management Manual Section 4.6 100 Green Roofs Vegetation Objective Establish dense plant growth of low-maintenance, low-water use succulent vegetation supplemented with some hardy perennials, grasses, and other native, non-woody vegetation. Plant Selection and Diversity Extensive green roofs should consist of a foundation of succulents (i.e. sedums, sempervirum) and other Crassulacean acid metabolism (CAM) species. CAM species are plants that photosynthesize, and thus evapotranspire, at night, which reduces the amount of water lost during hot, sunny days. These low- growing species also provide dense coverage to aid in erosion control and weed suppression. A minimum of 10 species should be included to promote microclimatic diversity and resilience to the roof, allowing for species to fill in if others are slow to perform. Use 10 percent deciduous species distributed throughout the roof to provide organic matter inputs through leaf litter. See Appendix H for a complete plant list with species for green roofs. Plant Quality & Space Cuttings Small pieces of sedums and other succulents broadcast onto soil surface to root in place, 100 to 200 pounds per 1,000 square feet is typical. Container Plants Plugs or 4-inch pots of pre-grown species (4 to 12 inches on center); should have a full crown and be free of weeds or other materials. If possible, pre-grow in green roof soil or install as bareroot plugs to avoid weed contamination and importation of organic soils to roofs. Conveyance and Outlet/Overflow Green roofs must include outlets to an approved location from roof drains, scuppers, and other drainage devices. Construction Ensure waterproofing is installed and protected prior to construction by a qualified roofing contractor. Install drainage layer, edging, filter fabric, and irrigation prior to soil installation. Do not install in heavy rains to avoid erosion. Install plants in spring or fall (or irrigate as needed to support plan establishment in summer). Do not install during freezing temperatures. Maintenance Proper maintenance is essential for a functioning green roof. Please refer to the operations and maintenance section (Chapter 7)for more information. See maintenance checklists (Appendix I)for further maintenance guidance for green roof vegetation. Herbicides should not be used as a weed control technique within stormwater facilities. Fungicides and insecticides should be used sparingly, if at all. Metals-based moss controls should not be used unless the green roof overflows to an infiltration facility that does not include an underdrain. 101 Lake Oswego Stormwater Management Manual h l ' . , , . . . 0 Ni pgrv— D)Evg . .. - T . . . , , , ? , , , s . . 4.6.6 Pervious Pavement i Definition Pervious pavement is a walking or driving surface designed to allow rainfall to percolate into the underlying soil or aggregate storage reservoir beneath the pavement. Introduction Applicability Table There are several options for the wearing course, or the surface layer of pervious pavement, including BMP Parameters porous asphalt, pervious concrete, or pavers. Appropriate for pretreatment • Porous asphalt is open-graded asphalt that allows Appropriate for onsite water to percolate or infiltrate into underlying soils. stormwater management V • Porous concrete omits fines in the aggregate to Provides flow control create stable air pockets that allow water to drain to the base below. There is an inverse relationship Provides water quality between porosity and strength. As porosity is treatment V increased, the structural strength is reduced. Appropriate for residential • Pavers are generally suitable for pedestrian areas UIC permit required and low traffic parking areas. They are available in a variety of configurations such as rigid concrete or Design professional durable plastic grid filled with gravel or a mixture of required (if vehicular traffic on gravel, sand, and topsoil suitable for vegetation. pervious surface) Pervious pavement is typically designed as a surface that receives only direct rainfall. Lake Oswego may allow pervious pavement facilities with extra aggregate storage reservoir depth to accommodate runoff from adjacent impervious surfaces but does not encourage this and may add additional conditions of approval as a result. Sediment must be controlled, and approval from the City is required prior to installing pervious pavement. While important for all stormwater facilities, proper installation and regular maintenance is critical for the functioning of pervious pavements. Figure 4.7. Pervious Pavement Section (Typ). Leveling Course 1/4"Aggregate A jir, Permeable Pavement Surface A 1 kJ$G e`t Aggregate Aggregate a• o Storage Storage Depth a Reservoir � o 0 A °®4 1 D p #57 Pavement Sub-Grade Lake Oswego Stormwater Management Manual Section 4.6 102 Pervious Pavement Site Requirements Pervious pavement is best designed to manage 11,1111k I precipitation falling directly on the pavement, without r'" additional runon. To be considered for pervious pavement, a site must meet the following design criteria: -� - - • Surface slope no greater than 6 percent. - ''� ` �~ - ' • Site does not receive high sediment loads. Runon from adjacent areas of standard pavement often comes f _ with elevated sediment loads; areas with high volumes of leaf litter can cause clogging, so avoid pervious pavement under large deciduous trees. In addition to porous concrete, porous asphalt, and pavers, there are structural • Elevated separation from underlying water table; frameworks developed by vendors that bedrock or other impermeable layer must be at least 4 contain aggregate in place, support traffic feet below ground surface. loads, and function as a pervious pavement • Pervious pavement is not appropriate for areas at system. elevated risk of hazardous spills such as gas stations. • Not appropriate for construction over fill soils unless evaluated and approved by geotechnical professional. Types Wearing Course Porous Concrete The wearing course of a porous concrete section shall meet the following: • 4-inch thickness for residential driveway, pedestrian only, private street, parking lot or fire lane • 7-inch thickness for public street Porous Asphalt • The minimum thickness of the wearing course of a porous asphalt - ` _�l "! 9li fir'•section shall be as follows: •5 _ T • 2.5-inches for residential driveway or pedestrian only path • 3 inches for private street, parking lot, or fire lane ^ • 6 inches for public street Aggregate Storage Reservoir 4 • Minimum depth for structural support must be 6 inches for vehicular loading • In addition to structural design considerations, design depth is Pavers are a goo op ion typically determined by storage depth needed to manage design for pedestrian and low traffic storm. locations. 103 Section 4.6 Lake Oswego Stormwater Management Manual Pervious Pavement Leveling Course Where pervious pavement installations are proposed over fine sediments, provide a 1- to 3-inch-thick leveling course if the pervious pavement surface is open-celled paving grids, interlocking concrete pavers, or porous asphalt concrete. Design Steps Pavement design is outside of the scope of these guidelines. Pavement design shall demonstrate that pavement structure has the structural strength for anticipated vehicle loadings. The following resources can provide guidance on structural pavement design: • National Asphalt Pavement Association Design, Construction and Maintenance Guide • AASHTO Guide for Design of Pavement Structures Step 1: Determine Site Infiltration Rate See Appendix B for information on how to determine the site infiltration rate. Step 2: Determine Design Flow and Volume for Storage (Qdesign, Calculate 10-year, 24-hour discharge and runoff volume using approved single-event methods. Step 3: Calculate Thickness of Storage Laye, Using an approved model, route the design storm through the pervious pavement storage area and determine the maximum ponding depth (considering infiltration and void ratio of storage layer material). The maximum ponding depth is the design storage layer thickness. Materials Leveling Course The leveling course should consist of 1 to 3 inches of 3/4-inch to 2-inch uniformly graded, washed aggregate (AASHTO No. 8 or ODOT Standard Specifications 02520). Aggregate Storage Reservoir The aggregate storage reservoir shall conform to ODOT Standard Specifications 00430.11 granular drain backfill material, AASHTO No. 57, or approved equal. Porous Asphalt The surface wearing course for porous asphalt shall conform to open-graded, 1/2-inch or 3/4-inch asphalt design from ODOT standard specification 00745 or approved equal. • Content: 6.0 to 6.5 percent by weight of total (dry aggregate) mix. Performance Grade (PG): 70-22. Do not use an asphalt cement performance grade less than 70-22 for open-graded, porous asphalt mixes. Lake Oswego Stormwater Management Manual Section 4.6 104 Pervious Pavement Construction For a pervious pavement installation to be approved in Lake Oswego, designers must demonstrate that construction will be performed by a qualified contractor— one who has prior experience with successful installations. Design professionals are encouraged to use performance-based specifications or require pre-qualifications such as certification through a national training program. During construction, the pervious pavement site must be protected from sediment and runoff. The subgrade must be protected from inappropriate compaction, from truck traffic, and construction equipment. Compaction will reduce the permeability of soils and should be done with caution. The subgrade shall be scarified prior to setting the aggregate storage reservoir. Compaction shall meet the following criteria: • Public roadways may require 95 percent compaction. Consult with City engineering department. • No compaction required for residential driveway or pedestrian-only surfaces over native fill. Maintenance Remove vegetation, debris, sediment, and other materials from the pavement surface promptly. If there is moss growth on surface of pervious pavement, pressure wash or vacuum sweep the surface during the dry season to preserve infiltration capacity. Review checklists in Appendix I for complete maintenance schedule. 105 Section 4.6 Lake Oswego Stormwater Management Manual ' - •• r .� ' •• -/APO , r - • R , ,.._.. .. a :wter. Ha.rvest !t • . _ , . , ` *` •sided •. 'ainBank 4.6.7 Rainwater Harvesting Definition Rainwater harvesting entails collection of stormwater in rain barrels or cisterns to use either for landscape irrigation or for non-potable household or building use, such as toilet flushing or cold water laundry. Introduction Applicability Table Rain barrels are BMPs that collect stormwater for reuse, BMP Parameters typically for landscape irrigation. Rain barrels typically have a small capacity, or storage volume, so they cannot Appropriate for completely meet onsite stormwater management or flow pretreatment control requirements. The usefulness of rain barrels for Appropriate for onsite flow control is also limited because of the nature of storms stormwater management V in the Pacific Northwest, where frequent storms occur in Provides flow control the fall, winter, and spring, when irrigation needs are much lower. Provides water quality treatment Cisterns are generally larger facilities that can supply storage for non-potable household or building uses as Appropriate for residential well as irrigation. Any stormwater reuse within a house UIC permit required or building must obtain the City's plumbing approval from Design professional the Building Department. System containment backflow required protection in the form of a reduced pressure (RP) type of backflow assembly must be provided. System containment RPs must be located on private property at the property line, immediately adjacent to the point of water service connection. . , - - •- ii _I ==__ f i iI " ' 1 "I' , , _ , , if i : :-. - r L ri .._ _ _ _ _ -_ � - � __ _- -' - -- -,y .4.i-c. r !' :-.;.-`•;,.- ti L V.• `hotti rovided I Rain$ank ese cisterns capture water from this building roof system. Lake Oswego Stormwater Management Manual Section 4.6 106 Rainwater Harvesting Site Requirements • To protect the water quality of the rainwater n n harvested, rainwater should not be harvested 1' II [ I from roofs containing copper or zinc, or _ 'I " materials treated with fungicides or herbicides. • Rainwater should be collected only from roofs - - . it.-: ' - and only if approved by the City's Building -,-_ Department. . 7 • Reuse of stormwater for non-potable uses is '''ry', -- restricted by the Oregon Plumbing Specialty ' Code and by the City's Building Department. • Rainwater harvested from rooftops with asphalt • rain .arre cap unng wa er rom a ome. shingles may not be suitable for watering vegetable gardens or edible crops. Verify that runoff from roof tops will be non-toxic prior to use on edible landscapes. Geometry—Types Cisterns may be installed either at grade, below ground, under a deck, or in a basement or crawl space. Multiple cisterns may be installed to provide increased storage capacity. i le,f ... ~ 44 ";-; r-Fill, 1.1 , "4 - r 4 _ Phot. ', ded b RainBan A cistern built into rock wall. Rain barrels capture water that can be used for supplemental irrigation. 107 Section 4.6 Lake Oswego Stormwater Management Manual Rainwater Harvesting Design Steps Supplemental calculations are not required for rain barrels. For cisterns, engineering may be required along with plumbing and other trade-specific work. For all cisterns that are proposed to meet onsite stormwater management or flow control requirements, the following information must be submitted: • Tank size and material • Water storage facility details and specifications • Pretreatment facility details • Pump and associated electrical details and specification • Piping size, material, and placement details and specifications • Bedding and foundation details and specifications • Average daily water use documentation • Hydraulic calculations demonstrating compliance with stormwater management requirements • Approved overflow system/location. Where cisterns are proposed to supplement domestic greywater uses, conduct a water balance calculation between domestic greywater uses and precipitation to determine the appropriate cistern size. See the Oregon Smart Guide to Rainwater Harvesting (BCD, 2015) for information on how to perform these calculations. Site conditions, overflow options, and other stormwater management measures proposed for the site will determine the proportion of mean annual precipitation (assume 40 inches) that must be stored to fulfill stormwater management requirements. Large cisterns may require geotechnical analysis and design of suitable base. Consult City Engineering Department staff for more guidance. Beddirn and Foundations Full cisterns can be very heavy. Given this, the soil bearing capacity or foundation upon which a cistern is placed should be evaluated to prevent settling or subsidence. This is particularly important for above-ground cisterns, as significant settling could cause the cistern to lean or be damaged. Bedding, foundations, and/or footers must be provided as warranted by system loading, geotechnical conditions, and manufacturer's recommendations. Design of these features must be performed by an appropriate design professional. Stone bedding is often used to support cisterns. Bedding thickness varies according to cistern system requirements, but should not be less than six inches. During bedding placement, over-excavation and replacement of loose or unstable subsurface material may be required if these conditions are encountered. A geotechnical engineer or other appropriate design professional should be consulted for additional guidance. Lake Oswego Stormwater Management Manual Section 4.6 108 Rainwater Harvesting Materials Rainwater harvesting includes the elements listed below. Collection System Rainwater is typically collected through gutters and downspouts along with any piping needed to route harvested rainwater to the cistern. Debris Screen/First Flush Diverter A leaf guard or screen should be provided to reduce sediment and other debris from entering the cistern. In addition to leaf screens, first flush or rain diverters mechanically "flush" off the first water of a storm before it enters the storage tank. This is the water that could be the most contaminated by particulates, bird droppings, or other materials lying on the roof, so eliminating these contaminants before they enter the storage and conveyance system is critical. Cisterns Cisterns can be constructed from a number of materials; including concrete, fiberglass, or plastic. Opaque containers must be used for aboveground cisterns to minimize algae growth. Delivery and Distribution System Delivery may be accomplished by a gravity system, or, it may include the pumps and pipes needed to move water from the storage system to the end use area. Consider designing a potable water back-up that can operate without electricity in emergency conditions. Water must be drawn from at least 4 inches above the bottom of the tank. Pretreatment Water quality treatment is typically required to protect the delivery and distribution system and to improve the quality of the collected water for use. The extent of water treatment depends on both the quality of the water entering the storage system and the intended water use. Systems must protect the functions of delivery valves and fixtures; they range from simple screens to cartridge filters, UV light, and chlorination. Screen systems and/or basic mechanical filtration are typically adequate for irrigation and toilet flushing reuse. Approval by the City's Building Department is required for any project routing harvesting water to an indoor plumbing system. Conveyance and Outlet/Overflow Each cistern must have a designated overflow when the volume of the tank meets capacity. The minimum overflow is 4 inches in diameter. Overflows must discharge to an approved location. 109 Section 4.6 Lake Oswego Stormwater Management Manual Rainwater Harvesting Construction All cisterns must be installed in accordance with manufacturer's installation instructions, the City's building code, and all applicable laws. Maintenance Cistern/storage systems must have both access points and drains to allow inspections and cleaning. Openings shall be designed to restrict entry from unauthorized personnel and/or appropriate signage shall be provided. Cleaning of any accumulated sediment on the bottom of the cistern must be possible by flushing through a drain, vacuuming, or another approved method. Cistern/storage systems that are buried below ground level must have a manhole riser that sticks out a minimum of 8 inches above the surrounding ground. Manhole covers must be secured and locked to prevent tampering. Operational Tips For rain barrels and cisterns to operate effectively as stormwater BMPs, there must be an appropriate balance between having sufficient water for the designated use and having sufficient empty storage to capture runoff and to meet or help meet flow control requirements. For rain barrels, owners should use collected rainwater as needed during dry periods following a storm. If the water is not needed for irrigation during the rainy season, consider finding alternate uses or releasing the water to a vegetated area between storms. This will help ensure that the barrels have sufficient capacity to capture runoff from the next storm. If routine use or active emptying of the rain barrel or cistern is not feasible, consider keeping the spigot open slightly at all times to allow a slow trickle of collected water to be released to a vegetated area over time. Unlike rain barrels, cisterns designed for non-irrigation uses are typically sized with some consideration for stormwater performance during the design phase. This would typically eliminate the need for tank emptying as part of maintenance activities. Nonetheless, if overflows are routinely occurring, a similar approach of finding alternate uses or actively emptying the tank between storms may need to be considered. Lake Oswego Stormwater Management Manual Section 4.6 110 This page is intentionally left blank. 111 Section 4.6 Lake Oswego Stormwater Management Manual 4 ,r il r 4.6.8 Filter Strips Definition A filter strip is a gently sloped vegetated strip that removes pollutants through filtration, sedimentation, and infiltration. Introduction A filter strip is a section of vegetated area adjacent to an I Applicability Table uncurbed, impervious surface. Stormwater flows at a low I BMP Parameters velocity and depth evenly across the entire width of the Appropriate for filter strip. Pollutants are removed through stormwater pretreatment contact with vegetation and underlying soils. Appropriate for onsite Filter strips are appropriate for meeting landscaping stormwater management requirements, but plants must be selected to be I Provides flow control consistent with their stormwater treatment function (see Appendix H). They must be protected from disturbance Provides water quality to preserve healthy vegetation and soil conditions in treatment V order to maintain water quality function. I Appropriate for residential Filter strips are a good choice for roads in Lake Oswego I UIC permit required and are complemented by a shoulder gravel strip that Design professional helps to uniformly distribute flow. They can also be good required choices for small projects; for example, to provide water quality treatment for roof or driveway runoff before it is discharged into a drywell, infiltration trench, or other facility. Figure 4.7. Filter Strip Schematic. ' k,. Aii ih.o,40 1 i H' \c''i Pollution generating Filter strip surface —Protect soils from disturbance - see construction section Lake Oswego Stormwater Management Manual Section 4.6 112 Filter Strips Site Requirements --. , r • Filter strips begin where flow is distributed evenly along the length of the strip. This may require .4 _ -, 4_ additional structures or design features to fully : spread point discharges along the length of the strip. tf ,� • There must be an approved overflow route. • • Filter strips are appropriate for all soil types. • Filter strips may be located on a range of site { conditions from full sun to full shade. Plant selection should match site conditions (see Filter strip monitoring. Appendix H). • Filter strips must be a minimum of 5 feet from the property line. • Filter strips must be a minimum of 5 feet from structures. • Filter strips must be a minimum of 50 feet from wetlands, rivers, streams, and creeks. Geometry Filter Strip Dimensions • Dimensions and slopes: Filter strips shall slope between 0.5 and 10 percent. If slope is greater than 5 percent, check dams are required. • Slope of pavement area draining to the strip shall be less than 6 percent. Steeper slopes with appropriate energy dissipation may be allowed with approval by City Engineer. • Filter strips shall have a minimum width of 5 feet, measured in the direction of flow. Figure 4.8. Filter Strip Diagram. <-1:' 50' minimum from , _ a._-=.� �o •(• J 1` streams 0 c� v _ l 1.1 � 21. 5' minimum �. width for _ all strips in i Photo courtesy of USDA NRCS. direction of flow (T) Filter strip between agricultural fields and stream. 113 Section 4.6 Lake Oswego Stormwater Management Manual Filter Strips Design Steps Following is the procedure to confirm that a proposed filter strip has adequate capacity to treat water quality flows. Step 1: Calculate Design Discharge Estimate peak discharge rate (()design) in cubic feet per second for water quality design storm. Parameter Units Value ()design design discharge cubic feet per second Step 2: Determine Preliminary Facility Parameters Determine width and slope of filter strip. Check dams are required perpendicular to flows where slopes are greater than 5 percent, unless approved by the City. Parameter Units Value T filter strip width (See Figure 4.8) feet S filter strip slope feet per feet 0.005 - 0.1 Step 3: Calculate Flow Depth Calculate flow depth (y in feet) using the following equation: () * n )06 _ design y 1.49* T* �S Parameter Units Value y flow depth feet maximum 0.083 n Manning's roughness coefficient (unitless) 0.25 T flow width feet S longitudinal slope feet per feet If flow depth is greater than one inch, adjust width and/or slope, and recalculate. Lake Oswego Stormwater Management Manual Section 4.6 114 Filter Strips Step 4: Check Velocity Calculate velocity of flow using the following equation (V in feet per second). Q V = T * y Parameter Units Value V flow velocity feet per second maximum 0.5 If velocity is greater than 0.5 feet per second, adjust width and/or slope and recalculate. 115 Section 4.6 Lake Oswego Stormwater Management Manual Filter Strips Materials Biofiltration Soil Mix Filter strip soil media must support long-term plant and soil health and provide treatment to water as it infiltrates. The City of Portland's specification for Standard Blend for Public and Private Facilities, Appendix F.3, should comprise at least the top 18 inches of soil depth and be placed on top of uncompacted native soil (Portland, 2013). This mixture can be achieved by amending native soils with compost, if soils are sandy loam or coarser. If site has finer grained native soils, a biofiltration soil mix may be needed to meet required infiltration rates (See Section 4.4 for information on amending onsite soils). Component Percent by Volume Sandy loam 60-70% Composted plant-based organic matter 30-40% Fines passing a size 200 sieve are restricted to 5 to 15 percent of the soil volume. Mulch Fine to medium hemlock bark or well-aged organic yard debris compost is recommended for filter strips. Mulch must be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It should not be over-applied. Vegetation Objective Establish dense plant growth of groundcovers, herbaceous plants, and shrubs, with a goal of 95 percent coverage. Plant Selection and Diversity Filter strips often experience moist soil conditions during the wet, rainy season and dry soil conditions during warm summers. Plant selection should respond to specific site conditions for each facility. See Appendix H for a complete plant list. Lake Oswego Stormwater Management Manual Section 4.6 116 Filter Strips Plant Quality Vegetation Type Recommended Size Sedges and rushes 10-inch deep container'; plug2; division; rhizome; or tuber 1 gallon or equivalent; plug; 4-inch pot allowed if the facility is left Grasses and forbs off-line for the first wet season, allowing plants an appropriate establishment period Shrubs 1 gallon; bareroot; or equivalent 110-inch deep container sedges and rushes provide a well-developed root system that more easily adapts to site conditions. 2 Plugs and container stock are more often grown from seed. Consider acquiring plants grown from seed in order to preserve genetic diversity and increase resiliency of vegetation on site. Grouping Plant Species Place smaller plants (1 gallon container or smaller) in odd-numbered clumps (3, 5, 7, etc.) of the same species throughout the planting areas. Avoid planting in rows. Planting Depth All plants should be buried as deep as they were grown in the nursery. Plant bare-rooted plants in a hole wide and deep enough so that roots are not bent, crowded, or exposed to air. Bareroot trees shall be planted so that their root flare is even with the soil surface. Conveyance and Outlet/Overflow Filter strips must drain to an approved outlet location. • If soils are suitable for infiltration, route flows to an infiltration trench, drywell, or other approved location (note that infiltration trenches with underdrains and drywells are UlCs and would need to be registered with DEQ). Construction Mark filter strip boundaries with stakes or flagging prior to construction and avoid any unnecessary soil compaction or disturbance. If amending onsite soil, till in soil amendments to a depth of at least 8 inches prior to planting. Maintenance Proper maintenance is essential for a functioning rain garden. Please refer to the operations and maintenance section (Chapter 7) for more information. Native grasses and sedges do not require cutting or mowing. See maintenance checklists in Appendix I for further maintenance guidance. Herbicides should not be used as a weed control technique within stormwater facilities. Insecticide and fungicide use should also be minimized. 117 Section 4.6 Lake Oswego Stormwater Management Manual : : ‘1133 -4 - II rti---54,-.; wk, - , - ,., ,..-, 4-k-,1 , . „ ; ..„ p„, 1 . . ..i. itolik ,.. ...ci. .‘,4-it...., ‘ „___,, -..... 4.1,.. ...., , mSWaI © 0. , � 4.6.9 Swales Definition A swale is a long, linear, gently sloped channel that removes pollutants through sedimentation and filtration. Introduction Applicability Table A swale is an open, gently sloped, vegetated channel designed for treating stormwater. The primary pollutant BMP Parameters removal mechanisms are filtration and sedimentation. Appropriate for These occur when vegetation intercepts stormwater and pretreatment V traps pollutants. I Appropriate for onsite stormwater management A swale is designed so that water will flow evenly across the entire width of a densely vegetated area. For best I Provides flow control performance and where feasible, offline swales are Provides water quality preferred. For small contributing areas, a swale may treatment V be designed for both treatment and conveyance of onsite stormwater flow. This combined use can reduce I Appropriate for residential development costs by eliminating the need for separate I UIC permit required conveyance systems. Note, however, that the design must satisfy both water quality and conveyance requirements. Design professional That is, it must have adequate retention time during required V * the water quality flow, and adequate stability such that Depending on site conditions. pollutants are not remobilized during the conveyance flow. In areas with steep slopes, weirs are used to retain flows and can maximize infiltration. Swales are best applied on a relatively small scale (generally less than 5 acres of impervious surface). They work well along roadways, driveways, and parking lots, and can be designed as beautiful public amenities. Figure 4.9. Swale Section (Typ). Facility Footprint See construction section for soil protection requirements p, Optional benching ,,, above design ponding 4 10' Width from edge of 0 m depth to integrate swale Bottom width g into the landscape S infiltration zone 1 Design „,„,,,.... 6" min. freeboard ponding - depth Mulch Native soil — /. Biofiltration soil mixL J Infiltration zone Lake Oswego Stormwater Management Manual Section 4.6 118 Swales Site Requirements • Sites must be large enough to incorporate a minimum of a 100-foot length. Shorter distances should be designed as a infiltration basins. `' - ," # s� r "�> • Swales may be designed for a range of site conditions; from full sun to full shade. Plant selection should match site conditions. e ' • Swale edges must be at least 5 feet from property lines. • The edges of swale design ponding depth (see Figure 4.9) must be at least 10 feet from buildings. • Swales are not appropriate adjacent to roads with slopes greater than 15 percent. Geometry Swales can be adapted to fit a wide variety of sites. Use the design steps and select from the plants and materials described in the following sections to ensure adequate treatment of stormwater. Swale Dimensions • Length: minimum 100 feet • Bottom width: minimum 3 feet. Swales intended to provide significant conveyance of offsite runon will be wider. See Chapter 5. • Side slopes: no greater than 4 horizontal: 1 vertical • Flow depth: no greater than 4 inches • Longitudinal slope: 0.5 to 4 percent (for steeper sites, use transverse/check dams or similar features to slow flow velocity) • Mulch depth: 2 to 3 inches Figure 4.10. Swale diagram. Jas\e eS Undevelope• "" s\o area •. Outflow 3'min. bottom width t. -� t Dispersed inflow = fir ' Pavement Inflow " 119 Section 4.6 Lake Oswego Stormwater Management Manual Swales Design Steps Following is the procedure to confirm that a proposed swale has adequate capacity to treat water quality flows. Step 1: Calculate Design Storm Calculate water quality (QwQ: 1 inch 24-hour) and conveyance design storm (Qpeak: Q25Peakpost- construction) using an approved method. (See Table 5.1 and Table 4.1 for design storms). Step 2: Select Manning's Coefficient Select Manning's roughness coefficient (n) based on planting type and density. Parameter Units Range n Manning's roughness coefficient (unitless) 0.2 to 0.3 Step 3: Establish Site Geometry Establish preliminary geometry for the following criteria, considering specific site conditions. Parameter Units Range SL longitudinal slope feet per feet 0.005 to 0.04 b bottom width feet 3 feet minimum Z side slopes H:V (e.g.,for 4H:1V side 4H:1 V or flatter slopes,Z=4) y flow depth feet 0.33 feet (4 inches) maximum Step 4: Calculate Wetted Perimeter, Hydraulic Radius, and Flow Area Calculate the wetted perimeter, hydraulic radius, and flow area using Manning's equation. Equations below are for a trapezoidal channel with equal side slopes. Parameter Equation P wetted perimeter P= b + 2y(1+Z2)°5 A cross-section area A= by + Zy2 A R hydraulic radius P Lake Oswego Stormwater Management Manual Section 4.6 120 Swales Step 5: Velocity Calculate velocity using Chezy-Manning equation. Parameter Units Equation V velocity feet per second (1.49/n)(R213)(SL112) Step 6: Calculate Residence Time Calculate required length for 9-minute residence time needed to adequately settle sediment. Parameter Units L length feet 60 sec L = Vx9 minutes ( 1 \ min Step 7: Confirm Capacity Confirm that swale has adequate capacity. Parameter Units cubic feet QSwale VA per second Is °Swale water quality design flow? Step 8: Check Stability Perform stability check using n=0.04 and °Peak (Step 1). Use trial and error to calculate the flow depth associated with Peak Calculate P,A, and R from Step 4 corresponding to the peak flow depth. Then, calculate VMax from Step 5 and confirm that V is less than 3 feet per second. This method is not applicable to swales with added check dams. Parameter Units Value ns Manning's n—stability check - 0.04 Peak conveyance stability flow cubic feet See Chapter 5 per second V maximum velocity for feet per 3 Max presumed stability at peak second design flow 121 Section 4.6 Lake Oswego Stormwater Management Manual Swales Materials Biofiltration Soil Mix Swale soil media must support long-term plant and soil health and provide treatment to water as it infiltrates. The City of Portland's specification for Standard Blend for Public and Private Facilities,Appendix F.3, should comprise at least the top 18 inches of soil depth and be placed on top of uncompacted native soil (Portland, 2013). This mixture can be achieved by amending native soils with compost, if soils are sandy loam or coarser. If site has finer grained native soils, a biofiltration soil mix may be needed to meet required infiltration rates (See Section 4.4 for information on amending onsite soils). Component Percent by Volume Sandy loam 60-70% Composted plant-based organic matter 30-40% Fines passing a size 200 sieve are restricted to 5 to 15 percent of the soil volume. Mulch Fine to medium hemlock bark or well-aged organic yard debris compost is recommended for swales. It should be placed in the facility only in areas above the designed flow depth. Keep mulch material out of the stormwater flow path to avoid clogging inlets or outlets. Mulch must be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It should not be over-applied. Vegetation Objective Establish dense plant growth of thin-stemmed, emergent, native vegetation within the swale treatment area, and a diversity of groundcovers, shrubs, and trees along edges and borders. Plant Selection and Diversity Swales often experience moist to saturated soil conditions during the wet, rainy season and dry soil conditions during warm summers. Plant selection should cater to specific site conditions for each facility. Select a minimum of three species for the treatment area. Select species that are suitable for the hydrologic, light, and soil conditions in the proposed swale. Swales should be designed so that they do not require mowing. See Appendix H for a complete plant list with species for swale bottoms (Zone A) and side slopes (Zone B). Plant Quality All plants shall meet most recent standards as defined by the American Standard for Nursery Stock. Lake Oswego Stormwater Management Manual Section 4.6 122 Swales Vegetation Type Recommended Size Sedges and rushes 10-inch deep container; plug2; division; rhizome; or tuber Grasses and forbs 1 gallon or equivalent; plug; 4-inch pot allowed if the facility is left off-line for the first wet season, allowing plants an appropriate establishment period Shrubs 1 gallon; bareroot; or equivalent Trees 3 gallon container or equivalent; minimum height 6 feet for conifers and minimum caliper 1.5 inches for deciduous species, unless approved by City 10-inch deep container sedges and rushes provide a well-developed root system that more easily adapts to site conditions. 2 Plugs and container stock are more often grown from seed. Consider acquiring plants grown from seed in order to preserve genetic diversity and increase resiliency of vegetation on site. Grouping Plant Species Place plants in odd-numbered clumps (3s, 5s, 7s, etc) of the same species throughout the planting areas. Planting Depth Plant bare-rooted plants 4 to 6 inches deep and plugs as deep as the pot or plug. Swale Side Slope Planting • Establish with low growing native groundcover and shrubs that allow adequate sunlight to the swale for plant growth. Locate shrubs to allow for maintenance access to the treatment area. • Establish groundcover vegetation that is low growing, will protect slopes from erosion, and will not compete with low shrubs planted on the side slopes. Conveyance and Outlet/Overflow Swales must include outlets to an approved location. Construction Mark filter strip boundaries with stakes or flagging prior to construction and avoid any unnecessary soil compaction or disturbance. If amending onsite soil, till in soil amendments to a depth of at least 8 inches prior to planting. Maintenance Proper maintenance is essential for a functioning rain garden. Swales that are intended to fill a conveyance role can be expected to require more frequent monitoring to assure that no excessive erosion has occurred, and to remove sediment buildup to maintain both pollutant removal and conveyance capacity. Refer to the operations and maintenance section (Chapter 7) for more information. Native grasses and sedges do not require cutting or mowing. See maintenance checklists in Appendix I for further maintenance guidance. Herbicides should not be used as a weed control technique within stormwater facilities. Insecticide and fungicide use should also be minimized. 123 Section 4.6 Lake Oswego Stormwater Management Manual Sa :,$ w 4.6.10 Sand Filters Definition Sand filters are structural landscaped reservoirs used to collect, filter, and infiltrate stormwater, allowing pollutants to settle and filter out as the water percolates through the sand and gravel. Note: This guidance was adapted from City of Portland Storm water Management Manual (Portland 2008). Introduction Applicability Table A sand filter includes a flow spreader, a sand bed, and may include an underdrain collection system. BMP Parameters Pretreatment is required through another facility. Sand Appropriate for filters can be constructed above, at, or below grade. pretreatment Depending on site conditions, sand filters can be Appropriate for onsite designed to completely infiltrate all the stormwater they stormwater management receive or be designed as flow-through facilities where only a portion of the flow is infiltrated, and overflow is Provides flow control directed to an approved discharge point. Provides water quality If plants are used, sand filters can be used to help treatment V fulfill a site's required landscaping area requirement Appropriate for residential and should be integrated into the overall site design. UIC permit required Numerous design variations of shape, wall treatment, Design professional and planting scheme can be used to fit the character of required a site. Figure 4.10. Sand Filter Section (Typ). Overflow 12" Maximum ponding depth • 1 Q i 1! li I. d i� N t � 1 "�l � 4il � tlit0 Sand filter depth is j .: 11l 12" Filter material 2" - ' .. -Aii ' * "°,� .tea EiMint 42411.61101 11 Washed drain rock 111 likOutlet to 8" 1lk=li 111` ppoved Native soil —/ \_testarb Perforated _ location underdrain Lake Oswego Stormwater Management Manual Section 4.6 124 Sand Filters Site Requirements i.' -4..�.os, • Infiltration sand filters must be at least 5 feet ,.t -- ' - - Ili- , r I from property lines. y F l -, _ '4- ip • Infiltration sand filters must be at least 10 feet !` 1 ; '` _. from building foundations. Y • No setbacks to building foundations are required ' - for lined flow-through sand filters where the height above finished grade is 30 inches or 9 less. Required setback of 5 feet to rights-of-way, utilities, and pipelines. .' - • 4 feet of hydraulic head is required from inlet to Outlet. Sand filter facility. • Pretreatment is required with a separate facility for the removal of trash and sediment. Geometry Sand Filter Dimensions • Width: >_ 18 inches (flow through) and 30 • Length to width ratio: 2:1 minimum inches (infiltration) • Slope: s 0.5 percent in all directions • Depth of sand filter bed: >_ 12 inches • For subgrade facilities, the filter medium must • Depth of filter material: 2 - 3 inches be 30 inches deep, with 8 inches of gravel above and below for conveyance. A minimum • Depth of drain rock (underlying sand filter and of 2 inches of freeboard (vertical distance filter material): >_ 8 inches between the overflow inlet elevation and • Ponding depth: where the facility is at or above overtopping elevation) shall be provided. grade, the storage depth must be at least 12 inches between the top of the filter medium and the base of the overflow Figure 4.11. Subsurface Sand Filter Section (Typ). Note: structural walls Downspout may be required. fr` „N 1...�L Trapped silt basin with lid , ,, t+ / Z.0 .1?-\3... ..'" . ir. y "1144lUk"A AiI, Vd1 A,,11j AA1 •Amy �_` `.. _.. -- 4" Perforated r-, distribution pipe Sandy loam viz, w �s A.- 2" to 3" Choker course ii In 8" of Drain rock 12" to30" of Sand 2" to 3" Filter material .6 ,;,,b�a8 g �neg& is g1 Fit:, a 1 Ct-C f Outfall to op 8" of Drain rock #- o -' it jug ' approved Native soil - Perforated underdrain location 125 Section 4.6 Lake Oswego Stormwater Management Manual Sand Filters Design Steps Sand filters can be designed in two ways: • Route stormwater hydrographs through a proposed facility to demonstrate that it meets the design criteria using an approved SBUH hydrologic model. • Calculate sand filter surface area using the simplified method described below. Step 1: Calculate Design Storm Size sand filters to treat the water quality design storm. Parameter Units Value Pdesign design storm inches 1.0 Step 2: Estimate Contributing Drainage Area Measure or estimate total drainage area that will discharge to the proposed sand filter. Parameter Units Value At drainage area square feet Step 3: Determine Facility Parameters Determine the sand bed depth and maximum ponding depth for the proposed sand filter. Parameter Units Value M sand filter media depth feet minimum 1.0 D maximum ponding depth feet Step 4: Calculate Hydraulic Gradient Calculate the hydraulic gradient (I [unitless]) across the sand filter media using the equation: ( D2 + M I = M Lake Oswego Stormwater Management Manual Section 4.6 126 Sand Filters Step 5: Calculate Sand Filter Surface Area Calculate the sand filter surface area (Asf[sf]) required using the equation: A *R t design Asf — K*I*T Parameter Units Value R routing factor (unitless) 0.7 T drawdown time hour 24 Method derived from Eastern Washington Stormwater Management Manual (Ecology 2004) Materials Sand filters should have vegetated side slopes (maximum 3H:1V slope) if space allows. If not feasible, vertical walls can be used around the filter. Sand Media The sand in a filter must consist of a medium sand meeting the size gradation (by weight) given in the table below. The contractor must obtain a grain size analysis from the supplier to certify that the No. 100 and No. 200 sieve requirements are met. Sand Media Specification U.S. Sieve Number Percent Passing 4 95-100 8 70-100 16 40-90 30 25-75 50 2-25 100 <4 200 <2 Source: King County 1998. Drain Rock Drain rock is required below the sand. For infiltration facilities where drain rock is specified to retain stormwater prior to infiltration, the specification is %-inch to 1%-inch washed drain rock (ODOT Standard Specification 00430.11,AASHTO No. 4, or equivalent).All flow-through facilities shall use Y2-inch to 3/4-inch washed drain rock (ODOT 00430.11, AASHTO No. 67, or equivalent). Drain rock and sand must be separated by a 2- to 3-inch layer of special filter material per ODOT Standard Specification 02610.10. 127 Section 4.6 Lake Oswego Stormwater Management Manual Sand Filters Walls Walls for the sand filter shall be made of stone, concrete, brick, or other durable material. Chemically treated wood that can leach out toxic chemicals and contaminate stormwater shall not be used. Waterproof Liners Flow-through facilities require a waterproof liner. There are many liner options, and installation methods vary. The liner shall be 30 mil PVC or equivalent. Underdrains Underdrains shall be perforated PVC pipe that meets ODOT Standard Specification 02410.70, or approved equal. Piping installation must follow current Uniform Plumbing Code. Pipe diameter shall be 4-inch minimum for private property and 6-inch minimum for public facilities. Vegetation Objective Planting is recommended for sand filters. Plants enhance infiltration, prevent erosion, and compete with weeds. Do not apply herbicides to manage weeds. Establish dense plant growth of groundcovers and shrubs with a goal of 95 percent coverage. See Appendix H for a list of species appropriate for sand filters. Plant Selection and Diversity Sand filters will experience extreme dry soil conditions during warm summers. Plant selection should respond to specific site conditions for each facility. See Appendix H for a complete plant list. Plant Quality Vegetation Type Recommended Size Sedges and rushes 10-inch deep container'; plug2; division; rhizome; or tuber 1 gallon or equivalent; plug; 4-inch pot allowed if the facility is left Grasses and forbs off-line for the first wet season, allowing plants an appropriate establishment period Shrubs 1 gallon; bareroot; or equivalent '10-inch deep container sedges and rushes provide a well-developed root system that more easily adapts to site conditions. 2 Plugs and container stock are more often grown from seed. Consider acquiring plants grown from seed in order to preserve genetic diversity and increase resiliency of vegetation on site. Grouping Plant Species Place plants in odd-numbered clumps (3s, 5s, 7s, etc) of the same species throughout the planting areas. Planting Depth Plant bare-rooted plants 4 to 6 inches deep and plugs as deep as the pot or plug. Lake Oswego Stormwater Management Manual Section 4.6 128 Sand Filters Conveyance and Outlet/Overflow Sand filters must be located off line. An overflow or bypass structure is required for larger storm events. For public sand filters, these additional criteria apply: • The sand filter must consist of an inlet structure, sand bed, underdrain piping, and liner. • The inlet structure shall spread the flow of incoming water uniformly across the surface of the filter medium during all anticipated flow conditions. The flow shall be spread in a manner that prevents nroiling or otherwise disturbing the filter medium. • Where a collector manifold with perforated lateral branch lines is used, lateral branch line spacing shall not exceed 10 feet. The underdrain laterals shall be placed with positive gravity drainage to the collector manifold. The collector manifold shall have a minimum 1-percent grade toward the discharge joint. All laterals and collector manifolds shall have cleanouts installed that are accessible from the surface without removing or disturbing filter media. 129 Section 4.6 Lake Oswego Stormwater Management Manual ►, r, •R ,11 , , fl.• Ij. I '1 �. ' .,, Il ' r IitStTd�} 7_ \\L . . :\Wétläfl.d rr, s 4.6.11 Constructed Wetland Definition: Constructed wetlands are stormwater treatment facilities that are designed to emulate natural wetlands, with natural, irregular shapes, shallow water that varies in depth, and varied side slopes. They are saturated or have standing water for part of the year, rather than draining over a short period of time as rain gardens are designed to do. They are typically shallower than ponds. Introduction Applicability Table Constructed wetlands present a wonderful opportunity to integrate wildlife habitat and a public amenity into BMP Parameters the landscape of a large residential or commercial Appropriate for development. Constructed wetlands with healthy, pretreatment thriving plants provide excellent water quality Appropriate for onsite treatment. They require a large surface area and stormwater management V a large contributing area to ensure that wetland conditions are maintained. Provides flow control Provides water quality Although this guidance describes a stormwater BMP treatment most applicable for large developments, planting and soil recommendations are also suitable for smaller I Appropriate for residential sites. If a residential or other small property has moist I UIC permit required soil conditions, consider designing to enhance those wetland conditions. Design professional required V Vector (mosquito) control is an important design ;, consideration for any facility that has standing water for extended periods of time. Bat boxes, diverse planting and other design strategies to encourage biological controls can help to keep mosquito populations in balance. ,. 4, . Figure 4.12 Constructed Wetland Section (Typ). A constructed wetland and ,./ r. ;:`= its surrounding site context showing varying topographic conditions and ` '. . plant communities. Jt _ Leaend t — — — Maximum ponding depth = 4 feet '?. - r.- • Average ponding depth <_ 2.5 feet , • • (6 Iico c (6 E I • C • • C C •,- . (6 • 0 (6 ,- (6• N _ N • _ • N _ ••Lu • • w • • Lu • • m . Lake Oswego Stormwater Management Manual Section 4.6 130 Constructed Wetlands t Site Requirements . •'� • Adequate space for wetland footprint and r�{ �: * + '" y' maintenance access. • I f j__ • • • • The edge of wetland must be located a minimum of " '1 • ';,� �,' '1 f+�;` 5 feet from property lines. 1 ti F' � i 1# I • The edge of wetland must be located a minimum of i I ,k 1, 10 feet from buildings. E, ; I ' • .y • Appropriate for Group C or D soils. • -�,, •.1} i 6,1x,. • • Appropriate site hydrology to sustain saturated , ;_ 1i , . conditions during part of the growing season to Constructed wetland vegetation facilitates filtration and maintain wetland vegetation. settling of pollutants. Geometry Constructed wetlands should be irregularly shaped, with a sinuous flow path and a variety of side slopes and benches incorporated to maximize plant establishment and diversity. See Figure 1 and Vegetation for an overview of wetland planting zone information. Constructed Wetland Dimensions • Two cells required, with the first cell, the sediment forebay, containing 10 percent of the design volume, unless pretreatment is provided in a different facility.A sediment forebay provides a clear visual indicator of when maintenance is needed. • Bottom width: at least 3 feet • Maximum ponding depth: 4 feet • Average ponding depth: <_ 2.5 feet • Slopes and depths should vary to provide a variety of habitat and maximize treatment. • Side slopes: no greater than 5 horizontal: 1 vertical below maximum ponding depth " • Side slopes: no greater than 3 horizontal: 1 • 3 ':L. '•i''i'• vertical above maximum ponding depth •fir ..�;....ti: p:4.: 'jt,. ._ • .'r ^:.:yam" sr'CRY. `:• Virrt, lk tky �j w•' as a constructed wetland. In addition to improving water quality, it provided habitat for endangered turtle species, as well as other amphibians and birds. 131 Section 4.6 Lake Oswego Stormwater Management Manual Constructed Wetlands Design Steps Constructed wetland sizing follows the same design steps as those for sizing a pond. However, constructed wetlands will have a larger surface area, varied side slopes, and shallower ponding areas to maximize water quality treatment. Step 1: Determine Drainage Basin Characteristics Calculate the drainage area and Natural Resource Conservation Service (NRCS) Curve Number separately for both pervious and impervious surfaces in the basin. See Appendix E for NRCS Curve Number determinations. Parameter Units Value Aimp and Aper drainage area (impervious and pervious) acres CN NRCS curve number (impervious and pervious) unitless Step 3: Calculate Runoff Volume Calculate the volume of runoff during a 10-year, 24-hour storm separately for impervious and pervious surfaces and then sum the results using the equations below: 1,000 * [ - 10] 2 V. + Vpery= 3,630 *A* ( Pdesign - 0L CN ) Pdesign + 0.8 r 1,000 - 10 L CN Parameter Units Values V. + Vpery runoff volume (impervious and pervious) cubic feet Pdesign design precipitation depth inches 1.0 Design varied side slopes, water depths, and planting zones to complement site characteristics. Step 3: Design Grading and Site Characteristics Step 4: Determine Wetland Permanent Pool Depths Permanent water depth in a wetland will vary in the different cells. Determine the average, maximum, and minimum depths for permanent pool in each cell of the wetland. Parameter Units Values dwetland permanent pool depth feet 0.5-4 Lake Oswego Stormwater Management Manual Section 4.6 132 Constructed Wetlands Step 5: Refine Wetland Grading Using design criteria as guidance, adjust the layout of the wetland cells, verifying that maximum depths do not exceed those in Step 4. Calculate the permanent pool volume from the preliminary grading layout and adjust if the volume is less than the design storm volume. Materials Growing Medium Constructed wetlands are best for Type C and D soils or in areas with a high groundwater table. Soils must be saturated for a long enough time to maintain wetland vegetation. Investigate the soil profile and determine whether appropriate site soils exist and if any soil amendments need to be added to assist with initial plant establishment. Mulch Fine to medium hemlock bark or well-aged organic yard debris compost is recommended for constructed wetlands. It should be placed on bare areas surrounding planted material in order to prevent weed establishment. Mulch must be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It should not be over-applied. If a groundcover is seeded to prevent weed establishment, provide a mulch ring of at least 2 feet surrounding the stems of planted trees and shrubs to prevent weed and plant competition. Vegetation Objective Establish dense plant growth of native submergent and emergent native vegetation within the constructed wetland and a diversity of groundcovers, shrubs, and trees along edges and borders above permanent pond depth. Intersperse submergent, emergent, and woody plant communities at variable heights between permanent pond depth and maximum design depth. Maintain a 20-foot minimum distance between water-seeking tree and shrub species (e.g., Oregon ash, alder, willows, and dogwoods) and inlets or outlets to prevent roots or stems from blocking structures or obstructing maintenance efforts. Plant Selection and Diversity Constructed wetlands often experience long periods of saturated soil conditions during the wet, rainy season, but may dry out during warm summers. Plant selection should cater to specific site conditions for each facility. Select a minimum of nine species for the treatment area. Select species that are suitable for the hydrologic, light, and soil conditions in the proposed constructed wetland. Constructed wetlands should be designed so that they do not require mowing. See Appendix H for a complete plant list of species appropriate for constructed wetlands. 133 Section 4.6 Lake Oswego Stormwater Management Manual Constructed Wetlands Plant Quality Vegetation Type Recommended Size Sedges and rushes 10-inch deep container'; plug2; division; rhizome; or tuber 1 gallon or equivalent; plug; 4-inch pot allowed if the facility is left Grasses and forbs off-line for the first wet season, allowing plants an appropriate establishment period Shrubs 1 gallon; bareroot; or equivalent 3 gallon container; bareroot or equivalent, minimum caliper should Deciduous trees be 1 1/2" at 6 inches above base. Mature size should be suited to site. Evergreen trees 3 gallon container or equivalent; minimum height 6 feet, unless approved by City. Mature size should be suited to site. 110-inch deep container sedges and rushes provide a well-developed root system that more easily adapts to site conditions. 2 Plugs and container stock are more often grown from seed. Consider acquiring plants grown from seed in order to preserve genetic diversity and increase resiliency of vegetation on site. All plants shall meet most recent standards as defined by the American Standard for Nursery Stock. Grouping Plant Species Place smaller plants (1 gallon container or smaller) in odd-numbered clumps (3, 5, 7, etc.) of the same species throughout the planting areas. Avoid planting in rows. Planting Depth All plants should be buried as deep as they were grown in the nursery. Plant bare-rooted plants in a hole wide and deep enough so that roots are not bent, crowded, or exposed to air. Bareroot trees shall be planted so that their root flare is even with the soil surface. Woody Vegetation Use woody vegetation to provide shade over standing water and to provide structural diversity within the constructed wetland. Shrubs and trees shall be sited to promote long-term health and survival, minimize maintenance, and protect lines of sight. Locate shrubs and trees to allow for maintenance access to the treatment area. Do not plant woody vegetation within 20 feet of inlet and outlet structures. Conveyance and Outlet/Overflow Constructed wetlands must overflow to an approved outlet structure. Lake Oswego Stormwater Management Manual Section 4.6 134 Constructed Wetlands Emergency Overflow Spillway All constructed wetlands shall have an emergency overflow spillway or other overland location that will safely pass runoff from the 100-year storm event over the pond embankment in the event of control structure failure or for storm events that exceed the design of the control structure. Spillways shall meet the following criteria: • Locate the spillway from the top of the berm embankment to a City-approved downstream conveyance system. • Design the spillway to carry the 100-year storm event for the total upstream drainage area. • The invert elevation of the spillway shall be at least 12 inches above the primary overflow elevation. • The minimum spillway depth shall be 12 inches from the top of the berm. • Provide a vegetated spillway designed to protect the spillway from erosion should an overflow event occur. Construction The planting designer shall place the plants during construction and create as-builts. Mark wetland boundaries with stakes or flagging prior to construction and avoid any unnecessary soil compaction or disturbance. When stockpiling soils, place them in shallow linear mounds to preserve soil microorganisms and vitality. See Chapter 3.3 for more information. Maintenance Maintain access routes to the constructed wetland for maintenance purposes.A public wetland facility should include a minimum 8-foot-wide access route with no greater than 10 percent slope. Staff gauges are required at inlets and outlets. Bat and bird boxes are encouraged to help with pest control. Pesticide use is strongly discouraged. Only compounds approved for aquatic environments are allowed. Pesticides and herbicides must be applied by a state licensed applicator and should be approved by the City prior to application. 135 Section 4.6 Lake Oswego Stormwater Management Manual W'''' lit- ''-' w nd . s , . _ _ _ ___.__._ , , #) 4.6.12 Ponds Definition Stormwater ponds approved for use in Lake Oswego include wet ponds, infiltration ponds, and detention ponds. Ponds detain and/or infiltrate stormwater. Ponds are a good choice where there is a large contributing area draining to a single regional facility, where there is adequate space to design a pond that can be integrated into the landscaping, and where it is accessible for maintenance. Introduction Applicability Table WET PONDS (RETENTION PONDS)* c C Wet ponds have a permanent pool of standing c o_ o_ water. Pollutants are removed through settling and BMP Parameters a_ o 0 biological processes. They are a good choice where c a soil infiltration rates are inadequate for an infiltration m c pond or rain garden, where there is a large enough 0 — contributing area that the pond will not be stagnant, Appropriate for and where they can be incorporated into the landscape as an amenity. pretreatment Appropriate for onsite INFILTRATION PONDS stormwater management V Infiltration ponds are vegetated depressions that Provides flow control temporarily pool stormwater before it percolates into Provides water quality underlying soils. Infiltration ponds may be solely treatment for flow control, if soils are granular, in which case a pretreatment facility is required. If soils have Appropriate for residential appropriate characteristics (organic content, soil sorptive capacity), infiltration facilities may also UIC permit required provide runoff treatment through physical filtration, Design professional adsorption, and precipitation. They are suitable required where infiltration rates are adequate, and where the contributing area exceeds 5 acres. DETENTION PONDS* Detention ponds are ponds that temporarily store water then release it slowly over time, through control structures that control the rate and time of release. There may be incidental infiltration, but generally soils are not adequate for complete infiltration. * Vector (mosquito) control is an important design consideration for any facility that has standing water for extended periods of time. Bat boxes, diverse planting and other design strategies to encourage biological controls can help to keep mosquito populations in balance. Lake Oswego Stormwater Management Manual Section 4.6 136 Ponds Site Requirements • Contributing area must be at least 5 acres. ;�_L • Site must have adequate space for pond and +� * �� maintenance access. a` t • or t• ; • Pond type selection should be appropriate for soil characteristics (e.g., if soils have adequate infiltration capacity, design an infiltration pond }�-� ' ( rather than a lined wetpond) • 01 1 • Edge of water surface shall be at least 20 feet tt �' f' from property lines and structures. This pond captures and treats neighborhood runoff. • Edge of water surface shall be at least 200 feet from tops of slopes greater than 15 percent. Geometry Pond Dimensions • Include two cells, with the first cell (forebay) containing approximately 10% of the design surface area. Forebays simplify maintenance and are strongly encouraged. Maintenance access must be provided to forebay. • Maximum side slopes: 3H:1V • Length to width ratio: >_ 3:1 Figure 4.13 Pond Section (Typ). Vegetated side slopes Edge of water surface Maximum ponding depth 1- olio nl=jlt— ! I11—Il1= + ;it, Permanent pool depth !!!' ! s111-111 Itlr �[q_1j (wet pond _III^lrl� -11112. 1 £I$Icll!_111�I11=11l3111 _ Infiltration (infiltration ponds) 137 Section 4.6 Lake Oswego Stormwater Management Manual Ponds Design Steps Detention Ponds: Use an approved model to route post-developed flows through the pond. See design steps for detention pipes and vaults (Section 4.6.13). Infiltration Ponds: Use method described for Infiltration Trench (Section 4.6.3) to determine pond size. Wet Ponds: The following steps apply to wet ponds. Step 1: Determine Drainage Basin Characteristics Calculate the drainage area and Natural Resource Conservation Science (NRCS) Curve Number separately for both pervious and impervious surfaces in the basin. See Appendix E for NRCS Curve Number determinations. Parameter Units Value Aimp and Apery drainage area (impervious and pervious) acres CN NRCS curve number (impervious and pervious) unitless Step 2: Calculate Runoff Volume Calculate the volume of runoff during a 10-year, 24-hour storm separately for impervious and pervious surfaces and then sum the results using the equations below: 1,000 * [ - 10] 2 Vimp + Vperv= 3,630 *A* ( Pdesign - 0'2 CN ) Pdesign + 0.8 r 1,000 - 10 [ CN Parameter Units Values V. + Vpery runoff volume (impervious and pervious) cubic feet Pdesign design precipitation depth inches 1.0 Step 3: Design Grading and Site Characteristics Design varied side slopes, water depths, and planting zones to complement site characteristics and transition site with surrounding context. Step 4. Determine Wetland Permanent Pool Depths Permanent water depth will vary in the different cells. Determine the average, maximum, and minimum depths for permanent pool in each cell. Parameter Units Values dpond permanent pool depth feet 2-8 Lake Oswego Stormwater Management Manual Section 4.6 138 Ponds Step 5: Refine Pond Grading Using design criteria as guidance, adjust the layout of the pond cells, verifying that maximum depths do not exceed those in Step 4. Calculate the permanent pool volume from the preliminary grading layout and adjust if the volume is less than the design storm volume. Materials Growing Medium Investigate the soil profile and determine whether appropriate site soils exist for healthy vegetation and if any soil amendments need to be added to assist with initial plant establishment. See Section 3.3 for soil testing and amendment requirements. Mulch Fine to medium hemlock bark or well-aged organic yard debris compost is recommended around the base of plantings in areas along the tops of banks and side slopes above the high water mark. Mulch must be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It should not be over-applied. Consider using rock or similar surfacing along the pond bottoms and side slopes that will experience prolonged inundation and that, may or may not dry out during the dry season. Mulch will help prevent weed establishment and soil erosion. Vegetation Objective Detention Ponds: It is difficult to establish vegetation in areas that are inundated for periods longer than 72 hours. Establish dense plant growth in areas along side slopes and tops of banks where water levels and soil conditions are amenable to vegetation. Infiltration Ponds: Infiltration ponds are designed to drain within a 24-hour period. Follow the vegetation guidance for rain gardens (Section 4.1.1) when designing an infiltration pond, while adjusting planting zones to the appropriate water depth tolerances along pond side slopes. Wet Ponds: Establish dense growth of native submergent and emergent vegetation along pond side slopes, and a diversity of groundcovers, shrubs, and trees along edges and borders of the pond. Added water quality treatment can be integrated into a pond system by adding a shallower wetland bench upstream of the pond outlet. Maintain a 20-foot minimum distance between water-seeking tree and shrub species (e.g., Oregon ash, alder, willows, and dogwoods) and inlets or outlets to prevent roots or stems from blocking structures or obstructing maintenance efforts. The remaining guidance for vegetation in ponds refers only to vegetation for wet ponds. Plant Selection and Diversity Wet ponds have fairly consistent soil moisture characteristics along side slopes. Depending on site soil and drainage characteristics, the side-slope and top-of-bank plantings may experience moist to saturated winter soil conditions and dry summer conditions. Plant selection should cater to specific site conditions for each facility. 139 Section 4.6 Lake Oswego Stormwater Management Manual Ponds Select a minimum of nine species for the treatment area. Select species that are suitable for the hydrologic, light, and soil conditions of the proposed pond. Ponds should be designed so that they do not require mowing. See Appendix H for a complete plant list of species appropriate for ponds. Plant Quality Vegetation Type Recommended Size Sedges and rushes 10-inch deep container'; plug2; division; rhizome; or tuber 1 gallon or equivalent; plug; 4-inch pot allowed if the facility is left off-line Grasses and forbs for the first wet season, allowing plants an appropriate establishment period Shrubs 1 gallon; bareroot; or equivalent Deciduous trees 3 gallon container; bareroot or equivalent, minimum caliper should be 1 1/2" at 6 inches above base Evergreen trees 3 gallon container or equivalent; minimum height 6 feet, unless approved by City 10-inch deep container sedges and rushes provide a well-developed root system that more easily adapts to site conditions. 2 Plugs and container stock are more often grown from seed. Consider acquiring plants grown from seed in order to preserve genetic diversity and increase resiliency of vegetation on site. All plants shall meet most recent standards as defined by the American Standard for Nursery Stock. Grouping Plant Species Place smaller plants (1 gallon container or smaller) in odd-numbered clumps (3, 5, 7, etc.) of the same species throughout the planting areas. Avoid planting in rows. Planting Depth All plants should be buried as deep as they were grown in the nursery. Plant bare-rooted plants in a hole wide and deep enough so that roots are not bent, crowded, or exposed to air. Bareroot trees shall be planted so that their root flare is even with the soil surface. Woody Vegetation Use woody vegetation to provide shade over standing water and to provide structural diversity surrounding the pond. Shrubs and trees shall be sited to promote long-term health and survival, minimize maintenance, and protect lines of sight. Locate shrubs and trees to allow for maintenance access to the treatment area. Do not plant woody vegetation within 20 feet of inlet and outlet structures. Lake Oswego Stormwater Management Manual Section 4.6 140 Ponds Conveyance and Outlet/Overflow Overflow must be to a City-approved, downstream conveyance system. For detention ponds, the primary overflow elevation shall be set at the level of the 10-year design release rate. Emergency Overflow Spillway Spillways shall meet the following criteria: All ponds shall have an emergency overflow spillway or other overland location that will safely pass runoff from the 100-year storm event over the pond embankment in the event of control structure failure or for storm events that exceed the design of the control structure. • Locate the spillway from the top of the berm embankment to a City-approved, downstream conveyance system. • Design the spillway to carry the 100-year storm event for the total upstream drainage area. • The invert elevation of the spillway must be at least 12 inches above the primary outlet elevation. • The minimum spillway depth must be 12 inches from the top of the berm. • A vegetated spillway must be provided. It must be designed to protect the spillway from erosion, should an overflow event occur. Construction The planting designer shall place the plants during construction so that proper submergence depth ranges can be maintained, and shall create as-builts. Encourage the use of onsite soils when constructing berms or surrounding pond topography. Mark pond boundaries with stakes or flagging prior to construction, and avoid any unnecessary soil compaction or disturbance. When stockpiling soils, place them in shallow linear mounds to preserve soil microorganisms and vitality. See Section 3.3 for more information. Maintenance Maintain access routes to the pond for maintenance purposes. A public pond facility should include a minimum 8-foot-wide access route with no greater than 10 percent slope. Consider using reject rock and/or low-growing native prairie species for maintenance access roads. Using reject rock for access roads will avoid hauling costs. Native prairie species will not need mowing, but they may be mowed prior to maintenance activities without experiencing excessive damage. Staff gauges are required at the pond inlet and outlet. Bat and bird boxes are encouraged to help with vector control. 141 Section 4.6 Lake Oswego Stormwater Management Manual ...., -4 0 `� D j % 0Q r.- _.e a anO . 4 ' ‘_ , . `: , --,.._.), ,-- . . `, ' ' A 4: 0ikk ;'''' a u It . \ 4 4.641.1i'- , \ 4.6.13 Detention Pipes and Vaults Definition Detention pipes and vaults are underground storage facilities that temporarily store water before releasing it. They reduce runoff rates from new development to meet design standards (typically to match predevelopment flow rates for specified design storms). Introduction Applicability Table Detention pipes and vaults are appropriate for commercial I BMP Parameters sites, industrial locations, and roads. They are used for large development projects on sites with inadequate Appropriate for space or unsuitable conditions for an infiltration pond, pretreatment constructed wetland with storage, or detention pond. Appropriate for onsite stormwater management Detention pipes and vaults are for flow control. They I Provides flow control V/ do not provide water quality treatment, so a separate water quality treatment BMP is necessary to meet water Provides water quality quality treatment requirements. Detention facilities must treatment be designed by an engineer. Designs must demonstrate I Appropriate for residential that the facilities have adequate maintenance access, can withstand vehicular and other structural loadings, will I UIC permit required be stable, have been designed to counteract buoyancy Design professional forces in areas of high groundwater, and that the materials required V can withstand chemical properties of soils on the site. To demonstrate that detention facilities will perform their primary flow control function, an engineer must submit calculations from an approved hydrologic model or method. Figure 4.14 Detention Pipe and Vault (Typ). Ladder steps 111 11-11— 'I ICI ii 111-111-111_ I I 1_Il l-' ' -'l I— 11- I I 11 111 11- f 2'max. 2'max--1i— 36" nin.storm drain - r Ladder steps _...., I i-_ I I \ — 1 �—� \— Detention pipe 7 36"min- storm drain Pollution/Flow Standard 18-inch control manhole sumped manhole Notes: 1. See City standard drawing for complete details. Lake Oswego Stormwater Management Manual Section 4.6 142 Detention Pipes and Vaults Site Requirements • Underground storage facilities must be located to avoid conflicts with other underground utilities. • Regular maintenance is essential to ensure continued function of underground detention facilities. Therefore, they must be accessible by maintenance vehicles. • Pipes and vaults must be placed on stable, consolidated native soil with suitable bedding. Pipes and vaults are not allowed in fill slopes, unless a geotechnical analysis is performed for stability and construction practices. Geometry Detention Pipe • Diameter: >_ 36 inches • Pipe bottom shall be flat or gently sloped: <_ 0.5 percent • Maximum distance between pipe bottom and finish grade: 20 feet • Sediment storage depth in upstream standard manhole: >_ 6 inches • Freeboard (distance between design headwater elevation and overflow elevation in control structure): >_ 6 inches Detention Vault • Vault bottom shall be flat or gently sloped to the center, forming a "V": <_ 0.5 percent • Sediment storage depth: >_ 6 inches • Freeboard: >_ 6 inches Pollution/Flow Control Manhole • The minimum allowable diameter for an orifice used to control flows in a public facility is 2 inches. Private facilities may use a 1-inch-diameter orifice if additional clogging prevention measures are implemented. • Orifice diameter shall be greater than or equal to the thickness of the orifice plate. • Orifices less than 3 inches shall not be made of concrete.A thin material (e.g., stainless steel, HDPE, or PVC) shall be used to make the orifice plate. The plate shall be attached to the concrete or structure. 143 Section 4.6 Lake Oswego Stormwater Management Manual Detention Pipes and Vaults Design Steps Detention facilities must be designed by an engineer, with calculations submitted to the City that demonstrate that the facility meets the following design criteria: • Post-development peak discharge rate must match the predeveloped peak discharge rate specified in Section 4.1 using an approved single-event hydrologic model. • In soils where groundwater may induce flotation and buoyancy, measures shall be taken to counteract those forces. Ballasting with concrete or earth backfill, providing concrete anchors, or other counteractive measures are required. Calculations demonstrating stability are required. • All vaults and pipes must meet structural requirements for overburden support and traffic loadings, if appropriate. H-20 live loads must be accommodated for tanks and vaults under roadways and parking areas. End caps must be designed for structural stability at maximum hydrostatic loading conditions. Step 1: Calculate Design Flows Calculate predevelopment peak flows (Qpfe) and post-development peak flows (Qpost, Q100) using approved model (see Section 4.1). Flow Control Design Standard: Maintain peak flow rates at their predevelopment levels for the 2-year, 5-year, and 10-year, 24-hour runoff events. Chapter 5 has information on approved methods and rainfalls for these events. 2 year, 24-hour storm 2.38 inches 5 year, 24-hour storm 2.85 inches 10 year, 24-hour storm 3.2 inches Step 2: Establish Preliminary Geometry Determine initial values for detention pipe or vault geometry parameters. Lake Oswego Stormwater Management Manual Section 4.6 144 Detention Pipes and Vaults Step 3: Establish Preliminary Outlet Structure Configuration Determine initial values for orifice diameter and overflow depth. Orifice Sizing Equation: Q = C A(2 g h)o.5 Parameter Units Value Q orifice discharge rate cubic feet per second C coefficient of discharge feet 0.60 for plate orifices A area of orifice square feet h hydraulic head feet g acceleration of gravity feet per second2 32.2 The diameter of plate orifices is typically calculated from the given flow. The orifice equation is often useful when expressed as an equivalent orifice diameter in inches. where: d - /36.88Q fh Parameter Units Value Q flow cubic feet per second d orifice diameter inches h hydraulic head feet Step 4: Route Design Storm Use an approved model to route the post development design storm (Qpost) through the detention facility, and compare peak outflow to the predevelopment design storm peak flow (0pre). Step 5: Refine Facility Adjust facility geometry, orifice size, and overflow elevation until design criteria are met. Step 6: Check 100-year Flow Check overflow design to ensure safe conveyance of the 100-year runoff discharge. 145 Section 4.6 Lake Oswego Stormwater Management Manual Detention Pipes and Vaults Materials Detention Pipe Galvanized metals leach zinc into the environment, especially in standing water situations. This can result in zinc concentrations that can be toxic to aquatic life. Therefore, galvanized materials shall not be used in stormwater facilities and conveyance systems. For public facilities, detention pipe materials and joints shall conform to technical and manufacturer's specifications. For private facilities, the pipe material shall conform to the Oregon Plumbing Specialty Code. Detention Vault Detention vaults shall be constructed of structural reinforced concrete (3000 psi,ASTM 405). All construction joints shall be provided with water stops. Flow Control Structure Orifices shall be protected within a manhole structure or by a minimum 18-inch-thick layer of 11/2 to 3-inch, evenly graded, washed rock. Orifice holes shall be externally protected by stainless steel with a mesh of 3/4 inch or less. Chicken wire shall not be used for this application. Orifices less than 3 inches shall not be made of concrete. A thin material (e.g., stainless steel, HDPE, or PVC) shall be used to make the orifice plate. The plate shall be attached to the concrete or structure. Conveyance and Outlet/Overflow Pollution/Flow Control Manhole Design Pollution/flow control manholes must be provided that regulate outflow from the underground detention facilities. Pollution/flow control manholes must comply with the specifications outlined in the City's Standard Details numbers B-1.08A and B-1.08B. The drainage report/plan set, in addition to documenting existing proposed conditions, must include, at a minimum, the flow control structure rim elevation, the storage pipe invert elevation, the outlet pipe invert elevation, the elevation of the top of the storage pipe, the elevation of the top of the overflow pipe, all pipe diameters, and any deviation in shear gate/lift assembly from the City's Standard Details. Maintenance Detention Pipe End Plates and Connections The upstream end of the detention pipe must have a maintenance hole or watertight end plate or plug of standard manufacture (not constructed in the field), which must be made from the same material as the detention pipe. Lake Oswego Stormwater Management Manual Section 4.6 146 Detention Pipes and Vaults Access The following access requirements apply to detention pipes: • Detention pipes more than 50 feet long must provide a cleanout. • Detention pipes more than 100 feet long must have access risers at each end to allow for maintenance and repair. • Detention pipes over 200 feet long must have an access riser at the upstream end and a cleanout at least every 100 feet. The following access criteria apply to both detention pipes and vaults: • Access openings shall be positioned no more than 50 feet from any location within the detention pipe or vault. • All detention pipe access openings shall have round, solid locking lids (usually 1/2 to 5/8-inch diameter, hexagonal-head cap screws). • 36-inch minimum diameter CMP riser-type manholes of the same gauge as the detention pipe material may be used for access along the length of the detention pipe and at the upstream terminus of the detention pipe in a backup system. The top slab is separated (1-inch minimum gap) from the top of the riser to allow for deflections from vehicle loadings without damaging the riser pipe. • All detention pipe and vault access openings must be readily accessible by maintenance vehicles. • Detention pipes and vaults must comply with the OSHA and Oregon OSHA confined space requirements, which include, but are not limited to, the preparation of ventilation plans and clearly marking entrances to confined space areas. • Access for detention vaults may be provided by use of removable panels, hatches, or ring and cover. • Internal structural walls of large vaults shall be provided with openings sufficient for maintenance access between cells. The openings shall be sized and situated to allow access to the maintenance "V" in the vault floor. • The recommended minimum internal height is 7 feet from the highest point of the vault floor (not sump), and the recommended minimum width is 4 feet. However, concrete vaults may be a minimum 3 feet in height and width if there are access manholes at each end, and if the width is no greater than the height. Minimum internal height requirements do not apply for any areas covered by removable panels. 147 Section 4.6 Lake Oswego Stormwater Management Manual +4, .�r •.1+ LIB tiEtFIow e :15r`; `= • ' ""`'1:' 7-r:- ,f . - Photo b David Haw.cod. 4.6.14 Sheet Flow Dispersion Note: This BMP guideline was adapted from Seattle Public Utilities (2009b) Definition Sheet flow dispersion is one of the simplest methods of runoff control. Sheet flow dispersion is the dispersion of concentrated flows from driveways, roadways, or other impervious surfaces through a vegetated pervious area. Because flows are already dispersed as they leave the surface (i.e., not concentrated), they need only traverse a narrow band of adjacent vegetation for effective flow attenuation and treatment. Applications and Limitations I Applicability Table Sheet flow dispersion is applicable for impervious surfaces I BMP Parameters with slopes less than 15 percent, such as driveways, sport Appropriate for courts, patios, roofs without gutters, recreational vehicle pretreatment pads, or other situations where concentration of flows can Appropriate for onsite be avoided. stormwater management Site Requirements I Provides flow control Provides water quality Minimum requirements associated with dispersion area treatment design include the following: Appropriate for residential • A 2-foot-wide transition zone to discourage I UIC permit required channeling must be provided between the edge of the contributing impervious area and the Design professional downslope vegetation. This may be an extension required of subgrade material (crushed rock), modular pavement, drain rock, or other material approved by the City. The transition zone may be narrowed for sidewalks and pathways, if approved by the City. • A 10-foot wide vegetated buffer must be provided for up to 20 feet of width of contributing impervious surface.An additional 5 feet of buffer width must be added for each additional 20 feet of width of contributing area or fraction thereof. For example, a 30-foot wide vegetated buffer would be required for a contributing impervious surface that is 50 feet wide. • The flow path must be covered with well-established lawn or landscape area (landscaping with well-established groundcover, or native vegetation with natural groundcover). The groundcover must be dense enough to help disperse and infiltrate flows and to prevent erosion. • Dispersion is typically not permitted within potential landslide areas. At the City engineer's discretion, a geotechnical report may be required. • Dispersion is typically not permitted within 10 feet of a steep slope (greater than 25 percent). • Dispersion is typically not permitted over contaminated sites or abandoned landfills. • For sites with septic systems, the discharge point must be downgradient of the drain field primary and reserve areas. This requirement may be waived if site topography clearly prohibits flows from intersecting the drain field. Lake Oswego Stormwater Management Manual Section 4.6 148 Sheet Flow Dispersion • Area receiving flow must be protected from compaction during construction, or substantial soil amendment may be required prior to final site stabilization. Overflow Conveyance Minimum requirements associated with overflow Wm. conveyance design include the following: • The dispersion area shall convey excess flow to an approved discharge point. Conveyance of a, j large storms shall be considered. w i IAAa • • No erosion or flooding of downstream cr • _;� ;., properties may result. - Photo by E jum.hry Bolton. This grassy field provides dispersion of water from the adjacent roadway. 149 Section 4.6 Lake Oswego Stormwater Management Manual i'y. ' .#' rP ti �y �G } 1. - �r�• . .. 1 r Z} -ter J ':- Z -v - ` r '3F Tree etention ... 4.6.15 Tree Retention Note: This BMP guideline was adapted from Ecology(2012). Definition Existing trees can be preserved or new trees can be planted to reduce stormwater runoff. Introduction Tree canopy can intercept a significant amount of rainfall G si, - -" before it becomes runoff, particularly if the tree canopy "" aye"•"''" • •+i 1. overhangs an impervious surface. Through the processes t - .' r , _ ' - . .•y..:l :• of evapotranspiration and nutrient uptake, trees have ".'--$ '4' ,'- I ., •,' ,,, the capacity to reduce stormwater runoff volumes and .� " p Y _ f . .aft improve water quality. Further, through root growth, trees ,.�, ,y •.. tr can improve the infiltration capacity of the soils in which '" they grow. Through these mechanisms, both tree planting .4` f and tree preservation can contribute to stormwater . �-7. ' ;, management on a site. Trees also provide additional '. •"� z ? environmental benefits including improved air quality, - - - - =" carbon sequestration, reduced heat island effect, and n:,, , {f ", ..,a_ habitat preservation or formation. ,,.4. o ' ; r,, f In the context of site development or redevelopment, f , • . '., "S ,,.- existing trees can be preserved or new trees can be ,_-y • "A $_ .-. planted. The degree of stormwater management provided4 - by a tree depends on the tree type (i.e., evergreen or ,I, t 'wart ' . •r, I deciduous), canopy area, and whether or not the tree _ •"' canopy overhangs impervious surfaces. Depending on -'' ' -;it,` these criteria, trees can provide impervious area reduction -a credits (See Appendix A-3) that are taken into account . - in calculating minimum stormwater management design r -- - -- requirements (e.g., 10-year 24-hour storm volume and ----____ peak flow rate). _� r _ - Tree retention must be coordinated with and abide by the City's tree code (LOC 55) to be considered a stormwater BMP. Trees used as a BMP and later removed may trigger need for replacement flow control measures. Lake Oswego Stormwater Management Manual Section 4.6 150 Tree Retention Site Requirements • Trees to be preserved must be in good health. • Both preserved trees and new trees must be compatible with proposed construction. • Ideal planting sites within a development are those that create interception opportunities around impervious surfaces. • Trees must be sited according to sun, soil, and moisture requirements. • Planting locations must be selected to ensure that sight distances and appropriate setbacks are maintained given mature height, size, and rooting depths. Design Steps Step 1: Inventory Existing Trees Inventory all existing trees on site by a licensed forester or arborist. For each tree, identify the species, the general health, and the diameter at breast height (DBH). DBH is defined as the outside bark diameter at 4.5 feet above the ground on the uphill side of a tree. Step 2: Identify Trees to Preserve Based on the inventory and the proposed site layout, identify trees that can be preserved. To be eligible for an impervious area reduction credit, existing trees to be preserved shall be in good health and have a minimum 6-inch DBH. For existing trees to be preserved smaller than this minimum size, an impervious area reduction credit for newly planted trees may be applied. To receive credit regardless of size, trees must be within 20 feet of new and/or replaced ground-level impervious surface, as measured from the tree trunk center. The preserved tree canopy area shall be measured as the area within the tree drip line. A drip line is the line encircling the base of a tree, which is delineated by a vertical line extending from the outer limit of a tree's branch tips down to the ground. If trees are clustered, overlapping canopies are not double counted. Step 3: Select and Site New Trees A list of approved tree species for stormwater credit is provided in Appendix G. Guidance for tree siting is presented in Appendix K. To receive impervious area reduction credit, new deciduous trees at the time of planting shall be at least 1.5 inches in diameter measured 6 inches above the ground. New evergreen trees shall be at least 4 feet tall. Similar to preserved trees, impervious area reduction credit for newly planted trees depends upon proximity to ground level impervious surfaces. To receive a credit, the tree must be planted within 20 feet of new and/or replaced ground-level impervious surfaces, as measured from the tree trunk center. To help ensure tree survival and canopy coverage, the minimum tree spacing for newly planted trees shall accommodate mature tree spread. In no circumstance shall impervious area reduction credit be given for new tree density exceeding 10 feet on center spacing. Step 4: Calculate Impervious Area Reduction Use the tables and formulas below to calculate the impervious area reduction that can be utilized in calculating minimum stormwater management design requirements (e.g., 10-year 24-hour storm volume and peak flow rate). 151 Section 4.6 Lake Oswego Stormwater Management Manual Tree Retention Impervious Area Reduction for Preserved Trees Tree Type Impervious Area Reduction Evergreen 20 percent of canopy area(minimum of 100 square feet per tree) Deciduous 10 percent of canopy area(minimum of 50 square feet per tree) Impervious Area Reduction = (Z Evergreen Canopy Area x 0.2) + (Z Deciduous Canopy Area x 0.1). Impervious Area Reduction for Newly Planted Trees Tree Type Impervious Area Reduction Evergreen 50 square feet per tree Deciduous 20 square feet per tree Impervious Area Reduction = Z Number of Trees x Credit (square feet). Impervious area reduction credits are not applicable to trees in native vegetation areas used for flow dispersion or other flow control practice. Trees planted to meet stormwater practice planting requirements cannot also receive impervious area reduction credit. The total tree credit for retained and newly planted trees shall not exceed 25 percent of impervious surface requiring mitigation. Construction The existing tree Perimeter Critical Root Zone (PCRZ), trunk, and canopy shall be fenced and protected during construction activities. The Perimeter Critical Root Zone is defined as the measurement of the inches of tree diameter at breast height (dbh), or 4.5 feet above grade, multiplied by 12 inches. PCRZ = diameter at breast height (inches) x 12 inches All protection barriers must remain in place throughout construction. New trees should be planted in accordance with City code and law. Maintenance Trees shall be maintained and protected on the site after construction and for the life of the development. During the life of the development, trees approved for stormwater credit shall not be removed without approval from the City. Trees that are removed or die shall be replaced within 6 months with like species or alternatives approved by the City. Provisions shall be made for supplemental irrigation during the first three to five growing seasons after installation to help ensure tree survival. Long-term irrigation is not required. Trees may be pruned for safety and arboricultural health purposes only; however, if a tree is planted near a building, pruning to protect the structure is recommended. Lake Oswego Stormwater Management Manual Section 4.6 152 Tree Retention This page is intentionally left blank. 153 Section 4.6 Lake Oswego Stormwater Management Manual y.: 4t I'istEucti1 . sQ„,_ . i . 5k611 ' ' truc. -... '-lit..'.5-..•.l. 1 rs.'-a.. ■J 1i'..'.:`k':1•I uality e ,: t t + \';r 4.6.16 Post Construction Soil Quality and Depth Note: This BMP guideline was adapted from Ecology(2012). Definition Establishment of soil quality and depth to regain greater stormwater functions in the post development landscape, provides increased treatment of pollutants and sediments that result from development and habitation, and minimizes the need for some landscaping chemicals, thus reducing pollution through prevention. Introduction Naturally occurring (undisturbed) soil and vegetation provide important stormwater functions including: water infiltration; nutrient, sediment, and pollutant adsorption; sediment and pollutant biofiltration; water interflow storage and transmission; and pollutant decomposition. These functions are largely lost when development strips away native soil and vegetation and replaces it with minimal topsoil and sod. Not only are these important stormwater functions lost, but such landscapes themselves become pollution generating pervious surfaces due to increased use of pesticides, fertilizers and other landscaping and household/industrial chemicals, the concentration of pet wastes, and pollutants that accompany roadside litter. Site Requirements Establishing a minimum soil quality and depth is not the same as preservation of naturally occurring soil and vegetation. However, establishing a minimum soil quality and depth will provide improved on-site management of stormwater flow and water quality. • Soil organic matter can be attained through numerous materials such as compost, composted woody material, biosolids, and forest product residuals. It is important that the materials used to meet the soil quality and depth BMP be appropriate and beneficial to the plant cover to be established. Imported topsoils should not have an excessive percent of clay fines. Do not use this BMP on soil slopes greater than 33 percent. Design • Soil retention. Retain, in an undisturbed state, the duff layer and native topsoil to the maximum extent practicable. In any areas requiring grading remove and stockpile the duff layer and topsoil on site in a designated, controlled area, not adjacent to public resources and critical areas, to be reapplied to other portions of the site where feasible. • Soil quality. All areas subject to clearing and grading that have not been covered by impervious surface, incorporated into a drainage facility or engineered as structural fill or slope shall, at project completion, demonstrate the following: 1.A topsoil layer with a minimum organic matter content of 10% dry weight in planting beds, and 5% organic matter content in turf areas, and a pH from 6.0 to 8.0 or matching the pH of the undisturbed soil. The topsoil layer shall have a minimum depth of eight inches except where tree roots limit the Lake Oswego Stormwater Management Manual 154 Post-Construction Soil Quality and Depth depth of incorporation of amendments needed to meet the criteria. Subsoils below the topsoil layer should be scarified at least 4 inches with some incorporation of the upper material to avoid stratified layers, where feasible. 2. Mulch planting beds with 2 inches of organic material 3. Use compost and other materials that meet these organic content requirements: a. The organic content for "pre-approved" amendment rates can be met only using compost meeting the compost specification for Bioretention (BMP T7.30), with the exception that the compost may have up to 35% biosolids or manure. The compost must also have an organic matter content of 40% to 65%, and a carbon to nitrogen ratio below 25:1. The carbon to nitrogen ratio may be as high as 35:1 for plantings composed entirely of plants native to the Puget Sound Lowlands region. b. Calculated amendment rates may be met through use of composted material meeting (a.) above; or other organic materials amended to meet the carbon to nitrogen ratio requirements, and not exceeding the contaminant limits identified in Table 220-B, Testing Parameters, in WAC 173- 350-220. The resulting soil should be conducive to the type of vegetation to be established. • Implementation Options: The soil quality design guidelines listed above can be met by using one of the methods listed below: 1. Leave undisturbed native vegetation and soil, and protect from compaction during construction. 2.Amend existing site topsoil or subsoil either at default "pre-approved" rates, or at custom calculated rates based on tests of the soil and amendment. 3. Stockpile existing topsoil during grading, and replace it prior to planting. Stockpiled topsoil must also be amended if needed to meet the organic matter or depth requirements, either at a default "pre- approved" rate or at a custom calculated rate. 4. Import topsoil mix of sufficient organic content and depth to meet the requirements. More than one method may be used on different portions of the same site. Soil that already meets the depth and organic matter quality standards, and is not compacted, does not need to be amended. Maintenance Establish soil quality and depth toward the end of construction and once established, protect from compaction, such as from large machinery use, and from erosion. Plant vegetation and mulch the amended soil area after installation. Leave plant debris or its equivalent on the soil surface to replenish organic matter. Reduce and adjust, where possible, the use of irrigation, fertilizers, herbicides and pesticides, rather than continuing to implement formerly established practices. 155 Lake Oswego Stormwater Management Manual 5 Conveyance and Detention Design Standards Note: Most of the design standards in this chapter have been adapted from Clean Water Services (2007). Lake Oswego's conveyance standards have been developed to protect public safety, to minimize flooding risk, and to protect water quality. 5.1 General Provisions - Conveyance General provisions regarding conveyance and detention standards are as follows: • The provisions of this chapter shall apply to all public and private surface water conveyance systems within the City's jurisdiction. Interpretations of such provisions and their application in specific circumstances shall be made by the City. • In addition to these rules, all surface water conveyances shall be designed and constructed according to applicable state and federal rules. • The City requires the applicant to provide an acceptable point of surface water discharge from the developed site. The acceptable point of discharge shall be deemed acceptable by the City prior to acceptance of the site assessment and feasibility analysis (described in chapter 3). • Except as otherwise provided, the extension of the public conveyance systems to serve any parcel or tract of land shall be done by and at the expense of the property owner or applicant and done in a manner that utilizes the natural drainage patterns while considering future planning concerns. The City reserves the legal right to perform the work or cause it to be performed and to bill the property owner for the cost or pursue special assessment proceedings as otherwise provided by City ordinance or permit conditions. • Refer to the City's technical specifications for conveyance systems, including all acceptable materials. • Design, including materials, size, and location of surface water mains and service lines, shall be in accordance with City Engineering Division's policies, design standards, technical specifications, and standard details and shall be approved by the City. Variances from the standard details and/ or technical specifications shall require written approval of the City. Please contact the City for the most current version of technical specifications and standard details. 5.2 Extension of Public Conveyance Systems A development requiring connection to the public surface water system shall extend the public storm conveyance systems to allow all adjacent uphill parcels to be served by the public systems. Under some circumstances, the City may waive this requirement for surface water conveyance if one of the following conditions is met: • The proposed connection to the public surface water conveyance system is for an existing building. • Topography prevents uphill parcels from being effectively served by the required conveyance extension. Lake Oswego Stormwater Management Manual Section 5.1 156 Conveyance and Detention Design Standards • An analysis demonstrates that the uphill parcels are likely to be served via another routing of the conveyance system, and the City agrees with this analysis. When the physical extension of the conveyance system is not required for reasons other than topography, the City may require an easement for future surface water conveyance. 5.3 Conveyance Easements 5.3.1 General Easements and public rights-of-way are outlined in LOC 42.18 and LOC 50.06. In general, the following conditions apply to surface water easements. • Public surface conveyance facilities, not located within public right-of-way, shall be located within an easement granted to the City. • The City may require that a minimum area of 7.5 feet in all directions from the edge of a manhole, catch basin, cleanout, or field inlet be encompassed in a public right-of-way or easement granted to the City. • Access easements shall be provided to manholes, where required by the City. • Proposed encroachments shall not prevent access to, cover, or block the flow of water to or into catch basins, ditches, or swales, and shall not otherwise alter the natural drainage pattern or adversely affect adjacent property. Where drainage is involved, the City Manager or his/her designee may set specific requirements. • Utility construction within easements shall minimize disturbance to existing conditions, especially trees and other vegetation. • Any disturbed areas within easements shall be restored to a condition similar to the condition prior to construction, including the replacement of plants of similar species as those removed or damaged. Replacement trees shall be of similar species and be a minimum of 1 '/2 inches in caliper. Applicants and reviewers shall review LOC 42.18 and LOC 50.06.008 for more detail. 5.3.2 Standard Conveyance Easement Width Standard utility easement widths are specified in LOC 50.06.008 (4). In general, when designing a surface water conveyance system and associated easements, designers shall consider the following: • For utility easements created on undeveloped real property and located outside public or private waterways or rights-of-way, widths shall be a minimum of 15 feet (7.5 feet on each side of the easement centerline). Widths shall be the minimum width possible to facilitate utility installation and maintenance. • For easements created over real property and located within or adjacent to public or private waterways, easements shall be a minimum of 10 feet (5 feet on each side of the easement centerline). Easements widths will consider many factors, including method of utility construction, ordinary depth of water over the conveyance, and the type of equipment used for utility construction, maintenance, and repair or replacement. 157 Section 5.2 Lake Oswego Stormwater Management Manual Conveyance and Detention Design Standards 5.3.3 Reduced Conveyance Easement Widths Conveyance easement widths may be reduced if all the following conditions are met to the satisfaction of the City: • A reduced easement width is needed due to the location of existing buildings that prevent a standard easement width. • Another conveyance route within the development site and public right-of-way is not possible due to topography. Furthermore, when reduced easement widths are allowed, the City may condition the reduced easement width on site-specific and other design factors. 5.3.4 Encroachments In general, the following encroachment conditions shall be considered in the design of the surface water conveyance system. The designer should consider, at a minimum, LOC 42.18, Public Rights-of- Way and Easements, when the possibility of easement encroachment exists. • Structures constructed within conveyance easements or vegetation or landscaping materials in, over or upon any dedicated public right-of-way or easement shall require an encroachment agreement with the City. Approval of the encroachment is at the discretion of the City. • The encroachment agreement shall allow or City to remove the structure, as needed, to access the conveyance system. Replacement of the structure shall be at the property owner's expense. • The City may require special protection for the conveyance system in the vicinity of the encroachment, including but not limited to: o Clearance to manholes and underground pipelines and storm drains shall be 7.5 feet. • When the City Manager or his/her designee determines that permitting the requested encroachment may subject the City to potential liability, a condition of permit issuance shall be the filing with the City Recorder of a policy of insurance and form of policy issued by an insurance company licensed to do business in Oregon. 5.4 Flow Determination for Surface Water Conveyance In general, the designer shall consider the following information regarding flows when designing the surface water conveyance system. 5.4.1 Land Use Assumptions for Flow Determination Ten- to 100-year flows for design of conveyance systems shall be based on full build-out of the upstream basin based upon the most recent approved county or City comprehensive land use plan and realistic estimates of development densities in areas included in recent additions to the urban growth boundary. Lake Oswego Stormwater Management Manual Section 5.4 158 Conveyance and Detention Design Standards 5.4.2 Computational Methods for Runoff Calculations Unless an alternative method is approved by the City in writing, calculation of storm runoff used for conveyance design shall be based on one of the following methods with the limitations on use of each listed. Rational Method The rational method is a simple equation that estimates peak runoff discharge. It is commonly used for sizing conveyance systems and stormwater facilities in small basins. The rational method formula is: Qpk=CfCIA Where: Qpk = peak flow, cubic feet per second (cfs) Cf = runoff coefficient adjustment factor to account for reduction of infiltration and other losses during large storm events C = runoff coefficient, dimensionless I = rainfall intensity (function of time of concentration), inches per hour (in/hr) A= drainage area (acres) Appendix D provides a table of runoff coefficients, rainfall intensity-duration-recurrence interval curves to determine rainfall intensity, and formulas for calculating the time of concentration. For a more detailed discussion of the rational method, refer to Appendix 7-F of the ODOT Hydraulics Manual (ODOT 2014). Santa Barbara Urban Hydrograph (SBUH) SBUH methods shall be based on the following information: • The rainfall distribution to be used within the City is the design storm of 24-hour duration based on the standard NRCS Type 1A rainfall distribution using the chart in Appendix E. • Soil types shall be derived from the NRCS soil survey for the appropriate county (Multnomah, Clackamas/Washington County). See Appendix G for reference. 5.5 Surface Water Conveyance Design Considerations In general, the designer shall consider the following when designing the conveyance system. 159 Section 5.4 Lake Oswego Stormwater Management Manual Conveyance and Detention Design Standards 5.5.1 Design for Full Build-Out Storm drainage facilities shall be designed and constructed to accommodate all future full build-out flows generated from upstream property. Future full build-out shall assume the maximum density, lot coverage, and impermeable area allowed in the upstream zoning at the time of development. Existing stormwater facilities may be included in the model, but no new facilities or facility retrofits shall be assumed. Hydraulic analysis of all conveyance facilities must be provided. 5.5.2 Stormwater Conveyance Design Criteria The design storm to be used for sizing of piped conveyance facilities depends on the contributing area. Refer to Table 5.1 for guidance. Table 5.1. Storm Recurrence Intervals for Conveyance Facilities: Open Channels, Culverts, and Bridges. Design Storm Location Contributing Area Recurrence Intervals Improvements on waterways with Any 100-year Runoff mapped FEMA 100-year floodplain Recurrence <40 acres 10-year Open channelsa 40- 640 acres 25-year >640 acres 50-year <40 acres 10-year Piped conveyanceb 40- 640 acres 25-year <40 acres 25-year Major Arterials >_ 40 acres 50-year <40 acres 10-year Collectors and Other Public Streets >_ 40 acres 50-year <40 acres 10-year Private Drives >_ 40 acres 25-year a Includes roadside ditches, drainage swales, streams. Flow must be contained within channel and adjacent undeveloped floodplain; no additional flood control structures are assumed. Bankfull dimensions of stream channels to be maintained through crossing. See Section 5.5.4 for more details. b Piped sections longer than typical normal culverted crossing of a single roadway. cAssume full build-out under current zoning to determine flow peaks. Recurrence interval may be adjusted downward at discretion of City Engineer if there is no significant change in flood damage risk. 5.5.3 Materials for Piped Conveyance Lake Oswego's complete design and construction specifications for storm sewers are available from the City and contained in Technical Specifications, section 02700. Table 5.2 summarizes approved materials for storm sewers. Lake Oswego Stormwater Management Manual Section 5.5 160 Conveyance and Detention Design Standards Table 5.2. Approved Storm Drain Materials. Material Specification Reinforced concrete pipe ASTM C-76, Class III Non-reinforced concrete pipe ASTM C-14, Class 3 (max. size 18") Ductile iron pipe Class 52. Additional detail in Technical Specifications, section 02610 PVC ASTM D3034, standard dimension ratio 35 HDPE AASHTO M294 Type S (12" to 36" diameter) or AASHTO 252 Type S (4" to 10" diameter) Minimum Dimensions Storm sewers shall conform to the following minimum dimensions: • 10-inch from inlets to catch basins and manholes, minimum slope 0.58 percent • 12-inch or greater for main lines, minimum slope 0.44 percent. Catch basins shall be located at the following spacings: • 300 feet maximum on slopes less than 6 percent • 200 feet maximum on slopes from 6 - 10 percent • 100 feet maximum on slopes greater than 10 percent. Manhole spacing interval shall be a maximum of 500 feet. Minimum Cleansing Velocity The pipe shall be sized and set to assure that a minimum cleansing velocity of 3 feet per second is maintained under full flow conditions. 5.5.4 Materials For Open Channels Open channels are an important component of the surface water system in Lake Oswego.Activities in natural channels should be assumed to require permitting through the U.S.Army Corps of Engineers and Oregon Department of State Lands removal/fill permits, at a minimum, until such time as those agencies have issued an opinion of non-jurisdiction. In some cases, more stringent requirements are associated with management of aquatic species under the Endangered Species Act may apply. Work in natural channels shall use appropriate best management practices to maintain water quality during construction, and a sustainable and functioning natural channel after construction. See the current Oregon programmatic Biological Opinion for transportation projects' or ODOT Routine Road Maintenance: Water Quality and Habitat Guide Best Management Practices (ODOT 2009) for examples. 1NOAA Fisheries.2014. Standard Local Operating Procedures for Endangered Species to Administer Maintenance or Improvement of Stormwater,Transportation or Utility Actions Authorized or Carried Out by the U.S.Army Corps of Engineers in Oregon (SLOPES V for Stormwater,Transportation or Utilities; March 14, 2014). 161 Section 5.5 Lake Oswego Stormwater Management Manual Conveyance and Detention Design Standards Channel Dimensions Oregon Department of Fish and Wildlife criteria for fish passage (physical and velocity barriers) shall be met on all perennial channels. Bridged crossings of open channels are preferred. For streams without a FEMA-designated 100 year floodplain, bridges should fully span 1.5 times the active channel width or the active channel plus floodplain wetted in a 10-year runoff event, whichever is less. For streams with a FEMA-designated 100 year floodplain, bridges must accommodate the 100 year event. Closed bottom culverts shall be embedded below channel invert to a depth of 30 to 50 percent of the culvert diameter, and shall be placed at grade to match the stream channel bed slope. Existing bankfull channel dimensions shall be maintained through the crossing. The bankfull channel is the channel sized to convey the 1.5- to 2-year flow event. Bankfull channel dimensions shall be measured at the location of the widest and deepest channel within 20 channel widths downstream of the proposed crossing or the influence of the existing channel crossing. Channel Crossing Materials Channel beds shall have stable, natural, bottom materials during runoff from the 10 year rainfall event. Bottom material shall be clean alluvium with similar angularity as the natural bottom material; no crushed rock is allowed. Grade control is required on channels with slopes higher than 3 percent. Riprap placed to protect abutments shall be placed only below bankfull depth and shall not constrict channel flow. Poured concrete shall be isolated from perennial flow until cured, at least 7 days. Treated wood shall be avoided. It may be used for minor repairs of existing wood bridges only, and not exposed to rainfall or placed where it would be submerged by bankfull flows. 5.5.5 Upstream Impacts Modifications to the existing onsite storm drainage facilities shall not restrict flows and, thereby, create backwater onto offsite property to levels greater than the existing situation, unless approved by the affected offsite property owner(s) and the City. When approved, the offsite property owner(s) shall agree to and sign a permanent easement legally describing the location of the backwater storage and authorizing the use of their property for stormwater drainage and detention purposes. The easement shall be in a form approved by the City. 5.5.6 Downstream Impacts At its discretion,the City may require the applicant to remove downstream restrictions that create backwater during the 25-year design storm in the current or post-development condition. Removal of downstream obstructions shall not be allowed if the removal will cause, contribute, or exacerbate damage from flooding to existing buildings or dwellings in the 100-year design storm or result in significant environmental impacts. When downstream restrictions are not removed, an onsite detention facility shall be required. See Section 5.8.2 below. Lake Oswego Stormwater Management Manual Section 5.5 162 Conveyance and Detention Design Standards 5.5.7 Cross Lot Drainage Developments shall not materially increase or concentrate runoff onto adjacent properties, except when the runoff is contained in an existing drainageway. Developments shall accommodate existing offsite drainage entering a development site so as to not negatively affect upstream property owners. 5.5.8 Dissipation of Runoff Discharge Runoff exiting a development site shall be discharged with adequate energy dissipaters to prevent downstream damage. 5.5.9 Separation Unless approved by the City, the minimum separation distance between parallel surface water conveyances shall be 5 feet. If vertical separation between utilities is less than 3 feet, additional horizontal spacing may be required to allow for maintenance access. 5.5.10 Alignment Public surface water utility conveyance shall be laid on a straight alignment and at uniform grade. See Section 5.5.3 for minimum slope for cleansing velocity. 5.6 Other Requirements for Public Conveyance Systems 5.6.1 Survey The applicant's engineer or surveyor shall be responsible for establishing the location of the surface water utility conveyance system by means of construction stakes offset along the utility centerline prior to commencement of construction. Moving upstream, there shall be a construction stake placed within 25 feet of each manhole, and at no more than 100-foot intervals along the mainline. Each lateral location shall be staked. 5.6.2 Railroad Crossings Crossing of railroad rights-of-way shall be done in conformance with the requirements of the railroad having jurisdiction. If bonds or certificates of insurance protection are required, they shall be furnished by the contractor or applicant to the railroad company concerned. The City shall be named as an additional insured. Actual permits or easements for such crossings shall be obtained by the applicant, and all terms for such permits or easements shall be met by the applicant and contractor. 163 Section 5.5 Lake Oswego Stormwater Management Manual Conveyance and Detention Design Standards 5.7 Surfacewater/Stormwater Laterals 5.7.1 General Provisions The specifications contained herein, together with the State of Oregon Uniform Plumbing Code and all other applicable requirements of federal, state, and local law, shall govern the installation of laterals. The provisions of City ordinances requiring permits, fees, and other requirements shall be complied with prior to the start of work on any portion of the sanitary or storm pipeline systems. 5.7.2 Planning Considerations Where a parcel requiring connection to a public surface water utility conveyance system cannot connect through a lateral meeting the requirements of this section, then extension of the public conveyance system • shall be required. This extension of the public system is in addition to the requirements of Section 5.2. Two adjacent single-family dwellings or two duplexes may be served by a single surface water utility upon II- consideration and written approval of the City. 5.8 Conveyance and Detention Design Standards 5.8.1 Purpose The purpose of these standards is to reduce the risk of flooding, prevent or reduce the risk to human life and property, and maintain the functions and values of stream channels and floodplains, such as providing aquatic habitat allowing for the storage and conveyance of stream flows through existing and natural flood conveyance systems. 5.8.2 Downstream Analysis It is the City's policy to prefer mitigation of downstream project impacts by requiring flow control via infiltration or flow control BMPs. In some locations, this may not be feasible. In addition, some projects may not be able to avoid exacerbating existing drainage problems. The downstream analysis, if required, is intended to address these projects. Downstream analysis shall follow the process in Figure 5.1. The burden shall be on the applicant to adequately demonstrate no significant impact. Lake Oswego Stormwater Management Manual Section 5.8 164 Conveyance and Detention Design Standards Figure 5.1. Downstream analysis process. /Is the project area >10% of drainage\ above a stable discharge location? AND/OR Are there existing flooding or erosion problems in \ downstream conveyance? j YES NO i i Does project meet flow control YES No further action requirements? mi NO YES Does the drainage system —) have adequate capacity at the 10% discharge point? NO v rImplementation flow control BMPs or modify conveyance to provide stability at peak flow indicated in Table 5.1 165 Section 5.8 Lake Oswego Stormwater Management Manual Conveyance and Detention Design Standards The analytical steps are: Step 1 Determine contributing area of the drainage system at the point of project discharge and compare to project area. Is the project area greater than 10% of the drainage area at point of discharge? If yes, go to Step 2. If no, go to Step 3. Step 2 Find the 10% discharge location: calculate contributing area at progressive downstream locations in the storm drainage/stream system until the point at which the project area is no more than 10% of the total upstream drainage area. Step 3 Assess existing flooding/erosion conditions: inquire of watershed council members and City staff regarding known flooding or erosion conditions. Look for overbank flooding, backwater at culverts, incision or bank erosion in ditches or stream channels at the 10% discharge location. If such conditions are reported or observed, attempt to determine peak runoff rates causing these issues (Step 4) and estimated downstream conveyance (Step 5). To assess impacts to natural stream systems, reconnaissance-level habitat characterization and analysis of bed and bank sediment entrainment based on estimated flows at geomorphic bankfull conditions will be required.Applicants may use channel data from the City, if available. Otherwise, applicants must obtain stream channel data using qualified professionals applying City-approved protocols. Site access is the applicant's responsibility. Step 4 Assess flow conditions: for the watershed upstream of the 10°/o discharge point, estimate the 2-year, 25-year, and 100-year peak discharge values under existing conditions (prior to development of the project site). Repeat the analysis for the developed condition of both the project site and the upstream watershed area. Use the runoff estimation methods described below. Step 5 Estimate downstream conveyance: use simple backwater calculations (for culverts, pipes) or slope-area calculation of the geomorphic bankfull channel using Manning's equation (for streams) to estimate hydraulic conveyance at the 10% discharge location. Demonstrate that the 2-year and 25-year peak discharges from the total contributing area in the developed condition are conveyed by the storm drainage/stream channel system at this location without impacts described in Section 5.5.5.Additional guidance for this step may be found in the current Oregon Department of Transportation Hydraulics Manual. Step 6 Design additional or larger flow control BMPs at the project site or mitigation measures in downstream conveyance. Instream measures will require significant coordination with City Planning and Engineering staff to determine appropriateness and feasibility. Lake Oswego Stormwater Management Manual Section 5.8 166 Conveyance and Detention Design Standards Runoff calculations for the downstream analysis shall use these assumptions • For the developed condition assessment, land use shall be full build-out of the upstream drainage basin using the most recently approved City Comprehensive Plan or current zoning, whichever produces greater runoff. • Runoff shall be calculated for the 2-year and 25-year 24-hour precipitation events. These values shall be derived from the most recent Precipitation-Frequency Atlas of the Western United States (published by the National Oceanic and Atmospheric Administration) or state-issued intensity-duration-frequency curve for the site. The rational method may be used for the site scale (Appendix E). Calculations at the 10% discharge location shall use SBUH (Appendix F), TR55, or SWMM methods unless approval for the rational method is obtained from the City Engineer. • Type 1A rainfall distribution is assumed (Appendix F). • Curve numbers shall be calculated as aerially-weighted averages for watershed land cover conditions, as shown in aerial photographs or GIS data (RLIS or City), based on guidance in Appendix F. • Hydrologic soil types shall be derived from the relevant county soil survey (see also Figure 3.1). • A maximum overland distance for sheet flow shall be 300 feet. 5.8.3 Flood Management Areas Defined Flood management areas shall include, but are not limited to, the following: • Land identified within the 100-year floodplain and floodway as shown on the Federal Emergency Management Agency (FEMA) flood insurance maps. See City website for details and maps. • Land identified in updated flood studies or any other authoritative data documenting flood elevations as approved by the City. • Applicants shall use the most recent and technically accurate watershed model information available from the City, or other updated data as approved by the City, to determine flood areas. 5.8.4 Flood Management Design Criteria The standards that apply to the flood management areas apply in addition to local, state, and federal restrictions governing floodplains and flood hazard areas. • All fill placed in a floodplain shall be balanced with an equal amount of removal and shall not decrease floodplain storage capacity at any stage of a flood (2-, 25-, or 100-year storm event). No net fill in any floodplain is allowed except when all of the following conditions are met: • When an area has received special protection from floodplain improvement projects which either lower the floodplain or otherwise protect affected properties; • Where the exceptions comply with adopted master plans, watershed management plans, or subbasin plans, if any; and • When all required permits and approvals have been obtained in compliance with FEMA rules and other local, state, and federal laws regarding fill in floodplains. • Large areas may not be excavated to gain a small amount of fill in a floodplain. Excavation areas shall not exceed the fill areas by more than 50 percent of the square footage, unless approved by the City. 167 Section 5.8 Lake Oswego Stormwater Management Manual Conveyance and Detention Design Standards • Any excavation dug below the winter"low water" elevation shall not count toward compensating for fill since the area would be full of water in the winter and not available to hold stormwater following a rain. Winter "low water" elevation is defined as the water surface elevation during the winter when it has not rained for at least three days and the flows resulting from storms have receded. This elevation may be determined from records, studies, or field observation. Any fill placed above the 100-year floodplain does not count toward the fill volume. • The excavated area shall be designed to drain if it is an area identified to be dry in the summer; for example, if it is used for a park or mowed in the summer. Excavated areas identified to remain wet in the summer, such as a constructed wetland, shall be designed not to drain. For areas that are to drain, the lowest elevation shall be at least 6 inches above the winter"low water" elevation and sloped to drain. One-percent slopes will be allowed in areas less than 1,000 square feet. • Excavation to balance a fill shall be located on the same parcel as the fill unless it is not reasonable or practicable to do so. In such cases, the excavation shall be in the same drainage basin, within points of constriction on the conveyance system, if any, as near as practical to the fill site, and shall be I constructed as a part of the same development project. • Short-term parking (motor vehicles remaining parked for less than 18 hours per day) in the floodplain may be located at an elevation of no more than one foot below the 10-year floodplain elevation as long as the parking facilities do not occur in a sensitive area. Long-term parking (motor vehicles remaining parked for more than 18 hours without being moved) in the floodplain may be located at an elevation of no more than one foot below the 100-year floodplain as long as the parking facilities do not occur in a sensitive area. • Temporary fills permitted during construction shall be removed upon completion of construction prior to the close of the in-water work window defined by the Oregon Department of Fish and Wildlife or other local, state, or federal authority. • Excavation and fill required for the construction of detention facilities or other facilities, such as levees, shall be specifically designed to reduce or mitigate flood impacts. Levees shall not be used to create vacant buildable land. • Excavation and fill required to restore or enhance floodplains, riparian areas, wetlands, uplands, and streams, including but not limited to the planting of vegetation and day-lighting of existing storm pipes, shall be permitted as long as the design complies with applicable federal, state, and local standards. • The floodplain may not be modified to increase water velocities such that stream bank erosion will be increased, unless the stream banks are protected to prevent the increased erosion. • Uncontained areas of hazardous materials are prohibited within flood management areas. • Existing nonconforming uses are allowed to continue in the flood management area. Existing nonconforming uses may be modified with approval from the City. • Any proposed work within or modification to a floodway shall be certified by an Oregon-registered professional engineer as to how it conforms to these standards and all other local, state, and FEMA regulations. • For streams, creeks, rivers, and other watercourses where the floodway has not been identified, the entire floodplain shall be treated as a floodway unless a study has been prepared by an Oregon- registered professional engineer and approved by the City to define the floodway limits for a stream section. Lake Oswego Stormwater Management Manual Section 5.8 168 Conveyance and Detention Design Standards This page is intentionally left blank. 169 Section 5.8 Lake Oswego Stormwater Management Manual cc,11titarim@[zilcDn ©EPU51@T PEY -Keigh _ii,D7@w@rri o©B and L C�� pc)7 ,g@ lirm@ER ©©17gaT©- This chapter describes construction practices that are required to ensure that construction activities do not cause unacceptable impacts on stormwater runoff and water bodies. 6.1 Legal Requirements 6.1.1 City Code found Erosion and sediment control is an integralpart of the City's water Moreh informationEsion may be permit g y in the Erosion Control permit quality program implementing both municipal stormwater permit and total application, available on the City maximum daily load implementation obligations. Temporary erosion and website at sediment control requirements can be found in Chapter 52 of the Lake htto://www.ci.osweao.or.us/ Oswego Code.An erosion control permit will be required for any projects publicworks/erosion-control- oermit-and-information that trigger stormwater management requirements. Smaller projects in proximity to surface water bodies also require an erosion control permit. and in Chapter 38 and Chapter 52 of the Lake Oswego Code, at The City's utility code (Lake Oswego Code [LOC] 38.29.930) prohibits the htto://www.ci.osweao.or.us/ dumping of"debris, soil, pollutants such as fuels, lubricants, bitumens, citvattornev/lake-osweao- sewage, paint and other harmful or hazardous substances into the surface codes water management system." For information on the Erosion Control program, please call: 6.1.2 MS4 Permit Erosion Control Inspector at (503)675-3991 Under the City's MS4 permit, contractors have the following responsibilities: For information on the Surface • Construction site operators must develop erosion prevention Water Management System, and must implement and please call: and sediment control site plansp Stormwater Quality maintain effective erosion prevention and sediment control Coordinator at(503)6'h-:s999 BMPs. Look for current materials at the • Construction site operators must prevent or control non- City's website: stormwater waste that may adversely affect water quality, such a htto://www.ci.osweao. or.us/publicworks/healthv- discarded building materials, concrete truck washout, chemicals, watersheds-oroaram litter, and sanitary waste. • Construction site operators shall clearly convey all permit requirements to all site personnel, including subcontractors. 6.1.3 1200-C Permit The 1200-C permit is Oregon's construction stormwater general permit, issued by DEQ. Coordination between this permit and the City's erosion control permit is described in the Erosion Control code (LOC Lake Oswego Stormwater Management Manual Section 6.1 170 Construction Phasing, Stormwater Pollution Prevention, and Erosion and Temporary Sediment Control Chapter 52). Under the NPDES 1200-C permit, the following observations are considered significant and should be documented with photographs and notes and reported to the City and DEQ: a. Earth slides or mud flows that leave the construction site and are likely to discharge to surface waters. b. Evidence of concentrated flows of water causing erosion when such flows are not filtered or settled to remove sediment prior to leaving the construction site and are likely to discharge to surface waters. Evidence includes the presence of rills, gullies, or channels. Flow to stormwater inlets or catch basins located on the site will be considered "leaving the site" if there are no sediment control structures downstream of the inlets or catch basins that are under the permittee's control. c. Turbid flows of water that are not filtered or settled to remove sediment prior to leaving the construction site and are likely to discharge to surface waters. Flow to stormwater inlets or catch basins located on the site will be considered "leaving the site" if there are no sediment control structures downstream of the inlets or catch basins that are under the permittee's control. d. Deposits of sediment at the construction site in areas that drain to unprotected stormwater inlets or catch basins that discharge to surface waters. Inlets and catch basins with failing sediment controls due to lack of maintenance or inadequate design will be considered unprotected. e. Deposits of sediment from the construction site on public or private streets outside of the permitted construction activity that are likely to discharge to surface waters. f. Deposits of sediment from the construction site on any adjacent property outside of the permitted construction activity that are likely to discharge to surface waters. 6.1.4 General Considerations Remain familiar with current erosion prevention and sediment control requirements. The contractor is responsible for knowing and following local, state, and federal requirements and to demonstrate this by having qualified persons responsible for project erosion control activities.. Most local regulations are mandated by the federal Clean Water Act; the City does not have the ability to waive these requirements. 6.2 Construction Practices 6.2.1 Good Construction Practices Minimizing land disturbance is one of the best ways to protect local water resources from the impacts of construction-and-development related stormwater runoff. For this reason, the City has assembled this chapter on construction phasing and sequencing and implementation of construction BMPs. Some important general principles that should govern erosion and sediment control plans and effective erosion and sediment control. They include: • Timing, scheduling, and staging of work to minimize overall impacts • Implementing procedures to ensure compliance with effective construction BMPs • Considering construction impacts during preliminary design and prior to onsite grading • Minimizing disturbances by establishing and protecting buffers and by reducing grading activities, 171 Section 6.1 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention, and Erosion and Temporary Sediment Control particularly around sensitive resources, including areas intended for use as stormwater infiltration facilities • Planning for containment of activities that could produce non-stormwater pollution, such as trash and construction debris, concrete washout and sawcut debris, and equipment fueling. Additional details can be found in Appendix D. • Preparing and maintaining documentation throughout the life of the project 6.2.2 Poor Construction Practices There are many construction and development practices that adversely affect stormwater runoff during and after development. They include: • Clearing all vegetation and exposing soils on a project site; � :. - ° t :� }!', fi _., ro � scraping the organic layer of soil and hauling off site 4-41 • Soil compaction of site and BMP areas from construction 11"`- equipment • Failing to protect existing and proposed locations of I. stormwater facilities and vegetated areas from construction - activities I.1 • • Using stormwater runoff and treatment facilities as temporary ` ' erosion and sedimentation BMPs or collection areas for construction debris ne"ec ive erosion con ro. • Stockpiling soils and other materials in unsuitable locations or without adequate protection and management These poor construction practices not only create construction site management issues (that cost time and resources) but also affect the long-term functionality of any stormwater facilities located on the site and affect the site's natural ability to treat and infiltrate stormwater. For example, failing to protect locations of stormwater BMPs from construction traffic can lead to overly compacted soils. Compacted soils, unless remedied at extra cost, lead to poorly or non-functioning stormwater facilities. Non-functioning or poorly functioning facilities become long-term, costly maintenance problems for the ultimate owner of the property. 6.3 Scheduling, Phasing, and Timing of Work If wet weather is forecast, grading 6.3.1 Scheduling activities should be postponed and disturbed areas should be stabilized Scheduling can be a very effective means of reducing impact as quickly as possible using afrom development and building activities, and of protecting site forecasts is includinge pual Weather forecasts expected features that are critical for infiltration and treatment of stormwater, precipitation amounts are available Construction activities should be planned and implemented to from the National Weather Service minimize the extent and duration of exposed soils and to maximize at http://www.wrh.noaa.gov/forecast/ the preservation and protection of site features that will be used wxtables/. The location of interest can be selected from a map. for stormwater management. Scheduling activities should take into account the season and daily weather forecast. Grading activities See Section 6.3.1 for more should occur during dry periods, and disturbed areas should be information. stabilized as quickly as possible. • Lake Oswego Stormwater Management Manual Section 6.3 172 Construction Phasing, Stormwater Pollution Prevention, and Erosion and Temporary Sediment Control 6.3.2 Phasing Furthermore, the site operator or developer should consider the following phasing when scheduling development and construction activities. • Schedule structural BMP installation and implementation activities before any site activity starts. The schedule should consider how runoff and preservation and protection areas will be managed during each phase of the development, and what elements should be installed and implemented before other activities are started. • Plan, site, and develop in a manner that protects areas that provide important water quality benefits or are particularly susceptible to erosion or sediment loss. • Consider water, sewer, and other utility connections at each phase of the project to minimize the amount of land disturbance. 6.3.3 Wet Weather Season Erosion and Sediment Control Requirements Soil and sediment are leading pollutants damaging the quality of rivers, streams, lakes, and other water bodies. These pollutants degrade water quality and reduce the ability of waterways to support aquatic life by encouraging higher water temperatures and by providing a vehicle that transports other pollutants into lakes and streams. Because the rate of erosion on construction sites is up to 500 times greater than on natural sites, the City of Lake Oswego has adopted stringent requirements meant to reduce erosion and sediment from construction sites while allowing construction to continue. The City requires contractors and developers to implement supplemental erosion and sediment control requirements if construction activities are planned between October 1 and May 31 and during all rainy periods. Wet weather measures must be fully implemented and remain effective throughout these periods. The approved erosion control plans contain details and information pertaining to wet weather measures that must be implemented on your site. Erosion control base measures, including sediment fence or wattles around the perimeter, catch basin inserts, and a construction entrance, do not constitute the additional required measures during the wet weather season. City inspection staff requires strict compliance with City ordinances and wet weather measures outlined in the erosion control plans for the project site. Stop work orders or citations may be issued for failure to install and/or maintain required erosion control measures, or for allowing sediment or other pollutants to enter the surface water system. To avoid a citation, contractors and developers must proactively maintain all erosion and sediment control measures, and educate subcontractors and crew members about City requirements. Information regarding approved measures and installation procedures is contained in the Erosion Prevention and Sediment Control Planning and Design Manual (Clackamas County et al. 2008). Hard copies of the manual may be obtained through Clean Water Services or Clackamas County Water Environment Services. Alternatively, an electronic copy can be downloaded free of charge from the City's website (http://www.ci.oswego.or.us/publicworks/erosion-control-permit-and-information) or Clackamas County Water Environment Services website (http://www.co.clackamas.or.us/wes/index.htm). 173 Section 6.3 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control The ACWA Construction Site Stormwater Guide (March, 2013) is a useful pocket resource illustrating the most common erosion control BMPs. It can be used at both the project planning stage and during onsite construction. Hard copies are available from the City. This document is available electronically at http:// www.oracwa.org/r-com-stormwater.html. Wet weather erosion prevention requirements are as follows (Lake Oswego 2015): • All stockpiled material must be fully covered with secured plastic sheeting and isolated with silt fencing or check dams/wattles at the toe of the slope unless being worked. • All exposed/disturbed soil must be covered at the end of each work day. Approved ground cover includes a 3-inch-minimum depth of straw, compost mulch, wood chips, or gravel. • All vehicles must be parked on the gravel construction entrance or driveway or other graveled parking pads. Do not park vehicles on soils protected with approved cover. • All concrete wash-out, mortar, and wet saw slurry, and all liquid waste must be dumped into leak- proof pans. No ground/pit dumping is allowed on any site. This is a year-round requirement. • All construction waste, garbage, and debris must be collected at the end of each work day and stored in containers or enclosed facilities until disposed of. This is a year-round requirement. • All erosion control facilities and practices described in the project erosion control plan must be properly installed, monitored for effectiveness, and maintained throughout the project. If the facilities and practices in the erosion control plan are not effective, additional measures must be implemented. Information regarding maintenance can be found in the Lake Oswego Code, section 52. • Other measures may be required by the inspector to address site conditions.All measures must be maintained in good working order, with accumulations of sediment removed as necessary. • To preserve ground cover, it is recommended that temporary plywood, plank, or gravel walkways be placed around the job site to provide construction access without further disturbing covered soils. • The "Wet Weather Requirements" are in addition to standard erosion and sediment control requirements and are incorporated by reference into the site erosion and sediment control plan. • All contractors, subcontractors, property owners, and anyone working on the construction site is expected to know local, state, and federal requirements related to erosion and sediment control and water quality.All regulations must be adhered to at all times. Wet weather guidance is periodically updated as new practices are shown to be effective. For the most up to date requirements, consult the City website at http://www.ci.oswego.or.us/publicworks/erosion-control- permit-and-information. Construction sequencing and phasing activities are simple and cost-effective ways of mitigating many of the stormwater impacts that originate with construction activities. Construction phasing and sequencing requires developers and contractors to follow and implement a scheduled plan and to implement non-structural BMPs during the development and construction phases of a project and after project construction. Developers and contractors should consider the procedures listed in Section 6.4, below, when performing construction sequencing. Lake Oswego Stormwater Management Manual Section 6.4 174 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control 6.4 Implementing Procedures 6.4.1 Communication Project success in meeting stormwater management objectives depends on communicating the intent of site and stormwater design, as well as the preservation of existing site features that help reduce the overall impact of stormwater runoff. Communication is also critical to ensuring that structural construction BMPs are implemented and maintained throughout the life of the project. Communication with subcontractors is an important step. Developers, builders, and contractors should not only install physical BMPs to mitigate construction and development stormwater impacts, they should also implement procedures in accordance with a planned schedule. Important procedural, or non-structural, BMPs are described below. Developers and contractors planning 6.4.2 Prohibition of Certain Activities construction activities should also Prohibiting activities that affect the long-term success of design and consult Appendix D for source control implementation of stormwater management on the site can greatly BMPs related to construction practices. influence the design success of stormwater facilities and the abilityof These include: g the entire site to manage stormwater. • Dust control To protect water bodies from construction impacts, some • Proper storage of solid wastes, construction-related activities are prohibited. They are discussed in including trash and concrete waste Section 6.3.3. • Spill prevention and cleanup 6.4.3 Managerial Practices • Proper disposal of fluids and wastes Managerial BMPs include administrative and procedural practices during development and construction. The applicant should consider, • Painting,finishing, and coating at a minimum, the following managerial practices when developing a of vehicles, boats, buildings, and equipment site. • Outdoor storage or transfer of • Train staff and contractors solid raw materials, by-products, or finished products • Ensure responsibility by implementing agreements with Other source control BMPs may subcontractors requiring them to adhere to legal requirements apply depending on the construction and permits, construction practices described in this chapter, project. Consult Appendix D and the the erosion control plan, and the Erosion Prevention and City's Erosion Control webpage for Sediment Control Planning and Design Manual (Clackamas more information. County et al. 2008) • Implement the project erosion control plan, stormwater pollution prevention plan (SWPPP), and other procedures before construction starts • Inspect and maintain structural BMPs, documenting these actions as they occur • Evaluate and update the project erosion control plan or SWPPP as needed For more information on each of these managerial BMPs, please see http://www.epa.gov/npdes/pubs/ sw_swppp_guide.pdf. 175 Section 6.4 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control 6.5 Consideration of Construction Impacts during Planning, Design, and Construction Erosion and sedimentation cause both environmental and economic impacts. Environmental impacts include introducing more nutrients (such as nitrogen and phosphorus) into already nutrient-limited waters within the City. Economic impacts include expensive construction delays because of sedimentation and site damage. This section provides an overview of some of erosion and sediment control principles. The Erosion Prevention and Sediment Control Planning and Design Manual (Clackamas County et al. 2008) should be reviewed in its entirety for more information on erosion prevention and sediment control considerations during all stages of project planning, design, and construction. Although this section is focused on typical construction practices, they are also applicable to any parcel where erosional issues may have become or could become an issue in the future. 6.5.1 Planning Considerations Land disturbance at a construction site accelerates erosion dramatically. Factors that influence accelerated erosional processes include: • Presence or absence of vegetation and ground cover • Rainfall and climate • Slope length and steepness • Surface texture • Soil type • Drainage basin area Vegetation is the most effective form of erosion control. Very little erosion occurs on soil covered with undisturbed natural vegetation. Minimizing site disturbance (described more fully in Section 6.6) immediately reduces the potential for site erosion. In general, soil particle size also influences erosional processes. Soil erodibility increases as the percentages of silt and fine sand increase. Soil erodibility also generally decreases as the percentages of clay and organic matter increase. Slope length and gradient also greatly influence soil erosion. Doubling erosional slope length increases erosional potential by four times. Doubling the slope gradient increases erosional potential by five times. 6.5.2 Design and Construction Considerations Designers should try to integrate existing trees and other natural vegetation into the site improvement plan and should clearly denote the vegetated areas that should be protected from disturbance. The City's website offers more information on the preservation and protection of trees during development. • Plan, site, and develop in a manner that avoids or minimizes impacts on areas that provide important water quality benefits or are particularly susceptible to erosion or sediment loss. Lake Oswego Stormwater Management Manual Section 6.5 176 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control • Plan grading to work with existing topography so that cut and fill slopes are as flat as practicable and consistent with soil stability. Slopes of 2:1 or steeper may require special design. • Where applicable and appropriate, locate construction pollutant sources (including sediment) away from drainage swales, wetlands, or water bodies. • Sediment removed from sediment control facilities should be placed in non-critical flat areas of the site. In no instance should the removed sediment be placed in a position where subsequent rainfall could return it to the sediment control devices. • Minimize land disturbance, such as clearing and grading and cut and fill, to reduce erosion and sediment loss. During construction, existing vegetation should be retained whenever possible and only areas necessary for construction activities should be cleared. 6.5.3 Recommended Construction Practices The Erosion Prevention and Sediment Control Planning and Design Manual (Clackamas County et al. 2008) is the primary reference for construction practices to minimize disturbed land area and to protect soils and sensitive areas from sediment during construction. 6.5.4 Selecting Construction Best Management Practices Temporary erosion and sediment control BMPs are implemented to protect and prevent soil disturbed by construction activities from moving off site and into the surface water management system. These construction BMPs are temporary and are not intended as a permanent practice to prevent stormwater runoff. BMPs should be selected in consideration of the site, the nature of construction activities, and the proposed sequencing of construction activities. A detailed discussion of these topics along with guidelines for design, installation, inspection, and maintenance of applicable BMPs are in the Erosion Prevention and Sediment Control Planning and Design Manual (Clackamas County et al. 2008). Construction Access and Parking The primary access point (s)for construction traffic should be identified on plans and stabilized before earthwork begins to prevent sediment and construction debris from leaving the site. If there will be a large amount of earthwork and construction vehicle traffic, a tire wash facility may be required. • BMP: construction entrance • BMP: tire wash facility • General considerations: Keep construction entrances (including the existing driveway) clean and swept free of debris. Sweep up any visible debris immediately; never rinse or power wash debris from the driveway into the street or into storm drains. Clean up any tracking on street surfaces at the end of each work day. 177 Section 6.5 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control • Keep parking limited to hard-surfaced or properly graveled areas. Do not block the construction entrance with trailers or materials. Crew parking areas are a common source of soil disturbance and tracking. • Put plywood or steel sheeting down on construction pathways and try to load and unload materials on the job site, not in the street. Even small "Bobcat"-style equipment can cause serious soil disturbance; running on wood or steel sheeting can reduce soil disturbance and limit damage to existing root systems. Perimeter and Sediment Controls Perimeter controls should be installed along borders of sensitive areas, around areas of proposed stormwater management facilities, and at the limits of clearing and construction activities. They should be installed downslope of proposed construction activities, and should be installed at a set elevation, rather than sloping, to avoid generating channelized runoff. If development substantially changes the natural drainage conditions on a site, merely protecting the drainage channels on the site may not be sufficient to prevent erosion. Sediment is required to be trapped on site. An example of a perimeter control BMP is clearly identifying locations of stormwater management facilities on construction plans and including construction notes on the plans similar to the following: • All stormwater BMP areas shall be clearly marked before site work begins to avoid soil disturbance during construction. No vehicular construction traffic, except that specifically used to construct the stormwater facility, shall be allowed within 10 feet of infiltration areas. Stormwater BMP locations should be appropriately protected during construction. If these locations cannot be appropriately protected from construction impacts and therefore loss of infiltration capacity, then the soil must be rehabilitated — uncompacted and potentially amended with organic material. Some erosion during construction is unavoidable. The function of a sediment barrier is to prevent sediment from leaving a site; the barrier detains sediment-laden stormwater on the site long enough for soil particles to settle out before the runoff enters receiving waters. Some types of sediment barriers are wattles, filter berms, and sediment fence. At a minimum, sediment control shall include: • Locate sediment basins and traps at low points below disturbed areas. • Use earth dikes or swales to route drainage from disturbed areas into acceptable discharge locations. • Place sediment barriers and sediment fences below small, disturbed areas on gentle to moderate slopes. The following BMPs, described in Clackamas County et al. 2008, are applicable: • BMP: buffer zone • BMP: wattles • BMP: preserve natural vegetation • BMP: filter berm • BMP: sediment fence Lake Oswego Stormwater Management Manual Section 6.5 178 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control Stabilize Slopes and Disturbed Areas Areas that have been cleared should be stabilized through use of seed and mulch as soon as possible after grading is completed. Mulch helps seedlings become established and protects the soil from raindrop splash until vegetation takes over. Soils may be planted with temporary or permanent vegetation. If the soil will be exposed during the winter months, protective measures other than vegetation must be used. See the Erosion Prevention and Sediment Control Planning and Design Manual (Clackamas County et al. 2008) for more information. Exposed slopes are a potential source of sediment in the event of storm events if they are not stabilized through one of the following methods: • Protect existing vegetation and limit soil disturbance whenever possible. Re-seed exposed soils as quickly as possible, and consider planting deep-rooted native vegetation to help re-stabilize soils. • BMP: seeding (temporary/permanent) • BMP: pipe slope drain • BMP: surface roughening Stockpile Areas Clearly identify stockpile areas and cover controls. • Designate places to "stage" and store construction materials. These areas should be protected with a gravel base, plywood, concrete, or another hard surface. Store piles of compost, bark dust, topsoil, or other amendments on tarps; try not to order these materials until immediately before they will be used on the site. • Cover stockpiles with plastic sheeting during the wet weather season, and in summer whenever the stockpiles will remain unworked for 14 days or longer. Cover exposed soils with 3 inches of straw, compost mulch, wood chips, gravel, or other ground cover to minimize disturbance and reduce the potential for tracking. • BMP: matting. Matting is a surface cover such as straw, jute, wood fiber, coir (coconut fiber), plastic netting, and Bonded Fiber Matrix that is used to provide erosion • BMP: plastic sheeting Runoff Control Runoff should be diverted from exposed soils or steep slopes. Construction changes the characteristics of runoff. The creation of impervious surfaces, removal of plant cover, and compaction of soil by construction traffic allows less water to percolate into the soil and, therefore, increases the volume of runoff. Drainage ways and outlets must be prepared to handle concentrated or increased runoff. See the Erosion Prevention and Sediment Control Planning and Design Manual (Clackamas County et al. 2008) for more information. 179 Section 6.5 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control Where construction stormwater concentrates, the following can be used to reduce runoff velocities: • BMP: check dam • BMP: diversion dike/swale • BMP: grass-lined swale • BMP: outlet protection • BMP: sediment basin • BMP: sediment trap Control Pollutants • Provide adequate, well-marked garbage disposal containers. Litter, debris, scrap materials, and other waste are considered pollutants and must be contained on the site. Containers should be leak-proof. Small containers should be emptied weekly; larger containers/dumpsters should be emptied as needed. • Provide leak-proof pans for concrete rinse water and left-over mortar and masonry materials. New regulations require these materials be contained and hauled off site. Small "kiddie swimming pools" work well for this purpose. Remember: these substances are harmful to landscaping and trees, and may pollute the water table. • Do not rinse any material into the storm drain. Remember: "Only rain goes down the drain!" Temporary and Permanent Stormwater Facilities • Know where the nearest storm drains are located, and keep them protected with non-woven filter inserts. Clean up and properly dispose of any material that accumulates in the filter, and remove and dispose of all filters at the end of the project. BMP: biofilter bag BMP: inlet protection 6.5.5 Maintenance and Inspection Procedures Maintenance and inspection procedures are also critical non-structural BMPs. Site operators should consider the following, at a minimum, when identifying construction stormwater maintenance procedures. • Outline all structural BMP maintenance and inspection procedures in the erosion control plan, SWPPP, or other BMP documents. Implement the maintenance procedures to keep physical BMPs in good working order at all times. If an erosion control permit has been issued for the site (either issued by the City or an NPDES 1200-C) the permit requires that you conduct regular inspections and document the findings of those inspections. Personnel selected to conduct inspections should be knowledgeable in the principles and practices of erosion and sediment controls and preservation and protection controls. They should also possess the technical skills to Lake Oswego Stormwater Management Manual Section 6.5 180 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control assess conditions at the construction site that could impact stormwater quality, and to assess the effectiveness of any sediment and erosion control measures selected. • Ensure that maintenance and inspection of construction BMPs occurs regularly (that is, as outlined on a schedule as described in this chapter) • Implement corrective action whenever an inspection (formal or informal) identifies a problem or potential issue. • Document all inspection and maintenance activities. At a minimum this should include the date, BMP, location, and maintenance performed. City Inspection City inspectors will visit construction and development sites to ensure that the approved erosion prevention and sediment control plan is properly implemented and that additional physical BMPs are in place, as necessary. At a minimum, inspectors will observe and document: • Disturbed areas of the site • Material and waste storage areas • Stockpile areas • Construction site access and parking areas • Sensitive areas • Tree protection • Discharge locations to public storm drains • Discharges to receiving waters, if appropriate Inspections will be documented with an inspection form, photographs and monitoring results as appropriate. Please see the City's website for more examples of erosion prevention and temporary sediment control tools. 6.5.6 Post Construction Before removing temporary erosion and sediment control measures, ground cover or permanent landscaping shall be established. For multi-building developments, erosion control measures need to remain in place until construction of the last building is completed. 181 Section 9.5 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control 6.6 Documentation 6.6.1 Erosion and Sediment Control Drawings The erosion and sediment control drawing is required by both the r City's erosion control permit, and as a component of DEQs 1200- _. C permit application. Plan should include a drawing set that will be • • : *ay s; part of the contract documents for the project. Applicable BMPs • '' are in the Erosion Prevention and Sediment Control Planning and Design Manual (Clackamas County et al. 2008) and summarized in III Section 6.5. The drawing set can be on a single sheet for smaller projects, but may require multiple sheets for larger projects. At • a minimum, the scaled drawing set must include the following ' • information: r= • • • Primary access point(s) for construction traffic. Access '&- • _. - points must be stabilized with crushed rock (reject rock f. f — from site construction is acceptable if it meets gradation requirements). • Limits of clearing and construction activities. Show perimeter controls that will be used to prevent sediment or construction debris from leaving the site. n control. • Sensitive areas, such as streams or wetlands. Show buffer protection measures that will be installed prior to any land-disturbing activities. • Areas of proposed stormwater management facilities. The location of stormwater BMPs shall not be subject to compaction prior to, during, and after the construction of the facility. Plan should delineate protection buffer and method of protecting from construction traffic. • Show sediment control measures that will be used on and at toes of slopes. • Clearly identify stockpile areas and cover controls. • Show existing and proposed stormwater facilities, including inlets, catch basins, and how they will be protected. • Show temporary stormwater collection, conveyance, and treatment facilities. • Include general notes for erosion and sediment control. (Appendix C provides a current list of required construction notes. Please see the City's website for any updates and more examples and tools.) Lake Oswego Stormwater Management Manual 182 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control 6.6.2 Erosion Control Plan Approval of a construction erosion Erosion control plans commonly consist of: a narrative describing and sediment control plan by the the proposed project and general operating procedures during City does not relieve the applicant construction, plan sheets outlining existing and proposed conditions, from the responsibility to ensure that erosion control measures are locations and types of erosion and sediment control BMPs, and constructed, implemented and staging, details showing typical installations, and associated maintained to contain sediment on specifications. Developers or site operators may be required to the construction site. submit an erosion control plan with their City permit application or • NPDES 1200-C application to DEQ, depending on the type of project and area of disturbance. Although erosion control narratives are common with larger developments, they are not as common for smaller projects. However, the plan elements and procedural BMPs are relevant to all projects, so developers or site operators of smaller projects should consider drafting and implementing an erosion control narrative. This plan will help the site operator manage the site and avoid damage to areas that should be protected. Erosion control plans typically include good housekeeping information for the site operators and contractors.An erosion control plan is designed to identify prohibited activities as well how to conduct activities with the correct physical BMPs. At a minimum, site operators and developers should consider the following prohibited activities when drafting procedural BMPs. • Activities that are violations of local, state, or federal laws • Activities that damage or alter physical BMPs Erosion control plans are referred to as stormwater pollution prevention plans (SWPPP) in Washington state and some other jurisdictions. SWPPP guidance and templates can be found at: http://cfpub.epa.gov/ npdes/stormwater/swppp.cfm#model Please see the City's website for more erosion prevention and temporary sediment control tools. 6.6.3 Inspection Log Contractors should maintain an inspection log that documents that all BMPs are intact and functioning well and that notes rainfall, observed problems, corrective actions, and results. Prior 24-hour rainfall amounts within in the City are measured by City weather stations, accessed on the City website at http://www. ci.oswego.or.us/publicworks/weather-stations-and-climate-information. Be sure to note which weather station is used as a reference. 183 Section 6.7 Lake Oswego Stormwater Management Manual ,lacpmw,,E[R@T D3-6- p NErlEagHEHce Regular and appropriate inspection and maintenance of stormwater management facilities is critical to their continued function. This chapter describes maintenance requirements for stormwater facilities, including access, submittals, reporting, and penalties in the event of violations. Detailed checklists with required maintenance activities are provided for each of the flow control, water quality, and LID BMPs described in Chapter 4. The specific maintenance activities during the calendar year and following storm events depend on the facility type and its design. 7.1 Legal Requirements The City surface water code (LOC Chapter 38) outlines legal requirements related to maintenance of stormwater facilities, including inspection, access, and considerations in the event of transfer of property ownership. Requirements are briefly summarized in this chapter, but property owners and managers should review the code for detailed legal requirements. 7.1.1 Maintenance Responsibilities Public Facilities Lake Oswego's Public Works, Operations Division, maintains public storm drain, ditches, storm drainage structures (e.g., catch basins and inlets), and both vegetated and non-vegetated stormwater facilities, including rain gardens and planters located within street rights-of-way. Private Facilities Maintenance of private facilities is the responsibility of the property owner, unless a different responsible party is identified in the Operations and Maintenance Plan (OMP). Under Lake Oswego's surface water code, responsibilities include: • Maintain stormwater management facilities in good condition, with facilities operating at design capacity and performing the function for which they were designed while in continuous working order. • Inspect and maintain at an appropriate frequency and level to avoid nuisance conditions in or adjacent to the stormwater management facility that suggest that the facility is not in good working order, such as uncontrolled runoff and overflow, stagnant water with concomitant algae growth, insect breeding, odors, discarded debris, or safety hazards created by the facility's operation. • Inspect stormwater facilities according to the schedule included in the maintenance checklists provided in this chapter or a facility-specific OMP. • Promptly repair and restore stormwater management facilities in accordance with the maintenance checklists provided in this chapter. • Provide and maintain all necessary access routes from the public right-of-way in accordance with this chapter or the OMP. Lake Oswego Stormwater Management Manual Section 7.1 184 Maintenance 7.1.2 Inspection Under Lake Oswego's surface water code, when stormwater management facilities are privately owned and maintained, the City shall be granted reasonable access rights by license or easement to the facilities. The City reserves the right to inspect private facilities to confirm that they are in conformance with Lake Oswego's stormwater management requirements. Examples of inspections may include but are not limited to: • Routine inspections • Random inspections • Inspections based upon complaints or other notice of possible violations • Inspection of areas or sites generating higher than typical sources of sediment or other pollutants • Inspections of businesses or industries with discharges that are more likely than the typical discharge to cause violations of state or federal water or sediment quality standards or the City's MS4 stormwater permit based on the City's evaluation of businesses and industries • Joint inspections with other agencies inspecting under environmental or safety laws • Inspection of Facilities Operating Under Partial Letter of Acceptance 7.1.3 Maintenance Access All stormwater BMPs (except those that serve only a single-family residence) shall have maintenance access that meets the following criteria: • Provide a maintenance access road that allows maintenance vehicles to perform needed maintenance activities. Needed maintenance activities include vacuum cleaning of pollution control manholes and similar structures, removal of debris from major debris control structures, removal of sediment collected in the facility, and grading needed to restore proper function of the facility. • Provide a maintenance access path that allows a person with hand equipment to perform maintenance and inspection services. These services typically require access to inlets and outlet structures. Provide a flat area adequate for a person to stand above all outlet debris control structures. Access by the City All stormwater management facilities, whether located on private or public property, shall be accessible at all times for City inspection. When stormwater management facilities are accepted by the City for maintenance, access easements shall be provided at a width meeting the criteria in the following sections to allow access by maintenance and inspection equipment. Maintenance Access Roads All ponds shall have an access road and ramp that meet the following criteria: • The road grade shall be 15 percent, maximum. • The road cross slope shall be 3 percent, maximum. • The road width shall be 12 feet on straight sections and 15 feet on curves. 185 Section 7.1 Lake Oswego Stormwater Management Manual Maintenance • The outside turning radius shall be 40 feet, minimum. • The road surface load capacity shall be H-20 loading with an assumed California bearing ratio of 3 or greater. • Road surfaces shall be sloped no more than 6 percent, with a surface of pervious pavement (see BMP design guidelines in Section 4.6.6) or gravel. Gravel examples include: 2 inches of%-0 crushed rock over 10 inches of 2-0 crushed rock over Mirafi 550X geotextile fabric. • For access road surfaces, consider using reject rock from site construction or a low-growing native prairie seed mix that does not need to be mowed but can withstand mowing prior to sediment removal/earthwork operations. Paver stones may be incorporated into access roads at larger facilities. Maintenance Access Paths Maintenance access paths shall meet the following design criteria: • The path grade shall be 33 percent, maximum, or have approved steps. • The path cross slope shall be 3 percent, maximum. • The path width shall be 3 feet, minimum. • The path surface shall provide an all-weather, mud—free, walkable access. • Acceptable path surface examples include grass, gravel, and wood chips. • Depending on the site, more restrictive ADA requirements may apply 7.1.4 Routine Maintenance Activities Sediment and pollutants, such as oil, debris, and sludge, can accumulate over time in various components of drainage collection, conveyance, and treatment systems such as catch basins, ditches, storm drains, and oil/water separators. When a storm event occurs, the pollutants can become mobilized and carried into the receiving streams or lake. Regular maintenance of the drainage system decreases the amount of pollutants available to contaminate the stormwater, and improves the efficiency of most stormwater facilities to trap influent pollutants. Stormwater facilities can include both structural and non-structural BMPs, all of which require routine maintenance to ensure they remain functional. Frequency and level of maintenance vary according to a facility's location, function, and exposure to impact. For most facilities, maintenance requirements should be described in the OMP.Additional requirements are described in this section. Checklists in Appendix K provide more details about operation and maintenance procedures for specific types of facilities. Many properties in Lake Oswego have privately owned catch basins or other stormwater management facilities—such as rain gardens, drywells, pervious pavement, and green roofs. Property owners are responsible for maintaining those facilities. If a property owner is unsure about whether a catch basin or other facility is public or private, they can contact the City of Lake Oswego Operations Division to find out. Catch Basins Routine cleaning of catch basins is one of the most important stormwater source control measures. Catch basins should be cleaned when they are more than half full or when the sediment is within 18 inches of the bottom of the outlet pipe. Scheduled maintenance (for example, cleaning a catch basin once per year) is a cost-effective way to maintain catch basins. Lake Oswego Stormwater Management Manual Section 7.1 186 Maintenance Rain Gardens Rain gardens require more maintenance during the first 2 to 3 years after they are constructed than after they are established. Typical maintenance activities during the first years include mulch replenishment, slope stabilization, weeding to remove invasive or nuisance vegetation, and watering during the dry season to allow new plants to establish a deep root system. Watering will also need to occur during prolonged dry periods to ensure plant survival. Once established, rain gardens should be inspected and maintained seasonally and within 24 hours after a major storm event. Seasonal and post-storm inspections include checking the inflow and outflow areas to ensure they are intact and are not clogged. If ponding occurs beyond the designed drawdown period of 24 hours after a rain event or ponds during small storm events, the soil may be clogged with fine sediment. Areas that collect sediment should have the clogged portion of the soil profile removed and replaced with new biofiltration soil media. This is usually needed only top 2 to 3 inches of soil within a limited portion of the facility footprint. Salvage or replace plants disturbed during the sediment removal process. Remove leaf litter and organic debris if it is effecting the facility function. Maintain pipes and ensure they are not clogged or damaged. Pipes should maintain at least 50 percent conveyance capacity at all times. Repair damaged pipes or replace them if the repairs are insufficient. If the facility has an underdrain and excessive ponding occurs (facility does not draw down within 48 hours), check to see if there is clogging. Rotary or jet clean roots and debris from underdrains. Vegetation should cover a minimum of 90 percent of a rain garden. Replace dead or diseased plants with like species or alternate species approved by the City. Remove weeds manually. If weeds cover more than 10 percent of a facility, they should be removed and bare areas replanted. Do not use pesticides or herbicides. Limit fertilizer use as well. Base use on soil tests, and opt for slow release fertilizer. Planters Planters require more maintenance during the first 2 to 3 years after they are constructed than after the plants are established. Maintenance activities during the first years may include erosion control, sediment removal, weeding to remove invasive or nuisance vegetation, and watering during the dry season to allow new plants to establish deep roots and become drought-resistant. Watering and irrigation will need to occur during extended dry periods to ensure plant survival. Once established, planters should be inspected and maintained annually and within 24 hours after a major storm event. Annual and post-storm inspections include checking inflow and outflow areas to ensure they are intact and are not clogged. To treat clogged or broken inlets or outlets, remove sediment and debris from pipes to maintain at least 50 percent conveyance capacity at all times, and repair or replace broken or damaged components. Planters must be watertight to protect abutting foundations from moisture damage. Check the planter liner; it should be secured above the planter's high water mark. If it is not, extend and secure it to planter walls above the high water mark. Cover all drain pipes with 6 inches of growing medium to prevent damage from freezing and thawing, and from ultraviolet (UV) rays (i.e., sun damage). 187 Section 7.1 Lake Oswego Stormwater Management Manual Maintenance Vegetation should cover a minimum of 90 percent of a rain garden. Replace dead or diseased plants with like species or alternate species approved by the City. Remove weeds manually. If weeds cover more than 10 percent of a facility, they should be removed and bare areas replanted. Do not use pesticides or herbicides. Limit fertilizer use as well. Base fertilizer use on soil tests, and opt for slow release fertilizer. Infiltration Trench Infiltration trenches should be inspected annually during wet weather to verify detention times are met. Water-efficient irrigation should be applied during plant establishment and during extended dry periods. Maintenance should be done at least once per year, at the time of inspection.Annual maintenance includes repairing clogs, the uneven distribution of flows, eroded filter bottoms, and any cracking that may have occurred. Remove unwanted plant growth and sediment accumulation around outlets. To treat clogged or broken inlets or outlets, remove sediment and debris from pipes to maintain at least 50 percent conveyance capacity at all times, and repair or replace broken or damaged components. Drywell Drywells should be inspected and maintained annually during wet weather to verify detention times are met.Annual maintenance includes repairing clogs, the uneven distribution of flows, eroded filter bottoms, and any cracking that may have occurred. Remove unwanted plant growth and sediment accumulation around outlets. To treat clogged or broken inlets or outlets, remove sediment and debris from pipes to maintain at least 50 percent conveyance capacity at all times, and repair or replace broken or damaged components. Greenroofs (Green roofs, in general, are low maintenance. However, scheduled seasonal maintenance is required. Maintenance activities vary in frequency depending on soil depth, vegetation type, and location. Structural components, including the waterproof membrane, should be operated and maintained in accordance with the manufacturer's and design specifications. Repair clogged drains and tears or perforations in the membrane by removing sediment and debris, repairing leaks and structural deficiencies, and replacing broken components. The waterproof membrane should be protected during all maintenance activities. The growing medium must be maintained at design depth to sustain a healthy plant cover and infiltrate within 48 hours. Planting should cover 100 percent of soils. Plant exposed soils, amend soils as necessary, and clear drains. Due to the low level of organic materials, fertilizers may be required for plant growth, but pesticides and herbicides should not be used. Minimal irrigation may be necessary to maintain vegetation health and ecological function. During the first growing season, remove weeds and other undesirable plants once a month. In subsequent years, remove weeds and undesirable plants during the spring and fall months. For desired plant species, replace dead or diseased plants with like species or City-approved alternates. During the rainy winter months, check drains monthly and remove accumulated debris. Pervious Pavement Pervious pavement should be maintained at least twice per year and requires regenerative, air style vacuuming to remove fine particulates from the infiltration spaces.After vacuuming pavers, restore lost material within the seams after vacuuming out sediment. Remove leaf litter and sediment prior to the Lake Oswego Stormwater Management Manual Section 7.1 188 Maintenance beginning of the rainy season and after large storm events to prevent surface clogging and ponding. Prevent large root systems from damaging structural components, and manually remove weeds. Ilf moss prevents infiltration from occurring or creates a public slip hazard, do not use herbicides. Limit fertilizer use as well. Base fertilizer use on soil tests, and opt for slow release fertilizer. Over time, settling may occur, and aggregate base, washed sand, and/or pavers may need to be replaced or repaired. Restoration and maintenance procedures will vary, depending on the surface material present (porous concrete, porous asphalt, or pavers). Refer to manufacturer's recommendations for detailed maintenance procedures. Pervious pavement must not be sealed. Rainwater Harvesting General maintenance for rainwater harvesting should be conducted twice each year. General maintenance activities include removing debris from roof and gutters, repairing or replacing damaged or missing structural components, removing debris from clogged pipes, and cleaning flow apparatus. The schedule and type of maintenance activities for rainwater harvesting vary based on the intended use and the size and model of cistern. For example, system designs for interior uses must have a monthly water balance that demonstrates adequate capacity for each month and reuse of all stored water annually. For system-specific rainwater harvesting Filter Strips Filter strips should be inspected monthly during the rainy season to ensure adequate drainage and spreading of water across the strip. Water-efficient irrigation should be applied during plant establishment and during extended dry periods. Seasonally evaluate vegetation for damage or disease, and replace dead plants as necessary to ensure at least 90 percent coverage. Manually remove invasive or nuisance weeds. Do not use herbicide or pesticides. Limit fertilizer use as well. Base fertilizer use on soil tests, and opt for slow release fertilizer. Swales Water-efficient irrigation should be applied during plant establishment and during extended dry periods. In subsequent years, swales should be inspected twice a year (spring and fall) at a minimum. Routine maintenance, to be done at the time of inspections, includes evaluating and replanting landscapes to achieve a minimum of 90 percent facility coverage. Remove weeds manually. Do not use herbicides or pesticides. Limit fertilizer use as well. Base fertilizer use on soil tests, and opt for slow release fertilizer. Trash and debris can impede water flow and clog the system. Remove all trash and debris and ensure at least 50 percent conveyance capacity at all times. Check inlet pipes and outlet structures for damaged or missing components, and cover with at least 6 inches of growing medium to prevent freeze/thaw and UV damage. Remove obstructions. Make sure stormwater is traveling the length of the treatment area and repair as necessary to ensure stormwater is not short-circuiting the treatment area. Sand Filters Sand filters should be maintained seasonally. In summer, make the necessary structural repairs and rake to remove accumulated sediment and debris. In winter, monitor flow-through rates. In fall, winter and spring, clear inlets and outlets/overflows to maintain conveyance. Structural components, including inlets, outlets/overflows, liners, and walls must freely convey stormwater. Remove sediment and debris from clogged structural components to maintain at least 50 percent 189 Section 7.1 Lake Oswego Stormwater Management Manual Maintenance conveyance capacity at all times. Manually remove sediment accumulation. Repair and seal all cracks, and replace damaged components if repairs are insufficient. Restore any splash blocks if missing or not installed. The facility must be water tight to protect abutting foundations from moisture damage. If necessary, extend and secure the liner to planter walls above the high water mark. Manually remove all weeds. Do not use pesticides or herbicides. Limit fertilizer use as well. Base fertilizer use on soil tests, and opt for slow release fertilizer. Prevent large roots from damaging structural components. The filter medium, including sands, gravels, and other materials, should allow adequate infiltration. If ponding occurs, rake and remove a layer of oil and sediment to restore infiltration rate. To reduce erosion, fill and lightly compact sediment or plant vegetation to disperse flow. Constructed Wetlands Water-efficient irrigation should be applied during plant establishment and during extended dry periods. (Constructed wetlands should be inspected during the first 3 years during both the growing and non- growing seasons during the first 3 years of plant establishment. During inspections, observe plant species presence, abundance, and condition; bottom contours and water depths relative to plans; and sediment, outlet, and buffer conditions. Plants may require watering during the dry season. Replace or replant dead and damaged plants, plant vegetation over bare patches, and manually remove weeds. Do not use herbicides or pesticides. Investigate any evidence of clogging or rapid release in the wetland, and correct any subsidence, erosion, cracking, unwanted vegetation growth, or over- accumulation of sediment around forebays. Ponds Water-efficient irrigation should be applied during plant establishment and during extended dry periods. Ponds should be maintained and inspected seasonally and within 24 hours following a major storm event. Maintain and repair or replace the liner if it is damaged or leaking. Maintain inlets and outlets by removing all trash and vegetation that may be blocking or clogging the pipes. If there is evidence of erosion or scour around either the inlet or outlet, determine and remedy the cause. Make sure energy-dissipating devices, such as rocks, are located at outlets, and replace if necessary. Check the structural integrity of the spillway, and repair or replace if necessary. If piping or erosion is visible in the berm, consult a civil or geotechnical engineer. Remove all trees, shrubs, and vegetation over 4 feet high from the spillway. Maintain healthy vegetation cover on side slopes to prevent erosion of side slopes. Detention Pipes and Vaults Detention pipes and vaults should be maintained seasonally. During summer, make all necessary structural repairs. During winter, monitor flow-through rate. In spring, inspect the pipes, vaults, and tanks for structural concerns and sediment accumulation. As needed, clean all gutters and rain drains. When over one-third full, remove oil, sediment, and debris from the conveyance system and pipes to maintain at least 50 percent conveyance capacity. Prevent large roots from damaging subsurface structural components. Note: Only professionals with confined space certification from the Occupational Safety and Health Administration (OSHA) should enter below-ground stormwater systems. Lake Oswego Stormwater Management Manual Section 7.4 190 Maintenance Sheet Flow Dispersion Sheet flow dispersion facilities should be inspected monthly during the rainy season (November through April) to ensure adequate drainage. Water-efficient irrigation should be applied during plant establishment and prolonged dry periods Seasonally evaluate vegetation for damage or disease, and replace dead plants as necessary to ensure 100 percent coverage. Manually remove invasive or nuisance weeds. Do not use herbicide or pesticides. Limit fertilizer use as well. Base fertilizer use on soil tests, and opt for slow release fertilizer. Trees Trees should be maintained for long-term health and survival. Routine maintenance activities include: • Adding mulch or amending soils • Pruning trees to removed damaged limbs or protect sight lines • Irrigating • Weeding Trees should be inspected regularly for damage and disease. Dead trees must be replaced within 6 months with the same species or alternatives approved by the City. Stressed trees should be assessed and treated according to their needs. Care for stressed trees may include: • Adjusting irrigation • Adding or amending soils • Pruning damaged or dead limbs • Removing weeds near their base. Do not use pesticides. Limit fertilizer use as well. Base fertilizer use on soil tests, and opt for slow release fertilizer. 7. 1.5 Transfer of Property Ownership An agreement with the City shall be required when the stormwater management facility will be owned, operated, or maintained by the City. When ownership of stormwater management facilities are transferred to and accepted by the City, the transfer shall include access easements of a width acceptable to the City Engineer that is sufficient to allow access by maintenance and inspection equipment. If property is transferred to another owner, the responsible party must inform the new owner(s) or responsible party of the existence of stormwater management facilities on the site, of restrictions of the stormwater management facilities (including design capacity and setbacks), and of the requirements for inspection and maintenance of the stormwater management facilities. If the responsible party changes due to the transfer of the property, an updated OMP should be submitted to the City. 7.2 Failure to Provide Adequate Maintenance In the event the City Engineer has reason to believe that the stormwater management facility has not been maintained or that it has become a threat to public safety or health, the City Engineer shall notify the responsible party by certified mail. The notice shall specify the suspected non-maintenance or conditions giving rise to the threat to public safety or health, and the measures the City Engineer deems needed to comply with the OMP. The notice shall specify that the responsible party has thirty (30) days, or a shorter 191 Section 7.4 Lake Oswego Stormwater Management Manual Maintenance time period established by the City Engineer as needed under the circumstances to protect the public safety and health, within which time such measures shall be completed. The surface water code (LOC Chapter 38.25) establishes enforcement provisions in the event of failure to provide adequate maintenance or respond to a notice of violation. Enforcement may include being charged for the cost of repair work, a lien on the property in the Municipal Lien Docket, and/or fines according to the current fine schedule. 7.3 Submittals and Reporting 7.3.1 Operations and Maintenance Agreement/Plan An OMP that has been developed for a specific facility or site, such as the example provided in Appendix A, must be provided. For projects that create or replace 1,000 square feet or more of impervious surface (large projects), an OMP is required. The OMP must describe how to properly maintain the facility, the frequency of maintenance required, and the party responsible for maintaining the facility. Owners/operators of facilities that manage stormwater from large projects must file a deed restriction or recorded maintenance covenant in the County in which the property or facility is located. This step needs to be completed before the City will issue a Certificate of Occupancy.A copy of the deed restriction is provided in Appendix A. 7.3.2 Records of Maintenance Activities The owner or responsible party identified in the OMP is required to keep all records of maintenance activities as described in this chapter or the OMP. The responsible party shall make records of the installation, and of all maintenance and repairs, and shall retain the records for at least five years. These records shall be made available to the City Engineer during inspection of the stormwater management facility and at other reasonable times upon request. In the event of transfer of property ownership, the initial responsible party shall transfer maintenance records to the new owner/responsible party. 7.4 Maintenance Checklists Maintenance checklists for each facility are included in Appendix I. The checklists include descriptions of items to inspect during specific times of the year and following storms, and corrective action items to address problems observed during inspection. Lake Oswego Stormwater Management Manual Section 7.4 192 Maintenance This page is intentionally left blank. 193 Section 7.4 Lake Oswego Stormwater Management Manual 8 References ASTM D2488-09a. 2009. Standard Practice for Description and Identification of Soils (Visual-Manual Procedure).ASTM International, West Conshohocken, PA. ACWA. 2013. ACWA Construction Site Stormwater Guide. Illustrated Best Management Practices. Accessed August 12, 2015. <http://www.oracwa.org/r-com-stormwater.html> Barth, Carol A., et al. 2000. 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Accessed August 12th, 2015. <http://www.ci.oswego.or.us/sites/default/files/ fileattachments/planning/webpage/11841/tree_removal.pdf> Lake Oswego, City of. 2003. City of Lake Oswego Surface Water Management Design Workbook. December 2003 Edition. Leopold, L.B., and T. Maddock. 1953. The Hydraulic Geometry of Stream Channels and Some Physiographic Implications. US Geological Survey Professional Paper, 252. Linsley, R.K.; Max A. Kohler; Joseph L. Paulhus. 1982. Hydrology for Engineers, 3rd ed. McGraw-Hill Companies. New York. NOAA Fisheries. 2014. Standard Local Operating Procedures for Endangered Species to Administer Maintenance or Improvement of Stormwater, Transportation or Utility Actions Authorized or Carried Out by the U.S. Army Corps of Engineers in Oregon (SLOPES V for Stormwater, Transportation or Utilities; March 14, 2014). ODOT. 2014. Hydraulics Manual. Oregon Department of Transportation. Revised April 2014. Accessed June 18, 2015. <http://www.oregon.gov/ODOT/HWY/GEOENVIRONMENTAL/pages/hyd_manual_info. aspx>. 195 Section 8.0 Lake Oswego Stormwater Management Manual References Oregon Building Codes Division. 2012. Oregon Specialty Codes Online. Accessed April 27, 2012. <http:// www.cbs.state.or.us/bcd/programs/online_codes.html> Otak. 2009. City of Lake Oswego Clean Streams Plan. Prepared for the City of Lake Oswego, by OTAK, Inc., Lake Oswego, Oregon. Portland, City of. 2014. Stormwater Management Manual. Environmental Services.Accessed August 12, 2015. <http://www.portlandoregon.gov/bes/64040> Portland, City of. 2008. Stormwater Management Manual.Accessed July 30, 2015. <http://www. portlandoregon.gov/bes/47952> Portland, City of. 2006. Sewer and Drainage Facilities Design Manual. July 30, 2015. <http://www. portlandonline.com/auditor/index.cfm?a=135777&c=28044> Portland, City of. 2004. Stormwater Management Manual. Seed Specifications.Adopted July 1, 1999. Revised September 2004. Hinman, C. 2012. Low Impact Development: Technical Guidance Manual for Puget Sound. Puget Sound Action Team and Washington State University Pierce County Extension. WSU and PSP. Accessed July 30, 2015. <http://www.psat.wa.gov/Publications/LID_tech_manua105/lid_index.htm> Seattle Public Utilities. 2009a. Director's Rules for Seattle Municipal Code Chapters 22.800—22.808. Volume 1: Source Control Technical Requirements Manual. Department of Planning & Development. Seattle Public Utilities. 2009b. Director's Rules for Seattle Municipal Code Chapters 22.800—22.808. Volume 3: Stormwater Flow Control & Water Quality Treatment Technical Requirements Manual. Department of Planning & Development. SCS. 1986. Stormwater Solutions. Oregon State University extension webpage.Accessed July 30, 2015. <http://extension.oregonstate.edu/stormwated> USDA. 2013. Web Soil Survey. Natural Resources Conservation Service. Accessed August 12th, 2015. <http://www.psat.wa.gov/Publications/LID_tech_manua105/lid_index.htm> USGS. 2013. Water Resource Studies in Oregon and Washington. Oregon Water Science Center. Accessed August 12th, 2015. <http://www.psat.wa.gov/Publications/LID_tech_manua105/lid_index.htm> Lake Oswego Stormwater Management Manual Section 8.0 196 References This page is intentionally left blank. 197 Section 8.0 Lake Oswego Stormwater Management Manual Dpgprinbi[a@n, 303(d) list A list developed in conformance with Section 303(d) of the federal Clean Water Act that identifies waters that do not meet water quality standards and where a total maximum daily load (TMDL) needs to be developed. Adsorption Adsorption is the process by which dissolved pollutants adhere to suspended particulates, bottom sediments, vegetation surfaces, or other media (such as activated carbon). Some filtration media help remove charged pollutant particles, such as metal cations, by adsorption. Anion exchanae A form of sorption involving the exchange of negatively charged ions between dissolved pollutants and media such as vegetation. Bedrock The native, contiguous, consolidated rock underlying the surface of the Earth. Above bedrock is usually an area of broken and weathered unconsolidated rock, usually called sediment. Bedrock is sometimes exposed on the surface, indicating that sediment has been removed by streamfiow or some other sediment transport process (e.g., landslides). Best manaaement practices (BMPs) The schedule of activities, controls, prohibition of practices, maintenance procedures, and other management practices designed to prevent or reduce pollution. BMPs also include treatment requirements, operating procedures and practices to control stormwater runoff. Non-structural BMPs are practices (rather than facilities) designed to prevent pollution. Procedural (non-structural) BMPs are procedures, such as training, agreements, and plans, that are implemented to mitigate construction and development stormwater impacts. Built out Most land is developed rather than vacant. Capacity See design capacity. Cation exchanae (also cation exchanae capacity ICEC1 Cations are positively charged elements such as calcium, magnesium, and ammonium (anions are negatively charged elements). The cation exchange capacity (CEC) is the total negative charge of soil, measured in millequivalents per 100 grams of soil (meq/100g). The CEC helps determine a soil's ability to attract or repel certain elements. Channel The land features (bed and banks) that confine a stream. Channel straiahtenina The modification of the natural drainage patterns of a channel and elimination of sinuosity, braiding, and other natural features. Lake Oswego Stormwater Management Manual Section 9.0 198 Definitions Desianated Management Aaencv Public agency that possesses the legal authority, technical competence, organizational ability, and financial resources to carry out all or part of the nonpoint source control program as stipulated in an agreement with the Department of Environmental Quality. Desian capacity The flow volume or rate that a stormwater management facility is designed to safely contain, receive, convey, reduce pollutants from, or infiltrate to meet a specific performance standard. Detention The temporary storage of stormwater runoff in a facility (typically a pond, vault, or large pipe), which is used to control the peak discharge rates. The entire stormwater volume is ultimately released, but at a lower discharge rate. Direct discharae Stormwater that enters receiving water through a point source, such as a pipe, outfall, or ditch. Discharae As defined in 40 CFR 122.2, "discharge" when used without qualification means the "discharge of a pollutant." "Discharge of a pollutant" means: (a) Addition of any "pollutant" or combination of pollutants to "Waters of the State"from any point source, or (b) Addition of any pollutant or combination of pollutants to the waters of the "contiguous zone" or the ocean from any point source other than a vessel or other floating craft which is being used as a means of transportation. Drvwell Perforated precast concrete cylinders that discharge stormwater into underlying soils. Easement An interest in land owned by another that entitles the holder of an easement to a specific limited use. Also an area of land covered by an easement. Entrain To carry. Erodible or leachable materials Erodible or leachable materials, wastes, or chemicals are those substances which, when exposed to rainfall, measurably alter the physical or chemical characteristics of the rainfall runoff. Examples include erodible soils that are stockpiled, uncovered process wastes, manure, fertilizers, oily substances, ashes, kiln dust, and garbage dumpster leakage. Fish saawnina and rearina habitats Habitats associated with vulnerable life stages of important fish species. The timing of construction activities that could potentially affect these areas is typically restricted. 199 Section 9.0 Lake Oswego Stormwater Management Manual Definitions Groundwater Subsurface water that occurs in soils and geological formations that are fully saturated. Groundwater fluctuates seasonally and includes perched groundwater. Hvdroaraoh A hydrograph graphically displays the discharge of stormwater runoff over time. The area under the hydrograph represents the total volume of stormwater runoff during that storm. Hvdromodification Modification of stream channels resulting from changes in stream flow, volume, and duration due to stormwater discharges. These may include bank erosion, incision, channel scour, and channel straightening. Hydromodification can result in loss of aquatic habitat and poor water quality. Hvdroaraoh A graphical representation of the distribution of rainfall over time. Illicit discharae Any discharge to a municipal separate storm sewer system that is not composed entirely of stormwater except discharges authorized and permitted under a NPDES permit or other state or federal permit, or otherwise authorized by DEQ. Impervious surface Any surface that prevents the infiltration of water or results in more runoff than in the undeveloped condition. Common impervious surfaces include: building roofs, traditional concrete or asphalt paving on walkways, driveways, parking lots, gravel roads, and packed earthen materials. Indirect discharae Stormwater that enters receiving waters via surface runoff or via groundwater. Industrial activities Activities such as manufacturing, transportation, mining, and steam electric power industries; scrap yards, landfills, certain sewage treatment plants, and hazardous waste management facilities. Infiltration The downward movement of rainwater or surface water through the soil. Infiltration pond A vegetated depression that temporarily pools stormwater before it percolates into underlying soils. Infiltration trench A linear, gravel-filled trench that distributes stormwater to underlying soils. Integrated oest management (IPMl A natural, long-term, ecologically based systems approach to controlling pest populations. This system uses techniques either to reduce pest populations or maintain them at levels below those causing economic injury, or to so manipulate the populations that they are prevented from causing injury. Invert elevation The elevation of the lowest part of the inside of a pipe, culvert, or ditch. Lake Oswego Stormwater Management Manual Section 9.0 200 Definitions Land disturbance Any activity that alters the land surface in a way that modifies characteristics affecting rainfall runoff or erosion potential. Larae Droiect New or redevelopment project that creates or replaces 3,000 square feet or more of effective impervious surface area. Larae woody debris An accumulation of trees and large branches. In water ways, large woody debris serves many purposes that are vital to life history of many native species of fish, plants, and animals. Low impact development (LIDI A stormwater management approach that seeks to mitigate the impacts of increased runoff and stormwater pollution using a set of planning, design, and construction approaches and stormwater management practices that promote the use of natural systems for infiltration, evapotranspiration, and reuse of rainwater and can occur at a wide range of landscape scales (e.g. regional, community, and site) Maximum extent practicable (MEPI The regulatory definition of an MEP is based on the Clean Water Act Section 402(p)(3)(B)(iii).According to Oregon DEQ, "the statutory standard that establishes the level of pollutant reductions that operators of regulated MS4s must achieve". The standard usually requires implementation of BMPs to comply with required control measures. Municipal separate storm sewer system (MS41 The regulatory definition of an MS4 (40 CFR 122.26(b)(8)) is "a conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains): (i) Owned or operated by a state, city, town, borough, county, parish, district, association, or other public body (created to or pursuant to state law) including special districts under state law such as a sewer district, flood control district or drainage district, or similar entity, or an Indian tribe or an authorized Indian tribal organization, or a designated and approved management agency under section 208 of the Clean Water Act that discharges into waters of the United States. (ii) Designed or used for collecting or conveying stormwater; (iii) Which is not a combined sewer; and (iv) Which is not part of a Publicly Owned Treatment Works (POTW) as defined at 40 CFR 122.2." In practical terms, operators of MS4s can include municipalities and local sewer districts, state and federal departments of transportation, public universities, public hospitals, military bases, and correctional facilities. National Pollutant Discharae Elimination System (NPDESI The NPDES program was initiated by the U.S. Congress in 1972 and amended in 1987 as part of the Clean Water Act. It regulates discharges of stormwater pollutants into navigable or regulated waters from municipalities and many types of industrial sites. Over-bank veaetation Vegetation that extends over the channel, providing shading and wood supply. Overflow elevation The elevation that water is allowed to rise in a pond before it flows into an outlet pipe. 201 Section 9.0 Lake Oswego Stormwater Management Manual Definitions Peak flow The maximum rate of flow of water during or after a precipitation event. Pervious Surface Pollutant generating impervious surface. Pilot infiltration test (PITl A relatively large-scale field procedure developed by Ecology's Technical Advisory Committee that estimates of infiltration rates of stormwater facilities while reducing scaling errors. The procedure involves excavation of a test pit with a horizontal surface area of about 100 square feet and subsequent measurement of infiltration rate. Pollutant Pollutant means dredged spoil, solid waste, incinerator residue, filter backwash, sewage, garbage, sewage sludge, munitions, chemical wastes, biological materials, concrete wash water, paint, radioactive materials (except those regulated under the Atomic Energy Act of 1954, as amended [42 U.S.C. 2011 et seq.]), heat, wrecked or discarded equipment, rock, sand, cellar dirt and industrial, municipal, and agricultural waste discharged into water. Potential severe erosion hazard area Surface areas where erosion can be easily caused by removal of vegetation cover, stripping topsoil, or placing fill, whether by natural causes such as streams or surface runoff, or by development activities. The placement of any new fill in such an area shall be considered as creating a potentially severe erosion hazard. (Known Potential Severe Erosion Hazard Areas are described and mapped in the Engineering Geology chapter of the Lake Oswego Physical Resources Inventory, March, 1976, on file at City Hall; specifically in Table II, "Characteristics and Limitations of Earth Materials" and "Engineering Geology" map, and the Relative Slope Instability Hazard Map of the Lake Oswego Quadrangle, prepared by the State of Oregon Department of Geology and Mineral Industries (DOGAMI), published in 1995.) Potential severe landslide hazard area Areas where earth movement or failure, such as slumps, mud flows, debris slides, rock falls or soil falls, are likely to occur as a result of development activities. These activities include excavation which removes support of soils by changes in runoff or groundwater flow or vibration loading such as pile driving or blasting. Predeveloament Conditions expected to have been present prior to development or land alteration by Anglo-Europeans (i.e. conditions present at the time of the Lewis and Clark expedition). For sizing of detention facilities, use a CN of 70, for forested conditions and Type C soils, to model predevelopment conditions. Prohibited material Prohibited materials include pollutants or other material that is not explicitly identified as authorized discharges per Schedule A(4)(a)(xii) of the City's NPDES MS4 permit that are discharged to the surface water management system. Public nuisance Nuisance conditions include improper function resulting in uncontrolled runoff and overflow; stagnant water with concomitant algae growth, insect breeding, and odors; discarded debris; and safety hazards created by a facility's operation. Lake Oswego Stormwater Management Manual Section 9.0 202 Definitions Replace or replacement The removal of an impervious surface that exposes soil followed by the placement of an impervious surface. Replacement does not include repair or maintenance activities on structures or facilities taken to prevent decline, lapse or cessation in the use of the existing impervious surface as long as no additional hydrologic impact results from the repair or maintenance activity. Receiving water A receiving water is the ultimate destination for stormwater leaving a particular site. Virtually all receiving waters are Waters of the State, and include "lakes, bays, ponds, impounding reservoirs, springs, wells, rivers, streams, creeks, estuaries, marshes, inlets, canals, the Pacific Ocean within the territorial limits of the State of Oregon, and all other bodies of surface or underground waters, natural or artificial, inland or coastal, fresh or salt, public or private (except those private waters that do not combine or effect a junction with natural surface or underground waters) that are located wholly or partially within or bordering the state or within its jurisdiction." (ORS 468B.005(10)). Redevelopment A project on a previously developed site that results in the addition or replacement of impervious surface. Riffles Shallow areas in a stream channel where the surface of flowing water is broken by waves or ripples. Rio ht-of-wav Public land used for public roads and utilities. Santa Barbara Urban Hvdroaraph (SBUH1 A hydrologic model that converts design storm incremental excess rainfall depths into instantaneous unit hydrographs and routes them through an imaginary reservoir. Sedimentation Deposition of sediment. Sensitive lands Those areas that the City has designated as sensitive lands within the sensitive lands zoning overlay. They include lands containing natural resources that have environmental significance within the Lake Oswego planning area (urban service boundary) including wetlands, stream corridors, and tree groves. Such lands are more sensitive or easily damaged by development impacts than non-resource lands. Sensitive lands have additional development restrictions regarding modification of vegetation and allowable land disturbance. Small proiect New or redevelopment project that creates or replaces greater than or equal to 200 square feet and less than 1,000 square feet of effective impervious surface area. Sorption Sorption is the process by which dissolved pollutants attach to suspended particulates, bottom sediments, vegetation surfaces, or other media (such as activated carbon). See absorption, adsorption and cation exchange for types of sorption. 203 Section 9.0 Lake Oswego Stormwater Management Manual Definitions Spillway An armored surface outlet from detention pond or other surface BMP to allow stormwater to discharge even in the event of outlet plugging or higher-than-design flows. Steep slope A steep slope is an average slope of 25 percent or more. Storm drain A pipe that transports stormwater. Stormwater The water that originates from precipitation, primarily rainfall and snowmelt. Stormwater manaaement facilities Facilities intended to collect or convey stormwater runoff, reduce pollutants from stormwater, or reduce hydrologic impacts associated with stormwater by detaining or infiltrating stormwater. Substrate Material underlying the portion of the streambed that receives direct flow. Surface water manaaement utility The surface water management utility is the entity that plans, designs, constructs, maintains, administers, and operates all City surface water conveyances and facilities, and the regulations for facility control. The surface water management utility also establishes standards for design and construction. Surface water manaaement system Includes all natural and manmade facilities utilized by the surface water management utility to regulate the quantity and quality of surface water, including drainage easements, culverts, storm drains, catch basins, stream corridors, rivers, ponds, wetlands and impoundments. Total maximum daily load (TMDLI DEQ develops TMDLs for"water quality limited" or"impaired" water bodies (on the 303(d) list) in accordance with OAR 340-042-0040, which defines how much of an identified pollutant a specified water body can receive and still meet water quality standards at critical locations or times of the year.. Lake Oswego Stormwater Management Manual Section 9.0 204