Loading...
Stormwater Management Manual 2020 . •t_ 4 , 214iP .. . A . . S %gig° ,44 . . - ipip:: 141L." . . ilirklift. .' 'qtr'lf66 =s1 .1 —.E. , IVrii - 1-ikj "'---`" , AIN. •---- y lots • 4 • .. 7 . .. , 1 . IF ...- .• nip .._.1110 ' E - (t4ilitht 0 0 0 .0111111, G October 1 2020 Acknowledgements Staff who contributed to the 2020 revised Stormwater Manual include: Stefan Broadus, PE, Asst. City Engineer; Todd Knepper, PE, Engineering Program Manager; Rob Amsberry, Engineering Staff; Sonja Johnson, Engineering Staff; Thomas Benson, Engineering Staff; Nancy Flye, Engineering Staff; and Ryan Lentz, Planning Staff. The City appreciates the assistance of citizen volunteers and staff on the 2016 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, former 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 (2016), City Council Liaisons David Donaldson (2012-2013) and Megan Phelan (2016), Assistant City Managers Rob Amsberry, Engineering Erica Rooney, PE, City Engineer Guy Graham, PE, former Public Works Director Evan Boone, Deputy City Attorney Jim Bateman, Public Works Jenny Slepian, Sustainability and Management Analyst Bonnie Hirshberger, Citizen Information Specialist Hamid Pishvaie, Assistant Planning Director David Gilbey, former Water Quality Coordinator Mary Larkin, Herrera Environmental Consultants Eryn Deeming Kehe, JLA, Inc. Kate Forester, PLA, Herrera Environmental Consultants Carol Slaughterbeck, Herrera Environmental Consultants Pam Peterson, Public Works Beautification Coordinator Anne MacDonald, CEG, former Stormwater Quality Coordinator Finally, City staff, project advisory committee members, and the consultant team have benefited 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. 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 15 2.1 Project Process 15 2.2 Activities That Trigger Stormwater or Permit Requirements 18 2.3 General Requirements For All Residents, Property Owners, and Business Owners 18 2.3.1 Prevent Illicit Discharges 18 2.3.2 Maintain Existing Drainage Patterns 19 2.4 Project Classifications and Development Types 19 2.4.1 Small Projects 20 2.4.2 Large Projects 20 2.4.3 Equivalent Stormwater Management 20 TOC- 1 2.5 Overview of Minimum Requirements: Design Requirements 20 2.5.1 Perform Site Assessment and Feasibility Analysis 20 2.5.2 Onsite Stormwater Management 21 2.5.3 Design for Water Quality 21 2.5.4 Design for Flow Control 25 2.5.5 System Designed by Licensed Professional 25 2.5.6 Underground Injection Control Review with Oregon Department of Environmental Quality 26 2.6 Overview of Minimum Requirements: Permits 26 2.6.1 City of Lake Oswego Erosion Control Permit 26 2.6.2 NPDES 1200-C Permit from Oregon Department of Environmental Quality 27 2.6.3 Street Opening Permit 27 2.6.4 Other Permits and Project Review 27 2.7 Overview of Minimum Requirements: Submittals 28 2.7.1 Drainage Report 28 2.7.2 Recorded Operations and Maintenance Plan and Deed Restriction 28 2.7.3 Landslide Hazard and Erosion Risk Areas Report 28 2.8 Project Exemptions, Adjustments, Exceptions, Waiver and Payment/ Fee In Lieu Information 29 2.8.1 Exempt Projects 29 2.8.2 Adjustments 29 2.8.3 Exceptions 29 2.8.4 Waiver/Fee In-Lieu Program Information 29 2.9 City Review of Development Permits 30 2.9.1 Engineering Department 30 2.9.2 Planning Department 31 2.9.3 Building Department 31 2.9.4 Public Works Department 31 2.10 Review Process for Different Types of Projects 31 Chapter 3 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 35 3.1 LID Principle #1: Understand the Site 35 3.1.1 Site Assessment 35 3.1.2 Site Assessment Components 37 3.2 LID Principle #2: Reduce Runoff through Design 40 3.3 LID Principle #3: Reduce Pollutants Carried by Runoff 41 TOC-2 3.4 LID Principle #4: Capture and Treat Runoff 41 3.4.1 Stormwater Facility Selection for Small Projects 43 3.4.2 Stormwater Facility Selection for Large Projects 43 3.4.3 Infiltration Facility Considerations 44 3.4.4 Use and Approval of Proprietary Stormwater Best Management Practices 45 3.4.5 Other Considerations in BMP Selection 46 3.4.6 Certification By Design Professional 46 Chapter 4 Stormwater Facility Design Guidelines 47 4.1 Performance Standards 47 4.2 Planting Design 48 4.2.1 Why Plants Matter in Stormwater Design 48 4.2.2 Vegetation Diversity and Its Effect on Water Quality 48 4.2.3 Plant Selection Process: Steps and Considerations 50 4.3 Stormwater Facility Design Methods and Computations 54 4.3.1 SBUH Method 54 4.3.2 PAC Tool 54 4.3.3 Equations 54 4.3.4 Prescriptive Sizing 54 4.3.5 Minimum Orifice Size 55 4.4 Soils 55 4.5 Pollution/Flow Control Manhole Design 56 4.6 Stormwater Facility Design Guidelines 56 4.6.1 Rain Gardens 57 4.6.2 Planters 65 4.6.3 Infiltration Trench or Gallery 73 4.6.4 Drywell 79 4.6.5 Green Roofs 85 4.6.6 Pervious Pavement 89 4.6.7 Rainwater Harvesting 93 4.6.8 Filter Strips 99 4.6.9 Swales 105 4.6.10 Sand Filters 111 4.6.11 Constructed Wetland 117 4.6.12 Ponds 123 4.6.13 Detention Pipes and Vaults 129 4.6.14 Sheet Flow Dispersion 135 4.6.15 Proprietary Stormwater Treatment Devices 137 TOC-3 Chapter 5 Conveyance and Detention Design Standards 139 5.1 General Provisions - Conveyance 139 5.2 Extension of Public Conveyance Systems 139 5.3 Conveyance Easements 140 5.3.1 General 140 5.3.2 Standard Conveyance Easement Width 140 5.3.3 Reduced Conveyance Easement Widths 141 5.3.4 Encroachments 141 5.4 Flow Determination for Surface Water Conveyance 141 5.4.1 Land Use Assumptions for Flow Determination 141 5.4.2 Computational Methods for Runoff Calculations 142 5.5 Surface Water Conveyance Design Considerations 142 5.5.1 Design for Full Build-Out 143 5.5.2 Stormwater Conveyance Design Criteria 143 5.5.3 Materials for Piped Conveyance 143 5.5.4 Materials For Open Channels 144 5.5.5 Upstream Impacts 145 5.5.6 Downstream Impacts 145 5.5.7 Cross Lot Drainage 146 5.5.8 Dissipation of Runoff Discharge 146 5.5.9 Separation 146 5.5.10 Alignment 146 5.6 Other Requirements for Public Conveyance Systems 146 5.6.1 Survey 146 5.6.2 Railroad Crossings 146 5.7 Surfacewater/Stormwater Laterals 147 5.7.1 General Provisions 147 5.7.2 Planning Considerations 147 5.8 Conveyance and Detention Design Standards 147 5.8.1 Purpose 147 5.8.2 Downstream Analysis 147 5.8.3 Flood Management Areas Defined 150 5.8.4 Flood Management Design Criteria 150 TOC—4 Chapter 6 Construction Phasing, Stormwater Pollution Prevention, and Erosion and Temporary Sediment Control 153 6.1 Legal Requirements 153 6.1.1 City Code 153 6.1.2 MS4 Permit 153 6.1.3 1200-C Permit 153 6.1.4 General Considerations 154 6.2 Construction Practices 154 6.3 Scheduling, Phasing, and Timing of Work 155 6.3.1 Scheduling 155 6.3.2 Phasing 155 6.3.3 Erosion and Sediment Control Requirements 156 6.4 Implementing Procedures 157 6.4.1 Communication 157 6.4.2 Managerial Practices 158 6.5 Consideration of Construction Impacts during Planning, Design, and Construction 158 6.5.1 Planning Considerations 158 6.5.2 Design and Construction Considerations 159 6.5.3 Recommended Construction Practices 159 6.5.4 Selecting Construction Best Management Practices 160 6.5.5 Maintenance and Inspection Procedures 163 6.5.6 Post Construction 164 6.6 Documentation 164 6.6.1 Erosion and Sediment Control Drawings 164 6.6.2 Erosion Control Plan 165 6.6.3 Inspection Log 165 Chapter 7 Stormwater BMP Maintenance 167 7.1 Legal Requirements 167 7.1.1 Maintenance Responsibilities 167 7.1.2 Inspection 168 7.1.3 Maintenance Access 168 7.1.4 Routine Maintenance Activities 169 7.1.5 Transfer of Property Ownership 174 7.2 Failure to Provide Adequate Maintenance 174 TOC-5 7.3 Submittals and Reporting 174 7.3.1 Operations and Maintenance Plan 174 7.3.2 Records of Maintenance Activities 174 7.4 Maintenance Checklists 175 Chapter 8 References 177 Chapter 9 Definitions 181 Appendix A Submittals A A-1 Site Assessment and Feasibility Resources A-2 Drainage Report Template A-3 Impervious Surface Area Reduction Table A-4 Operations and Maintenance Agreement A-5 Sample Operations and Maintenance Plan A-6 Stormwater Certification Form Appendix B Infiltration Testing Guidance B B-1 Infiltration Testing Report B-2 Infiltration Testing Guidance Appendix C Reading the Soil C Appendix D Biofiltration Soil Mix Specification D Appendix E Rational Method Specifications E Appendix F Facility Sizing Methods F F-1 Santa Barbara Urban Hydrograph (SBUH) Method Input Data F-2 Guide to Using Portland's PAC Tool for Lake Oswego Appendix G Right Tree, Right Place G TOC -6 Appendix H Approved Plant Lists and Seed Mixes H H-1 Approved Plant List H-2 Approved Seed Mixes Appendix I Erosion Prevention and Sediment Control Plan Information I I-1 Erosion Prevention and Sediment Control General Notes 1-2 Erosion Prevention and Sediment Control Application Appendix J Source Control J Appendix K Maintenance Checklists K 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 shall 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 shall 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 • Epp MEMORANDUM h °REGO‘-‘ TO: Martha Bennett, City Manager FROM: Erica Rooney, City Engineer Engineering SUBJECT: Stormwater Management Manual Revision DATE: October 1, 2020 Engineering staff and consultants completed the original Stormwater Management Manual (SWMM) in March 2016 which was adopted by Council under Ordinance 2695. With technical advancements, and based on additional experience working with developers and stormwater management, the SWMM was recently revised to create an easier design framework for developers and clarify inconsistencies. The City Manager is authorized to amend the SWMM from time to time: 38.25.110 Stormwater Management Manual. The City Manager shall 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 (Manual). The City Manager is delegated authority to adopt, revise, and update the Manual as necessary. The Manual may be modified from time to time, at the discretion of the City Manager, based on improvements in engineering, science, monitoring and local experience.The Manual shall describe the best management practices (BMPs) appropriate for use in the City. The Engineering Department recommends approval of the revised 2020 Stormwater Management Manual. I hereby adopt the revisions and updates as stated in the 2020 Stormwater Management Manual, effectivft upon my signature. 9/1 /Z-d • Mart Be ett,City Manager Date ii Lake Oswego Stormwater Management Manual 503.675.3984 380 A Avenue PO BOX 369 Lake Oswego, OR 97034 www.ci.oswego.or.us Effective Date: March 17, 2016 ORDINANCE NO_ Z6 5 AN ORDINANIX OF TH E CITY COUNCIL OF THE CITY OF LAKE OSW EGO AMENDING LOC SECTIONS 38.O6.2.c.I,38.24.5, 50.06,OD5, 5160-004,50,01.003,50-07.004,50,10-003,2 AND ADDING ARTICLE 38,25, REGARDING STORMWATER MANAGEMENT;AND ADOPTING FINUINES (Ir1.I 15-0050). WHEREAS, the regulation of stormwater runoff reduces threats to public health and safety and the environment by decreas.ing discharge of pollutants to receiving waters;and 4 +HEPFAS, creeks, rivers and strums within the jurisdiction of Lake Oswego experience bank erosion and contribute sediment and sedirnent-baur+d pollutants to downstream waters;and WHEREAS, inadequate surface and subsurface drainage planning and practice can dead to erosion and property damage and risk to life;and ig th refora in the public intent;and WHEiiL t r regulation Di gormwater runoff is required by the I lunlclpaf Separate Storm Sewer System(IVI54)permit Issued to the City by the Oregon Department of Environmental Quality(DEO) under the rational Pollutant Discharge Elimination System(14PDES) program; and WHEREAS, starmwater Is locally discharged to the subsurface through Underground Injection control facilities (dry wells); pollutants in this dEicharge, 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{1 Mb Ls}that require the city to further reduce sediment discharges and WHEREAS,other regulations of stormwater may-be appfietion a sit pecfic basis to development activitig*within water of the state where compliance with the City s starmvater requirements may be presumed to fulfill all or part of the pe Milt requirements of the regulating agencies.; arid WHEREAS, illicit and non-stormwater discharges to the storm drain system cars contribute a wide variety of pollutants to waterways, and the control cif 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 cons[ftute a potential hazard to the health, safety, and property of the citizens of the City (LQc38.24.505)and for effective facility management to occur,the facility must also be protected from excess sediment; Lake Oswego Stormwater Management Manual iii WHEREAS.,EAS, notice of the public hearing#or consideration of this Ordinance was duly given in the manner required by law; a n d WHEREAS, the Planning Commi55iLn has recommended that LU 15.0050 be approved by the City Council; and WHEREAS, a publ5k hearing vn LU 15-0050 was held before the City Council of the City of Lake Oswego on February 2, 201E. The C`rtyof Lake Oswego ordains as follows: Section 1, The City.Council hereby adopts the Finding5 and Conclusions (LU 15-0050 attached as Attach ment C. Section 2. LOC Chapter 50,Articles 50.E_044, 50,06.00C 50,07,00,50.00.004,and 50.10.002 are hereby amended as shown in Attachment A (new text shown in pkLdouble-undorllrred ty s; deleted text shown tIt strik tith-typo). Section 3, LOC Chapter 38..Artitles 38.05.38.24, and 38.21S are hereby amended; and a new Article 38.25 is as added as shown in Attachment B (new text shown in imit double-underlined tvqe; deleted teat shown In 4r4kethretig:1440pe)_ Section 4. The City adopts the Lake Oswego Stormwater Management Manual as the primary said once document For sturmvwater management, 5ogtiloin..5. 5emerabIlit+r. The provisions of this ordinance are severable. If any portion of this ordinance is for art reason held to be invalid, such decision shall not aFfett 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 16tih day of February, 2016. AYES; Mayor Studebaker, Buck,Glad man,Gustafson,Calllfl$,O'I,oIIP, Martin, NOES_ None. ABSTAIN:None. EXCUSED. None. 111 Kent Studeba eer,Mayor Dated; .4.71.4A,€ 4d Ordinance No. 2695 Page 2 of 3 iv Lake Oswego Stormwater Management Manual ATTEST_ a4114T71(litt Anne•Mane Simpson,City Recorder A D TO ;Mf David Powell, City Atto nay Ordfnanta No. 2595 Page 3 of 3 Lake Oswego Stormwater Management Manual v vi Lake Oswego Stormwater Management Manual 0 0 0 A bb Te 0 0 El 0 0.1 Abbreviations and Acronyms AASHTO -American Association of State Highway OMP— operations and maintenance plan and Transportation Officials ORS - Oregon Revised Statute ASTM -American Society for Testing and Materials PIT- pilot infiltration test BMP - best management practice psi - pounds per square inch CFR — Code of Federal Regulations PVC - polyvinyl chloride cfs - cubic feet per second ROW - right of way CN — Runoff curve number (NRCS TR-55) SBUH — Santa Barbara Urban Hydrograph DEQ — Oregon Department of Environmental SWPPP— stormwater pollution prevention plan Quality TAPE —Technology Assessment Protocol — ESC — erosion and sediment control Washington Department of Ecology FEMA— Federal Emergency Management Agency TMDL—total maximum daily load GIS —geographic information system UIC — underground injection control GULD - general use level designation US EPA— United States Environmental Protection Agency HDPE - high-density polyethylene USGS — United States Geological Survey IPM — integrated pest management LID — low impact development LOC — Lake Oswego Code MS4 - municipal separate storm sewer system NPDES - National Pollutant Discharge Elimination System NRCS — Natural Resources Conservation Service OAR— Oregon Administrative Rule ODOT— Oregon Department of Transportation Lake Oswego Stormwater Management Manual Section 0.1 vii viii Lake Oswego Stormwater Management Manual 1 lEirodaigo .._____, 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, 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 and 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 or as well as other legal requirements. In addition to addressing the such ruas tseaws,and .They gardens, n constructed wetlands.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 J. 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. III I �_ ,. ► i ' CII1 ._ . --jitokiapilia ......, --4i16.1 ..„...., - ::. .---'1';=:-'--.-fr.,tsti ., :,-_,- ;:: -,...1::-..-'-:4---' _ = --.- - - :. ' - -- ____ _ - - - . • • - - A6 --- `T T �V °'C : r .` .4, • a.,,y�.. . .. fVf • Oswego Lake Lake Oswego Stormwater Management Manual Section 1.1 1 Introduction Table 1.1 NPDES MS4 Permit Requirements Addressed by this Manual. c c O _ = o 0 c •E y Cl)GI c cv vs a) Ica a) a) ra r G NO= y 4- '5 — E a aci aciu iiac) -o 2 • 0 MS4 Permit Requirement a '3 y 3 ns co C c 0 � s rnE 3 0 0 C) Q L E > u c E C r L = c ,O L 1- L .- C 0 L 0 fA 0 , cn � Cr) C, . c� O. L � 13 I � Iy CO I � I �, ;, I0 N E M c 0 in 4- CD +' L N. c O L N O L c L (I) L R co it L it 0 �, a) 7) 0 • a) c a) 3 0 a> 0 Qy Q >+ 7 Q� Q0) aE 0_ aE co -0 co To _m co •— ca cn ca E ca •— s c .cc as s 3 s CU t ° 0 .cca O ca c.) < u) C) CD O Ci 0 v) H 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 11111 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 I 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. il 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 1 • that maximize reduction in discharge of pollutants. City shall review, approve, and verify proper 6 implementation of post-construction site plans for new • • • development and redevelopment projects. 2 Section 1.1 Lake Oswego Stormwater Management Manual Introduction c v; 42 a) O �' E v, m ° ° v o 6 ° m ai >+ c .r a a = 0 �noaa, cn . ' c.* _ CO E a a• ci E �i •( c ad '5 2 cis E 6- a ° c > m m V7 ram+ L O i I" L MS4 Permit Requirement a 3 y 3 m _ _ c m r a w 3 i ° a a°' Q " E > cn N o E O L _ Oi N = y O o - O — N r a4 v) . v) O . Uao u) I3 IF IN I ; I �, W I8 N E c y c Nr a> In N co N. co d d 'w ° m = O c d 3 L m` _ w 4 r I O Tir aN a(.' a� a•a' a E a a v— as -0 cc ca _ ca .— cC cn cC O E co .- . c tcd .0 tag .C �, a, tca O cC UQu) OW U0 1 U0/) I— U :E 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 • • 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 !Introduction 1.2 Organization of this Manual Chapter 0.0 lists abbreviations 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, shall 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 shall 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. Chapter 9 provides definitions of key terms used 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. I 7 4 jot:t.46 Rwilivil aro.� • f a+a a.t f ',, yr ` -011110 i O swegoo {j 1� , 4 i .ri 7) . .,, iir 44 17 I IEEE (iti lc . Loki 3 —Rrri i. d,a Nigkar c /./ • PIANIANol -.-: of.trii Simtp l AaR Al.--. riN. '� If1 hilitioam. 1166 401 ii,1.44 lain ■ Far mire Iniomruu.g*to www.danwricktruernsinewiterrelroMarimarklm 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 ' 1 • Changing the timing at which the Some areas of Lake Oswego have f 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, or are open drainage ditches. • Increasingdurations(the time - During periods of heavy rainfall . -�;=_ =:��:�# : - period during which a receiving on areas with a large amount of ...,_' - - - water experiences elevated flood ji� • ,=•�L4 _c,- t- ` y ,. flows) impervious surface, local flooding _ <-_' •<;,,s.,.— - ' may occur, along with increased • Increasing the runoff volume associated risks to health, public Flooding in Lake Oswego 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 shall 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 Introduction 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 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 shall encompass riparian protection, increased shade protection, and overall resiliency measures that will adapt to changing climate patterns. 8 Section 1.3 Lake Oswego Stormwater Management Manual 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 is required to 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 shall 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. Lake Oswego Stormwater Management Manual Section 1.3 9 Introduction Table 1.2 Impaired Water Quality in Receiving Waters of the City of Lake Oswego. Waterbody H Y v N d O d > d 'a N O O U 0) e V C E v co 01 07 O = - c ++ lC d 7 Parameter 3 3 c L 0 0 __ > w y E. To = § ie .c Op u_ N H I— Aquatic Weeds or Algae • Biological Biological Criteria • • Chlorophyll a Ammonia • General Dissolved Oxygen • •' •2 • • • Chemistry Phosphorus • • • • • Arsenic • Copper • • • Iron • • I • Metals Lead • • • Mercury • • • • • • Thallium • Zinc • • E. coli • • • Microbial Fecal Coliform • Dioxin _i • Hexachlorobenzene • Other Organics PAHs • PCBs • Tetrachloroethylene • Aldrin • Chlordane • I Pesticides Cyanide I • I DDT/DDE • I Diedrin • • pH • Physical Temperature • • J • I • I I • • 1 Year round nonspawning River Mile(RM)0 to 11.0. 2 Spawning RM 0 to 13.9 Sources: Water Quality Assessment Database (DEQ 2012);Tualatin and Willamette TMDL(DEQ 2001; DEQ 2006). 10 Section 1.3 Lake Oswego Stormwater Management Manual 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 J 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 shall be managed at its source by minimizing effective impervious surface area and by using vegetation, amended soils, and infiltration practices to minimize the amount of 1 atil � stormwater runoff that shall be managed off site. Effective source control measures that prevent pollutants from coming into contact with stormwater are also important for protecting receiving waters. ' Ah Once stormwater runoff leaves a site, it is much more costly and less . • 1' ,, effective to transport, treat, and control. . LID: uses site design practices to reduce the amount of stormwater f to be managed off site. These practices include reducing impervious Car wash kits are used to prevent surface area and preserving vegetation and soil characteristics contaminated water that results that promote infiltration and biological treatment processes, among from washing our cars from entering others. LID can also utilize site-scale BMPs that are integrated into a storm drain 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 Lake Oswego Stormwater Management Manual Section 1.4 11 Introduction Examples of LID techniques Green roof •s 7144 Alk k Q] 1:T ti • Pervious pavement Cistern to collect and Linear rain gardens Riparian buffer i . 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 facilities shall 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, vaults, or other the best way to manage runoff and facilities that temporarily store stormwater and then release it at rates meet City requirements. to match predevelopment flow rates and/or durations. Flow control BMPs attempt to reduce the flooding and hydromodification impacts of development. In this manual, Structural Stormwater BMPs are referred to as both facilities and BMPs. 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). Non-structural BMPs also include maintenance practices (Chapter 7) and source control measures described in Appendix J. Additional non-structural BMPs include education and outreach, illicit discharge detection and elimination, public involvement, and adaptive management. The City's website provides more information on these ongoing stormwater programs. 12 Section 1.4 Lake Oswego Stormwater Management Manual 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 I 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. v '40...1 ,-,, • ` 4 _1• . . - A,r• q ti - .off 7���� •--' tee.. . r ilt — F� agikk S \ w y� m1 rY. y1I :J." ti fir ' . ..' t _-,t' _ 1-11 -- "' 4 F 7'�: � .'4.eY lz ; • . ! ,iis , .a y,;:.-:..t •,,—" • ; - Since land uses in the Cityare mostlyresidential, retrofit-type BMPs that �,. are appropriate at the small site scale are and will be a priority. a, .-;.'- : r. *R' V ' The City of Lake Oswego is addressing stormwater runoff at different scales of land use, including integrating stormwater management practices into those different scales to the maximum extent practicable. At the regional scale,this Commercial development in Lake means working within the City and neighboring jurisdictions to preserve open Oswego tends to include landscaping space and critical ecological features, encourage development in areas under and community amenities. Well-selected redevelopment pressure, and using land efficiently.At the neighborhood scale, and designed stormwater BMPs can this means applying green street principles where practicable, minimizing enhance the landscaping of a commercial impervious surfaces, and making stormwater features public and private site in addition to providing an important amenities. function. Lake Oswego Stormwater Management Manual Section 1.5 13 Introduction 1.6 Updates This manual was developed between 2013 and 2016 and updated in 2019/2020. 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. • BMP sizing and analysis tools to streamline sizing of BMPs. Current BMP selection and sizing guidance is described in Chapters 3 and 4 and Appendix D. As the City addresses its hydromodification requirements in the MS4 permit, new sizing tools will likely be developed. • 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 shall 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 shall they be adopted. 14 Section 1.6 Lake Oswego Stormwater Management Manual 2 Project Planning ,J Permits , and Stormwater Mang1ement Reqjirements 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 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. Figure 2.1 City Project Planning, Design, and Permitting Processes. Project Begiirriig: Candud Site gent& Feasbiity Analysis iii Planing: Deli: Construction: Post- ( MOM Assms al remit and Begs project Consirud Const udion moons and design regiments design Pmject Maintenance._.. 0000O 11 wih Engiieering Sluff Engineering Engileering Find Review. Mgr S1otf Flaming SWF Inspeckci Inapecik i PROJECT TIMELINE __ =_ •Preiimiray arrir� Rep ot port • Final drainage Import •I eorded W • Aroruat •Preimiray 3i�P i • Cuesi uetio.Ptau Plan milli as.-6ui specfrom and •Impervious Palm Reduction Farm • 4 A.Y Plan *males �pjr.l.•..arp •3ernnraf<•r recuds t10 yrs1 •UEU LAC approval ILand-Use Application Land Use Approval Other Agency Permits Building Permit Erosion Cenhol Permit Sheet Opening Permit =Project Stage When AciPerty Ccarocnty Orris Lake Oswego Stormwater Management Manual Section 2.1 15 Project Planning, Permits, and Stormwater Requirements Figure 2.2 Project Planning Steps. Step I Review inifibraffri plied requirerneres table acid description (Chapter 2)t� dekrrhre whether the project is smaR er lama,and which regraernisls HWY- Step 2 fiSewiew LOP/Inratf Development epproeCren and ao npide sde diSSESS- ment andora for be sir(Chapter 3)_ step 3 Cao it wiitr Cif stilt- Planr aig and Engineering (Semen 2 9) Step 4 Plan the primed to reduce runoff end reduce pollutands carried by runoff(LID PilePriiciplos 12 and 13 hr Chaff 3)._ Step 5 Select Deeds.wider gu .and flow ccrrtd Ellffis the are appl:aule Irwfhe sil>r (Chwler Step 6 Design sdrrn■aler ronnar it fardfrties(Chapter 4)and cornptele sr.hndtats rieccreasury(ApperidocA) 16 Section 2.1 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements Plarrrirg a Prllct? step 7 Mae MEC you have al perrnin. have tralaled your erosion ccarldals,and called run your rest aosiotr odd iispecfen BEFORE siardrg construdien or olearixg the sib See ampler 6 for conslrucfen B 1Ps_ See C ler 2 for an overview of pent and Appendix A for subnitthk Step 8 mud your pnct If st enw r tackles are prafessinatly designed.design nand lisped and cerliy their canstrrxhai(Seam 34_14 Subrrg shaved and sired ceriiirafon bo the City poor to the foal erosion control trispedicn. Step Subrri embodier'ID DEQfor wry UlCs coristuded arsile.&na a copy of the UIC's DEQ regialudim to the City prim-to tie frail erosini control Step 10 Sebrrl a copy ofthe C)MA Plan to the Ct4F for re ierrI and approval_ Record OW plan n■dh the County and mama a copy of the J:.1 OW plan b the City price to the frail erosion aoirtrol iesp on_ Step 11 Request intpecbm 1py City slak =wide as-t lit plans.and pay rnairrteruixe and landscape f■umicid searifos as appropiala (Chamber Step 12 Illtdrrtatrr storm rater raoliies 'Malabo-71 Employ Pd115:111 prrve nti ri thdrirea In prolong the fire of sthrrrwr er far$e� (Chapher6 Lake Oswego Stormwater Management Manual Section 2.1 17 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 • Re-routing onsite drainage • Creation or replacement of impervious surface areas • Repair of large areas of impervious surface (excluding roofs), 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 shall 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 many people who work or live in Lake Oswego. ; Regardless of classification or development type, every project shall • ;: `M prevent illicit discharge and avoid creating or exacerbating drainage ` problems. 2.3. 1 Prevent Illicit Discharges All residents, property owners, and business owners in Lake Oswego shall prevent illicit discharges in accordance with the Stormwater Utility provision of Lake Oswego Code (LOC) 38.25.150 and shall Soap leaving a driveway and implement source control BMPs (Appendix J) to avoid such discharges. flowing toward storm drain that Illicit discharges include pollutants resulting from a spill or deliberate will ultimately reach Oswego Lake. 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 contained in a TMDL approved by the US Environmental Protection Agency (US EPA), or c. Cause or contribute to a violation of a city, state, or federal law or regulation, or d. Cause or contribute to the endangerment of public health, safety, or welfare; the environment; or public or private property. 18 Section 2.3 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements ;-- w ,H r + _° Commercial car washing causing an An illicit discharge from construction Oil and grease illicit discharge illicit discharge 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 shall 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 adverse drainage shall be prioritized during design. For the uphill property owner, "properly discharging the water" means Oregon drainage law,which keeping the location of the discharge the same, avoiding substantial originates from common law or increases in the acceleration and concentration of stormwater, and case law, has developed without preserving areas where water infiltrates, ponds, and/or evaporates legislative action, and it is embodied in court decisions.Therefore,there rather than flows onto adjacent property. are no Oregon Revised Statues to cite pertaining to Oregon drainage The City's updated stormwater code (LOC 38.25) contains additional law. For more information on this information on rights and responsibilities related to stormwater topic, consult the current version of management. Enforcement and penalties for violations of stormwater the ODOT Hydraulics Manual. requirements are summarized in LOC 38.25.180. 2.4 Project Classifications and Development Types For the purposes of determining stormwater management requirements, Lake Oswego categorizes projects based on impervious surface area created or replaced (small and large) and the type of development. Applicants are encouraged to implement site planning principles to minimize land disturbance and to reduce impervious surface area. To determine the project classification, applicants shall complete the impervious surface area reduction table in Appendix A. This manual provides a brief overview of the development process only. It is not intended to provide an in-depth discussion of planning requirements. Additional permit and review requirements depend on the development type. For information on permit and review requirements, see Section 2.10. Lake Oswego Stormwater Management Manual Section 2.4 19 Project Planning, Permits, and Stormwater Requirements 2.4.1 Small Projects Small projects are defined as development projects that create greater than or equal to 1,000 square feet and less than 3,000 square feet of impervious area. 2.4.2 Large Projects Large projects are defined as development projects that create and/or replace greater than or equal to 3,000 square feet of impervious surface area. 2.4.3 Equivalent Storm water Management 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 on the site may be managed to achieve an equivalent pollutant reduction. 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 Astormwater management into the landscape in a way that will assessments applicants andpe er i the site feasibility preserve onsite drainage, soils, and native vegetation.Another analysis,they shall keep in mind important goal of the site assessment is to identify suitable these LID principles: locations for stormwater facilities before design begins on a project. The applicant will perform an infiltration test and document soils, 1. Understand the site seasonally high groundwater elevations, existing topography, 2. Reduce runoff through current hydrologic conditions and natural features, onsite design vegetation, land use and zoning, and existing utilities. Completing 3. Reduce pollutants carried by the site assessment and feasibility analysis involves printing out a runoff site map from LOMap (an interactive map website with geographic 4. Capture and treat runoff information system (GIS) data for these features), conducting a site visit, performing technical assessments (i.e., infiltration testing), and These principles are discussed in gathering additional information as available. greater detail in Chapter 3. Section 3.1 and the drainage report template in Appendix A have more guidance on performing the site assessment and feasibility analysis. 20 Section 2.5 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements 2.5.2 Onsite Storm water Management The goal of onsite stormwater management is to capture, treat, and infiltrate stormwater to mimic natural hydrologic conditions. The City Feasibility of infiltration, or lack of such feasibility, shall requires onsite stormwater management using infiltration facilities be determined by pre-existing (see See Table 3.1 for approved facilities). This requirement complies site conditions such as soil with the MS4 permit requirement to "incorporate site-specific permeability,topography, depth management practices that target natural surface or predevelopment to groundwater, or landslide hydrologic functions as much as practicable. The site-specific susceptibility.A lack of available surface area on the parcel management practices shall optimize onsite retention based on the following the placement of site conditions." the desired structure(s)is not grounds alone for determining See Chapter 4 for design criteria and options to meet onsite that infiltration-based stormwater stormwater management requirements. management is not feasible. • 2.5.3 Design for Water Quality The City's MS4 permit requires that City- and development-related activities "capture and treat" 80 percent of the average annual runoff. LID and onsite stormwater management practices shall be given high priority. Water Quality Design Standard Capture and treat 80 percent of the average annual runoff(1.0 inch in 24 hours) 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. • Infiltration rain garden • Infiltration planter • Sheet flow dispersion Where sites are not suitable for infiltration, water quality treatment facilities that could be implemented include: • Flow-through rain garden • Constructed wetland • Alter strip • Flow-through planter • Wet pond • Proprietary facilities • Swale • Sand filter See Chapter 4 for stormwater facility selection guidance, and design criteria and guidelines. Lake Oswego Stormwater Management Manual Section 2.5 21 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 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 runoff shall be maintained at their predevelopment levels for the 2-year, 5-year, and 10-year, 24-hour runoff events. Predeveloped areas shall be modeled with a CN of 70. Downstream Analysis Demonstrate adequate conveyance / / capacity in accordance with V V Section 5.8. 22 Section 2.5 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater 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 System Designed by Licensed Professional ✓ ✓ See Section 2.5.5 for specific requirements. Underground Injection Control (UIC) Facilities Review with DEQ UlCs shall either be rule authorized or registered with DEQ.A copy of the DEQ approval shall be V V submitted to the City. Drywells shall conform to Oregon Plumbing Code and City requirements. 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. Lake Oswego Stormwater Management Manual Section 2.5 23 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 or more. Other City Permits A street opening permit is required for driveway approaches, work in right-of- ✓ ✓ way, and work in public easements.A traffic control plan is required for lane closures. Submittals Drainage Report See Section 2.7.1 and Appendix A [LOC 38.25.120]for Drainage Report Requirements. If the project requires V V a downstream analysis, it shall be completed by a registered Civil PE with hydrological experience. Recorded Operations and Maintenance Plan Stormwater facilities shall have a recorded maintenance agreement ✓ ✓ for 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. 24 Section 2.5 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements 2.5.4 Design for Flow Control Flow control is required for large projects. Projects that are able to meet onsite stormwater management requirements (infiltration of the 10-year, 24-hour storm event)will meet flow control requirements without additional measures. Flow control is not required for projects that discharge directly to the Willamette River, the Tualatin River, or Oswego Lake, provided the following conditions are met: • The project site is drained by a conveyance system that is comprised entirely of man-made conveyance elements (e.g., pipes, culverts, outfall protection, etc.) and extends to the ordinary high- water line of the exempt water body. • The conveyance system between the project site and the exempt receiving water shall have sufficient hydraulic capacity to convey discharge from the proposed development of the site, and the existing development condition from the remaining drainage area contributing to the conveyance system, based on the conveyance standards outlined in Chapter 5. • Any erodible elements of the man-made conveyance system must be adequately stabilized to prevent erosion under the conditions noted above. Flow Control Desian Standard Maintain peak flow rates at their predevelopment levels for the 2-year, 5-year, and 10-year, 24-hour runoff events. Section 4.1 has information on design storms, and Chapter 5 describes approved methods for calculations. Predeveloped areas shall be modeled with CN = 70. 2.5.5 System Designed by Licensed Professional Because Lake Oswego is largely built out, only a limited amount of land is available for surface stormwater facilities such as constructed wetlands and ponds. Since such facilities require dedicated land area 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 most stormwater facilities to be designed by a licensed professional engineer or landscape architect that understand how to incorporate the facility into the community context, improve aesthetics, and ensure water quality and flow control function. The following facilities, do not require design by a registered engineer or landscape architect when designed using prescriptive sizing factors (see Chapter 4): • Rain garden Certification of Stormwater Design • Planter Lake Oswego code requires that designers certify stormwater • Pervious pavement facilities after construction. See Section 3.4.6 for more information. • Lake Oswego Stormwater Management Manual Section 2.5 25 Project Planning, Permits, and Stormwater Requirements 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 UIC, refer to OAR 340-44 and likely UlCs that shall be registered with DEQ: 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 shall complete the or gallery UIC General, Industrial and Commercial Stormwater UIC facilities shall be registered with DEQ (See DEQ UIC webpage). Registration Application available aww. A copyof the DEQ registration shall be submitted to the Cityprior to d the DEQ website: c/dochttp:// / g deq.state.or.us/wq/uic/dots/ issuance of the Certificate of Occupancy. forms/GenlndComSW.pdf. 2.6 Overview of Minimum Requirements: Permits F 441. 2.6.1 City of Lake Oswego Erosion Control Permit .; A All projects that disturb soil shall 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 is 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 any pond, lake, river, stream . °' corridor, canal, or wetland except as noted in the City code. For more information, consult the following sources: 74106 - • LOC Chapter 52 for specific erosion and sediment submittal requirements, and required content for an erosion control plan. Stormwater art in the landscape Erosion Prevention and Sediment Control Planning and Design helps express local character. • Manual(Clackamas County et al. 2008) for planning and design information for specific erosion and sediment control BMPs. 26 Section 2.6 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements 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/stornnwater/constappl.htm 2.6.3 Street Opening Permit Street opening permits are needed during development or redevelopment of driveway approaches, work in the public right-of-way, work in public easements, connection to a public utility, 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 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 shall always verify project status initially with the Planning Department. Lake Oswego Stormwater Management Manual Section 2.6 27 Project Planning, Permits, and Stormwater Requirements Project Review All projects shall include 2 sets of the construction documents that have an engineering scale and dimensions and include: • Site address, building permit number, and LU case number (if applicable) in the title block • Property lines and lot dimensions with the location and size of all trees on the property • Right-of-way, easements, 10-ft contours, stormwater flow direction, and public utilities within 100 ft of the project, • Existing and proposed improvements and AASHTO sight distance requirements (engineering scale and dimensions) with distances to property lines indicated • A separate sheet with the dimensioned and scaled stormwater system (plan view and profile view). Include the existing stormwater system if it will be retained and indicate the overflow route for projects using onsite stormwater management. • A separate sheet with a composite utility plan showing the wastewater, water, and stormwater systems. • A separate sheet with the landscaping sheet, plant list, and all retained trees. 2.7 Overview of Minimum Requirements: Submittals 2.7.1 Drainage Report A drainage report shall be submitted for all small and large projects. The drainage report shall document which minimum project requirements from Table 2.1 apply and shall demonstrate that the project will meet applicable requirements. Calculations and drawings shall be submitted as appendices to the report. Appendix A contains a list of information that shall be considered and included in preparing the drainage report. A preliminary drainage report to demonstrate feasibility with all minimum requirements shall be submitted with the land-use application. 2.7.2 Operations and Maintenance Plan and Deed Restriction An operations and maintenance (O&M) plan is required for all stormwater facilities, with the level of detail and requirements scaled to the facility type. The O&M Plan shall include a dimensioned site plan (engineering scale) that clearly depicts all stormwater facility components, detail drawings of the stormwater facilities, maintainance requirements (including maintenance frequency), and the party responsible for maintaining the facility. Chapter 7 and Appendix K provide resources for the O&M Plan and an example is shown in Appendix A.5. The O&M Plan must be recorded in the county in which the facility is located. This step needs to be completed before the City will issue a Certificate of Occupancy. 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. 28 Section 2.7 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements 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 shall 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 this manual 2. Minor repair or maintenance activities that are not considered to be replacement of impervious surfaces under the MS4 permit, are part of regular maintenance, and do not result in additional hydrologic impacts. Major structural repairs, facility restorations, and dredging must be reviewed and approved by the City. 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 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 Waiver/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 Lake Oswego Stormwater Management Manual Section 2.7 29 Project Planning, Permits, and Stormwater Requirements "equivalent pollutant reduction measures" for sites that cannot meet their stormwater management requirements on site. Options identified in the MS4 permit include a fee-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 fee-in-lieu or offsite mitigation option. 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 shall 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 • Providing information on the locations of existing City-owned infrastructure, stormwater discharge locations, and driveways • Questions regarding capacity of existing storm drains or drainage ditches • Clarifications on technical specifications and performace standards related to stormwater facilities or conveyance facilities • Stormwater submittals (e.g., Drainage Report, Operations and Maintenance Plan) • Slope stability • Questions regarding impervious area reduction credits 30 Section 2.8 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements 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 is the main point of contact for the following: • Questions regarding tree protection and removal, including the best locations and drainage conditions to protect trees and meet setback requirements • Zoning and associated development limitations • Setbacks (including special street setbacks) • Overlay districts (neighborhood, sensitive lands) • Qustions regarding stormwater for individual building permits. The designer needs to verify that stormwater facilities are sited based on soils, topography, and other information from the site assessment and feasibility analysis. 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 2014). 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 Department The Public Works Department maintains public storm drains (inlets and catchbasins), ditches, storm drainage structures (e.g., catch basins and inlets), and public stormwater facilities such as ponds, planters, and raingardens. 2.10 Review Process for Different Types of Projects When the applicant visits the City Planning counter, the Planning Department will review the proposed project to determine land use and development review requirements. Land use and development review is not required for ministerial developments (less than 1,000 square feet of new impervious surface and less than 10 feet in height), unless one of the permits noted in Section 2.6.4 is required. 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. Lake Oswego Stormwater Management Manual Section 2.9 31 Project Planning, Permits, and Stormwater Requirements r Table 2.2 Lake Oswego Review Process: Application and Design Phase DESIGN PHASE ApplicantII City Phase 1: Preapplication Conference Schedule p eopplieffion oorrlererrcewtith Planning Planning Department wall coordinate with atie- Deparbnert_ department represent vas as appropriate. Ciorne prepared to discuss the site as smart snot Where stc rnwaler mariagernert recpiremens apply feasbity analysis(Section 25.1 and lWha r 3) (see Tubas 21).a representative*Ian tie and other nirwnwn recpirerrreris(see Table 21)_ Engineering Deparbmerrt wi= • Review stc rnwaler management approach • Explain storrnwaler standards and discuss stnrmwaler management dons • Discuss site dimign options that can mirirnize slornarmter management regiE nerrls and prod native sods • Discuss cormeoiii7ris to City adios • Discuss right-cl-way and sidewa&ertensFon requirements and permit • Disci of raims eri I makienar a reqki-err►enI •lfisit the sib if appropriate ApplicantCity Phase 2: Project Application Submission The Appicarit is nesponstle for subrrittir}g the The Planning Depaitmentwill assign a laird use following cesefile number arid prepare a staff report for • Prefrninary drainage report(see Sedition 2_7 and rrirrQ artd major deveIepmen1. Appentix A) ■ Preiriinary site plan shoving slormir rter fe�dities The Engineering Departrruerrt Sri review oiler applioetions. Applicant I City Phase 3: Land Use Decision If appication is Approved: Prmeed with fmal design The Planning Coordinator vnl nobly the Appicant of project ❑msutt S1irmivaler Management Manual of whether the upplicaion is approve, deried.or for submittal and design requirements related to approved tiifi miditioris Inc rrirror or major shrrnersder. dial!pmerds Engiraeemg Departrnerrt staff wit ff is A rived v*ith crdilicns= Review nobly Applicant cri deltas has of other project type s. pad pP conditions re approval_Contact Engineering Deparbnerit if any of the conditions of approval relaled to stomrwater are unclear. If project is Denied: Consul)with Planning arndfor Engineer Coordinator'or options. 32 Section 2.10 Lake Oswego Stormwater Management Manual Project Planning, Permits, and Stormwater Requirements Table 2.3 Lake Oswego Review Process — Construction Phase CONSTRUCTION PHASE ApplicantII City Phase 1: Construelion Pled Review Appicarrt submilsannsirucbvn plans and Engineering Development Coordinator will: drainage repertlo Engineering. • Review plans and applicable .: standards . Submit two plan sels(legible and drawn to scale) • Issue construction permit for public infrastruchre That iidudeal plan elements • Sdiedule pre-construction meeting with ▪ Submitdrafl Operations rind Maintenance Plan oontraotors and owner (OMP)(see Section 2.7.2 and Appendix A) ■ Evidence of UIG .f bn (if applicable} Appbcarri City Phase Appicant pays 120%construction bond Engineering will: (en eers pubic improvement oast estimate- EP E) • Ensure facility constructed per plans and sign otf on completion •Submit 9%project plan review fee •Bond rBquired to be posted b ekwe sign off by My • Lamle duration of consiniction phase •Oolfect 12D%construdinn band •Covers: I : A_r resporrsibirties • Issuedfreleased by: developer, includes letter of partial acceptance by Engineering Development Coordinator in developerfappiearts insurance cvmpenyr fr release of bond Applicant City Phase 3: Post-Consirmlion Applicant submits aa-buift drawiigs of far ihtyr. Engineering Development Coordinator will: Applicant pays 1D%Iwo-ysar mat name bond. • Review as-bust drawings of_,... = facilities •Length:tsao years from issuance of Letter of Partial • Engineering and Building Inspectors wi Acceptance coon:k ale final walk-trough and punch list for • Comers:public improvements including siorrrrwater ccnnpliariae with condii3r5 of approval faciilies • Issue Letter of Partial „ ▪ Issuedfreleas€d by: Errgiieering Department • Release 12D%construction bond Applicant pays 5%per year and submits ietfer from •Collect 10%maintenance bond and 5%landscape landscaper io Planning Department(general bond, as appicable landscaping) Engineering Department (siormmrater faciliy L• L��-ring}. Engineering Staff wil update project file with bond dates and as-buil numbers_ • Length:two years from issuance of Partial Letter of Acceptance Engineering Staff wil issue final (10D%) Letter of •Covers:vegetation estabishnrent, compost A.-- -- mitering and functionality of the facility. • Issuedfreleased by: Planning or Engineering Department,as applicable Lake Oswego Stormwater Management Manual Section 2.10 33 Project Planning, Permits, and Stormwater Requirements Table 2.4 Lake Oswego Review Process — Operations and Maintenance Phase OPERATIONS AND MAINTENANCE PHASE Applimnt II City Phase 1: Operabons and Maintenance Agreement1Phar Appicant submis trial OMP to{ _ Anal appeared Maier facility OMP shall be recra ded with Co FrLy Clerk by applicant before the certificate of lEICCUpancy is issued. Applicant City Phase One Year Idler Issuance of Leifer of Partial Accepharce Owner submits fist-der irspecion and Engineering Inspector will: rnai-lenience rewords in accordance wilh OMP. •Perform me-year infrastruolime iispecion and Owner has 3D days ID need and prepare a needs and correcionslist ▪ r:. notice(sy Engineering Development Co rdinator will: • Issue facility needs and •:11".ris notice Applicant City Phase 3: Second Year After Issuance of Leifer of-Partial Accepharce Owner subrriils second-year lisped:ion and Engineering Development Coordinator will: maiilenance records r accordance wilh OMP. • Issue featly needs grid corrections notice Owner has 3D days to respondtrediiy need and ■Approve corrections ▪ r:. nolice(sy • Release bond Applicant City Phase 4: Ongoirg Diner submits iispection and maintenance records iti accordance wilh OMP. 34 Section 2.10 Lake Oswego Stormwater Management Manual °Qe ° me -t LeaIm - A _I ' a 0 T-Ph- _ e -2 a D E e ecC 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 shall 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 shall 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 shall conduct the Step 1: Review information needed for all steps of analysis site assessment and feasibility analysis prior to consulting with City Step 2: Create site maps using LOMaps Building, Engineering, 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 35 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. Where possible, maps should extend 100 feet beyond the project limits to show adjacent property areas. The site map shall include the following elements, where applicable: • 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), including address and/or cross streets • Stormwater infrastructure management subbasin (available on LOMap's Surface Water Layer; see below) • Total impervious area (TIA) of project site • Existing and proposed improvements with dimensions and distance from property lines • Existing and 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. LOMap 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 36 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, 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. If sandy or gravelly soils are 3.1.2 Site Assessment Components observed, this may be a good site for a drywell or infiltration trench. Sandy Soils or gravelly soils often have infiltration rates that are too high to provide Opportunities for infiltration are constrained by soil conditions. As effective treatment, so a water noted in the Clean Streams Plan (Otak 2009), nearly half of the soils quality treatment BMP is typically in Lake Oswego are classified as belonging to Natural Resources required prior to the infiltration BMP. Conservation Service (NRCS) Hydrologic Soil Group C. Another 30 Note that registration of a BMP with percent of the soils are classified as GroupD, and the remainder of DEQ may be required if the facility is considered a ulc. 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 slower 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 37 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 may not be 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. Test infiltration rates (Appendix B) and soil quality to assess suitability for infiltration and to pick the best location for LID BMPs. Infiltration testing is required 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 3.4.3 of this manual. Figure 3.1 Lake Oswego Soils Map _.....-L .,,. ..1::7 Kr-L, -r\,_ „i4L.5 Q o 0, Soil Type 11 TICORI Pt4.17U Fil r m A t 4 p 1 -'1 —7 j"4 1. 41:-..‘ ,,0 1 C. T1H �FK° e0 r 1 fLi ji . IfiaN •-, ....* ---IITP;41 li _ii.. . . A. •lUaleUn 1. e Y�. 11- i ---4144111111 4 In. I \ Rtrergmver um „r„.� City Of Lake Oswego !I - Soil Hydrologic Groups. / 0 025 as Ors 1 1 IE .� �'�'®1 , 38 Section 3.1 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection See Appendix C for more information on how to read characteristics of site soils. City may require consultation with a geotechnical engineer. Topography Identify and map breaklines or changes in slope to help characterize flat, sloping, and steeply sloping topography. To reduce construction and post-construction impacts, work shall be conducted within the Are there areas of protected native existing topography to the maximum extent practicable. soils and/or amended soils that have been added for stormwater The design shall be developed in a way that works with existing management function?These soils grades. might be evident by the presence Plan the project to minimize land disturbance such as clearing of healthy native vegetation. • They would also be loose and and grading and cut and fill. uncompacted and infiltrating stormwater. • Keep cut and fill slopes as flat as practicable to help preserve soil stability. Hydrology Several facilities are not suitable near or on steep slopes. Confirm Where is the water draining? This will be easier to confirm on a rainy safe drainage route to prevent day, but even in dry conditions, general drainage patterns shall be concentrated stormwater from evident. draining down steep slopes. A geotechnical engineering analysis Note the following: and report may be required. • Locations of catch basins, inlets, ditches, and other stormwater facilities • Depressions where water can pond and/or infiltrate during If ponding water is observed on storms areas of clay soils(or saturated soils during otherwise dry periods), • Offsite areas that slope toward the property and may confirm that the area is not a contribute runoff to the property jurisdictional wetland (consult with Planning Department). If • Areas of the site that drain to neighboring properties. not, and if a stormwater facility is required, consider this area for a constructed wetland or wet pond. If Depth to Groundwater an onsite stormwater management or water quality treatment BMP is Information on the approximate depth to groundwater can be found on not needed, consider incorporating the NRCS website and from review of well logs held by Oregon Water this wet area into landscaping Resources Department database. plans through use of wetland plantings. Depth to groundwater varies seasonally; assume seasonal high groundwater conditions for design purposes. Seasonal high groundwater for the purpose of calculating separation distance to stormwater facilities does not include perched groundwater. Lake Oswego Stormwater Management Manual Section 3.1 39 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection Vegetation Confirm setbacks from sensitive Note location and size (diameter at breast height) of all trees on the areas. Select a vegetated facility property (City 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 location and impervious surface areas of existing structures, developed and site history including parking areas, buildings, sheds, etc. Also document is unknown, test soils for any existing utilities, easements, or other site features that could contamination. influence feasibility. 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. Such information may help inform stormwater facility selection. 3.2 LID Principle #2: Reduce Runoff Through Design LID can reduce the adverse ecological impacts associated with development. When there is less impervious area due to careful site In addition to minimizing impervious planning and design, and native soils are protected or amended to surface area and reducing runoff preserve or promote infiltration, less stormwater enters receiving waters through design, careful site design as surface runoff. Consequently, hydrologic impacts to channels and techniques that work with natural other downstream receiving waters, such as increased peak flows or topography, sas, and places that reducevegeation help create flow durations, are reduced. impacts on the environment, are public amenities, increase habitat Reducing runoff through design means designing a project that is for native species, and beautify appropriate for and tailored to a particular site. It requires knowledge Lake Oswego. 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 • 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. 40 Section 3.2 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 3.3 LID Principle #3: Reduce Pollutants Carried By Runoff The goal of principle #3 is to protect healthy native soils and provide infiltration where possible. Contaminated soils impair habitat and water quality when transported downstream (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 shall be protected. The applicant shall flag these areas so construction vehicles do not compact soils. The applicant shall consider the following when preserving soil depth The Lake Oswego Public Works and quality: Stormwater Program webpage provides resources and links to stormwater information relevant to • Do not disturb native vegetation and soil, and protect these the City. areas from compaction during construction. Visit: • Amend existing site topsoil or subsoil based on site soil http://www.ci.oswego.or.us/ testing results. publicworks/stormwater-program • Stockpile existing duff and topsoil during grading and earthwork, and replace it prior to planting. Stockpiled topsoil shall 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. • 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. 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. 3.4 LID Principle #4 — Capture and Treat Runoff Lake Oswego is required to prioritize LID stormwater practices and reduce pollutant loading associated 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 shall 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 runoff from the proposed development, including overflows from BMPs. This information shall 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 Lake Oswego Stormwater Management Manual Section 3.3 41 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 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.) Table 3.1 Stormwater Management Best Practices Minimum Project Applications Site Requirements Requirements 4- _ a) r E ai >+ t c o .*x co > CD , O v * a) a) a) �, - _ _ a) o N a oc LL C m a) N a,Q> y . m aFw o a> >. ; .c r}, L 5 a in > o ++ • a) y 3 °: N L L 5 L > z+ v > a; a to tll i = 0 cC in O 0 0 O d ✓ Q 0 C u a) d Oct d 0 a s r tv ,8 y i d i i > +- to = r p co Z c O O O O 4- E .2 Q i c -a c — E 3 EO _ = a1 = _ •C 0- 0- 0 0. E, a = = i4 t) C1 0 a O al a5 2 2 3 2 0 E g E 2 ce ce d Q a R N N >y o. o. 0 o. d •= OL •= = ru Facility Type co 0 0 0 a Q Qom° Q e: 2 C� E E in u) Rain garden, infiltration 4.6.1 . . . . . • 3 0.25 >_5 >_10 Rain garden, flow through 4.6.1 . . . . . • NA** — >_5 NA Planter, infiltration 4.6.2 • • • • • • 3 0.25 >_5 >_10 Planter, flow through 4.6.2 . . . . • . NA** — >_5 NA Infiltration trench or gallery (proprietary infiltration 4.6.3 • • • 5 0.25 >_5 >_10 chamber) Drywell 4.6.4 • • • • 5 2 .5 >_10 Green roof 4.6.5 • • • NA NA NA NA Pervious pavement 4.6.6 • . . • 3 0.25 >_5 >_10 Rainwater harvesting 4.6.7 • • NA NA NA NA Filter strip 4.6.8 • • • • NA NA >_5 >_10 Swale 4.6.9 • • • • NA NA >_5 >_10 Sand filter 4.6.10 • • • NA** NA >_5 NA Constructed wetland 4.6.11 • • • • • NA NA >_5 >_10 Wet pond (retention pond) 4.6.12 • • • • • NA** NA >_5 >_10 Infiltration pond 4.6.12 • • • • • 3 0.25 >_5 >_10 Detention pond 4.6.12 • • . NA** NA >_5 ?10 Detention pipes and vaults 4.6.13 • . NA NA NA NA Sheet flow dispersion 4.6.14 • • • • 3 0.25 >5 >10 Proprietary treatment BMP 4.6.15 • . • NA NA NA NA Notes NA= not applicable *All listed facilities are allowed on private property **Facilities located in high groundwater may need waterproof liners to provide groundwater separation. ***Setback from structures is measured from edge of excavation for facility 42 Section 3.2 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 3.4.1 Stormwater Facility 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 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 unsuitable for infiltration. Where rain gardens and planters are not feasible due to site constraints, Registration forms for UICs can be infiltration trenches, drywells, or proprietary infiltration chambers may be found on DEQ's website at: used. Since these technologies are considered UlCs, they may need to 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 for Sheet Flow Dispersion design).. 3.4.2 Stormwater Facility Selection for Large Projects Large projects shall meet requirements for water quality and flow control, in addition to onsite stormwater management requirements, where applicable. Best management practices to minimize impervious surfaces shall be considered first. These include: • Installation of green roof • Installation of pervious pavement • Rainwater harvesting • Other as approved by the City engineer Stormwater from remaining impervious surfaces shall be managed with facilities 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 shall 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 shall be sized to infiltrate the water quality design storm (1 inch in 24 hours.) Lake Oswego Stormwater Management Manual Section 3.4 43 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 3.4.3 Infiltration Facility Considerations Infiltration facilities shall meet a minimum infiltration rate standard and have additional requirements for facility setbacks. In addition, if the proposed facility is a UIC, it shall 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) or other industry-standard infiltration test method. Guidance for conducting a PIT test is included in Appendix B.A correction factor of 2.0 shall 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, as measured from the outside top edge of the stormwater facility: • 10 feet between the top edge of the facility and the edge of the building foundation • 100 feet from contaminated sites • 500 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 • 5 feet from right-of-way • 7 feet from new utility trenches and 5 feet from other existing utilities • A geotechnical report is required to determine setbacks from slopes for infiltration facilities within 200 feet of a steep slope hazard area or landslide hazard area (LOC 50.06.006(2)) or at the request of the City Engineer. Figure 3.2 Setback Diagram ®ICI®I 5' 10' Infiltration = % ' �, _ _ Zone n � 0 CIF do •r �,k,, WyysY 44 Section 3.4 Lake Oswego Stormwater Management Manual Site Assessment, Feasibility Analysis, and Stormwater Facility Selection Underground Injection Control Devices If an applicant proposes a UIC, the applicant shall 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 residential roofs only is automatically rule authorized. Additionally, if the UIC owner/operator cannot obtain DEQ approval, the UIC system shall 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. Contaminated Soils Some 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 If an infiltration facility is proposed within identified contaminated soils and 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. 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). Lake Oswego accepts proprietary technologies that have General Use Level Designation (GULD) for basic, dissolved metals, or phosphorus treatment as water quality treatment BMPs. Facilities with Pilot Use Level Designation (PULD) or Conditional Use Level Designation (CULD) are not permitted. The City may require pretreatment facilities to improve the performance of proprietary stormwater BMPs. 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 Lake Oswego Stormwater Management Manual Section 3.4 45 Site Assessment, Feasibility Analysis, and Stormwater Facility Selection 3.4.5 Other Considerations in BMP Selection In addition to the previously discussed feasibility factors, consider the following when selecting or designing BMPs for a particular site: • Ease of maintenance —All facilities will require some form of routine inspection and maintenance. Selection of a BMP shall consider how easy or difficult the BMP will be to maintain, including frequency, accessibility, and need for specialized training or equipment. Select or design facilities to minimize the level of effort and expense associated with these factors. Pay attention 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 facilities can have high levels of community acceptance, this is due primarily to the fact they are "out-of-sight, out-of-mind," which can also lead to maintenance neglect and cause long-term performance issues. • 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. • Climate change adaptability—stormwater facilities 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. To the extent feasible, consider oversizing the facility, minimizing bypass during intense storm events, and creating adaptable planting plans to adapt to climate change. 3.4.6 Certification By Design Professional In order to ensure that facilities are constructed as designed and provide the intended stormwater function, the City requires that the design professional inspect and certify that the stormwater facilities have been constructed in accordance with the design, meets requirements outlined in the approved Drainage Report, and can be maintained in accordance with the operations and maintenance plan (LOC 38.25.130). The certification form is in Appendix A-6, Submittals. 46 Section 3.4 Lake Oswego Stormwater Management Manual 4 Stormwater FacilityDesign Guidelines This section provides an explanation of the performance standards and design criteria necessary for stormwater facilities to meet Lake Oswego's stormwater requirements. Several BMPs in this section are intended to reduce the impervious surface subject to water quality and flow control requirements, reducing the area required to be treated by other BMPs. To reduce cost and size of stormwater BMPs, consider incorporating green roofs and pervious pavement to reduce the size of impervious area associated with sizing a stormwater facility. See Appendix A-3 for tracking of the impervious area reduction BMPs. 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 Design Storm* Onsite stormwater Infiltrate the runoff volume from the management (All projects) 10-year, 24-hour storm event within 24 3.2 inches hours Water quality Capture and treat 80% of average 1.0 inch (Large projects only) annual runoff Maintain peak flow rates at their 2 year, 24 hour storm: 2.38 inches Flow control predevelopment levels for the 2-year, 5 year, 24-hour storm: 2.85 inches (Large projects only) 5-year, and 10-year, 24-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. Many onsite stormwater facilities provide water quality treatment without additional measures (see Table 3.1). Water quality BMPs shall 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. Flow control BMPs are required for large projects, unless the site drains through a man-made conveyance system directly to Oswego Lake, the Tualatin River, or the Willamette River. See Section 2.5.4 for details. The flow control requirement may be met through detention and slow release of stormwater, or through infiltration of some or all of the required design storms. Lake Oswego Stormwater Management Manual Section 4.1 47 Stormwater Facility Design Guidelines 4.2 Planting Design 4.2.1 Why Plants Matter in Stormwater Design Plants play several important roles in the function of stormwater "Picking the Right Plant facilities while also contributing to the aesthetic quality of the for the Right Place" landscape in Lake Oswego. Planting design shall respond to In order for plants to thrive and fora variable environmental conditions (e.g., soil moisture conditions, stormwater facility to achieve long- sun exposure, existing infrastructure, setbacks and sight distances, term success,planting design shall pedestrian use and interaction, existing plant communities and carefully consider the placement of invasive species control, and visual buffering) in conjunction with each plant within and around a facility. p 1 The design shall consider the mature surrounding community aesthetics and values. For infiltration height and spread of each plant and facilities, it is important to choose plants that do not require the use respond to environmental and cultural of fertilizers or pesticides to thrive. Plants shall also be adapted to site conditions. . local micro-climate conditions and require little to no extra watering 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. 4.2.2 Vegetation Diversity and Its Effect on Water Quality . `r. Diversifying vegetation in stormwater facilities can significantly improve water " . ; , t:ii y. quality treatment effectiveness by enhancing the ecological function and unit `•� , ,.f . ;A treatment processes occurring in stormwater facilities. 'y� '' ' ,,u ; ,,,,, �a ,, }' fiAi F1 vF fi t f ' , Diverse, healthy native or site-adapted vegetation improves water quality by: �, ' ti , L ,, F +P ,� F, ti +:g75�ii 6 • Reducingsediment load. The stems and leaf blades of vegetation ,..9 , �' ; `{�� g � ��� ,� � `� ti F vlA r Nr�� � ,l #� ' k/, Irk , �i. intercept stormwater and act as physical filters bycapturing sediment z,, :+� i.;�,�� 0' ''� s" ' '.'"' p p Y p g �, t 4 ,i Alt� f�; � f� � ti �� and associated pollutant particles. Emergent plant species are '_ , ,!i �r�1,.a'+ , :; ¢' t;'1;��,g 1:# Al particularly suited to reducing sediment load because of their plant structure and adaptation to water inundation and saturated soils. Plants play an important role in filtering sediment and pollutant particles from • Reducing water velocity. Vegetation reduces the velocity of stormwater. water moving through a stormwater facility, allowing more time for pollutants to settle to the bottom and minimizing turbulence that may The physical filtration otherwise induce re-suspension of sediments previously deposited in process results in removal the pond bottom. of sediment from runoff, Increasing ability of soil to absorb and filter pollutants. Root while the biological uptake • systems increase the potential for water to filter through soil, which component can result increases the adsorption, sorption, and anion exchange interactions in removal of metals, that remove pollutants in stormwater. phosphorus, nitrogen, and some hydrocarbons from • Absorbing pollutants into plant matter. Plants will absorb some the stormwater. 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. 48 Section 4.2 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines Dense, healthy vegetation will provide good filtration and biological • uptake in a stormwater facility. Indicators of degraded filtration and biological uptake include: • Low plant density: bare soil and sparse vegetation on facility ;x, -, bottom and side slopes "' r"4 ` 0" ' - ` ` • Unhealthy plant community: poor apparent health of the plant �, � Y• .�,,0:'�;,� community (e.g. signs of disease, stress, and lack of vigor) 4 . ."�' j --: `, • Monocultures: a planting comprised or dominated by a single species Himalayan blackberries dominate this plant community and are an indicator of • Dominated by invasive species: invasive species spread poor plant community health. rapidly and suppress the establishment, growth, and health of the intended plant community. 4 _• 1. ,yam 4`y' 2. , .. 'S.: L. + }.J ` '.1.' .`.max 3. .. n:ram •',. ..- ,. + ':1, ::* ; F °RAC f. ,.14'd i:"2 v max..„xiifttl .. • ..watm.... nto _.:.k:::r V, , .. .,�,�� x, . This constructed wetland has a healthy - , s 'a v Y " variety of native trees, shrubs, and '•' -- : groundcover species simultaneously •• : : V , `0 } Clean Water.Services .•_ providing water quality benefits and a • -' 'a ! .,. _ neighborhood amenity. 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 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 (see 2019 Oregon Department of Agriculture Noxious Weed List): Knotweed (e.g., Japanese, giant, etc.) Garlic mustard Purple loosestrife Butterfly bush Teasel Toadflax(e.g.yellow and dalmation) Thistles(e.g., Canadian, bull, Italian, etc.) Yellow-flag iris Hawkweed (several varieties) Scotch broom Tansy ragwort English ivy Blackberries(Himalayan and evergreen) St. Johnswort Clematis(Old man's beard) Bamboo Kudzu Lake Oswego Stormwater Management Manual Section 4.2 49 Stormwater Facility Design Guidelines 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 stormwater facility plant lists. Figure 4.1 Plant Selection Process. lamination Gathering SIBp 1 Ccrrdud aria assessment and feasibibi analysis mid ailed relerrart proiect nkurimixt _ W (F.càty Revlon nod Phad Seier:tion Sdertappropr plarts Inriadily and vile lxatirn_ 1 REVUE PLANT SEa E1 TIK r Check Aaraitty of Plant Selection with Loeal Nurseries NO Sip 3 Are the selecled plantar available. or is there enough time ID contract with a maser!ID grown or acquie the p ? REVUE l PLPMT YES SEI Ef 1T KM Review Y�rti narroe Regi&e• arch of Plant Seiedion and PL: -_ - 1 NO SIEp 4 Does the plardselerhon afgr with Lang-term nrrwiterrmioe Y 6:a� ,14 aapabil e k and resource 1YES Update Doctaner Pim a,mod Step 5 -5peciicahorrs To Gramme tnstallarion and Establishment of1 istiorr 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 shall influence design. Each stormwater facility shall 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. 50 Section 4.2 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines Information gathering shall 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? • Are there any potential sources of contaminants from surrounding areas 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 habitat and Natural Areas System Plan (Lakconnectivity or value for a fragmented natural area? information on the City's paa rksse • Are there particular species that may be attracted to the site and natural areas and how they relate to stormwater and (bird populations, amphibians, butterflies, etc.) and that may habitat management goals for benefit or be harmed by its location? Lake Oswego. • 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? • Shall 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 landscape facilities to ensure stewardship and not marginalization due to perceived "messiness"? Lake Oswego Stormwater Management Manual Section 4.2 51 Stormwater Facility Design Guidelines Step 2: Facility Design and Plant Selection After gathering enough information about the site being designed, it is time to begin plant selection. As a rule, the applicant shall select A spe Nat species that occurs naturally in a plants that will provide for year-round water quality and aesthetic particular region, ecosystem, and functions. Native plants shall always be considered first to meet habitat. design goals. If native species cannot meet the design needs of a site due to specific environmental or cultural goals, horticulturally Acceptable non-native or hortappropriate s ecies (that is, species that are well adapted to local Annual or perennial p p p Annual or perennial species that is site conditions and reliably perform well in stormwater facilities) not persistent or competitive with can be used to supplement a native plant selection. All plant lists native vegetation. and species selections shall be created based on species available and obtained from local nurseries. The justification for following this Naturalized species: Nonnative species that were protocol is explained below. introduced by humans to a region, ecosystem, and habitat, but have Vegetation coverage within the planted facilities must be shown on now become a part of natural native the site plan, with a diagram and calculation for a minimum of 100% plant communities. coverage at maturity. 100% mature coverage must be achieved within one year following construction and shall never drop below Undesirable plant species: Plant species that are on the region's 90% coverage. noxious weed list and plants that out-compete and dominate native Some non-native, horticultural species are suitable for stormwater plant communities. facilities. These species are usually chosen when a site is located within a highly developed area where an applicant has a specific design goal in mind. They are also beneficial when the mature size of native species is too large to be accommodated in a facility (e.g., narrow parking swales, flow-through planters alongside buildings, etc.) and a specific type of species is desired. For some facilities, such as green roofs, there are only a few native species easily found in nurseries, and it is useful to augment the planting palette with non- native, horticultural species. In these instances, plant lists shall be reviewed by a landscape architect or horticultural professional experienced in native natural systems. Native versus Non-Native Vegetation Planting native species shall 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). 52 Section 4.2 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines Step 3: Check Availability of Plant Selection with Local Nurseries It is important to make sure that the plants selected for a design are available at a local nursery. If a plant list contains species that are not typically available or are requested in a large quantity, a nursery can often help find the plants needed if given enough time. Some nurseries can even grow species not normally within their selection if they are given at least a year of advance notice. If the project timeline is shorter than a year between design and installation, which is often the case, the need to verify availability of plant species is even more 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 Fy , consider the maintenance needs, limitations, and consequences of design decisions. • z z • Review operations and maintenance guidance for your ° facility design (See Appendix K . � �• 5,� , • Assess available maintenance capabilities and resources , " o "n to maintain the site. Determine whether additional training , � '` 40a or information is needed to properly maintain the facility. • Evaluate the maintenance needs of various planting design alternatives This planting border has been overtaken by weeds and does not show clear signs • Will vegetation border walkways, driveways, or other of care. 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)? 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. Lake Oswego Stormwater Management Manual Section 4.2 53 Stormwater Facility Design Guidelines 4.3 Stormwater Facility Design Methods and Computations Facilities shall be sized for the full contributing impervious area. The City's current guidance for sizing and hydraulic analysis of stormwater facilities 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). ri4.3.1 SBUH Method Table 4.2 City of Lake Oswego Precipitation The SBUH method is appropriate for sizing Design Storm/ 24-Hour Rainfall most BMPs included in this manual. There are proprietary stormwater models available Recurrence Interval Depth that may be used to conduct SBUH modeling (years) (inches) (functionally equivalent algorithms in proprietary Water Quality 1.0 programs are also allowed).Alternatively, a 2-year 2.38 spreadsheet may be used. See Appendix F for 5-year 2.85 further information. 10-year 3.20 The City uses the SBUH method to verify sizing 25-year 3.73 of facilities. Precipitation is one of the input 50-year 4.13 parameters for the SBUH method. The City 100-year 4.54 currently uses the NRCS Type 1A, 24-hour 500-year 5.55 rainfall distribution resolved into 10-minute time Source: Otak 2009 intervals as the standard design hydrograph. Table 4.2 summarizes precipitation for various design storms. When sizing flow control facilities, pre-development conditions shall be modeled using a CN of 70. 4.3.2 PAC Tool Another method approved by the City is the City of Portland Presumptive Approach Calculator (PAC). For water quality facilities sized using the PAC, the facility surface area shall be increased by 25 percent. No correction factor is required for onsite stormwater facilities sized using the PAC with flow control criteria. 4.3.3 Equations Equations specific to the volume-based sizing of stormwater facilities are included in Section 4.6. The equations do not include routing or infiltration, and will therefore result in larger facilities than those sized using the SBUH or the PAC. 4.3.4 Prescriptive Sizing For project sites with limited infiltration capabilities, stormwater facility sizing to meet water quality and flow control requirements can use the following prescriptive facility sizing factors. Prescriptive sizing factors may be used for rain gardens or stormwater planters with up to 6,000 of contributing impervious surface area per facility. Note that design of facilities based on this prescriptive sizing methodology requires an underdrain and orifice. 54 Section 4.3 Lake Oswego Stormwater Management Manual Stormwater Facility Design Guidelines Table 4.3 provides facility surface area sizing factors as a percentage of the contributing impervious area. Facilities designed with prescriptive sizing must follow the geometry notes below Table 4.3 in order to meet the performance standards for water quality treatment and flow control outlined in Section 4.1. Table 4.3 Prescriptive Sizing Method Design Infiltration Rate Raingarden Planter 0.0 - 0.24 in/hr 9% 6% 0.25 - 0.50 in/hr 8% 5% 1.Facility design assumes 12 inches of surface storage, 18 inches of growing media,and 12 inches of drain rock.Facility is designed with an underdrain,3H:1 V side slopes and a single 0.5-inch orifice outlet,located at an elevation equivalent to the bottom of the drain rock. 2.The design infiltration rate should be based on tested site conditions,with appropriate safety factors applied. 3.Facilities should be designed with an open bottom to allow infiltration unless site conditions require a lined facility(e.g.,high groundwater,contaminated soils, building setback requirements).Lined facilities must assume a design infiltration rate of 0.0 in/hr. 4.3.5 Minimum Orifice Size When designing a rain garden or planter for flow control, a minimum orifice size of 0.5 inches is required. When designing a detention pond, vault, or pipe for flow control, a minimum orifice size of 1.0 inches is required. 4.4 Soils Soil health is fundamental to a functional stormwater facility. Soils play the most important role in pollutant removal, 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 shall be treated as a community of living organisms. It needs to be protected and handled carefully during construction. Consider methods to use and amend onsite soil first. If onsite soil is not appropriate for the facility design goals, consider a sustainable way to salvage soil for use at another local site. If soil or amendments need to be brought into a site from an outside source, research where the soil is 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/backfill material needs to be stockpiled during construction, implement appropriate erosion control procedures (plastic covering, silt fence, etc.). If topsoil needs to be moved and stockpiled during construction, place topsoil in shallow linear mounds and sow a temporary cover crop over the mounds. Cover crops will stimulate soil activity and protect topsoils from erosion. The best protection for topsoil during construction is to move it from shallow mounds into its target area as quickly as possible. Leaving topsoil in large piles and covering it with plastic for long periods will result in sterile soil, devoid of beneficial organisms that treat water quality and aid in plant establishment. Lake Oswego Stormwater Management Manual Section 4.4 55 Stormwater Facility Design Guidelines 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) r • 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 facility design and construction practices based on research findings. 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 guidelines in Section 4.6.13 for pipes and vaults. 4.6 Stormwater Facility Design Guidelines The following section includes BMP facility design guidelines for facilities approved for use in Lake Oswego. Section BMP Facility Type Section BMP Facility Type 4.6.1 Rain garden 4.6.9 Swale 4.6.2 Planter 4.6.10 Sand filter 4.6.3 Infiltration trench or gallery 4.6.11 Constructed wetland 4.6.4 Drywell 4.6.12 Ponds 4.6.5 Greenroof 4.6.13 Detention pipes and vaults 4.6.6 Pervious pavement 4.6.14 Sheet flow dispersion 4.6.7 Rainwater harvesting 4.6.15 Proprietary Stormwater 4.6.8 Filter strips Treatment Devices 56 Section 4.5 Lake Oswego Stormwater Management Manual _ � mi. f r - L .: ail l IAF• . - L ir • - •. ' + {- -- �I' Kg:-' .11% r . e n s ._.: _ .. . ... • ... f r . S • - . 16 e 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 This facility is applicable for: phytoremediation through contact with vegetation, Impervious Area Reduction and biodegradation and adsorption of pollutants through contact with soil organisms and chemical Onsite Stormwater soil processes. Rain gardens are ideal for residential Management V and small commercial sites, within parking lots, and along roadways. They can help fulfill landscaping Flow Control requirements. Water Quality Treatment Linear rain gardens may look similar to swales, but they have a flat bottom and pond water to infiltrate UIC Pretreatment it vertically through treatment soils. Swales have a gently sloping bottom and provide treatment through Public Property filtration by vegetation as water is conveyed through the swale. Rain gardens can have multiple distinct Single-Family Residential depressions, called cells, which can be linked Development hydraulically via overflow structures or berms. With thoughtful plant selection and design, a rain Prescriptive Sizing garden can become a community and wildlife asset Methods V 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 (Typical and not for construction). rFacility Footprint Optional benching above -Design ponding design pondingdepth to depth 9 p � Bottom width � integrate rain gardens into the landscape Freeboard Native soil _/ . + I Overflow Biofiltration soil mix L Mulch (18 inches std.) f' i[ = - _ • Drain rock Outlet to approved 1 > (12-24 inches) discharge location Infiltration zone Lake Oswego Stormwater Management Manual Section 4.6 57 Rain Gardens Site Requirements • Pretreatment not required. �. • For infiltration rain gardens, the facility bottom shall be at least 3 feet above seasonal high groundwater elevation or other layer that limits infiltration (e.g., bedrock, clay lens). • Maximum 24-hour draw down time. • Infiltration rate shall be at least 0.25 inches per hour for an y`i ' Ir'' �r infiltration rain garden. Otherwise, the rain garden shall pond water for longer than a 24-hour period. If infiltration rate is less Rain garden at Glencoe Elementary, Portland, OR than 0.25 inches per hour, a flow-through rain garden with an underdrain shall be used and additional flow control measures may apply. • Infiltration rain gardens shall meet requirements and setbacks listed in Section 3.4.3. • Setbacks from top edge of facility: From property line > 5. From structures >_ 10 feet for infiltration and N/A for flow-through rain gardens. • Rain gardens should be dispersed across a project site to manage runoff from small drainage area(s). • Water quality raingardens shall not be located downstream of detention. • Infiltration raingardens are not allowed on steep slopes. Refer to Section 3.4.3 for additional setback requirements. Design Requirements Dimensions • Maximum drain rock depth is 24 inches. Minimum drain rock depth is 12 inches. • Minimum bottom width: 2.0 feet • Planted side slopes: no steeper than three horizontal to one vertical (3H: IV). Rock or concrete walls may be used for areas that require steeper side slopes. • Minimum orifice size: 0.5 inches • Minimum freeboard above overflow: 2.0 inches, if contributing area is less than 3,000 square feet; 6.0 inches for larger contributing areas • Maximum ponding depth: 12.0 inches • Depth of biofiltration mix: 18 inches minimum Conveyance and Outlet/Overflow Rain gardens shall include overflows (e.g. pipe, V-notch or broad-crested weir) that discharge to an approved location. In areas where it is inappropriate to infiltrate (e.g. where infiltration rates are less than 0.25 inch per hour or underlying soils are contaminated) underdrains shall be provided along the length of the raingarden. 58 Section 4.6 Lake Oswego Stormwater Management Manual Rain Gardens ...' + I 1 _ 4:1 1 :, -t; ..ititrli: .,, iiii • 04* Rain water is directed from this home's rooftop to a rain garden in the front yard Design Steps Design Performance Goal Capture and store the design storm in a surface pond for infiltration through the biofiltration soil medium. Rain gardens may be sized either using the prescriptive sizing methodology, equations found in this section, the SBUH method, or Portland's Presumptive Approach Calculator (PAC) Tool with adjustment factor. The simplified equations in this section are conservative and may result in larger facilities. See Section 4.3 for more information and requirements on alternative sizing methods. 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 Vimp or Vperv= 3,630 *A* ` Pdesign - 0'2 CN ) ( Pdesign + 0.$ 1,000 ) — 1 0] CN Lake Oswego Stormwater Management Manual Section 4.6 59 Rain Gardens Parameter Units Values Vimp + VpeN runoff volume (impervious and pervious) cubic feet Pdesign design precipitation depth inches water quality: 1.0 onsite: 3.20 rff 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 Dpond rain garden ponding depth inches maximum 12 Dmedia rain garden media depth inches minimum 18 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 runoff volume (Vimp + Vpem), 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 Dpond ) (Amedia Dmedia media 12 12 Where nmedia applies to subsurface storage layers. Parameter Units Values ns porosity (soil media) unitless 0.25 rig porosity (gravel layer) unitless 0.40 60 Section 4.6 Lake Oswego Stormwater Management Manual Rain Gardens 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 + Vperv), 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 design infiltration rate inches per hour Materials Waterproof Liners When required, waterproof liners shall be 30 mil PVC membrane. Drain Rock Drain rock is required below the biofiltration soil mix (i.e., growing medium; see below). For infiltration planters, use 0.75 inch to 1.5 inches of 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. Underdrain If native soil infiltration rates are less than 0.25 inch per hour, planters shall 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 prefabricated 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 ODOT Standard Specification 02415.50 for polyvinyl chloride pipe. All flow-through facilities shall use 0.75 inch to 1.5 inches of washed drain rock around underdrain. Choker Course Drain rock and biofiltration soil mix shall be separated by a 2-inch to 3-inch choker course layer. Choker course shall conform to ODOT Standard Specifications 00430.11. Lake Oswego Stormwater Management Manual Section 4.6 61 Rain Gardens Biofiltration Soil Mix Rain garden soil media shall 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 D. 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. Vegetation Mulch Fine to medium-sized hemlock bark or well-aged organic yard debris compost is recommended for rain gardens. It shall 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 shall not be over applied. 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 shall 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 five types of vegetation for each facility. Select species that are suitable for the hydrologic, light, and soil conditions in the proposed rain garden and reflect a mix of vegetation types. Rain gardens shall be designed so that they do not require mowing. See Appendix H for a list of plants appropriate for rain gardens. 62 Section 4.6 Lake Oswego Stormwater Management Manual Rain Gardens Plant Quality Vegetation Type Recommended Size Sedges and rushes 10-inch deep container', plug2, division, rhizome, or tuber Grasses and forbs 1 gallon or equivalent; plug; Shrubs 1 gallon, bareroot, or equivalent Deciduous trees 3 gallon container, bareroot or equivalent, minimum caliper shall be 1.5 inches at 6.0 inches above base 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 to minimize flow channelization. Planting Depth All plants shall 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 shall be a minimum of 3 feet from sidewalks and roadways. 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 shall maintain vehicular and pedestrian lines of sight near street crossings. Lake Oswego Stormwater Management Manual Section 4.6 63 Rain Gardens Construction Mark rain garden boundaries with stakes or flagging prior to construction and avoid soil compaction or disturbance. If soil is compacted, it is critical to 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 shall be protected from sedimentation during construction. 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 K for further maintenance guidance. Herbicides shall not be used as a weed control technique within stormwater facilities. Insecticide and fungicide shall not be used. 64 Section 4.6 Lake Oswego Stormwater Management Manual {• h • f - i I I p d o ers f ' 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 phytoremediation This facility is applicable for: through contact with vegetation; and biodegradation and adsorption of pollutants through contact with soil Impervious Area Reduction organisms and chemical soil processes. Planters are Onsite Stormwater essentially rain gardens with structural walls. The Management V following considerations may help you select the best facility for your site: Flow Control • Planters require less space than rain gardens to treat Water Quality Treatment the same contributing area providing they are flat bottomed. UIC Pretreatment • Planters are more costly than typical rain gardens to construct because they have structural walls. Public Property • Planters typically require a structural engineer to design (because of their structural walls). Planters Single-Family Residential need energy dissipation at inlets and planters Development receiving water from the street need a sumped pollution control catch basin at inlets or a sediment Prescriptive Sizing forebay. Methods V • 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 shall be designed to consider safety issues (including pedestrian safety). Curbing, fencing, or placing planters above grade may be necessary. Figure 4.3 Planter Section Facility footprint (Typical and not for construction). See construction section f) ` Alternate inlet/building downspout <soil protection requiremen Overflow Design Walkway Surface inlet ponding depth Mulch Liner i.. Y+ a . Planter n Biofiltration soil mix • 0 r = Choker course To approved outlet location F! - I LL1 - Drain rock II i -- Underdrain to run length of planter for flow-through facilities- Native soil _=,i-= Optional bottom and liner(for flow-through facilities) Lake Oswego Stormwater Management Manual Section 4.6 65 Planters Site Requirements • Pretreatment not required. . • For infiltration planters, the facility bottom shall be .' , at least 3 feet above seasonal high ground water T>, l� ,• elevation or other layer that limits infiltration (e.g., • bedrock, clay lens, perched groundwater). • • 1 • Maximum 24-hour draw down time. • R-i • The infiltration rate shall be at least 0.25 inch '} z 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 the infiltration rate is less than 0.25 inch per hour, a flow-through planter with an underdrain shall be used and additional flow control measures may apply. • Infiltration planters shall meet requirements listed in Section 3.4.3. • Consider pedestrian safety and access when designing planters. Ensure that planters located next to parking areas have clearance for pedestrians exiting vehicles. • Setbacks from top edge of facility: From property line >_ 5 feet. From structures >_ 10 feet for infiltration and N/A for flow- through planters. • Planters should be dispersed across a project site to manage runoff from small drainage area(s). • Water quality planters shall not be located downstream of detention. • Infiltration planters are not allowed on steep slopes. Refer to Section 3.4.3 for additional setback requirements. Design Requirements Planter Dimensions • Maximum drain rock depth is 24 inches. Minimum drain rock depth is 12 inches. '+ • Width (interior): I ; - at least 30 inches for infiltration planter - at least 18 inches for flow-through planter ma. .+JimagaE / • Maximum ponding depth: 12.0 inches Arm • Minimum freeboard above overflow: 2.0 inches if contributing impervious area is less than 3,000 square Red osier dogwood providing year-round aesthetic feet; 6.0 inches for larger contributing areas interest • Minimum orifice size: 0.5 inch Depth of biofiltration mix: 18 inches minimum • De p R. Conveyance and Outlet/Overflow 4 Planters shall include overflows that discharge to an approved location. In areas where it is inappropriate to infiltrate (e.g., where , k , infiltration rates are less than 0.25 inch per hour or underlying �Ui soils are contaminated), underdrains shall be provided along *' the length of the planter. Street right-of-way planters 66 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. Planters may be sized either using the prescriptive sizing methodology, equations found in this section, the SBUH method, or Portland's Presumptive Approach Calculator (PAC) Tool with adjustment factor. The simplified equations in this section are conservative and may result in larger facilities. The PAC Tool includes additional guidance for sloped facilities. See section 4.3 for more information and requirements on alternative sizing methods. 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 Aimp 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 Vamp or VpeN= 3,630 *A* ( Pdesign - 0.2 * CN ) 1 000 ( Pdesign + 0.8 * I — 10. ) CN Parameter Units Values Vimp + VpeN runoff volume (impervious and pervious) cubic feet Pdesign design precipitation depth inches water quality: 1.0 onsite: 3.20 Step 3: Determine Planter Treatment Depth Select the design depth for ponding in the planter before overflow. Parameter Units Values °pond planter ponding depth inches maximum 12 Dmed.a planter media depth inches minimum 18 Lake Oswego Stormwater Management Manual Section 4.6 67 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). 12 (V,.mp + Vperv) A - s pond Dmedia media, Where nmedia applies to subsurface storage layers. Parameter Units Values AS surface area of planter square feet media porosity (soil media) unitless 0.25 nmedia porosity (gravel layer) unitless 0.40 Materials Planter Walls and Bottom Planter walls (minimum 6-inches thick) and bottom shall be made of concrete. Chemically treated wood that can leach out toxic chemicals and contaminate stormwater shall not be used. Waterproof Liners When required, waterproof liners shall be 30 mil PVC membrane. Drain Rock Drain rock is required below the biofiltration soil mix (i.e., growing medium; see below). For infiltration planters, use 0.75 inch to 1.5 inches of 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. Underdrain If native soil infiltration rates are less than 0.25 inch per hour, planters shall 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 prefabricated 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 ODOT Standard Specification 02415.50 for polyvinyl chloride pipe. All flow-through facilities shall use 0.75 inch to 1.5 inches of washed drain rock around underdrain. 68 Section 4.6 Lake Oswego Stormwater Management Manual Planters Choker Course Drain rock and biofiltration soil mix shall be separated by a 2-inch to 3-inch choker course layer. Choker course shall conform to ODOT Standard Specifications 00430.11. Biofiltration Soil Mix Planter soil media shall 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 D. 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 shall 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 shall not be over applied. Vegetation Objective Establish dense plant growth with a diversity of groundcovers. Careful consideration shall 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. Lake Oswego Stormwater Management Manual Section 4.6 69 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 shall 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 shall be designed so that they do not require mowing. I See Appendix H for a plant list. Plant Quality Vegetation Type Recommended Size Sedges and rushes 10-inch deep container, plug2, division, rhizome, or tuber Grasses and forbs 1 gallon or equivalent, plug Shrubs 1 gallon, bareroot, or equivalent 3-gallon container, bareroot, or equivalent; minimum caliper shall be Deciduous trees 1.5 inches at 6.0 inches above base. Mature size shall be suitable for location. Evergreen trees 3-gallon container or equivalent; minimum height 6 feet, unless approved by City. Mature size shall 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 to minimize flow channelization. Planting Depth All plants shall 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. 70 Section 4.6 Lake Oswego Stormwater Management Manual 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 shall 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. Construction Mark planter boundaries with stakes or flagging prior to construction and avoid soil compaction (including from material stroage). 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 shall not be cut or mowed. See maintenance checklists (Appendix K) for further maintenance guidance. Herbicides shall not be used as a weed control technique within stormwater facilities. Insecticide and fungicide use shall not be used. Lake Oswego Stormwater Management Manual Section 4.6 71 This page is intentionally left blank. 72 Section 4.6 Lake Oswego Stormwater Management Manual • re n c � t orGailer • er 4.6.3 Infiltration Trench or Gallery Definition An infiltration trench is a linear, gravel- Applicability Table filled trench that distributes stormwater to underlying soils. This facility is applicable for: An infiltration gallery includes Impervious Area Reduction underground chambers to increase subsurface stroage. Onsite Stormwater Management Introduction Flow Control Infiltration trenches that receive only surface runoff(no underdrains or subsurface pipe) are not classified as Water Quality Treatment underground injection control devices (UICs). UIC Pretreatment N/A Infiltration trenches with underdrains and infiltration galleries are "Class V Injection Wells" under the UIC Public Property program (OAR Division 44). UICs are classifieds either as: exempt (no registration required), authorized by Single-Family Residential rule, or authorized by permit. Infiltration trenches and Development galleries do not provide water quality treatment, so water quality treatment is required before stormwater Prescriptive Sizing Methods discharges into the facility. Infiltration trenches or galleries that receive stormwater runoff must be registered and rule authorized with DEQ. Infiltration trenches and galleries used exclusively for single family residential roof runoff are exempt from registration with DEQ and only require pretreatment such as a silt trap catch basin with an 18-inch minimum sump. Designers are encouraged to review current regulations and UIC registration materials from DEQ. I Figure 4.4. Infiltration Trench or Gallery Section (Typical and not for construction). Note: Outlet to Perforated distribution Observation well (required) approved point of pipe discharge may be I (6" PVC pipe) Finish grade required. I • I I ti M1k'II•I I I I I I I I I I I l + r f 14k ; Min. 18° 11 I MMMM '� • •` •, ,. ,p ► �. L. ' • * •- — ^ Minimum 6" Sump � , ., • w ""• # " ate• washed drain '� .z %-4 rock below • - C1' ems=•.' x• .1;?.,_v v �• ; � r°�r � . � '� i. .I .. � i I I . . Pipe inlet 5I � •,\I,k y./f} I+I I x4 ',4�kY.f! I+II I x4�I•��kY.f III II*4 JI•�k k.,Jf� III II� �I,1 ti�J II Lake Oswego Stormwater Management Manual Section 4.6 73 Infiltration Trench or Gallery Site Requirements = • • f - • Trenches or galleries are not approved for steep slopes. Refer to Section 3.4.3 for additional setback requirements. , - _ • Trenches or galleries shall be located at least 10 feet ;-,,,-., ;" - mo from building foundations. Y • Trenches or galleries shall be located at least 5 feet from property lines. mispilignsigisios— • Infiltration trenches or galleries are not allowed in the Infiltration trench also serving as an outdoor gathering public right-of-way. space _ • Trenches shall not be located where they will be subject to vehicular traffic. • Soil surrounding trenches or galleries shall be native, uncompacted soil. • Bottom of trench or gallery shall be at least 5 feet above seasonal high groundwater elevation. • Native soil design infiltration rate shall be at least 0.25 inch per hour. • Infiltration trenches or galleries shall be located outside of tree protection zone or at least 10 feet from the base of newly planted trees and large shrubs. • Pretreatment is required for infiltration galleries unless the gallery is receiving only single-family residential roof runoff. Pretreatment options include: • Water Quality BMPs listed in Table 3.1 o Proprietary Stormwater Treatment Devices approved under the TAPE program with a General Use Level Designation (GULD) for basic treatment (See Section 4.6.15). o Catch basins with a 36-inch sump and trapped outlet (snout). • Not within 500 feet of drinking water wells. • Maximum 24-hour draw down time. Design Requirements Infiltration trenches or galleries are rectangular in shape and can be any length. Infiltration Trench Dimensions • Depth of drain rock: 12 inches minimum • Width: 2 feet minimum • Length: varies Infiltration Gallery Dimensions • Per manufacturer specifications 74 Section 4.6 Lake Oswego Stormwater Management Manual Infiltration Trench or Gallery Conveyance and Outlet/Overflow An observation well is required for infiltration trenches constructed for large projects (>_ 3,000 square feet of new or replaced impervious surface). The well allows confirmation of proper drainage.A cleanout is required for infiltration galleries to allow for ongoing maintenance. See Chapter 3. Outlets that overflow to an approved discharge point may be required. Design Steps The required storage capacity of an infiltration trench or gallery 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 (Section 3.4.3) and perform infiltration test. A geotechnical report is required for infiltration facilities 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 2 ( Pdesign - 0.2 * [ - 101 ) Vimp + Vpery= 3,630 *A* CN 1,000 ( Pdesign + 0.8 * [ - 10] ) 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 75 Infiltration Trench or Gallery Step 4: Establish Preliminary Geometry Establish infiltration facility depth, width, and length for preliminary evaluation. Parameter Units Values H depth feet 1 min. W width feet 2 min. L length feet Step 5: Calculate Infiltration Volume Calculate the volume that can infiltrate from the infiltration facility to groundwater in a 24-hour period using the facility geometry and the design infiltration rate. 1 Vi = Ait *Rinf * T 12 Parameter Units Values Rinf design infiltration rate inches per hour 0.25 min Ait 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 facility. 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. 76 Section 4.6 Lake Oswego Stormwater Management Manual Infiltration Trench or Gallery Materials Drain Rock Drain rock shall be 0.75 inch to 1.50 inch of 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 granular drain backfill material of 1.50-inch to 0.75-inch crushed gravel meeting the following gradation requirements: Seive Size Percent Passing (inches) (by Weight) 2.00 100% 1.50 95-100% 1.25 - 1.00 - 0.75 0-15% 0.50 0-2% 0.25 - Perforated Distribution Pipe The distribution pipe shall be perforated, 6-inch-diameter PVC pipe that conforms to ODOT Standard Specification 02410.70. The invert elevation shall 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. Construction The location of the proposed infiltration facility shall 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 facility shall be level. Infiltration rates shall be tested after construction to confirm that they are consistent with the design infiltration rates. Lake Oswego Stormwater Management Manual Section 4.6 77 Infiltration Trench or Gallery Maintenance If ponding is observed in the observation well or at the surface of the facility, 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 facility. See Chapter 7 for additional maintenance guidance. Er • , 1.- . * Mill eillk 0 - • 41149P--.1"Llib"- le" .. lit . .:4'. .."1 : ' 1....1 illikmpor 'VII ir , .4 o Iiiritw hit ' - fildlilit • . Washed gravel used for construction of infiltration trenches 78 Section 4.6 Lake Oswego Stormwater Management Manual . , -�_ 0 - - I AAy r . . . . . el �,��� }�_._.A-i ..._....... _ 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 federal This facility is applicable for: Underground Injection Control Program (OAR Division 44). UlCs are classified as exempt (no registration required), Impervious Area Reduction authorized by rule, or authorized by permit. Onsite Stormwater Drywells do not provide water quality treatment, so water Management quality treatment is required before stormwater discharges Flow Control into drywells. Drywells used exclusively for single-family residential roof runoff are exempt from registration with DEQ and only require pretreatment such as a silt trap catch Water Quality Treatment basin with an 18-inch minimum sump. Drywells used for other types of stormwater treatment, such as driveways, are UIC Pretreatment N/A required to obtain DEQ approval and to provide a copy of the approval to the City. Where space is available, rain Public Property gardens are preferred to manage residential runoff because they provide both treatment and flow control (and Single-Family Residential are not considered UlCs). Development Designers are urged to review current regulations and UIC Prescriptive Sizing registration materials from DEQ. Methods Figure 4.5. Drywell Section (Typical and not for construction). See also City standard detail SD1-07. T 1 1 I Drywell I inlet pipe I I Accessible cap I or lid with lock 1 \ ■■■ — — — — — _ _ .sue 12"Soil laver ..r�g?!e„y` " . Perforated pipe section Sump . n�':o. `a ;¢ �•�It ;Q : m Stone fill material Building foundation : 5r'5 ':$=i'". pirliv:-.1i-;?_ ,t, ( ) 10'from building foundation Lake Oswego Stormwater Management Manual Section 4.6 79 Drywell Site Requirements • Native soil design infiltration rate shall be at least 2.0 inches per hour. • Bottom of drywells shall be at least 5 feet above seasonal groundwater or impermeable layer. s drywell shall be at least 10 feet from building e''• ,, ' 1 j foundations. • The edge of excavation for the drywell shall be at - - _ - .1• g rY "q_: .w least 5 feet from property lines. • The top of drywell shall be lower than the floor ` •r " elevation of basements in immediately adjacent ° • buildings and/or properties. {Irr • Drywells are not allowed on steep slopes. Refer to Section 3.4.3 for additional setback requirements. • Private drywells are not allowed in the public right- uic evaluation of-way. • Soil surrounding drywells shall be native, uncompacted soil. • Not allowed within 500 feet of drinking water wells. Design Requirments Drywell Dimensions f — t, _ _ ; • Diameter: 2 feet minimum Pretreatment 4 A 4 g q * e. 0 • • • # # I Pretreatment is required for drywells. # # The following BMPs/facility are approved for I`s i gip �IN 0 . providing pretreatment where required: 0,s ; igo • 0 I • Water Quality BMPs listed in Table 3.1 i A % iilk # 0 - • 0 • I •# al �=a • Proprietary Stormwater Treatment Devices �_ ili � ,t approved under the TAPE program with a General Use Level Designation (GULD) for basic treatment Large drywell (See section 4.6.15). • Catch basins with a 18-inch sump and trapped outlet (snout) for single family residential applications. A 36-inch sump is required for all other applications 80 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 3.4.3), and perform an infiltration test (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 Apery 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 2 ( Pdesign - 0.2 * [ - 10] ) Vimp or Vpem= 3,630 *A* CN * r 1,000 1 (Pdesign + 0.8 I. CN - 10] ) 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 81 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). Vi = AdW * Rinf T * ) * N 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. 82 Section 4.6 Lake Oswego Stormwater Management Manual Drywell Materials Drain Rock Place 12-inch minimum layer of 1.50-inch to 0.75-inch round rock that conforms to ODOT Standard Specifications 00430.11 or AASHTO 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. Sumps require maintenance when sediment has accumulated up to 1/3 of the sump depth, regardless of the drainage type leading to the catch basin. The City also requires a sumped catch basin for roof drainage leading to drywells. Refer to the operations and maintenance section (Chapter 7) for more information. Lake Oswego Stormwater Management Manual Section 4.6 83 This page is intentionally left blank. 84 Section 4.6 Lake Oswego Stormwater Management Manual -y . . ree n r _ _ 'T• • -• �.ti -'fr. - - L _ _ ram' 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. r w Introduction Applicability Table A green roof includes the following elements: a thin, This facility is applicable for: layered system of waterproofing, drainage layers, Impervious Area Reduction growing media, and planting to cover impervious roof areas and allow water to be absorbed, detained, and Onsite Stormwater N/A evaporated back into the atmosphere. Management Green roofs are often employed on sites where other Flow Control N/A stormwater management strategies are not feasible Water Quality Treatment N/A due to lack of available space Green roofs are mostly "extensive," meaning they UIC Pretreatment are low-maintenance and lightweight, and primarily Public Property designed for stormwater management, with aesthetics as a secondary goal. Semi-intensive green roofs have Single-Family Residential somewhat deeper soils or more expansive plantings. Development Green roofs are not designed for foot traffic or recreation. They shall be differentiated from accessible Prescriptive Sizing "roof gardens" and "terraces," which include deeper Methods 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 (Typical and not for construction). Approved Gravel plantings mulch Drainage layer with Metal filter fabric edgingiiii' f � '� F M N. kr i i t r = I' 5 it - 7. _', 4. *• 'i. • Growing a a -- media �. 4 F a r ' 4#min. ;. � Waterproofin• / \- Protection/ root barrier Building structure Drain to approved_Z( outfall location Lake Oswego Stormwater Management Manual Section 4.6 85 Green Roofs Site Requirements • Flat or slightly sloped roofs on large institutional, .0_- commercial, or residential projects. Green roofs ,. ti +(ar - }..._ work on sloped roofs upto a maximum of 4:12 `• '.`` roof pitch without additional engineering - and � � r -'y.5 `{:ir -- can be steeper with intermediate structural Wit' , �` ,�3.- *--- ° ''h 'g ",r jN. y irf� support of soils. Z 'or*Oil � '~ ';. • Structural considerations: shall be able to carry L. additional loads as determined by a structural engineer (15 to 30 pounds per square foot is typical) to support fully saturated conditions. Graphic representation of a green roof These load recommendations do not include snow load. • Access to roof via crane, lift or other device is recommended to load heavy, and bulky materials up to rooftop surface. Design Requirements 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: 0.25 inch per foot • Maximum roof slope: 4:12 roof pitch (greater slopes allowed with engineering/stabilization) • Perimeter edges and access pavers shall 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 — _ . µ ,. :is?, , p 4 y4 is�..i p��.,�r�y .4.:M,:. Y:. .� _ � ��` ,. pi ca , 6 t ,a ,��. _ -- — — r� •-. .'.- .- - = ,- .'I i r r d �zi ..,. F* , ' ` +ate ;,,,:51,14,f,-,!: ooi .1,,LP •• '---4,t Kit 4 •� a _ t �v y e - 'rt- _Vei‘' ,' A . ..,..;,..,. 11.., ,,,„ --,,..,li,t_ "or ,. .,.. .4,,_,.. ...k et• • -,- 0 Joa. ., ,.i ,,, . Flat commercial/mixed use green roof Sloped residential green roof 86 Section 4.6 Lake Oswego Stormwater Management Manual Green Roofs Conveyance and Outlet/Overflow Green roofs shall include outlets to an approved location from roof drains, scuppers, and other drainage devices. 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 Units 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 shall 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 shall be placed over the protection layer to allow for water movement under the growing media. The drainage layer shall 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 shall be placed over the filter fabric to a depth of at least 4 inches to meet requirements. It shall 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 shall consist of: Pumice/Lightweight Aggregate 80-90% Composted plant-based organic matter 10-20% Lake Oswego Stormwater Management Manual Section 4.6 87 Green Roofs Mineral Mulch A mulch layer of washed gravel or non-decompostable material (no fines) that will not be moved by wind or water movement shall be placed on top of the growing media. 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 shall 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 shall 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); shall 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. 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 K) for further maintenance guidance for green roof vegetation. Herbicides shall not be used as a weed control technique within stormwater facilities. Fungicides and insecticides shall be not be used. Metals-based moss controls shall not be used unless the green roof overflows to an infiltration facility that does not include an underdrain. 88 Lake Oswego Stormwater Management Manual ■ erv � ous aveme 4.6.6 Pervious Pavement 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 This facility is applicable for: porous asphalt, pervious concrete, or permeable Impervious Area Reduction pavers. Onsite Stormwater N/A • Porous asphalt is open-graded asphalt that allows Management water to percolate or infiltrate into underlying soils. Flow Control N/A • Porous concrete omits fines in the aggregate to create stable air pockets that allow water to drain Water Quality Treatment N/A to the base below. There is an inverse relationship between porosity and strength. As porosity is UIC Pretreatment N/A increased, the structural strength is reduced. Public Property • Pavers are generally suitable for pedestrian areas and low traffic parking areas. They are available in Single-Family Residential a variety of configurations such as rigid concrete or Development durable plastic grid filled with gravel or a mixture of Prescriptive Sizing gravel, sand, and topsoil suitable for vegetation. Methods Pervious pavement is designed as a surface that receives only direct rainfall. Sediment shall 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 (Typical and not for construction). leveling Course 1l4'Aggregate Permeable Pavement Surface • A Aggregate AggregalP Storage al'' � ReserwStarageir V t‘ 4E57 � spy` r' _ • Pavement Sub-Grade Lake Oswego Stormwater Management Manual Section 4.6 89 Pervious Pavement Site Requirements Pervious pavement shall be designed to manage precipitation falling directly on the pavement, without - - • additional run-on. To be considered for pervious ~_ r ' ' 3-''.'- - ' pavement, a site shall meet the following design criteria: ,xr. ..„ • • Surface slope no greater than 6 percent. ' • Site does not receive high 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 frameworks developed by • Elevated separation from underlying water table; vendors that contain aggregate in place, support traffic bedrock or other impermeable layer shall be at least 3 loads, and function as a pervious pavement system feet below ground surface. • Pervious pavement is not appropriate for any site that may handle, store, or dispose of hazardous materials. • Minimum infiltration rate of 0.25 inch per hour. • Not appropriate for construction over fill soils unless evaluated and approved by geotechnical engineer. • No stormwater run-on allowed. Design Requirements Wearing Course • Porous Concrete r 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 v44; • 7-inch thickness for public street Porous Asphalt The minimum thickness of the wearing course of a porous asphalt == _ section shall be as follows: Pavers are a good option for pedestrian • 2.5 inches for residential driveway or pedestrian only path and low traffic locations • 3.0 inches for private street, parking lot, or fire lane • 6.0 inches for public street Pavers Pavers shall be designed according to manufacture's recommendations. 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. 90 Section 4.6 Lake Oswego Stormwater Management Manual Pervious Pavement Aggregate Storage Reservoir • Minimum depth for structural support shall be 6 inches for vehicular loading. • In addition to structural design considerations, design depth is typically determined by storage depth needed to manage design storm. • Prescriptive sizing: pervious pavements designed with 6 inches of aggregate storage and a minimum design infiltration rate of 0.25 inches per hour are assumed to meet the requirement to provide onsite stormwater management for the 10-year design storm. 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 Layer 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 shall consist of uniformly graded, washed aggregate that conforms to AASHTO No. 8 gradation. 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 concrete pavement (ACP) 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 91 Pervious Pavement Construction For a pervious pavement installation to be approved in Lake Oswego, designers shall 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 shall be protected from sediment and runoff. The subgrade shall be protected from inappropriate compaction, from truck traffic, and construction equipment. Compaction will reduce the permeability of soils and shall 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 allowed 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 K for complete maintenance schedule. 92 Section 4.6 Lake Oswego Stormwater Management Manual • •- _ _ _A. ._ _R *Inwà4er. ._ . a rve' 4 -o Sided" ainBank 4.6.7 Rainwater Harvesting Definition Rainwater harvesting entails collection of stormwater in cisterns to use either for landscape irrigation or for non-potable building use, such as toilet flushing or cold water laundry. Applicability Table Introduction This facility is applicable for: Cisterns are large facilities that can supply storage I Impervious Area Reduction for non-potable building uses as well as irrigation. Any stormwater reuse within a building shall obtain Onsite Stormwater the City's plumbing approval from the Building Management V Department. System containment backflow protection in the form of a reduced pressure (RP) Flow Control type of backflow assembly shall be provided. System I Water Quality Treatment containment RPs shall be located on private property at the property line, immediately adjacent to the point I UIC Pretreatment of water service connection. Rain barrels are not stormwater management facilities. Public Property N(* Rain barrels are BMPs that collect stormwater for Single-Family Residential reuse, typically for landscape irrigation. Rain barrels Development typically have a small storage volume, so they Prescriptive Sizing cannot be used to meet stormwater management Methods requirements.The usefulness of rain barrels for stormwater management is also limited because of *Requires City pre-approval. the nature of storms in the Pacific Northwest, where frequent storms occur in the fall, winter, and spring, when irrigation needs are much lower. . _ b -- r ��. —m , ! , i 1 - :il s . iii. _____ } t r }l i U --7IF _ • T ,'4.�L -� �• •��f. _ . ,�-+14^`•ter. - - � - - - •- tier. . • -. isholO:provided'15S7•RanBank These cisterns capture water from this building roof system Lake Oswego Stormwater Management Manual Section 4.6 93 Rainwater Harvesting Site Requirements • To protect the water quality of the rainwater harvested, rainwater shall not be harvested from roofs containing copper or zinc, or materials treated with fungicides or herbicides. • Rainwater shall be collected only from roofs and only if approved by the City's Building Department. • Reuse of stormwater for non-potable uses is regulated by the Oregon Plumbing Specialty Code and by the City's Building Department. • Rainwater harvested from rooftops with asphalt 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. Design Requirements 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 i -. +. 1 A cistern built into rock wall Rain barrels capture water that can be used for supplemental irrigation, but are not stormwater management facilities. Conveyance and Outlet/Overflow Each cistern shall have a designated overflow when the volume of the tank meets capacity. The minimum overflow is 4 inches in diameter. Overflows shall discharge to an approved location. 94 Section 4.6 Lake Oswego Stormwater Management Manual Rainwater Harvesting Design Steps For cisterns, engineering may be required along with plumbing and other trade-specific work. Use an ARCSA-certified professional for design. For all cisterns that are proposed to meet onsite stormwater management or flow control requirements, the following information shall 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. System designs for interior uses shall have a monthly water balance that demonstrates adequate capacity for each month and reuse of all stored water annually. See the Oregon Smart Guide to Rainwater Harvesting (BCD, 2015) for information on how to perform these calculations. Use ARCSA design guidelines and an ARCSA-certified professional. 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 shall 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. Bedding and Foundations Full cisterns can be very heavy. Given this, the soil bearing capacity or foundation upon which a cistern is placed shall be evaluated to prevent settling or subsidence. This is particularly important for aboveground cisterns, as significant settling could cause the cistern to lean or be damaged. Bedding, foundations, and/or footers shall be provided as warranted by system loading, geotechnical conditions, and manufacturer's recommendations. Design of these features shall be performed by an appropriate design professional. Stone bedding is often used to support cisterns. Bedding thickness varies according to cistern system requirements, but shall 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. Consult a geotechnical engineer for additional guidance. Lake Oswego Stormwater Management Manual Section 4.6 95 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 shall 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 shall 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 shall 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 shall 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. 96 Section 4.6 Lake Oswego Stormwater Management Manual Rainwater Harvesting Construction All cisterns shall be installed in accordance with manufacturer's installation instructions, the City's building code, and all applicable laws. Maintenance Cistern/storage systems shall 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. Cisterns shall be designed to accommodate cleaning of any accumulated sediment on the bottom of the cistern by flushing through a drain, vacuuming, or another approved method. Cistern/storage systems that are buried below ground level shall have a manhole riser that sticks out a minimum of 8 inches above the surrounding ground. Manhole covers shall be secured and locked to prevent tampering. Lake Oswego Stormwater Management Manual Section 4.6 97 This page is intentionally left blank. 98 Section 4.6 Lake Oswego Stormwater Management Manual .-,-•,l • • Filter Strl 4.6.8 Filter Strips Definition A filter strip is a gently sloped vegetated strip that removes pollutants through filtration, sedimentation, and infiltration. Introduction Applicability Table A filter strip is a section of vegetated area adjacent to This facility is applicable for: an uncurbed, impervious surface. Stormwater flows at a low velocity and depth evenly across the entire Impervious Area Reduction width of the filter strip. Pollutants are removed through Onsite Stormwater stormwater contact with vegetation and underlying soils. Management Filter strips are appropriate for meeting landscaping I Flow Control requirements, but plants shall be selected to be I Water Quality Treatment consistent with their stormwater treatment function. They shall be protected from disturbance to preserve I UIC Pretreatment healthy vegetation and soil conditions in order to maintain water quality function. Public Property Filter strips are a good choice for roads in Lake I Single-Family Residential Oswego and are complemented by a shallow gravel Development strip that helps to uniformly distribute flow. They can also be good choices for small projects; for example, Prescriptive Sizing to provide water quality treatment for roof or driveway Methods runoff before it is discharged into a drywell, infiltration trench, or other facility. Figure 4.7. Filter Strip Schematic. ___Iii °A'`°'"- I Pollution generating Filter strip surface —Protect soils from disturbance - see construction section Lake Oswego Stormwater Management Manual Section 4.6 99 Filter Strips Site Requirements • Flow shall be distributed evenly along the length "'" ,, of the strip. This may require additional structures or design features to fully spread point discharges at: __ along the length of the strip. • There shall be an approved overflow route. sa , • Filter strips are appropriate for all soil types. • Filter strips may be located on a range of site 11 conditions from full sun to full shade. Plant selection shall match site conditions (see Appendix H). Filter strip monitoring • Filter strips shall be a minimum of 5 feet from the property line. • Filter strips shall be a minimum of 5 feet from structures. • Filter strips shall be a minimum of 50 feet from wetlands, rivers, streams, and creeks. Design Requirements 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. �� 50' minimum from _� — �� streams Qom _ w 5' minimum = " =- _ width for M all strips in Photo courtesy of USDA NRCS. direction of flow (T) Filter strip between agricultural fields and stream 100 Section 4.6 Lake Oswego Stormwater Management Manual Filter Strips Conveyance and Outlet/Overflow Filter strips shall 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). 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 (Qdesign) in cubic feet per second for water quality design storm. Parameter Units Value Qdesign 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 T filter strip width (See Figure 4.8) feet S filter strip slope feet per feet 0.005 - 0.1 Lake Oswego Stormwater Management Manual Section 4.6 101 Filter Strips Step 3: Calculate Flow Depth Calculate flow depth (y in feet) using the following equation: Q *=\ design n 0.6 y ) 1.49*T* JS 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. Step 4: Check Velocity Calculate velocity of flow using the following equation (V in feet per second). Q V = T * 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. 102 Section 4.6 Lake Oswego Stormwater Management Manual Filter Strips Materials Biofiltration Soil Mix Filter strip soil media shall support long-term plant and soil health and provide treatment to water as it infiltrates. The soil mix shall comprise at least the top 18 inches of soil depth and be placed on top of uncompacted native soil. 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 shall be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It shall 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 shall respond to specific site conditions for each facility. See Appendix H for a complete plant list. Lake Oswego Stormwater Management Manual Section 4.6 103 Filter Strips Plant Quality Vegetation Type Recommended Size Sedges and rushes 10-inch deep container, plug2, division, rhizome, or tuber Grasses and forbs 1 gallon or equivalent; plug 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 shall 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. 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 Native grasses and sedges do not require cutting or mowing. See maintenance checklists in Appendix K for further maintenance guidance. Herbicides shall not be used as a weed control technique within stormwater facilities. Insecticide and fungicide shall not be used. 104 Section 4.6 Lake Oswego Stormwater Management Manual 5 _mow '�• `.. ,.„IiA 4. �� _. .wa - _ i.7 V 4.6.9 Swales Definition A swale is a long, linear, gently sloped channel that removes pollutants through sedimentation and filtration. Introduction A swale is an open, gently sloped, vegetated Applicability Table channel designed for treating stormwater. The This facility is applicable for: primary pollutant removal mechanisms are filtration and sedimentation. These occur when vegetation Impervious Area Reduction intercepts stormwater and traps pollutants. Onsite Stormwater A swale is designed so that water will flow evenly Management across the entire width of a densely vegetated area. Flow Control For best performance and where feasible, offline swales are preferred. For small contributing areas, I Water Quality Treatment a swale may be designed for both treatment and conveyance of onsite stormwater flow. This combined UIC Pretreatment use can reduce development costs by eliminating the need for separate conveyance systems. Note, Public Property however, that the design shall satisfy both water quality and conveyance requirements. That is, it shall Single-Family Residential have adequate retention time during the water quality Development V flow, and adequate stability such that pollutants are Prescriptive Sizing not remobilized during the conveyance flow. In areas Methods 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 (Typical and not for construction). Facility Footprint 111 See construction section for soil protection requirements o, Optional benching above design ponding I f •5 depth to integrate swale Bottom width 10' Width from edge of m 4 Po into the landscape into zone 1 / E1 ! " mn1. . freeboard ing , ---- - Mulch Native soil —/ -..`"--- <,..Biofiltration - - - - -,d '- -- soil mix Lake Oswego Stormwater Management Manual Section 4.6 105 Swales Site Requirements • Sites shall be large enough to incorporate a , . minimum of a 100-foot length. Shorter distances shall be designed as rain gardens. # A. •- . cr. . . ,,rir • Swales may be designed for a range of site - — — conditions; from full sun to full shade. Plant s selection shall match site conditions. s . ,L '• _ 4 • Swale edges shall be at least 5 feet from property - . lines. • The edges of swale design ponding depth (see �'�''� Figure 4.9) shall be at least 10 feet from buildings. • Swales are not appropriate adjacent to roads with slopes greater than 15 percent. • Swales shall not be located downstream of detention. Design Requirements 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. • Side slopes: no steeper 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) • Residence time: 9 minutes minimum o L-IlConveyance and Outlet/Overflow 4. ; - a -' Swales shall include outlets to an approved f/' r .\6e location. Undeveloped i,, a` " ''" s\O e area al Outflow • 3'min. bottom width r ) _z——. nen .ffr,... 1.- . ,i. Dispersed inflow �F 1 r Pavement Inflow L " u Figure 4.10. Swale diagram 106 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: Q25Peak post- construction) using an approved method. (See 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:1 V 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 + Z 2) 0.5 A cross-section area A = by + Zy 2 A R hydraulic radius P Lake Oswego Stormwater Management Manual Section 4.6 107 Swales Step 5: Velocity Calculate velocity using Chezy-Manning equation. Parameter Units Equation V velocity feet per second (1.49 / n) (R 2/3) (Si_1/2) Step 6: Calculate Residence Time r. Calculate required length for 9-minute residence time needed to adequately settle sediment. Parameter Units L length feet 60 sec L = Vx9minutes ( ) 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 108 Section 4.6 Lake Oswego Stormwater Management Manual Swales Materials Biofiltration Soil Mix Swale soil media shall support long-term plant and soil health and provide treatment to water as it infiltrates. Biofiltration soil mix shall meet the requirements of Appendix D and shall comprise at least the top 18 inches of soil depth and be placed on top of uncompacted native soil. 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 shall 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 shall be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It shall 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 shall 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 shall 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 4.6 109 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 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 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 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. 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 swale. 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 K for further maintenance guidance. Herbicides shall not be used as a weed control technique within stormwater facilities. Insecticide and fungicide shall not be used. 110 Section 4.6 Lake Oswego Stormwater Management Manual I t. • � /I , • ' ;. , . _:IIiii _ SthidTiIters - ti • _i. ... .. _ 1 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. Introduction Applicability Table A sand filter includes a flow spreader, a sand bed, and may include an underdrain collection system. I This facility is applicable for: Pretreatment is required through another facility. I Impervious Area Reduction Sand filters can be constructed above, at, or below grade. Depending on site conditions, sand filters can Onsite Stormwater be designed to completely infiltrate all the stormwater Management they receive or be designed as flow-through facilities where only a portion of the flow is infiltrated, and I Flow Control overflow is directed to an approved discharge point. I Water Quality Treatment If plants are used, sand filters can be used to help fulfill a site's required landscaping area requirement UIC Pretreatment and shall be integrated into the overall site design. Numerous design variations of shape, wall treatment, Public Property and planting scheme can be used to fit the character I Single-Family Residential of a site. Development V Guidance provided for sand filters has been adapted Prescriptive Sizing Methods from the City of Portland Stormwater Management Manual (2008). Figure 4.10. Sand Filter Section (Typical and not for construction). Overflow 12" Maximum ponding depth .foL 14 `l l � 1 5 f` ■ 4 hfll 1 ': '_.7 Sand filter —'� ,, I I� i r I' i 'iI I ! i,#� '= depth is >_ 12" — - • ' Filter material 'at, my 5, 42p41 I L 2" - 3" 1-13 Washed drainJ= I _ *M [ a errock > 8 111—f!M � Outlet to approved Native soil Perforated location underdrain Lake Oswego Stormwater Management Manual Section 4.6 111 Sand Filters Site Requirements IIIMMIIIIIIIIIIIPI __ T • Infiltration sand filters shall be at least 5 feet from .0- + , - - Ni property lines. !i t....." • Infiltration sand filters shall be at least 10 feet from building foundations. i' ' ___ • No setbacks to building foundations are required - . _ for lined flow-through sand filters where the - height above finished grade is 30 inches or ',, less. Required setback of 5 feet to rights-of-way, 'Cr utilities, and pipelines. �� £ <._� :-s . r; �4b} 4 - • 4 feet of hydraulic head is required from inlet to Outlet. Sand filter facility • Pretreatment shall be provided with a separate facility for the removal of trash and sediment. Design Requirements Sand Filter Dimensions • Width: >_ 18 inches (flow through) and 30 • Length to width ratio: 2:1 minimum inches (infiltration) • Slope: <- 0.5 percent in all directions • Depth of sand filter bed: >- 12 inches • For subgrade facilities, the filter medium shall • 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 shall be at least 12 • Design to drain in 24 hours inches between the top of the filter medium and the base of the overflow Figure 4.11. Subsurface Sand Filter Section (Typical and not for construction). Note: structural walls Downspout • • - - may be required. Trapped silt basin with lid , - r • _ 4" Perforated goidistut.r distribution pipe •. + +Sandy loam • -CIPM. .•, • 2" to 3" Choker course . a. 8" of Drain rock --- _. - . - ;r • - 12" to 30" of Sand 2" to 3" Filter material - lei lam- _-4 7 .1 Outfall to 4 8" of Drain rock a approved location Native soil Perforated underdrain 112 Section 4.6 Lake Oswego Stormwater Management Manual Sand Filters Conveyance and Outlet/Overflow Sand filters shall 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 shall 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 roiling 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. 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 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 Lake Oswego Stormwater Management Manual Section 4.6 113 Sand Filters 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 p maximum ponding depth feet Step 4: Calculate Hydraulic Gradient Calculate the hydraulic gradient (I [unitless]) across the sand filter media using the equation: (D 2 + M ) I = Step 5: Calculate Sand Filter Surface Area Calculate the sand filter surface area (Asf[sf]) required using the equation: A t Pdesign A sf K * I * T Parameter Units Value R routing factor (unitless) 0.7 T drawdown time hour 24 K hydraulic conductivity of sand bed inch/hour 1.0 Method derived from Eastern Washington Stormwater Management Manual (Ecology 2004) 114 Section 4.6 Lake Oswego Stormwater Management Manual Sand Filters Materials Sand filters shall have vegetated side slopes (maximum 3H:1V slope) if space allows. If not feasible, vertical walls can be used around the filter. The sand in a filter shall consist of a medium sand meeting the size gradation (by weight) given in the table below. The contractor shall 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 1.50-inch to 0.75-inch washed drain rock (ODOT Standard Specification 00430.11, AASHTO No. 4, or equivalent). All flow-through facilities shall use 0.75- inch to 0.50-inch washed drain rock (ODOT 00430.11,AASHTO No. 67, or equivalent). Drain rock and sand shall be separated by filter material per ODOT Standard Specification 02610.10. Filter Material Filter material shall be a specially graded mixture of course sand and crushed or uncrushed rock per ODOT Specification 2610.10. 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. Lake Oswego Stormwater Management Manual Section 4.6 115 Sand Filters Underdrains Underdrains shall be prefabricated, perforated PVC pipe that meets ODOT Standard Specification 02410.70, or approved equal. Piping installation shall 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 shall 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 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 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. 116 Section 4.6 Lake Oswego Stormwater Management Manual r F p�; r`+ i 7 k • . r r : 6 �,}yii_ �� !'' l it:S1. 4.,A .C. :11 .." . 14, .•%' I yid' .. i we • • .„ 1 i y-r Sy ., t�4,i ;ii h •Rid JO. •4. . ,tip', '�: d,' '� , �ti i \ 1 V. J. • t(.. y � l l .11t &�����,' 1 f ' it t _ n.d'A ' d \ %. S. � ����, . c.,A., 1 ,1 . illiliiiWil' r�. ... `a'1F iPal. i L `" J-Ai7-i •, t ,_rr,:-•• 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. Applicability Table Introduction Constructed wetlands present an opportunity to This facility is applicable for: integrate wildlife habitat and a public amenity into Impervious Area Reduction the landscape of a large residential or commercial development. Constructed wetlands with healthy, Onsite Stormwater thriving plants provide excellent water quality Management treatment. They require a large surface area and a large contributing area to ensure that wetland Flow Control V conditions are maintained. Water Quality Treatment Although this guidance describes a stormwater facility UIC Pretreatment most applicable for large developments, planting and soil recommendations are also suitable for smaller Public Property sites. If a residential or other small property has moist soil conditions, consider designing to enhance those Single-Family Residential wetland conditions. Development V Vector (mosquito) control is an important design Prescriptive Sizing consideration for any facility that has standing water Methods . 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. Q'"7 Figure 4.12 Constructed Wetland Section (Typical and not for construction). , , - Lr:i Leaend '' ~r .�S — — — Maximum ponding depth = 4 feet '" Average ponding depth <_ 2.5 feet : I • • !j ; : . c) . g c' ; 0) ; c' • 0) • ri • x . ▪ D • u) •w • • w • • w • cn • D • Lake Oswego Stormwater Management Manual Section 4.6 117 Constructed Wetlands Site Requirements -`'.ri6 . • .:.. • • Adequate space for wetland footprint and -, ' ` • ' ,, '° rl i 4:I " 1 maintenance access. • l r '' '', �' i ' .ti., a 1�a1'r • The edge of wetland shall be located a minimum of r ;'. Ij r `t ' ,'' • ; .t': +' 'k;' 5 feet from property lines. 1 - -' , • :r: r r�• , t i � ��' �r • The edge of wetland shall be located a minimum of .i ' - ; II-- , ._- ' f 10 feet from buildings. ! �s � �, -I. -. Iffy '' +` , � • Appropriate for Group C or D soils. t • Appropriate site hydrology to sustain saturated ` A. �. i conditions during part of the growing season to Constructed wetland vegetation facilitates filtration and maintain wetland vegetation. settling of pollutants Geometry Constructed wetlands shall 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 3.0 feet • Maximum ponding depth: 4.0 feet • Average ponding depth: <_ 2.5 feet • Slopes and depths shall vary to provide a variety of habitat and maximize treatment. • Side slopes: no steeper than 5 horizontal: 1 , 4ti, vertical below maximum ponding depth '4.' ,• • f-' ~ y` . - • Side slopes: no greater than 3 horizontal: 1 ;f {;" '' vertical above maximum ponding depth -.= sk.` , —��'� - { $ _- _. K$r 4t•'� LL ter' • .+. .y. Sr 3' .:. 11'Y4 Ski'.._ (*i.fi",,1 - �'}_' .a.� am _ 4 Clean Water Services'Spyglass pond was retrofitted to function as a constructed wetland. In addition to improving water quality, it provided habitat for endangered turtle species, as well as other amphibians and birds 118 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 ( Pdesign - 0.2 [ - 101 ) 2 Vimp + Vpery= 3,630 * A * CN 8 * [ 1,000 + O ( design - 10 ) CN Parameter Units Values Vimp + 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. 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 4.6 119 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 shall 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. Soil amendments shall meet the requirements of Appendix D. Mulch Fine to medium hemlock bark or well-aged organic yard debris compost is recommended for constructed wetlands. It shall be placed on bare areas surrounding planted material in order to prevent weed establishment. Mulch shall be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It shall 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 shall 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 shall be designed so that they do not require mowing. See Appendix H for a complete plant list of species appropriate for constructed wetlands. 120 Section 4.6 Lake Oswego Stormwater Management Manual Constructed Wetlands Plant Quality Vegetation Type Recommended Size Sedges and rushes 10.0-inch deep container'; plug2; division; rhizome; or tuber 1.0 gallon or equivalent; plug; 4.0-inch pot allowed if the facility is Grasses and forbs left off-line for the first wet season, allowing plants an appropriate establishment period Shrubs 1.0 gallon; bareroot; or equivalent 3.0 gallon container; bareroot or equivalent, minimum caliper shall Deciduous trees be 1.5 inches at 6.0 inches above base. Mature size shall be suited to site. Evergreen trees 3.0 gallon container or equivalent; minimum height 6.0 feet, unless approved by City. Mature size shall 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 shall 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 shall overflow to an approved outlet structure or discharge location. Lake Oswego Stormwater Management Manual Section 4.6 121 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. • A spillway shall be designed by a professional engineer and include energy dissipation or direct connection to the City's public stormwater system to avoid scouring and erosion. 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 Section 3.3 for more information. Maintenance Maintain access routes to the constructed wetland for maintenance purposes.A public wetland facility shall 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 shall be applied by a state licensed applicator and shall be approved by the City prior to application. 122 Section 4.6 Lake Oswego Stormwater Management Manual ,. ,- ;+ iiir 7.40,111kE 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 WET PONDS (RETENTION PONDS) Applicability Table Wet ponds have a permanent pool of standing This facility is applicable for: water. Pollutants are removed through settling and biological processes. They are a good choice where Impervious Area Reduction soil infiltration rates are inadequate for an infiltration Onsite Stormwater pond or rain garden, where there is a large enough Management Infiltration Pond contributing area that the pond will not be stagnant, and where they can be incorporated into the Flow Control landscape as an amenity. Water Quality Treatment Wet Pond INFILTRATION PONDS UIC Pretreatment Wet Pond Infiltration ponds are vegetated depressions that temporarily pool stormwater before it percolates into Public Property underlying soils. Infiltration ponds may be solely for flow control, if soils are granular, in which case Single-Family Residential a pretreatment facility is required. If soils have Development appropriate characteristics (organic content, soil Prescriptive Sizing sorptive capacity), infiltration facilities may also Methods provide runoff treatment through physical filtration, adsorption, and precipitation. They are suitable 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. Wet ponds may be combined with detention ponds to provide water quality treatment and flow control in a single facility. Lake Oswego Stormwater Management Manual Section 4.6 123 Ponds Site Requirements • Contributing area shall be at least 5 acres. ;a, • Site shall have adequate space for pond and -"'* — Ill • Pond type selection shall be appropriate for _ soil characteristics (e.g., if soils have adequate - 1 infiltration capacity, design an infiltration pond -' � f ; IIIN rather than a lined wetpond) , • Edge of water surface shall be at least 20 feet w 'a `� � ~ #riW from property lines and structures. This pond captures and treats neighborhood runoff • Edge of water surface shall be at least 200 feet from steep slopes. • 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. 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 shall be provided to forebay. • Maximum side slopes: 3H:1V • Length to width ratio: >_ 3:1 Figure 4.13. Pond Section (Typical and not for construction). Vegetated side slopes Edge of water surface 14 4. 1 .f a ••••• Maximum ponding depth 1 , • Ip ��� V • —- I� Permanent pool depth • — n w-t .ond - il i Infiltration (infiltration ponds) 124 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: ( r 1,000 1 ) 2 Pdesign - 0.2 L - 10 J Vimp + Vpery = 3,630 * A * CN ( design + 0.8 * [ 1,000 - 10] ) CN Parameter Units Values Vimp + 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 4.6 125 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. Soil amendments shall meet the requirements of Appendix D. 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. Keep mulch material out of the stormwater flow path to avoid clogging inlets or outlets. Mulch shall be weed-free and applied 2 to 3 inches thick to cover all soil between plants. It shall 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. 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 Infiltration Ponds: in areas along side slopes and tops of banks where water levels and soil Infiltration ponds are conditions are amenable to vegetation. designed to drain within a 24-hour period. Follow Wet Ponds: Establish dense growth of native submergent and emergent the vegetation guidance vegetation along pond side slopes, and a diversity of groundcovers, for rain gardens (Section shrubs, and trees along edges and borders of the pond. Added water 4.6.1) when designing quality treatment can be integrated into a pond system by adding a an infiltration pond, while shallower wetland bench upstream of the pond outlet. adjusting planting zones to the appropriate water depth tolerances along Plant Selection and Diversity pond 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 shall cater to specific site conditions for each facility. 126 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 shall 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 10.0-inch deep container'; plug2; division; rhizome; or tuber Grasses and forbs 1.0 gallon or equivalent; plug Shrubs 1.0 gallon; bareroot; or equivalent Deciduous trees 3.0 gallon container; bareroot or equivalent, minimum caliper shall be 1.5 inches at 6.0 inches above base Evergreen trees 3.0 gallon container or equivalent; minimum height 6.0 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 shall 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. 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. Lake Oswego Stormwater Management Manual Section 4.6 127 Ponds Conveyance and Outlet/Overflow Overflow shall 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 shall be at least 12 inches above the primary outlet elevation. • The minimum spillway depth shall be 12 inches from the top of the berm. • A spillway shall be designed by a professional engineer and include energy dissipation or direct connection to the City's public stormwater system to avoid scouring and erosion. Construction The planting designer shall place the plants during construction so that proper submergence depth ranges can be maintained, and shall create as-builts. 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. Maintenance Maintain access routes to the pond for maintenance purposes. A public pond facility shall 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. Proper maintenance is essential for a functioning pond. Ponds 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 K for further maintenance guidance. Herbicides shall not be used as a weed control technique within stormwater facilities. Insecticide and fungicide shall not be used. 128 Section 4.6 Lake Oswego Stormwater Management Manual . , _ 0 ti e . �: y s , . i fls.. , ' s .. "has �rl mil =r • •auitsF 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 Detention pipes and vaults are appropriate for Applicability Table commercial sites, industrial locations, and roads. They are used for large development projects on sites This facility is applicable for: with inadequate space or unsuitable conditions for an infiltration pond, constructed wetland with storage, or Impervious Area Reduction detention pond. They are prohibited from use for single- Onsite Stormwater family residential developments. Management Detention pipes and vaults are for flow control. They Flow Control do not provide water quality treatment, so a separate water quality treatment BMP is necessary to meet water I Water Quality Treatment quality treatment requirements. Detention facilities shall be designed by an engineer. Designs shall demonstrate UIC Pretreatment that the facilities have adequate maintenance access, can withstand vehicular and other structural loadings, Public Property will be stable, have been designed to counteract buoyancy forces in areas of high groundwater, and that Single-Family Residential the materials can withstand chemical properties of soils Development on the site. Prescriptive Sizing Methods To demonstrate that detention facilities will perform their primary flow control function, an engineer shall submit calculations from an approved hydrologic model or method. Figure 4.14. Detention Pipe and Vault (Typical and not for construction). Laller slew gi`�liiiill' 7 r il 1 Rill-Ell IMII - Leif rr 3? _ — mai moos Fr�F-r. — mirim I I •11 Ca - - -r 1 L 3ITni<�n P am. L Milli arkr Isile IIIII 1_ See Ciy slindird draft tr Lake Oswego Stormwater Management Manual Section 4.6 129 Detention Pipes and Vaults Site Requirements • Underground storage facilities shall be located to avoid conflicts with other underground utilities. • Regular maintenance is essential to ensure continued function of underground detention facilities. Therefore, they shall be accessible by maintenance vehicles. • Pipes and vaults shall 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: >_ 36.0 inches • Pipe bottom shall be flat or gently sloped: <_ 0.5 percent • Maximum distance between pipe bottom and finish grade: 20.0 feet • Sediment storage depth in upstream standard manhole: >_ 6.0 inches • Freeboard (distance between design headwater elevation and overflow elevation in control structure): >_ 6.0 inches Detention Vault • Vault bottom shall be flat or gently sloped to the center, forming a "V": <_ 0.5 percent • Sediment storage depth: >_ 6.0 inches • Freeboard: >_ 6.0 inches Pollution/Flow Control Manhole • The minimum allowable diameter for an orifice used to control flows in a public facility is 2.0 inches. Private facilities may use a 1.0-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 3.0 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. 130 Section 4.6 Lake Oswego Stormwater Management Manual Detention Pipes and Vaults Design Steps Detention facilities shall 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 shall match the predeveloped peak discharge rate specified in Section 4.1 using an approved single-event hydrologic model. The CN for predevelopment is 70. • 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 shall meet structural requirements for overburden support and traffic loadings, if appropriate. H-20 live loads shall be accommodated for pipes and vaults under roadways and parking areas. End caps shall 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 3.20 inches Step 2: Establish Preliminary Geometry Determine initial values for detention pipe or vault geometry parameters. Lake Oswego Stormwater Management Manual Section 4.6 131 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) 0.5 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 (Qpfe). 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. 132 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 18.0-inch-thick layer of 1.5 to 3.0-inch, evenly graded, washed rock. Orifice holes shall be externally protected by stainless steel with a mesh of 0.75 inch or less. Chicken wire shall not be used for this application. Orifices less than 3.0 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 shall be provided that regulate outflow from the underground detention facilities. Pollution/flow control manholes shall comply with the specifications outlined in the City's Standard Details numbers B-1.08A and B-1.08B. The drainage report and design drawings, in addition to documenting existing and proposed conditions, shall 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 shall have a maintenance hole or watertight end plate or plug of standard manufacture (not constructed in the field), which shall be made from the same material as the detention pipe. Lake Oswego Stormwater Management Manual Section 4.6 133 Detention Pipes and Vaults Access The following access requirements apply to detention pipes: • Detention pipes more than 50 feet long shall provide a cleanout. • Detention pipes more than 100 feet long shall have access risers at each end to allow for maintenance and repair. • Detention pipes over 200 feet long shall 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 shall be readily accessible by maintenance vehicles. • Detention pipes and vaults shall 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. 134 Section 4.6 Lake Oswego Stormwater Management Manual ee owDispersion . Photo b David Haw.00d. 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 Applicability Table Sheet flow dispersion is applicable for impervious surfaces with slopes less than 15 percent, such as This facility is applicable for: driveways, sport courts, patios, roofs without gutters, recreational vehicle pads, or other situations where Impervious Area Reduction concentration of flows can be avoided. Onsite Stormwater Site Requirements Management Native soil design infiltration rate shall be at least Flow Control • 0.25 inches per hour if being used for onsite stormwater management. Water Quality Treatment • Dispersion is not permitted within potential UIC Pretreatment N/A landslide areas. At the City engineer's discretion, a geotechnical report may be required. Public Property • Dispersion is not permitted within 10 feet of a Single-Family Residential steep slope (greater than 25 percent). Development Prescriptive Sizing • Dispersion is not permitted over contaminated Methods sites or abandoned landfills. • For sites with septic systems, the discharge point shall 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. • Area receiving flow shall be protected from compaction during construction, or substantial soil amendment may be required prior to final site stabilization. Design Requirements • A 2-foot-wide transition zone to discourage channeling shall be provided between the edge of the contributing impervious area and the downslope vegetation. This may be an extension 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 shall be provided for up to 20 feet of width of contributing impervious surface. An additional 5 feet of buffer width shall be added for each additional 20 feet of width of contributing area or fraction thereof. Lake Oswego Stormwater Management Manual Section 4.6 135 Sheet Flow Dispersion • The flow path shall be covered with a well-established landscape area (landscaping with well- established groundcover, or native vegetation with natural groundcover). The groundcover shall be dense enough to help disperse and infiltrate flows and to prevent erosion. Overflow Conveyance Minimum requirements associated with overflow conveyance design include the following: • The dispersion area shall convey excess flow - to an approved discharge point. Conveyance of large storms shall be considered. •, > id 0 A . IV Ti r" • No erosion or flooding of downstream - properties may result. � '��=`` Photo by Humphry Bolton. This grassy field provides dispersion of water from the adjacent roadway 136 Section 4.6 Lake Oswego Stormwater Management Manual tr--•• ' *" ' • ,.... :q: f - 4 a ó r w a r . ,: . • a V Y r.+ a L •. -A" 11 t Devitfes T } a. 4.6.15 Proprietary Stormwater Treatment Devices Definition Proprietary Stormwater Treatment Devices are manufactured technologies used to address the stormwater quality impacts of land development, including removing pollutants through physical, chemical or biological treatment processes. These stormwater systems rely upon a variety of mechanisms to remove pollutants. Introduction Applicability Table Proprietary Stormwater Treatment Devices include These types of facilities are applicable for: hydrodynamic separators, cartridge filters, and other Impervious Area Reduction emerging treatment technologies that are designed Onsite Stormwater to remove pollutants from stormwater. Proprietary Management BMPs/facility are generally grouped by their use for pretreatment, oil removal, enhanced treatment, basic Flow Control treatment, phosphorus removal and construction pollutant Water Quality Treatment ✓ management. There are numerous manufacturers that that build proprietary stormwater treatment devices. UIC Pretreatment ✓ Stormwater treatment technologies are reviewed and Public Property ✓ certified by several agencies. Lake Oswego follows the Technology Assessment Protocol - Ecology— better Single-Family Residential known as the TAPE program — administered by the Development Washington State Department of Ecology (Ecology). Lake Prescriptive Sizing Oswego accepts proprietary stormwater treatment devices Methods that have General Use Level Designation (GULD) for basic, dissolved metals, or phosphorus treatment as water quality treatment BMPs. Pilot Use Level Designation (PULD) or Conditional Use Level Designation (CULD) are not permitted. The City may require pretreatment facilities to improve the performance of proprietary stormwater treatment devices. The proprietary stormwater treatment devices that have been tested and approved under Ecology's TAPE program are regularly updated on the TAPE program website. https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-permittee- guidance-resources/Emerging-stormwater-treatment-technologies Site Requirements • Proprietary stormwater treatment devices may be located on a range of site conditions. Site requirements vary by type of system. Review the manufacturer's restrictions and recommendations when selecting an appropriate treatment device and configuration for the development and site conditions. • Proprietary stormwater treatment facility shall be a minimum of 5 feet from structures. Lake Oswego Stormwater Management Manual Section 4.6 137 Proprietary Stormwater Treatment Devices • There shall be an approved overflow route. • Provide maintenance access in accordance with manufacturer recommendations. Design Requirements • Proprietary stormwater treatment devices may be configured as an inline system or an offline system with high flow bypass, in accordance with manufacturer specifications. • To meet water quality standards, the proposed proprietary stormwater treatment device must have the Ecology approval under the General Use Level Designation (GULD) for basic, dissolved metals, or phosphorus treatment as water quality treatment BMPs. Pilot Use Level Designation (PULD) or Conditional Use Level Designation (CULD) are not approved to meet water quality treatment standards. • Proprietary stormwater treatment devices shall be designed to treat the peak flow or total volume from the water quality storm event, as defined in Table 4.1. • Calculations to determine the required size, number, or configuration of the proprietary stormwater treatment device must be based on the design guidelines specified in the GULD approval documents from Ecology. Maintenance Provide access for the maintenance equipment and personnel required to perform the maintenance recommended by the manufacturer. See Chapter 7 for additional maintenance guidance. 138 Section 4.6 Lake Oswego Stormwater Management Manual 5 Conveyance and Detention DesIgn_Standards 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 approved 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. Public lines shall be extended to the far side of the property, i.e. "to and through". 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 engineering design standards for conveyance systems and specifications not covered in this manual, including all acceptable materials. In the event that a discrepancy occurs, the more stringent and protective of water quality shall apply. • 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. • Foundation drains shall not be directed to vegetated stormwater facilities. 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. 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 Lake Oswego Stormwater Management Manual Section 5.1 139 Conveyance and Detention Design Standards extension. 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 structure's midpoint 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.Access specifications are covered in Chapter 7 and in the City's Engineering Design Standards. If a discrepancy occurs, the more stringent and protective of water quality will apply. • 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 required by the City's municipal code regarding Sensitive Lands and/or Tree code.At a minimum, the disturbed area must be restored to the condition prior to development. 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. 140 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 shall 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 the 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: 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 estimates of development densities in areas included in recent additions to the urban growth boundary. Lake Oswego Stormwater Management Manual Section 5.4 141 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 C 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 F. • Soil types shall be derived from the NRCS soil survey for the appropriate county (Multnomah, Clackamas/Washington County). See Appendix F for reference. 5.5 Surface Water Conveyance Design Considerations In general, the designer shall consider the following when designing the conveyance system. 142 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 future build out flows generated from an upstream basin. 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 shall be provided including maximum pipe capacity reached in the analysis. 5.5.2 Stormwater Conveyance Design Criteria The design storm to be used for sizing of 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, including bridges, on Any 100-year Runoff waterways with mapped FEMA 100-year Recurrence floodplain <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 Culverts on Major Arterials >_ 40 acres 50-year <40 acres 10-year Collectors and Other Public Streets >_ 40 acres 50-year <40 acres 10-year Culverts on Private Drives >_ 40 acres 25-year a Includes roadside ditches and drainage swales. Flow shall 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. Instream work shall comply with USACE, DSL, and ODFW requirements.Any discrepancy between the agency requirements shall use the more stringent and protective of water quality. b Piped sections longer than typical normal crossing with a culvert 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 the public stormwater system are available from the City's Engineering Design Standards. Table 5.2 summarizes approved materials for the public stormwater system.Any discrepancy shall use the more stringent and protective of water qulaity. Lake Oswego Stormwater Management Manual Section 5.5 143 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 Stormwater pipes 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 and street inlets shall be located, at a minimum, on the upstream side of an intersection and at sags in roadways. Maximum spacing between catchbasins shall be: • 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. Manholes shall be installed at directional changes with a maximum spacing of 500 feet. Minimum Cleansing Velocity The pipe shall be sized to ensure a minimum velocity of 3 feet per second (fps) and a maximum velocity of 10 fps 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 shall be assumed to require permitting through the U.S. Army Corps of Engineers (USACE) and Oregon Department of State Lands (DSL) 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. '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). 144 Section 5.5 Lake Oswego Stormwater Management Manual Conveyance and Detention Design Standards Channel Dimensions Oregon Department of Fish and Wildlife (ODFW) 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 shall 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 shall 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, placed at grade to match the stream channel bed slope, and match the natural stream channel orientation. 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 Materials Conveyance ditches shall have open graded rock sized to be stable under the hydraulic design events listed in Table 5.1. Beds of natural streams shall follow current ODFW, DSL, and USACE requirements with any discrepancies defaulting to the more stringent and protective of water quality. 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 is prohibited. 5.5.5 Upstream Impacts Modifications to the existing onsite storm drainage patterns 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 recorded with the County, 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 5.5 145 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 without negatively affecting upstream property owners. 5.5.8 Dissipation of Runoff Discharge Runoff exiting a development site shall be discharged with adequate energy dissipaters that comply with la the City's Engineering Design Standards. At a minimum, energy dissipaters shall have a 6-inch depth, a width 3 times the diameter of the pipe, and a length 4 times the diameter of the pipe 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 stormwater conveyance shall be laid on a straight alignment and at uniform grade. Manholes shall be constructed at changes in direction. 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. 146 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, the City's Engineering Design Standards, 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 pipeline systems. 5.7.2 Planning Considerations Where a parcel requiring connection to the public stormwater 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. 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 require mitigation of downstream project impacts by requiring flow control via infiltration or flow control facilities. 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 that project impacts are not significant to the downstream system. If additional mitigation is required, identify additional or larger flow control facilities at the project site or other mitigation measures in downstream conveyance. Instream measures, if necessary or proposed, will require significant coordination with City Planning and Engineering staff to determine appropriateness and feasibility. Lake Oswego Stormwater Management Manual Section 5.8 147 Conveyance and Detention Design Standards Figure 5.1. Downstream analysis process. Does the project provide onsite yEs stormwater management for the —' No'hither acforr 41 1 0-yea r design storm? i 4NO Are there crg capadyF or errnicr problems n dowrrrearn conveim ce? YES IID Coe;project m eet iliac control ,ES —D. to further acorn ) requkermuht? FID Lii, " Perform beam eratrals- ) 11 YES ( Does the dar ruteafn ) — drainage sum Fiore adequide cm way for peak hair couveyarroe? ilo NG Irnplement inuu antral I IPs or madly ccureeyance to provide capactly at peak new ivicated mTable 5_1 148 Section 5.8 Lake Oswego Stormwater Management Manual Conveyance and Detention Design Standards The analytical steps are: Step 1 Determine the limits of the downstream analysis. For large projects, the limit is the receiving water body (go to Step 4). For small projects, the limit is determined in Step 2 and 3. Step 2 Determine contributing area of the public 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 3. If no, set the limit of the downstream analysis at 1,500 feet downstream from the project site and go to Step 4. Step 3 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 less than 10% of the contributing drainage area. Set the limits of the downstream analysis at the 10% dischrage location or 1,500 feet downstream of the project site, whichever is greater. Step 4 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. If such conditions are reported or observed, attempt to determine peak runoff rates causing these issues (Step 5) and estimated downstream conveyance (Step 6). 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 shall obtain stream channel data using qualified professionals applying City-approved protocols. Site access is the applicant's responsibility. Step 5 Assess flow conditions: for the watershed upstream of the drainage system, estimate the 2-year, 25-year, and 100-year peak discharge values for the developed condition of the project site and existing the upstream watershed area. Use the runoff estimation methods described below. Step 6 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 in the downstream system. Demonstrate that the 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. Lake Oswego Stormwater Management Manual Section 5.8 149 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 precipitation events listed in Table 5.1. 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 100 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. 150 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 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. • Hazardous materials are prohibited within flood management areas. • Existing nonconforming uses are allowed to continue in the flood management area. • 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 5.8 151 Conveyance and Detention Design Standards This page is intentionally left blank. 152 Section 5.8 Lake Oswego Stormwater Management Manual 6 Ta ��° ® � P a u° g � Th. j aCe Er PollutEs r Peets � 0 , TO 0 ci , )0 0 la - Lir) 0 E TO 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 Cit 's water Moreh informationErsion 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 sediment control requirements can be found in Chapter 52 of the Lake ErosOswego Code.A city erosion control permit is required for any projects discuss Control is also discussed in Chapter 38 that disturb 500 square feet or more. Smaller projects within 50 feet of and Chapter 52 of the Lake surface water bodies also require an erosion control permit. Oswego Code The City's utility code (Lake Oswego Code [LOC] 38.29.930) prohibits the For information on the Erosion dumping of"debris, soil, pollutants such as fuels, lubricants, bitumens, Control program, please call: sewage, paint and other harmful or hazardous substances into the surface Erosion Control Inspector at water management system." (503)675-3991 For information on the Surface 6.1.2 MS4 Permit Water Management System, please call: Under the City's MS4 permit, contractors have the following Stormwater Quality responsibilities: Coordinator at(503)675-3999 • Construction site operators shall develop erosion prevention and sediment control site plans and shall implement and maintain effective erosion prevention and sediment control BMPs. • Construction site operators shall prevent or control non- stormwater waste that may adversely affect water quality, such a discarded building materials, concrete truck washout, chemicals, 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 6.1 153 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 shall 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 Some important general principles 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 reducing grading activities particularly around sensitive resources, including areas intended for use as stormwater infiltration facilities 154 Section 6.1 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention, and Erosion and Temporary Sediment Control • Planning for containment of activities that could produce ,,,, non-stormwater pollution, such as trash and construction debris, concrete washout and sawcut debris, grout and/or 1I1 mortor waste, and equipment fueling (additional details can be found in Appendix J) • Preparing and maintaining documentation throughout the life of the project These poor construction practices not only create construction • —NC — site management issues (that cost time and resources) but also Ineffective erosion control 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. 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 shall be postponed and disturbed areas shall 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 ine 1pudingal BMPs.eWeather forecasts including expected features that are critical for infiltration and treatment of stormwater, precipitation amounts are available Construction activities shall 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 shall take into account the season and daily weather forecast. Grading activities See Section 6.3.1 for more shall occur during dry periods, and disturbed areas shall be stabilized information. as quickly as possible. 6.3.2 Phasing The site operator or developer shall consider the following phasing when scheduling development and construction activities. • Schedule structural BMP installation and implementation activities before any site activity starts. The schedule shall 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. Lake Oswego Stormwater Management Manual Section 6.3 155 Construction Phasing, Stormwater Pollution Prevention, and Erosion and Temporary Sediment Control 6.3.3 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. Standard erosion control measures, including sediment fence or wattles around the disturbed area, catch basin inserts, and a construction entrance constitute the required measures for every project. In addition, 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 shall be fully implemented and remain effective throughout these periods. The approved erosion control plans contain details and information pertaining to wet weather measures that shall be implemented on your site. 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 shall 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 City's Erosion and Sediment Control Manual.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) Wet weather erosion prevention requirements are as follows (Lake Oswego 2015): • All stockpiled material shall 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 shall 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 shall 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 shall 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 shall 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 shall 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 shall be implemented. 156 Section 6.3 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control • Other measures may be required by the inspector to address site conditions.All measures shall 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 shall 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 shall consider the procedures listed in Section 6.4, below, when performing construction sequencing. 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 shall not only install physical BMPs to mitigate construction and development stormwater impacts, they shall also implement procedures in accordance with a planned schedule. Important procedural, or non-structural, BMPs are described below. Lake Oswego Stormwater Management Manual Section 6.4 157 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control Developers and contractors planning 6.4.2 Managerial Practices construction activities shall also consult Appendix J for source control BMPs Managerial BMPs include administrative and procedural practices related to construction practices.These during development and construction. The applicant shall consider, include: at a minimum, the following managerial practices when developing a • Dust control site. • Proper storage of solid wastes, • Train staff and contractors including trash and concrete waste • Spill prevention and cleanup • Ensure responsibility by implementing agreements with subcontractors requiring them to adhere to legal requirements • Proper disposal of fluids and and permits, construction practices described in this chapter, wastes the erosion control plan, and the City's Erosion and Sediment • Painting,finishing, and coating Control Manual of vehicles, boats, buildings, and equipment • Implement the project erosion control plan before construction • Outdoor storage or transfer of starts. Once the erosion controls are installed, call for an solid raw materials, by-products, or erosion control inspection. finished products • Inspect and maintain structural BMPs, documenting these Other source control BMPs may apply actions as they occur depending on the construction project. Consult Appendix J and the City's • Evaluate and update the project erosion control plan Erosion Control webpage for more information. 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 pollutants 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 City's Erosion and Sediment Control Manual shall 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 158 Section 6.4 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control • 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 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 slope length increases erosion potential by four times. Doubling the slope gradient increases erosional potential by five times. 6.5.2 Design and Construction Considerations Designers shall try to integrate existing trees and other natural vegetation into the site improvement plan and shall clearly denote the vegetated areas that shall 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. • 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 shall be placed in non-critical flat areas of the site. In no instance shall 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 shall be retained whenever possible and only areas necessary for construction activities shall be cleared. 6.5.3 Recommended Construction Practices The City's Erosion and Sediment Control Manual is the primary reference for construction practices to minimize disturbed land area and to protect soils and sensitive areas from sediment during construction. Lake Oswego Stormwater Management Manual Section 6.5 159 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control 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 shall 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 City's Erosion and Sediment Control Manual. Construction Access and Parking The primary access point(s) for construction traffic shall be identified on plans and stabilized before earthwork begins to prevent sediment and construction debris from leaving the site. On projects with 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. • 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 shall be installed along borders of sensitive areas, around areas of proposed stormwater management facilities, and at the limits of clearing and construction activities. They shall be installed downslope of proposed construction activities, and shall 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. 160 Section 6.5 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control 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 shall be appropriately protected during construction. If these locations cannot be appropriately protected from construction impacts and therefore loss of infiltration capacity, then the soil shall 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 are also applicable: • BMP: buffer zone • BMP: preserve natural vegetation • BMP: wattles • BMP: filter berm Stabilize Slopes and Disturbed Areas Areas that have been cleared shall 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 it must be covered at the end of each day. 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. • 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. Lake Oswego Stormwater Management Manual Section 6.5 161 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control • 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 shall 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. • BMP: Bonded Fiber Matrix • BMP: plastic sheeting Runoff Control Runoff shall 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 shall be prepared to handle concentrated or increased runoff. Where construction stormwater concentrates, the following can be used to reduce runoff velocities: • BMP: check dam • BMP: outlet protection • BMP: diversion dike/swale • BMP: sediment basin • BMP: grass-lined swale • BMP: sediment trap Control Pollutants • Provide leak-proof, well-marked garbage disposal containers. Litter, debris, scrap materials, and other waste are considered pollutants and shall be contained on the site. Small containers shall be emptied weekly; larger containers/dumpsters shall 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. 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!" 162 Section 6.5 Lake Oswego Stormwater Management Manual Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control Temporary and Permanent Stormwater Facilities • Know where the nearest storm drains are located, and keep them protected with non-woven filter inserts or biofilter bags. 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 shall 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. Implement the maintenance procedures to keep physical BMPs in good working order at all times. Your permit requires that you conduct regular inspections and document the findings of those inspections. Personnel selected to conduct inspections shall be knowledgeable in the principles and practices of erosion and sediment controls and preservation and protection controls. They shall also possess the technical skills to 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 as scheduled in the approved erosion control plan. • 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 shall include the date, BMP, location, and maintenance performed. Maintenance inspection reports shall be kept on site and made available to City or DEQ inspectors upon request. 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. Inspections will be documented with an inspection form, photographs and/or monitoring results as appropriate. Lake Oswego Stormwater Management Manual Section 6.5 163 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control 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. 6.6 Documentation n; ':.;; - `; 6.6.1 Erosion and Sediment Control Drawings The erosion and sediment control drawing is required by . both the City's erosion control permit, and as a component - - • of DEQs 1200-C permit application. Plan shall include a 111 • drawing set that will be part of the contract documents • for the project. Applicable BMPs are in the City's Erosion and Sediment control Manual and summarized in Section 6.5. The drawing set can be on a single sheet for smaller -••..� projects, but may require multiple sheets for larger Y -- projects.At a minimum, the scaled drawing set shall include the following information: • Primary access point(s) for construction traffic. ` ! Construction entrances shall be stabilized with clean 1.5 inch rock. • Limits of clearing and construction activities. Show perimeter controls that will be used to prevent sediment or construction debris from leaving the site. • Sensitive areas, such as streams or wetlands. Effective erosion control Show buffer protection measures that will be installed prior to any land-disturbing activities. • 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 including existing stormwater facilities within 100ft of the property boundaries. • Show temporary stormwater collection, conveyance, and treatment facilities. • Include general notes for erosion and sediment control. • Show location of concreted washout • Include wet weather notes if soil will be disturbed between October 1 and May 31 164 Section 6.5 Lake Oswego Stormwater Management Manual 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 are required to submit an the construction site. erosion control plan with their City permit application or NPDES 1200- C application to DEQ. This plan will help the site operator manage the site and avoid damage to areas that shall 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. 6.6.3 Inspection Log Contractors shall 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. Be sure to note which weather station is used as a reference. The maintenance inspection log shall be maintained onsite and provided to City or DEQ inspectors upon request. Lake Oswego Stormwater Management Manual 165 Construction Phasing, Stormwater Pollution Prevention and Erosion and Temporary Sediment Control This page is intentionally left blank. 166 Section 6.7 Lake Oswego Stormwater Management Manual 11 or water BMP 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. 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 in Appendix K for each of the stormwater 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 shall review the code for detailed legal requirements. 7.1.1 Maintenance Responsibilities Public Facilities Lake Oswego's Public Works Department 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 manual or a facility-specific OMP. Lake Oswego Stormwater Management Manual Section 7.1 167 Maintenance • Promptly repair and restore stormwater management facilities in accordance with the maintenance checklists provided in this manual. • Provide and maintain all necessary access routes from the public right-of-way in accordance with this manual or the OMP. 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/Facility (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. • Refer to the City's Engineering Design Standards for additional requirements for access roads. 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. 168 Section 7.1 Lake Oswego Stormwater Management Manual Maintenance 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. • 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/Facility design guidelines in Section 4.6.6) or gravel. Gravel examples include: 2 inches of 3/4-0 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. • Applicable ADA requirements, if any, may apply 7.1.4 Routine Maintenance Activities Stormwater facilities can include both structural BMPs 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 shall be described in the OMP. Checklists in Appendix J provide more details about operation and maintenance procedures for specific types of facilities. Catch Basins Routine cleaning of catch basins is one of the most important stormwater source control measures. Catch basins shall be cleaned when the sump is more than one third full. 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 7.1 169 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 shall 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 shall 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 shall maintain 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 shall 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 shall be removed and bare areas replanted. Do not use pesticides, herbicides, or 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 shall 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 and repair or replace broken or damaged components. Planters shall be watertight to protect abutting foundations from moisture damage. Check the planter liner; it shall 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. Vegetation shall cover a minimum of 90 percent of a planter. 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 shall be removed and bare areas replanted. Do not use pesticides, herbicides, or fertilizer. 170 Section 7.1 Lake Oswego Stormwater Management Manual Maintenance Infiltration Trench or Gallery Infiltration trenches and galleries shall be inspected annually during wet weather to verify detention times are met. Water-efficient irrigation shall be applied during plant establishment and during extended dry periods. Maintenance shall 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 and underground chambers to maintain conveyance capacity and storage. Repair or replace broken or damaged components. Drywell Drywells shall 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. To treat clogged or broken inlets, remove sediment and debris from pipes and catch basins to maintain conveyance capacity at all times. Repair or replace broken or damaged components. Greenroofs Scheduled seasonal maintenance is required for greenroofs. Maintenance activities vary in frequency depending on soil depth, vegetation type, and location. Structural components, including the waterproof membrane, shall 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 shall be protected during all maintenance activities. The growing medium shall be maintained at design depth to sustain a healthy plant cover and infiltrate within 48 hours. Planting shall 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 shall 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 shall 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 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. If moss prevents infiltration from occurring or creates a public slip hazard, use physical removal measures. Do not use herbicides or other moss removing chemical application. Lake Oswego Stormwater Management Manual Section 7.1 171 Maintenance 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 shall not be sealed. Rainwater Harvesting General maintenance for rainwater harvesting shall 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 shall have a monthly water balance that demonstrates adequate capacity for each month and reuse of all stored water annually. For system-specific rainwater harvesting maintenance schedules, refer to manufacturer orARCSA recommendations. Filter Strips Filter strips shall be inspected monthly during the rainy season to ensure adequate drainage and spreading of water across the strip. Water-efficient irrigation may 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, pesticides, or fertilizer. Swales Water-efficient irrigation may be applied during plant establishment and during extended dry periods. Swales shall 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, pesticides, or fertilizer. Trash and debris can impede water flow and clog the system. Remove all trash and debris and ensure conveyance capacity at all times. Check inlet pipes and outlet structures for damaged or missing components. 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 shall 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 shall freely convey stormwater. Remove sediment and debris from clogged structural components to maintain 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 shall 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, herbicides, or fertilizer. Prevent large roots from damaging structural components. 172 Section 7.1 Lake Oswego Stormwater Management Manual Maintenance The filter medium, including sands, gravels, and other materials, shall 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 may be applied during plant establishment and during extended dry periods. Constructed wetlands shall be inspected 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, pesticides, or fertilizer. 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 shall be applied during plant establishment and during extended dry periods. Ponds shall 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. Restore the integrity of energy-dissipating devices. 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 shall 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. When sediment accumulation exceeds 12 inches, remove oil, sediment, and debris from the conveyance system and pipes to maintain conveyance and storage capacity. Prevent large roots from damaging subsurface structural components. Note: Only professionals with confined space certification from the Occupational Safety and Health Administration (OSHA) shall enter below-ground stormwater systems. Sheet Flow Dispersion Sheet flow dispersion facilities shall be inspected monthly during the rainy season (October 1 through May 1) to ensure adequate drainage. Water-efficient irrigation may 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, pesticides, or fertilizer. Lake Oswego Stormwater Management Manual Section 7.4 173 Maintenance 7.1.5 Transfer of Property Ownership When ownership of stormwater management facilities are transferred to and accepted by the City, the transfer shall include all maintenance and access easements. If property is transferred to another owner, the responsible party shall 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. 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 30 days, or a shorter 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 Plan An OMP is required for all projects (small projects and large projects) subject to this manual. The OMP shall describe how to properly maintain each stormwater management facility, the frequency of maintenance required, and the party responsible for maintaining the facility. Owners/operators of facilities that manage stormwater shall file a record and O&M Plan 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. 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. 174 Section 7.4 Lake Oswego Stormwater Management Manual Maintenance 7.4 Maintenance Checklists Sample maintenance checklists for each facility type are included in Appendix K. 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 7.4 175 Maintenance This page is intentionally left blank. 176 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. The Practice of Watershed Protection 1(4): 210-213. Article 92. Technical Note #53. Pollutant Dynamics Within Stormwater Wetlands: Plant Uptake. Accessed July 30, 2015. <http:// stormwatercenter.net/Library/Practice_Articles.htm#cite> BCD. 2015. Oregon Smart Guide, Rainwater Harvesting. State of Oregon, Department of Consumer & Business Services, Building Codes Division. Accessed June 11, 2015. <http://www.bcd.oregon.gov/ pdf/3660.pdf> Center for Watershed Protection. 2011. Solutions for Clean Water and Healthy Natural Resources. Accessed July 30, 2015 <http://www.cwp.org/> Clackamas County, Water Environment Services; Clean Water Services; Oak Lodge Sanitary District; City of Gladstone; City of Happy Valley; City of Lake Oswego; City of Milwaukie; City of Oregon City; City of West Linn; and City of Wilsonville. 2008. Erosion Prevention and Sediment Control Planning and Design Manual. Revised December 2008. Clean Water Services. 2009. The Low Impact Development Approaches Handbook. Accessed August 12, 2015. <http://www.cleanwaterservices.org/PermitCenter/NewsAndResources/LIDAHandbook.aspx> Clean Water Services. 2007. Design and Construction Standards. Accessed August 12, 2015. <http://www. cleanwaterservices.org/PermitCenter/DesignAndConstruction/default.aspx> Debo, Thomas N. and Andrew J. Reese. 2003. Municipal Stormwater Management, 2nd ed. Lewis Publishers, CRC Press LLC. Boca Raton, Florida. DEQ. 2012. Water Quality Assessment Database. Water Quality Assessment- Oregon's 2012 Integrated Report Assessment Database and 303(d) List. Accessed August 12, 2015. <http://www.deq.state.or.us/wq/ assessment/rpt2012/search.asp#db> DEQ. 2013. Industrial Stormwater Best Management Practices Manual. State of Oregon Department of Environmental Quality, Water Quality Division, Surface Water Section, Dennis Jurries and Krista Ratliff, authors. February 2013 (last updated January 23, 2015). Ecology. 2012. Stormwater Management Manual for Western Washington. Publication Number 12-10-030. Washington State Department of Ecology. Olympia, Washington. Accessed July 30, 2015. <http://www.ecy. wa.gov/programs/wq/stormwater/manual.html> Ecology. 2004 Stormwater Management Manual for Eastern Washington. Publication Number 04-10-076. Washington State Department of Ecology. Olympia, Washington. Accessed July 30, 2015. <http://www.ecy. wa.gov/biblio/0410076.html> FHWA. 2007. Roadside Revegetation. An Integrated Approach to Establishing Native Plants. Report Number FHWA-WFL/TD-07-005. Federal Highway Administration. Western Lands Highway Division. Technology Deployment Program. Accessed July 30, 2015. <http://nativerevegetation.org/pdf/learn/ technical_guide.pdf> Lake Oswego Stormwater Management Manual Section 8.0 177 References Jurries, Dennis. January, 2003. Biofilters (Bioswales, Vegetated Buffers, & Constructed Wetlands) for Storm Water Discharge Pollution Removal. Oregon DEQ, Northwest Region Document. King County. 1998. King County, Washington Surface Water Design Manual. Department of Human Resources. September 1998. Kitsap County. 2012. Kitsap County Stormwater Pond Retrofit Design Guidance Manual. Kitsap County Public Works Department. Port Orchard, Washington. Kloster, T. Leybold, and C. Wilson. 2002.Green Streets: Innovative Solutions for Stormwater and Stream Crossings. In: Global Solutions for Urban Drainage. Edited by E.W. Strecker and W. C. Huber, pp. 1-18. Ninth International Conference on Urban Drainage ((ICUD). Portland, OR. Lane, E.W. 1955. Design of Stable Channels. Transactions of the American Society of Civil Engineers, Volume 120, pages 1-34. Lake Oswego, City of. 2015. Erosion Control Permit and Information. Lake Oswego Public Works. Accessed August 12, 2015. <http://www.ci.oswego.or.us/publicworks/erosion-control-permit-and- information> Lake Oswego, City of. 2013. Wet Weather Season Erosion and Sediment Control Requirements: Minimum effective dates: October 1st through May 31st and during all rainy periods. City of Lake Oswego Public Works Department. Revised May 2013. Lake Oswego, City of. 2012a. Parks Plan 2025: Lake Oswego Parks, Recreation and Natural Areas System Plan. Accessed October 4, 2012. <http://www.ci.oswego.or.us/parksrec/parks-plan-2025> Lake Oswego, City of. 2012b. Right Tree in the Right Place. Accessed August 12, 2015. <http://www. _ ci.oswego.or.us/sites/default/files/fileattachments/planning/webpage/12530/right_tree_right_place_sept12. pdf> Lake Oswego, City of. 2010. Surface Water Code/Surface Water Design Manual Revision: Project Charter. City of Lake Oswego Engineering-Surface Water. Lake Oswego, City of. 2007. Tree Removal, Protection & Care. Community Development Department Planning Division. 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>. 178 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_manual05/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 8.0 179 References This page is intentionally left blank. 180 Section 8.0 Lake Oswego Stormwater Management Manual 9 Definis 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 streamflow 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 exchange 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 straiahteninq 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 9.0 181 Definitions Curve number (CN) A factor included in hydrologic models using the SCS runoff equation, such as TR-55 and the Santa Barbara Urban Hydrograph, that is related to soil type, soil infiltration capability, and land use. Desianated Manaaement 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 or outfall. 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. 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 spawning 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. 182 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. Seasonal high groundwater, for the purposes of calculating separation distance to stormwater facilities, 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. 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; sport courts, traditional concrete or asphalt paving on walkways; driveways, parking lots or strips, gravel roads, pathways, and driveways; and packed earthen materials. Artificial turf and sport courts are considered impervious surfaces. Indirect discharae Stormwater that enters receiving waters via surface runoff or via groundwater. Industrial activities Activities covered by DEQ's industrial stormwaer (1200-Z) program or which are permitted uses for industrial or industrial park zones in the City's municipal code. Infiltration The movement of water through the soil matrix. Integrated pest management (IPM1 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. Land disturbance Any activity that alters the land surface in a way that modifies characteristics affecting rainfall runoff or erosion potential. Lake Oswego Stormwater Management Manual Section 9.0 183 Definitions Larae proiect New or redevelopment project that creates and/or replaces 3,000 square feet or more of effective impervious surface area. Large woody debris A collection of at least 5 pieces of interconnected dead woody material consisting of at least one piece which is 2ft in diameter and 33 ft in length. Low impact development (LIDS 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 (MEP1 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 shall 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 (NPDES), 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 at which water flows into the overflow or outlet of a facility. Peak flow The maximum rate of flow of water during or after a precipitation event. 184 Section 9.0 Lake Oswego Stormwater Management Manual Definitions Pilot infiltration test (PIT) 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. Pondina Death The depth of water allowed in a facility before stormwater enters the overflow structure. 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. Predevelooment 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). See Section 4.3 for guidelines on simulating predevelopment conditions for sizing flow control facilities. Pretreatment Treatment of stormwater to remove sediment that is included upstream of a primary stormwater treatment facility or BMP. Examples include sumped catch basins located upstream of drywells, proprietary BMPs, such as hydrodynamic separators located upstream of bioretention facilities, or a settling forebay included in the design of a constructed wetland or pond. 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. Lake Oswego Stormwater Management Manual Section 9.0 185 Definitions 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. 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. Retention The process of collecting and holding surface and stormwater runoff with no surface outflow Riffles Shallow areas in a stream channel where the surface of flowing water is broken by waves or ripples. Riaht-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. Seasonal Hiah Groundwater See Groundwater. 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. 186 Section 9.0 Lake Oswego Stormwater Management Manual Definitions Small project New or redevelopment project that creates 1000 sq ft of impervious area but less than 3000 sq ft. 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. Spillway An outlet designed by a registered Civil PE professional that will safely pass flows exceeding the design capacity of a pond or other large stormwater facility including the 100-yr 24-hr design storm event. Steep slope A steep slope is an average slope of 15 percent or more. Storm drain A pipe that transports stormwater. Stormwater The water that originates from precipitation, primarily rainfall and snowmelt. Stormwater management 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 management 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 management 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 (TMDL1 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 9.0 187