F-011 Brown and Caldwell Stormwater Analysis LU 23-0002, Land Use Review
Attachment A. Land Use Review Findings,LU 23-0002 I
LOC Section
and Title Summary of Requirements Applicant Information Findings
LOC 38.25.12O.1.d.i
A.Site As- Provide site assessment and feasibility infor- The applicant submitted a Drainage Report(Emerio, The site assessment information demonstrates feasibil-
sessment mation,including maps,soils information,and revised 9/15/23,Infiltration Testing(Rapid Soil So- ity to meet the guidelines in the LOSWMM,Section
and Feasibil- field testing. lutions,dated 7/8/21),preliminary site and utility 2.5.1 and Appendix A.
ity plan sheets,and other supporting information. This standard is met.
The submittal includes a description of the existing
site conditions and project overview.A vicinity map
and proposed utility plan are provided.The reports
and plan sheets document soil conditions,proposed
on-site drainage patterns,existing and proposed
conditions,and proposed stormwater management
facility locations.
B.On-Site Size onsite stormwater management facilities to This land use review is only for the stormwater for the The runoff calculations and stormwater facility sizing
Stormwater infiltrate the 10-year,24-hour design storm south access lane(not the associated five lots)be- calculations for the drywells in the applicant's Drainage
Manage- event to the maximum extent practicable(see cause a"flag lot"is being created through the lot- Report(Appendix C:Drywell Sizing)are not consistent
ment LOSWMM Table 2.1). line adjustment process. with LOSWMM,Section 4 and indicate it is feasible to
The Infiltration Testing performed by Rapid Soil Solu- fully manage the 10-year,24-hour storm onsite using a
tions measured an infiltration rate of 720 private drywell.The design infiltration rate used is cor-
inches/hour and identified groundwater at a depth of rect.
40 feet below the surface. As a condition of approval,if the applicant chooses to
For the south access lane,the applicant prepared use a surface infiltration system,the final facility place-
preliminary plans and calculations to demonstrate ment must adhere to setback requirements from build-
the feasibility of installing one private drywell to ing foundations(10')and property lines(5')per
manage the 10-year,24-hour storm event.Pre-treat LOSWMM Table 3.1. If the applicant chooses to use a
ment is identified for runoff prior to being conveyed private drywell system in the final design,the applicant
to the drywell using a lynch style catch basin with an is required provide pretreatment prior to the drywell via
18-inch sump,which is smaller than the required an acceptable method in the LOSWMM and show that a
sump depth listed in the LOSWMM. design infiltration rate of 2.0"/hr can be met at the fa-
Runoff calculations are based on 8,095 square feet cility location.If a private drywell is proposed the appli
of new impervious area associated with the new ac cant must meet the pretreatment requirements in
cess lane. LOSWMM 4.6.4.
As conditioned,this standard is met.
IBrown AND Caldwell
1
Use of contents on this sheet is subject to the limitations specified at the beginning of this document.
LU 23-0002 LandUseReview 091823 AttachmentA.docx
LU 23-0002, Land Use Review
EMIMr Attachment A. Land Use Review Findings,LU 23-0002
LOC Section
and Title Summary of Requirements Applicant Information Findings
The drywell is sized based on a design infiltration rate
of 360 inches per hour,with an applied safety factor
of 2.The proposed drywell was proposed to be 28
inches in diameter and 5 feet in height.
C.Erosion Obtain an Erosion Control Permit when develop- The Drainage Report indicates that the land disturb- The applicant has demonstrated feasibility to meet this
and Sedi- ment disturbs more than 500 square feet or is ance area will exceed 500 square feet.The prelimi- standard.
ment Control within 50 feet of waters of the state.The Erosion nary plan set does not include an overall grading The proposed development will disturb more than 500
Control Permit is based on an approved Erosion plan and site disturbance areas. square feet.As a condition of approval,the applicant is
Control Plan,submitted with the building appli- required to submit an Erosion Control Plan with the final
cation. design report and obtain an Erosion Control Permit from
the City.
D.Under- Demonstrate that proposed UlCs will be rule au- �The private drywell for the south access lane is classi- Under OAR 340-44-0018,injection of stormwater runoff
ground Injec- thorized under OAR 340-44-008 or will obtain a Pied as an underground infiltration system,which is a from residential roof tops is automatically rule author-
tion Control DEQ-issued UIC Permit. UIC.The Drainage Report identifies pretreatment of ized and does not require registration or a DEQ-issued
(UIC)Review runoff prior to being conveyed to the drywell with a UIC permit.Injection of runoff from driveways and park-
with DEQ lynch style catch basin with an 18-inch sump. ing areas requires DEQ rule authorization and registra-
tion.
As a condition of approval,if the applicant chooses to
use a private drywell system,the applicant is required to
register and obtain DEQ authorization for use of the pro-
posed UIC system and provide pretreatment in accord-
ance with the LOSWMM prior to the drywell.
As conditioned,this standard is met.
IBrown AND Caldwell
2
Use of contents on this sheet is subject to the limitations specified at the beginning of this document.
LU 23-0002 LandUseReview 091823 AttachmentA.docx
LU 23-0002, Land Use Review
Attachment A. Land Use Review Findings,LU 23-0002
LOC Section
and Title Summary of Requirements Applicant Information Findings
E.Storm- Submit a stormwater system design plans and A Drainage Report has been prepared,along with As condition of approval,demonstrate that the grading
water Sys- specifications in accordance with the LOSWMM. site and utility plans(Sheet 4),showing the concep- of the south access lane drains all new and replaced im-
tem Design tual stormwater management facility locations(one pervious surface area to the proposed stormwater man-
drywell).The preliminary site and utility plan does not agement facility and ensure that the correct contributing
identify pretreatment of runoff prior to being con- drainage area is reflected in the stormwater manage-
veyed to the drywell. ment facility design calculations.
The Drainage Report has been prepared by a li- Certification by the engineer of record shall be required
censed professional engineer to address the require- prior to issuance of final occupancy permits that all fa-
ments of the LOSWMM. cilities were constructed as approved,to the satisfaction
of the City Engineer.
As conditioned,this standard is met.
F.Down- Submit a downstream analysis in accordance The applicant does not propose offsite discharge Under LOSWMM,Section 5.8.2 and Figure 5.1,no addi-
stream Anal- with the LOSWMM.The City's policy is to prefer during the 10-year storm event. tional analysis or project action is required.
ysis Require- mitigation of downstream project impacts This standard is met.
ment through onsite infiltration and flow control BMPs
over increasing the capacity of the downstream
system(LOSWMM 5.8.2).
G.Drainage The Preliminary Drainage Report must document The Drainage Report has been prepared by a regis- The applicant has demonstrated feasibility to meet this
Report Re- the Site Assessment and Feasibility Analysis,ex- tered professional engineer.The report includes all standard.
quired plain how site conditions have influenced the se- the required elements. A final storm drainage report,prepared by a registered
lection of stormwater facilities,and present type, professional engineer,in accordance with the Storm-
size,and location of proposed stormwater man- water Management Code and LOSWMM,shall be sub-
agement facilities and discharges,along with mitted for review at the time of building permit submit-
supporting calculations(LOSWMM 2.7.1a). tal.
H.Opera- A draft operations and maintenance plan(OMP) The applicant is proposing to install a drywell to man- As a condition of approval,an OMP needs to be pro-
tions and is required for each proposed stormwater man- age the south access lane.The applicant did not pro- vided for all the proposed stormwater management fa-
Maintenance agement facility. vide an OMP for the drywell. cilities.
Plan A recorded OMP is required for each stormwater facility.
The OMPs must describe how to properly maintain the
facilities,the frequency of maintenance required and
the party responsible for maintaining the facilities.Proof
of the recorded OMP shall be provided prior final occu-
pancy.
IBrown AND Caldwell
3
Use of contents on this sheet is subject to the limitations specified at the beginning of this document.
LU 23-0002 LandUseReview 091823 AttachmentA.docx
LU 23-0002, Land Use Review
Attachment A. Land Use Review Findings,LU 23-0002
LOC Section
and Title Summary of Requirements Applicant Information Findings
As conditioned,this standard is met.
I.Responsi- The applicant's designer shall be responsible for The applicant has provided materials to document a The applicant has demonstrated feasibility to meet this
bility of the all stormwater system design,Site Assessment stormwater system design,the Site Assessment and standard.
designer and Feasibility Analysis,Drainage Report,and Feasibility Analysis,and a Drainage Report.The ap- As a condition of approval,an OMP needs to be pro-
OMP,as applicable. plicant has not provided an OMP. vided for all the proposed stormwater management fa-
cilities.
LOC 38.25.120.1.d.ii
A.Require- All requirements for small projects under LOC See items above. N/A
ments and 38.25.120.d.ii apply to large projects.
controls ap-
plicable to
small pro-
jects
B.Water Design stormwater facilities to capture and treat See On-site Stormwater Management described Under LOSWMM,Section 4.1,projects that meet the on-
Quality 80%of the average annual runoff volume. above. site stormwater management requirement,will meet wa-
LOSWMM Section 2.5.3 defines the water qual- ter quality requirements without additional measures.
ity design event as 1.0 inch in 24 hours. This standard is met.
C.Flow Con- Design stormwater facilities to maintain post-de- See On-Site Stormwater Management described Under LOSWMM,Section 4.1,projects that meet the on-
trol velopment peak flow rates at their pre-develop- above. site stormwater management requirement,will meet
ment(c. 1850)levels for the 2-year,5-year,and flow control requirements without additional measures.
10-year,24-hour runoff events.Flow control is This standard is met.
not required for projects that discharge directly
to the Willamette River,Tualatin River,or Oswego
Lake(LOSWMM,Section 2.5.4.)
D.NPDES Projects over 1 acre of disturbance must obtain a The total area of construction disturbance of the pro- The applicant has demonstrated feasibility to meet this
1200-C Per- DEQ Construction Stormwater Permit(NPDES posed development(five lots and the south access standard.
mit 1200-C)prior to construction. lane)is expected to exceed 1 acre(project site's area As a condition of approval,the applicant will need to ob-
of analysis is 1.29 acres). tain a NPDES 1200-C Permit.
IBrown AND Caldwell
4
Use of contents on this sheet is subject to the limitations specified at the beginning of this document.
LU 23-0002 LandUseReview 091823 AttachmentA.docx