APPROVED STM PLN2020-0033-Stormwater Report-12.31.2020Walnut Short Plat
4-Lot
1012 Walnut Street
Edmonds, WA 98020
PLN 2020-0033
Stormwater Site Plan Report
Prepared for:
Select Homes
Date: December 31, 2020
12/31/2020
Prepared by: Rob Long, PE
RAM Engineering, Inc.
16531 13th Ave W, Suite A108, Lynnwood WA 98037
(425) 678-6960 RAMengineeringinc.com Job No. 19-032
RECEIVED
CITY OF EDMONDS
DEVELOPMENT
SERVICES DEPARTMENT
DEC 31 2020
APPLIES WITH APPLICABLE
CITY STROMWATER CODE
01/29/2021
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Walnut Short Plat
1012 Walnut Street
Edmonds, WA 98020
Introduction: This summary report provides site design information for a single family 4-lot short
plat by Select Homes. This report includes storm drainage analysis to support permit review and
approval. The property is located on the south side of Walnut Street between 10th Ave S and 96th
Ave W, in the east 1/2 of Section 25, T 27 N, R 3 E, W.M.
Site Address: 1012 Walnut Street
Edmonds, WA 98020
Tax Parcel Number: 27032500100500 and
00434203500400
Applicant: Select Homes
16513 13th Ave W, Suite A108,
Lynnwood WA 98037
Contact: Kayla Clark
(425) 742-6044
TABLE OF CONTENTS
SECTION PAGES
A. Project Summary ............................................................................................................................... 4
B. Parcel (Vicinity) Map .......................................................................................................................... 1
C. Site Plan ............................................................................................................................................. 1
D. Aerial Photo ....................................................................................................................................... 1
E. Stormwater Control System Summary ............................................................................................. 14
F. Drainage Basin Description and Maps ............................................................................................... 3
G. Grading and Erosion Control Summary (SWPPP Evaluation) ............................................................ 3
H. Operation and Maintenance Guidelines .......................................................................................... 10
I. Geotechnical Engineering Evaluation .............................................................................................. 16
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page A-1
A. PROJECT SUMMARY:
This report provides engineering information for the proposed construction of a 4-lot single family residence
short plat; the project is located on the south side of Walnut Street between 10th Ave S and 96th Ave W
(1012 Walnut Street) in the City of Edmonds. The applicant, Select Homes, proposes to remove the existing
residence and construct four single family residence on the site. This report provides the evaluation for the
proposed 4-lot short plat.
Existing Conditions. The total site consists of about 32,297 sf (0.741 acres) and is currently occupied by an
existing single-family residence with a loop access drive between Walnut St and 10th Ave S. The easterly
loop access driveway to Walnut is shared with three additional single family lots east of the subject site.
The existing residence has a detached garage, shed and swimming pool associated with the existing site
development. The existing house, garage and pool are surrounded by grass lawn, landscape areas and
several large trees. The site is surrounded by single family residences to the on all sides (see attached aerial
photo- section D).
The site generally slopes in a northerly direction; a majority of the site’s drainage runoff would sheet flow
northerly to the stormwater collection system in Walnut St; a small portion of the site slopes off in a westerly
direction and runoff would sheet-flow westerly down Cedar St. No stream or wetlands were discovered on
or immediately adjacent to the site. The stormwater system along Walnut St and Cedar St flow wester to
9th Ave and then north in the Shell Creek watershed basin. Additional discussion of the local drainage basin
and downstream path is discussed in section F of this report.
Developed Conditions. The proposed project includes the construction four single residence and associated
driveway and utilities. All four lots will utilize the existing shared private road and access Walnut St; three
of the lots (Lots 2, 3 and 4) will utilize a shared driveway off the existing private road. All existing structures
and impervious surface onsite will be removed/replaced with the site development; landscaping and grass
lawn around the new residence will stabilize the site upon building construction. New infrastructure
improvements include a stormwater conveyance and separate infiltration trenches for each lot; water;
sewer; power; and communications. The four individual lot infiltration trench systems will be designed and
sized to meet compliance with the LID performance standard of the Edmonds Community Development
Code (ECDC). All disturbed pervious (lawn and landscape) areas will preserve or restore the health and
water-holding capacity of the soils by compost-amending (Per DOE BMP T5.13: Post-Construction Soil
Quality and Depth). Due to the fact, the pollution generating driveway area are less than 5,000 sf (+/-4,635
sf) no water quality treatment system will be provided. A full overview of the proposed storm drainage
control facilities, offsite/downstream analysis and grading and erosion control measures are discussed in
the following sections of this report.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page A-2
Summary of Minimum Requirements for Category 2 Project:
5.1 Minimum Requirement #1 – Preparation of Stormwater Site Plan. The proposed site development
consists of disturbing about 0.8 acres of land and creating/replacing about 15,360 sf of hard surface (4 lots
15,595, private shared driveway 1,360 sf, and private road expansion 1,000 sf). Thus, the project is classified
as a Category 2 project per the City’s classification system. The civil site development plans and this report
have been prepared to address the projects impacts.
5.2 Minimum Requirement #2 – Construction Stormwater Pollution Prevention. A construction
Stormwater Pollution Prevention Plan (SWPPP) has been incorporated into the site development plans. A
summary of the site’s erosion control measures that evaluates the typical 13 elements of a SWPPP are
included in section G. The total site disturbance area of the project is less than one acre, thus a formal
Notice of Intent application for NPDES coverage will not be made to the Department of Ecology.
5.3 Minimum Requirement #3 – Source Control of Pollution. Specific source controls are not required for
single family residential sites. General requirements for these sites include preventing the discharge of
pollutants to the City’s storm drainage system per Edmonds City Code Chapter 7.200 (Illicit Discharges). This
includes common household items such as pesticides, herbicides, fertilizers, detergents and fluids from
vehicle maintenance.
Single family residences shall incorporate DOE’s S411 BMPs for landscape and lawn vegetation
management. Lawn and vegetation management can include control of objectionable weeds, insects, mold,
bacteria, and other pests with pesticides. Examples include weed control on golf course lawns, access roads,
and utility corridors and during landscaping; sap stain and insect control on lumber and logs; rooftop moss
removal; killing nuisance rodents; fungicide application to patio decks, and residential lawn/plant care. It is
possible to release toxic pesticides such as pentachlorophenol, carbamates, and organometallics to the
environment by leaching and dripping from treated parts, container leaks, product misuse, and outside
storage of pesticide contaminated materials and equipment. Poor management of the vegetation and poor
application of pesticides or fertilizers can cause appreciable stormwater contamination.
5.4 Minimum Requirement #4 – Preservation of Natural Drainage Systems and Outfalls. The site generally
slopes in a northerly direction towards Walnut St or west towards Cedar St. The proposed development will
install infiltration trenches and amend the disturbed site soils; thus, reducing the site developments runoff
to a de minimis amount. Each infiltration system will have a catch basin or trench drain that will allow
overflow flows discharge towards Walnut St; stormwater flows entering Walnut St flow westerly towards
9th Ave and then north within the Shell Creek watershed, thus maintaining the natural outfall watershed
basin. The site is located within the Shell Creek watershed basin and eventually discharge to the Puget Sound
(via Willow Creek) per the City of Edmonds maps (see section F).
5.5 Minimum Requirement #5 – Onsite Stormwater Management. The developed site conditions include
new/replaced hard surface for the roof tops, driveways, and walkway/patio areas for the proposed
residences. All rooftop and exterior area collection drains shall convey stormwater run-off to the on-site
infiltration trench systems on each lot. To accommodate the widening of the existing private road, the
existing infiltration trenched (sized for the existing private road) will be expanded. Four individual lot
infiltration trench systems will be designed and sized to meet compliance with the LID performance
standard (MR #5) of the Edmonds Community Development Code (ECDC). Additionally, all disturbed
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page A-3
pervious (lawn and landscape) areas will preserve or restore the health and water-holding capacity of the
soils by compost-amending (Per DOE BMP T5.13: Post-Construction Soil Quality and Depth). See LID BMP
feasibility evaluation table (below); civil site development plans; Section C for additional stormwater BMP
evaluation; and the geotechnical evaluation include in Section I.
5.6 Minimum Requirement #6 – Runoff Treatment. The project does not propose to create/replace more
than 5,000 sf of pollution-generation impervious surface onsite. The project will create about 4,635 sf
pollution generation hard surfaces (driveways); most the site’s hard surface is rooftop and walkway/patio
areas (non-pollution generation surfaces). Thus, no basic or enhanced water quality treatment system is
required. The site’s stormwater control system will include a sump and a floatable tee that will provide
some water quality and oil/water separation of the collected runoff.
5.7 Minimum Requirement #7 – Flow Control. The project will not exceed any of the required thresholds
to provide flow control onsite. In accordance with ECDC 18.30.060.D.7 there are three thresholds:
1) Projects in which the total of effective impervious surfaces is 10,000 square feet or more in a threshold
discharge area, or
Each individual lot will contain infiltration trench system that will infiltrate 100% of the proposed
lot hard surface areas. Additionally, an additional 1,000 sf of hard surface will be added to the
existing private road. The existing private road infiltration system will be expanded to
accommodate the addition hard surface and will be sized to infiltrate 100% of the stormwater
runoff. Thus, the project total new and replace effective impervious surfaces is will be zero.
2) Projects that convert 0.75 acres or more of vegetation to lawn or landscape, or convert 2.5 acres or
more of native vegetation to pasture in a threshold discharge area, and from which there is a surface
discharge in a natural or man-made conveyance system from the site, or
The site is currently developed with an existing single-family residence and less than 0.75 acres of
areas on site will be converted to vegetation lawn or landscape areas.
3) Projects that through a combination of hard surfaces and converted vegetation areas cause a 0.10
cubic feet per second (cfs) increase or greater in the 100-year flow frequency from a threshold
discharge area as estimated using the Western Washington Hydrology Model (WWHM) or other
approved model and one-hour time steps (or a 0.15 cfs increase or greater using 15-minute time
steps).
The individual lot infiltration systems and the expansion of the existing private road infiltration
trench will all be sized to infiltrate 100% of the modeled storm events (WWHM); therefore, there
will be no increase of the 100-year flow frequency from the site.
5.8 Minimum Requirement #8 – Wetland Protection. No wetlands are known to exist on or adjacent to the
site.
5.9 Minimum Requirement #9 – Operation and Maintenance. An operation and maintenance summary is
provided in section H.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page A-4
LID BMP Performance Standards Evaluation
Lawn and landscaped areas:
BMP Viable Limitations / Infeasibility Criteria
BMP T5.13: Post-Construction Soil Quality and Depth
(Volume V, Chapter 5) Yes
Limitation: Considered infeasible on slopes greater than 33
percent are present., BMP T5.13 shall be applied to the site
post construction.
Roofs:
BMP T5.30: Full Dispersion
(Volume V, Chapter 5)
No
Infeasibility: Comprehensive Plan and Zoning minimum
density requirements do not allow for large native
vegetation retention and flow path areas (100 ft) need to
meet full dispersion requirements.
BMP T5.10A: Downspout Full Infiltration Systems
(Volume III, Chapter 3) Yes 100% infiltration on the individual lots and the expanded
private road is proposed.
Bioretention
(Volume V, Chapter 7) Yes However, 100% infiltration is proposed.
BMP T5.10B: Downspout Dispersion Systems
(Volume III, Chapter 3) Yes However, 100% infiltration is proposed.
BMP T5.10C: Perforated Stub-out
(Volume III, Chapter 3) Yes However, 100% infiltration is proposed.
Detention vaults or pipes in accordance with the
Edmonds Stormwater Addendum Yes However, 100% infiltration is proposed.
Other Hard Surfaces:
BMP T5.30: Full Dispersion
(Volume V, Chapter 5) No See BMP T5.30 dispersion infeasibility discussion above.
BMP T5.15: Permeable Pavement
(Volume V, Chapter 5) Yes However, 100% infiltration is proposed.
Bioretention (Volume V, Chapter 7) Yes However, 100% infiltration is proposed.
BMP T5.12: Sheet Flow Dispersion
BMP T5.11: Concentrated Flow Dispersion
(Volume V, Chapter 5)
No No area is feasible for dispersion once the site is developed.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page B-1
B. PARCEL (VICINITY) MAP
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Page C-1
C. SITE PLAN
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page D-1
D. AERIAL PHOTO (City of Edmonds GIS, 2017 photo)
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-1
E. STORMWATER CONTROL SYSTEM SUMMARY AND CALCULATIONS:
The project proposes to create and/or replace 15,595 sf of hard surface with the complete site
development (4 lots 13,235 sf, private shared driveway 1,360 sf, and private road expansion 1,000 sf). To
mitigate the development of the site, individual lot stormwater infiltration trench systems and an
expansion of the existing private road infiltration trench system will be constructed to control the
stormwater runoff (flow control). The project applicant has elected to meet the LID Performance Standard
to comply with ECDC 18.30.060.D.b (ii) and (iii), stormwater discharges shall match developed discharge
durations to pre-developed durations for the range of pre-developed discharge rates from 8 percent of the
2-year peak flow to 50 percent of the 2-year peak flow from the project site. The following is a summary of
the proposed new/replaced hard surface areas:
Hard Surface Summary:
4 Lots (Lot 1= 3,267 sf; Lot= 3,524 sf; Lot 3=3,542 sf; Lot 4= 2,902 sf) 13,235 sf
Expansion of Existing Private Road to Walnut St = 1,000 sf
Shared Driveway for Lots 2, 3 and 4 = 1,360 sf
Total New/Replaced Hard Surface = 15,595 sf (0.358 ac)
A site specific geotechnical evaluation was made to assess the use of infiltration and LID measures onsite.
The evaluation determined infiltration is feasible onsite and recommends a long-term infiltration design
rate of 1.1 in/hr for the new onsite systems.
As stated above the infiltration trenches have been designed for the total hard surface for each individual
lot and shall be designed/sized to infiltrate 100% of the WWHM modeled storm events. The attached
calculations size each individual lot’s infiltration system based on the design infiltration rate of 1.1 in/hr.
Below is a summer of each individual lot system:
Hard Surface Trench Size
Lot 1 3,267 sf 27 ft x 12 ft x 3.0 ft
Lot 2 3,524 sf 22 ft x 16 ft x 3.0 ft
Lot 3 3,542 sf 25 ft x 15 ft x 3.0 ft
Lot 4 2,902 sf 285 sf x 3 ft
Total = 13,235 sf
Tract 999 Private Shared Driveway = 1,360 sf 47 ft x 5 ft x 3.0 ft
The existing private road will be widened to accommodate the additional lots (4 new lots) on the existing
private road. The minimum width will be expanded from 16 feet to 20 feet for the first (northerly) portion
of the private road that provides access to more than four lots. The expanded private road will create
1,000 sf (0.023 ac.) of new hard surface. To accommodate the additional hard surface the existing
infiltration trench for the private road will be expanded by 23 ft. The expansion of the trench is based on
the original design trench (4 ft width, 1.67 ft depth, and an infiltration rate of 2.3 in/hr). Included below
are the expansion calculations (WWHM input/output) and details of the original design criteria.
All proposed infiltration systems shall include a basin that will allow the extraordinary storm events to
overflow into an adjacent driveway and/or private road area that will direct the stormwater runoff
northerly to Walnut Street.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-2
Lot 1 Infiltration Trench WWHM Input/output:
Hard Surface Summary:
Roof Area = 2,417 sf (0.056 ac.)
Driveway Area = 800 sf (0.018 ac.)
Sidewalk/Patio = 50 sf (0.001 ac.)
Total = 3,267 sf (0.075 ac.)
Lot 1 Trench Size: 27 ft x 12 ft x 3.0 ft
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-3
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-4
Lot 2 Infiltration Trench WWHM Input/output:
Hard Surface Summary:
Roof Area = 2,474 sf (0.057 ac.)
Driveway Area = 1,000 sf (0.023 ac.)
Sidewalk/Patio = 50 sf (0.001 ac.)
Total = 3,524 sf (0.081 ac.)
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-5
Lot 2 Trench Size: 22 ft x 16 ft x 3.0 ft
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-6
Lot 3 Infiltration Trench WWHM Input/output:
Hard Surface Summary:
Roof Area = 2,417 sf (0.056 ac.)
Driveway Area = 1,075 sf (0.025 ac.)
Sidewalk/Patio = 50 sf (0.001 ac.)
Total = 3,542 sf (0.082 ac.)
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-7
Lot 3 Trench Size: 25 ft x 15 ft x 3.0 ft
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-8
Lot 4 Infiltration Trench WWHM Input/output:
Hard Surface Summary:
Roof Area = 2,502 sf (0.057 ac.)
Driveway Area = 400 sf (0.009 ac.)
Sidewalk/Patio = 0 sf (0.000 ac.)
Total = 2,902 sf (0.066 ac.)
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-9
Lot 4 Trench Size: 285 sf x 3 ft
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-10
Tract 999 Private Shared Driveway Infiltration Trench Expansion WWHM Input/output:
Tract 999 Private Shared Driveway = 1,360 sf (0.031 ac.)
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-11
Private Road Trench Expansion: 47 ft x 5 ft x 3.0 ft
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-12
Private Road Infiltration Trench Expansion WWHM Input/output:
Expansion of Existing Private Road to Walnut St = 1,000 sf (0.023 ac.)
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-13
Private Road Trench Expansion: 23 ft x 4 ft x 1.67 ft
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page E-14
Existing Private Road As-built Information:
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page F-1
F. DRAINAGE BASIN DESCRIPTION:
The site is located within the City of Edmonds Shell Creek watershed basin. The Shell Creek basin is a
centralized urban basin that collects stormwater runoff from primarily residential areas in the City Edmonds
and ultimately discharges flows northwesterly to Puget Sound via Shell Creek (see attached watershed map).
The proposed stormwater infiltration trench systems are designed to infiltrate most the site stormwater
flows into the underlying outwash soils. Flows that do not infiltrate into the soils will sheet flow off in a
northerly direction towards Walnut Street. The infiltration systems include a collection catch basin that will
overflow into Walnut St in the event of an extraordinary rainfall event. Any overflow would flow towards
Walnut St and enter the pipe and catch basin system of Walnut St. The stormwater system in Walnut St
directs runoff west to 9th Ave and then north. The flows continue northerly along 9th Ave and eventually
discharges to Shell Creek about 2,600 feet northwest of the project site.
Shell Creek continues in a northwesterly direction and eventually discharges into the Puget Sound about 1.3
miles northwest of the site
Most of the downstream conveyance system within a ¼ mile of the site consists of a man-made pipe and
catch basin system and Shell Creek stream channel. No apparent flooding or system inadequacies within a
¼ mile downstream of the site were discovered during site visits or research of the downstream system.
See attached watershed and downstream drainage facility map for the local downstream path.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page F-2
F-2. Watershed Basin Map
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Stormwater Site Plan Report RAM No. 19-032
Page F-3
F-3. Downstream Drainage Facility Map (City of Edmonds GIS)
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page G-1
G. GRADING and EROSION CONTROL SUMMARY:
The project will require grading to construct the proposed buildings and utilities (including the stormwater
infiltration trench systems). Standard erosion control measures are proposed to be used during
construction. The primary erosion and sediment control BMP during construction will be proper soil
stabilization methods. Exposed soils shall be stabilized by application of effective BMPs that protect the soil
from the erosive forces of raindrops, flowing water, and wind. Applicable practices include, but not limited
to, temporary and permanent seeding, sodding, mulching, plastic covering, erosion control fabrics, matting,
the early application of gravel base on areas to be paved, and dust control. The contractor shall select a soil
stabilization method best suited for the particular situation. Stock piles must be stabilized and protected
with sediment trapping measures. In addition, site containment of exposed soils shall be sustained by using
silt fence barriers along the down-slope boundaries of the site’s disturbance areas. See the site
development plan for details.
Soils: In accordance with the project’s site specific geotechnical engineering study by Earth Solutions NW,
LLC the on-site native soils encountered are characterized primarily as silty sand underlain by poorly graded
sand with and without silt. Based on the results of USDA textural analyses, the native sand soils at depth
were classified primarily as gravelly sand with fines contents of about 3 to 7 percent. City soil maps identifies
Everett very gravelly sandy loam (Map Unit Symbol: 17) as the primary soil units underlying the subject site.
Everett soils are well drained soil and are categorized as a hydrologic group A soil. Earth Solutions NW, LLC.
Recommends a long-term design infiltration rate of 1.1 inches per hour. Full soils description is included in
the project’s site specific geotechnical engineering evaluation by Earth Solutions NW, LLC.
Construction Stormwater Pollution Prevention Plan (SWPPP): The following is a summary of the site’s
erosion control measures that evaluates the typical DOE 13 elements of a SWPPP:
Element 1: Mark Clearing Limits: Clearing limits have been delineated on the engineering site development
plans.
Element 2: Establish Construction Access: A construction access has been delineated on the engineering
site development plans.
Element 3: Control Flow Rates: During construction silt fencing will provide attenuation of site runoff and
upon project completion and stabilization (established lawns and landscape of exposed soils),
the infiltration storm system will provide flow control.
Element 4: Install Sediment Controls: Filter fabric fence has been delineated and detailed on the
engineering site development plans. At a minimum, silt fence will be installed along the down
gradient perimeter of the disturbed area that will receive sediment-laden runoff.
Element 5: Stabilize Soils: Soils will be stabilized per the TESC notes listed on the engineering site
development plans.
Element 6: Protect Slopes: Exposed slopes shall be stabilized per the TESC notes listed on the engineering
site development plans.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page G-2
Element 7: Protect Drain Inlets: Drain inlet protection will be installed on all catch basins that will receive
sediment-laden runoff. See the engineering site development plans for locations and detail.
Element 8: Stabilize Channels and Outlets: This element is not applicable since there are no temporary
channels or outlets proposed.
Element 9: Control Pollutants: Pollutants shall be managed as described in the TESC notes listed on the
engineering site development plans.
Element 10: Control De-Watering: Highly turbid or contaminated de-watering water shall be handled
separately from stormwater. The water from all de-watering systems for trenches and
foundations shall be treated or disposed prior to discharging from the site.
Element 11: Maintain BMP’s: BMPs shall be maintained and removed at the end of the project as follows:
i. All temporary and permanent erosion and sediment control BMPs shall be inspected,
maintained and repaired in accordance with the Drainage Manual or as approved or
required by the City to assure continued performance of their intended function in
accordance with BMP specifications.
ii. The applicant may remove temporary BMPs when they are no longer needed.
iii. All temporary erosion and sediment control BMPs shall be removed within 30 days after
construction is completed and the City has determined that the site is stabilized.
Element 12: Manage the Project: The owner of the site is responsible for managing the installation and
maintenance of the site BMPs.
Element 13. Protect Low Impact Development BMPs: During construction the proposed infiltration trench
systems areas shall be protected from sediment accumulation, sediment-laden runoff and
soils, and compaction from construction equipment. Soils restoration and/or replacement
may be needed if soils are compacted or contaminated with sediment.
Conclusion: The final site development construction plans include specific grading and drainage
improvement notes and details. With proper installation, maintenance and inspections of the proposed
construction the project should have minimal impact to the surrounding environment.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page G-3
G-1. Soils Map
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Page H-1
H. OPERATION AND MAINTENANCE GUIDELINES:
These guidelines are intended to provide operation and maintenance instructions for the project’s storm
drainage control facilities. The owner is responsible for maintenance of storm drainage facilities within
the property (private property owner system); the owner is not responsible for maintenance within the
public right-of-way (City system).
Private Property Owners: Private property owners are responsible for properly maintaining the stormwater
infrastructure on their property to ensure it operates as designed. The City has developed an inspection
program to ensure private property owners are properly maintaining their stormwater systems.
City System: City crews perform maintenance activities on the entire storm drainage system, including
inspection and cleaning of catch basins, street sweeping, emergency flooding response, creek
maintenance, inspection and monitoring of private stormwater detention systems.
Operation and Maintenance Requirements for private property owner system:
This manual is not comprehensive; although it explains the intended operation of the various
components of the drainage system, and suggests a routine of inspection and maintenance, it cannot
anticipate every problem. Once a historical record of maintenance is established, it may be prudent to
alter the routine. It is recommended that maintenance records be kept, and that the records be
reviewed periodically.
Concept of Operation: The drainage design is shown and described in the site development engineering
plans and report. The approved site development plans and report should be retained by the owner and
used as a reference to identify drainage facilities outlined in this manual.
Conveyance Systems. The design objective in pipe sizing was to convey the large storm events without
the water surface exceeding the ground elevation. Frequent overtopping of the pipe system in a particular
area might indicate a downstream blockage. Overtopping of the drain systems at the catch-basins is an
indication that maintenance is required.
Stormwater Infiltration Trench System. The private onsite infiltration trench systems consist of excavated
trench filled with drain rock per the City Standard detail (SD-636). The infiltration trench system has been
designed to infiltrate most storm event; if an extraordinary event was to occur or multiple back to back
large storm events occurred the upstream catch-basins may overtop and overflow into the street system.
If regular overtopping of the catch basin is experienced the system shall be thoroughly cleaned and
inspected. Over a period of time siltation of the infiltration bed can occur (especially if the system and
upstream catch basin are not inspected and cleaned regularly) and replacement of the rock in the
infiltration system may be required.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page H-2
The following pages outline standard general maintenance criteria for the project’s drainage facilities.
For additional and updated maintenance information visit the Washington State Department of
Ecology’s web-site at: http://www.ecy.wa.gov/programs/wq/wqhome.html
General Infiltration Maintenance Criteria
Provision should be made for regular and perpetual maintenance of the infiltration basin/trench, including
replacement and/or reconstruction of the any media that are relied upon for treatment purposes.
Maintenance should be conducted when water remains in the basin or trench for more than 24 hours after
the end of a rainfall event, or when overflows occur more frequently than planned. For example, off-line
infiltration facilities should not have any overflows. Infiltration facilities designed to completely infiltrate
all flows to meet flow control standards should not overflow.
Removal of accumulated debris/sediment in the basin/trench should be conducted every 6 months or as
needed to prevent clogging, or when water remains in the pond for greater than 24 hours after the end of a
rainfall event.
RAM Engineering, Inc. Walnut Short Plat
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Page H-3
Appendix V-A: BMP Maintenance Tables
Ecology intends the facility-specific maintenance standards contained in this section to be
conditions for determining if maintenance actions are required as identified through inspection.
Recognizing that Permittees have limited maintenance funds and time, Ecology does not
require that a Permittee perform all these maintenance activities on all their stormwater BMPs.
We leave the determination of importance of each maintenance activity and its priority within the
stormwater program to the Permittee. We do expect, however, that sufficient maintenance will
occur to ensure that the BMPs continue to operate as designed to protect ground and surface
waters.
Ecology doesn’t intend that these measures identify the facility's required condition at all times
between inspections. In other words, exceedance of these conditions at any time between
inspections and/or maintenance does not automatically constitute a violation of these standards.
However, based upon inspection observations, the Permittee shall adjust inspection and
maintenance schedules to minimize the length of time that a facility is in a condition that
requires a maintenance action.
Table V-A.1: Maintenance Standards - Detention Ponds
Maintenance
Component Defect Conditions When
Maintenance Is Needed
Results Expected When
Maintenance Is Performed
General
Trash & Debris
Any trash and debris which
exceed 1 cubic feet per 1,000
square feet. In general, there
should be no visual evidence of
dumping.
If less than threshold all trash
and debris will be removed as
part of next scheduled
maintenance.
Trash and debris cleared from
site
Poisonous
Vegetation and
noxious weeds
Any poisonous or nuisance
vegetation which may constitute
a hazard to maintenance
personnel or the public.
Any evidence of noxious weeds
as defined by State or local
regulations.
(Apply requirements of adopted
IPM policies for the use of
herbicides).
No danger of poisonous
vegetation where maintenance
personnel or the public might
normally be. (Coordinate with
local health department)
Complete eradication of noxious
weeds may not be possible.
Compliance with State or local
eradication policies required
Contaminants and
Pollution
Any evidence of oil, gasoline,
contaminants or other pollutants
No contaminants or pollutants
present.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page H-4
Table V-A.1: Maintenance Standards - Detention Ponds
Maintenance
Component Defect Conditions When
Maintenance Is Needed
Results Expected When
Maintenance Is Performed
(Coordinate removal/cleanup
with local water quality
response agency).
Rodent Holes
Any evidence of rodent holes if
facility is acting as a dam or
berm, or any evidence of water
piping through dam or berm via
rodent holes.
Rodents destroyed and dam or
berm repaired. (Coordinate with
local health department;
coordinate with Ecology Dam
Safety Office if pond exceeds 10
acre-feet.)
Beaver Dams Dam results in change or
function of the facility.
Facility is returned to design
function.
(Coordinate trapping of beavers
and removal of dams with
appropriate permitting agencies)
Insects
When insects such as wasps
and hornets interfere with
maintenance activities.
Insects destroyed or removed
from site.
Apply insecticides in compliance
with adopted IPM policies
Tree Growth and
Hazard Trees
Tree growth does not allow
maintenance and inspection
access or interferes with
maintenance activity (i.e., slope
mowing, silt removal, vactoring,
or equipment movements). If
trees are not interfering with
access or maintenance, do not
remove
If dead, diseased, or dying trees
are identified
(Use a certified Arborist to
determine health of tree or
removal requirements)
Trees do not hinder
maintenance activities.
Harvested trees should be
recycled into mulch or other
beneficial uses (e.g., alders for
firewood).
Remove hazard Trees
Side Slopes of Pond Erosion
Eroded damage over 2 inches
deep where cause of damage is
still present or where there is
potential for continued erosion.
Slopes should be stabilized
using appropriate erosion
control measure(s); e.g.,rock
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page H-5
Table V-A.1: Maintenance Standards - Detention Ponds
Maintenance
Component Defect Conditions When
Maintenance Is Needed
Results Expected When
Maintenance Is Performed
Any erosion observed on a
compacted berm embankment.
reinforcement, planting of grass,
compaction.
If erosion is occurring on
compacted berms a licensed
engineer in the state of
Washington should be consulted
to resolve source of erosion.
Storage Area
Sediment
Accumulated sediment that
exceeds 10% of the designed
pond depth unless otherwise
specified or affects inletting or
outletting condition of the
facility.
Sediment cleaned out to
designed pond shape and
depth; pond reseeded if
necessary to control erosion.
Liner (if Applicable) Liner is visible and has more
than three 1/4-inch holes in it.
Liner repaired or replaced. Liner
is fully covered.
Ponds Berms
(Dikes)
Settlements
Any part of berm which has
settled 4 inches lower than the
design elevation
If settlement is apparent,
measure berm to determine
amount of settlement
Settling can be an indication of
more severe problems with the
berm or outlet works. A licensed
engineer in the state of
Washington should be
consulted to determine the
source of the settlement.
Dike is built back to the design
elevation.
Piping
Discernable water flow through
pond berm. Ongoing erosion
with potential for erosion to
continue.
(Recommend a Goethechnical
engineer be called in to inspect
and evaluate condition and
recommend repair of condition.
Piping eliminated. Erosion
potential resolved.
Emergency
Overflow/ Spillway
Tree Growth Tree growth on emergency
spillways creates blockage
Trees should be removed. If root
system is small (base less than
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page H-6
Table V-A.1: Maintenance Standards - Detention Ponds
Maintenance
Component Defect Conditions When
Maintenance Is Needed
Results Expected When
Maintenance Is Performed
and Berms over 4
feet in height
problems and may cause failure
of the berm due to uncontrolled
overtopping.
Tree growth on berms over 4
feet in height may lead to piping
through the berm which could
lead to failure of the berm.
4 inches) the root system may
be left in place. Otherwise the
roots should be removed and
the berm restored. A licensed
engineer in the state of
Washington should be consulted
for proper berm/spillway
restoration.
Piping
Discernable water flow through
pond berm. Ongoing erosion
with potential for erosion to
continue.
(Recommend a Geotechnical
engineer be called in to inspect
and evaluate condition and
recommend repair of condition.
Piping eliminated. Erosion
potential resolved.
Emergency
Overflow/Spillway
Emergency
Overflow/Spillway
Only one layer of rock exists
above native soil in area five
square feet or larger, or any
exposure of native soil at the
top of out flow path of spillway.
(Rip-rap on inside slopes need
not be replaced.)
Rocks and pad depth are
restored to design standards.
Erosion See "Side Slopes of Pond"
Table V-A.2: Maintenance Standards - Infiltration
Maintenance
Component Defect Conditions When Maintenance Is
Needed
Results Expected
When Maintenance Is
Performed
General
Trash & Debris See Table V-A.1: Maintenance
Standards - Detention Ponds
See Table V-A.1:
Maintenance
Standards - Detention
Ponds
Poisonous/Noxious
Vegetation
See Table V-A.1: Maintenance
Standards - Detention Ponds See Table V-A.1:
Maintenance
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page H-7
Table V-A.2: Maintenance Standards - Infiltration
Maintenance
Component Defect Conditions When Maintenance Is
Needed
Results Expected
When Maintenance Is
Performed
Standards - Detention
Ponds
Contaminants and
Pollution
See Table V-A.1: Maintenance
Standards - Detention Ponds
See Table V-A.1:
Maintenance
Standards - Detention
Ponds
Rodent Holes See Table V-A.1: Maintenance
Standards - Detention Ponds
See Table V-A.1:
Maintenance
Standards - Detention
Ponds
Storage Area Sediment
Water ponding in infiltration pond after
rainfall ceases and appropriate time
allowed for infiltration. Treatment basins
should infiltrate Water Quality Design
Storm Volume within 48 hours, and
empty within 24 hours after cessation of
most rain events.
(A percolation test pit or test of facility
indicates facility is only working at 90%
of its designed capabilities. Test every 2
to 5 years. If two inches or more
sediment is present, remove).
Sediment is removed
and/or facility is
cleaned so that
infiltration system
works according to
design.
Filter Bags (if
applicable)
Filled with Sediment
and Debris
Sediment and debris fill bag more than
1/2 full.
Filter bag is replaced
or system is
redesigned.
Rock Filters Sediment and Debris
By visual inspection, little or no water
flows through filter during heavy rain
storms.
Gravel in rock filter is
replaced.
Side Slopes of
Pond Erosion See Table V-A.1: Maintenance
Standards - Detention Ponds
See Table V-A.1:
Maintenance
Standards - Detention
Ponds
Emergency
Overflow Spillway
and Berms over 4
feet in height.
Tree Growth See Table V-A.1: Maintenance
Standards - Detention Ponds
See Table V-A.1:
Maintenance
Standards - Detention
Ponds
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page H-8
Table V-A.2: Maintenance Standards - Infiltration
Maintenance
Component Defect Conditions When Maintenance Is
Needed
Results Expected
When Maintenance Is
Performed
Piping See Table V-A.1: Maintenance
Standards - Detention Ponds
See Table V-A.1:
Maintenance
Standards - Detention
Ponds
Emergency
Overflow Spillway
Rock Missing See Table V-A.1: Maintenance
Standards - Detention Ponds
See Table V-A.1:
Maintenance
Standards - Detention
Ponds
Erosion See Table V-A.1: Maintenance
Standards - Detention Ponds
See Table V-A.1:
Maintenance
Standards - Detention
Ponds
Pre-settling Ponds
and Vaults
Facility or sump filled
with Sediment and/or
debris
6" or designed sediment trap depth of
sediment. Sediment is removed.
Table V-A.5: Maintenance Standards - Catch Basins
Maintenance
Component Defect Conditions When Maintenance is Needed
Results Expected
When Maintenance is
performed
General
Trash & Debris
Trash or debris which is located immediately in
front of the catch basin opening or is blocking
inletting capacity of the basin by more than
10%.
Trash or debris (in the basin) that exceeds 60
percent of the sump depth as measured from
the bottom of basin to invert of the lowest pipe
into or out of the basin, but in no case less than
a minimum of six inches clearance from the
debris surface to the invert of the lowest pipe.
Trash or debris in any inlet or outlet pipe
blocking more than 1/3 of its height.
No Trash or debris
located immediately in
front of catch basin or
on grate opening.
No trash or debris in
the catch basin.
Inlet and outlet pipes
free of trash or debris.
No dead animals or
vegetation present
within the catch basin.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page H-9
Table V-A.5: Maintenance Standards - Catch Basins
Maintenance
Component Defect Conditions When Maintenance is Needed
Results Expected
When Maintenance is
performed
Dead animals or vegetation that could generate
odors that could cause complaints or
dangerous gases (e.g., methane).
Sediment
Sediment (in the basin) that exceeds 60
percent of the sump depth as measured from
the bottom of basin to invert of the lowest pipe
into or out of the basin, but in no case less than
a minimum of 6 inches clearance from the
sediment surface to the invert of the lowest
pipe.
No sediment in the
catch basin
Structure Damage
to Frame and/or
Top Slab
Top slab has holes larger than 2 square inches
or cracks wider than 1/4 inch. (Intent is to make
sure no material is running into basin).
Frame not sitting flush on top slab, i.e.,
separation of more than 3/4 inch of the frame
from the top slab. Frame not securely attached
Top slab is free of
holes and cracks.
Frame is sitting flush
on the riser rings or top
slab and firmly
attached.
Fractures or
Cracks in Basin
Walls/ Bottom
Maintenance person judges that structure is
unsound.
Grout fillet has separated or cracked wider than
1/2 inch and longer than 1 foot at the joint of
any inlet/outlet pipe or any evidence of soil
particles entering catch basin through cracks.
Basin replaced or
repaired to design
standards.
Pipe is regrouted and
secure at basin wall.
Settlement/
Misalignment
If failure of basin has created a safety, function,
or design problem.
Basin replaced or
repaired to design
standards.
Vegetation
Vegetation growing across and blocking more
than 10% of the basin opening.
Vegetation growing in inlet/outlet pipe joints
that is more than six inches tall and less than
six inches apart.
No vegetation blocking
opening to basin.
No vegetation or root
growth present.
Contamination
and Pollution
See Table V-A.1: Maintenance Standards -
Detention Ponds No pollution present.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page H-10
Table V-A.5: Maintenance Standards - Catch Basins
Maintenance
Component Defect Conditions When Maintenance is Needed
Results Expected
When Maintenance is
performed
Catch Basin
Cover
Cover Not in
Place
Cover is missing or only partially in place. Any
open catch basin requires maintenance.
Cover/grate is in place,
meets design
standards, and is
secured
Locking
Mechanism Not
Working
Mechanism cannot be opened by one
maintenance person with proper tools. Bolts
into frame have less than 1/2 inch of thread.
Mechanism opens with
proper tools.
Cover Difficult to
Remove
One maintenance person cannot remove lid
after applying normal lifting pressure.
(Intent is keep cover from sealing off access to
maintenance.)
Cover can be removed
by one maintenance
person.
Ladder Ladder Rungs
Unsafe
Ladder is unsafe due to missing rungs, not
securely attached to basin wall, misalignment,
rust, cracks, or sharp edges.
Ladder meets design
standards and allows
maintenance person
safe access.
Metal Grates
(If Applicable)
Grate opening
Unsafe Grate with opening wider than 7/8 inch. Grate opening meets
design standards.
Trash and Debris Trash and debris that is blocking more than
20% of grate surface inletting capacity.
Grate free of trash and
debris.
Damaged or
Missing.
Grate missing or broken member(s) of the
grate.
Grate is in place, meets
the design standards,
and is installed and
aligned with the flow
path.
RAM Engineering, Inc. Walnut Short Plat
Stormwater Site Plan Report RAM No. 19-032
Page I-1
I. GEOTECHNICAL ENGINEERING REPORT
Geotechnical Evaluation
Prepared by: Earth Solutions NW, LLC
Revision Date: March 25, 2020
Pages: 15
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