Edmonds Westgate - Storm Drainage Report.pdfWestgate Mixed Use
Edmonds Way and 100th Ave W
Edmonds, Washington, 98020
Storm Drainage Report
1505 Westlake Ave. N T 206.522.9510
Suite 305 F 206.522.8344
Seattle, WA 98109 www.pacland.com
Prepared By: Luke Randles, P.E.
Reviewed By: Bill Fortunato, P.E.
Prepared: March 2017
Revised: July 2017
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 2
Section Page
EXECUTIVE SUMMARY .......................................................................................................................................................... 2
SECTION 1 - PROJECT OVERVIEW ......................................................................................................................................... 2
SECTION 2 - EXISTING CONDITIONS ..................................................................................................................................... 2
SECTION 3 - DEVELOPED CONDITIONS ................................................................................................................................ 2
SECTION 4 - OFF-SITE ANALYSIS ........................................................................................................................................... 2
SECTION 5 - CONDITIONS AND REQUIREMENTS ................................................................................................................. 2
SECTION 6 - HYDROLOGIC MODELING ................................................................................................................................. 2
SECTION 7 - CSWPPP ANALYSIS AND DESIGN ..................................................................................................................... 2
SECTION 8 - SPECIAL REPORTS AND STUDIES ..................................................................................................................... 2
SECTION 9 - OTHER PERMITS .............................................................................................................................................. 2
SECTION 10 - OPERATIONS AND MAINTENANCE MANUAL ............................................................................................... 2
APPENDICES: A- EXHIBITS
B - PLANS
C - OPERATIONS AND MAINTENANCE MANUAL
D - GEOTECHNICAL REPORT
E - HYDROLOGIC MODELING CALCULATIONS
Table of Contents
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 3
The proposed development is located at the southwest corner of Edmonds Way and 100th
Avenue West in Edmonds, Washington. The proposed development is bordered by a retail
building to the east, Edmonds Way to the north, a fast food restaurant to the northwest, and a
residential property to the southwest. The residential property to the southwest is separated
from the proposed development are by a steep vegetated slope in the southwest corner of the
site. The northwest corner of the site was previously developed as a residential area, but
currently contains only building foundations and a driveway. The eastern side and southeast
corner of the site are primarily paved. The project involves construction of a new mixed use
building and parking garage, along with associated above-grade parking, drive aisles, amenity
areas, utilities, storm drain management facilities, and landscaping.
Portions of the vegetated/forested hillside located in the southwest corner of the site are
classified as a Landslide Hazard Critical Area, and shall remain undisturbed by the development.
The project site lies within the Edmonds Watershed and the Edmonds Way drainage basin.
The property is zoned WMU (Westgate Mixed Use). The comprehensive plan for the site is
Community Commercial.
Design Criteria:
The City of Edmonds has adopted Stormwater Management Manual (SWMMWW), (2012, and as
amended in 2014) and Edmonds City Code (ECC) Chapter 18.30 Stormwater Management Code.
Per the requirements of SWMMWW, both flow control and water quality mitigation will be
required as a result of the proposed development. Site drainage is conveyed to the municipal
stormwater system in Edmonds Way, which flows to the northwest and discharges directly into
Puget Sound. Per the ECC, flow control requirements are per the Edmonds Way Specific
Requirements outlined in section 18.30.060.D.5.iv.
Table 1
Jurisdictional Requirements
Peak Runoff Control:
10-year and 100-year peak flows shall not
exceed 0.25 cfs and 0.45 cfs per acre of
impervious surface area, respectively.
Water Quality: 91st percentile, continuous runoff model
Conveyance Design: 25-year storm event
Executive Summary
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 4
Project Description
The proposed development is located at the southwest corner of Edmonds Way and 100th
Avenue West in Edmonds, Washington. The proposed development is bordered by a retail
building to the east, Edmonds Way to the north, a fast food restaurant to the northwest, and a
residential property to the southwest. The residential property to the southwest is separated
from the proposed development are by a steep vegetated slope in the southwest corner of the
site. The northwest corner of the site was previously developed as a residential area, but
currently contains only building foundations and a driveway. The eastern side and southeast
corner of the site are primarily paved. The on-site development area is approximately 1.23 acres
in size. The project involves construction of a new mixed use building and parking garage, along
with associated above-grade parking, drive aisles, amenity areas, utilities, storm drain
management facilities, and landscaping.
Site Location
Figure 2 - Site Location
Section 1 - Project Overview
PROJECT
SITE
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 5
Location: Edmonds Way and 100th Ave W 98020
Section/Township/Range: NW Quarter of Section 25, Township 27, Range 03
Parcel/Tax Lot(s): 00610700200600
00610700200601
00610700200701
00610700200702
Size: 2.13 acres
City, County, State: Edmonds, Snohomish County, Washington
Governing Agency: City of Edmonds
Design Criteria: 2012 Stormwater Management Manual for Western Washington, as amended
in 2014
Edmonds City Code Section 18.30 Stormwater Managment
Addendum to Edmonds Community Development Code Chapter 18.30
Drainage Basin: The project site is located within the Mcaleer Creek Subwatershed which is part
of the Lake Washington/Cedar River Watershed.
Soils
Websoil Survey
Figure 3 - Site Soils (USDA Web Soil Survey Map)
The site is underlain by 70.9% Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes
and 29.1% Urban land according to the USDA Natural Resource Conservation Service, Web Soil
Survey for King County Area, Washington.
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 6
Geotechnical Report
A geotechnical report was performed for the project by PanGEO, Incorporated on September 17,
2016. The report identified that the soils observed during site borings were generally consistent
with the Composite Geological Map of the Sno-King Area. Four different material types were
encountered during the on-site borings. Those types included Fill, Recessional Outwash,
Advanced Outwash, and Vashon Till. Fill consisting of a thin layer of medium dense to dense,
brown, fine to coarse gravel with silt and sand was encountered to a depth of 1 to 1-1/2 feet
was encountered in the area to the southeast of the proposed building. Recessional Outwash
consisting of loose, yellow brown, very silty, fine sand to sandy silt was encountered to depths of
roughly 4-1/2 feet on the western parcel. Advanced Outwash consisting of medium dense to
dense, brown to brown gray, fine to medium sand with silt and gravel was encountered to the
north of the proposed building, and extended to the full depth of the borings. Vashon Till
consisting of very dense, brown gray, massive, silty, fine sand with gravel was encountered on
the existing hillside. The geotechnical report is presented for reference in Appendix D.
Based on a grain size analysis of on-site soils, infiltration rates are estimated to range between
0.3 to 0.5 inches per hour. For preliminary design purposes, a long-term infiltration rate of 0.1
inches per hour has been assumed. Additional testing will be performed to provide final design
infiltration rates.
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 7
Existing Sub-basins
The existing site consists of two sub-basins, which are generally divided between the existing
residential development to the northwest (henceforth identified as Basin A) and the portion of
the existing commercial development to the southeast (henceforth identified as Basin B).
Stormwater runoff from Basin A sheet flows into Edmonds Way, where it is collected in catch
basins, which are part of a municipal storm system located in Edmonds Way. Basin B drains to
on-site catch basins, which collect runoff and convey it to a subsurface detention vault and
treatment structure prior to discharging to the municipal storm system in Edmonds Way. The
upstream and downstream analysis of these two sub-basins will be described in more detail in
Section 4. An exhibit showing the existing conditions of both sub-basins is provided in
Appendix A.
Section 2 - Existing Conditions
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 8
Developed Sub-basins
The developed site also consists of two sub-basins and preserves the same approximate
drainage patterns and outfalls as the existing site. The upstream and downstream analysis of
these two sub-basins will be described in more detail in Section 4. An exhibit showing the
developed conditions is provided in Appendix A.
The proposed development will utilize a combination of Low Impact Design (LID) Best
Management Practices (BMPs) and conventional stormwater facilities to manage runoff on the
site. The project will be divided into two separate sub-basins, with one portion of the site
draining toward the northwest boundary and the other draining toward the southwest.
Runoff from pollutant-generating surfaces in the northwest basin (henceforth identified as Basin
A), consisting of a small portion of parking located behind the proposed building, will be routed
toward a bioretention cell located along the edge of the property. The bioretention cell will
provide water quality treatment, as well as a small amount of infiltration. Non-pollutant-
generating surfaces, including the roof and sidewalks, will be routed directly to a subsurface
stormwater detention system. The system will consist of arched chambers surrounded by a
gravel bed. The outlet for this system will be directed to a flow control structure, which will
utilize an orifice to control the release of runoff existing the system. Downstream of the flow
control structure, the system will combine with the overflow from the bioretention cell prior to
discharging to the existing municipal system located in Edmonds Way.
Runoff from pollutant-generating surfaces in the southeast basin (henceforth identified as Basin
B) consisting of asphalt pavement drive aisles to be repaved for grading purposes, will drain to
permeable concrete parking areas, which will provide water quality treatment as well as a small
amount of infiltration. Non-pollutant-generating surfaces, including the roof and sidewalks, will
be routed directly to a subsurface stormwater detention system. The system will consist of
arched chambers surrounded by a gravel bed. The outlet for this system will be directed to a
flow control structure, which will utilize an orifice to control the release of runoff existing the
system. Downstream of the flow control structure, the system will combine with the overflow
from the permeable pavement surfaces prior to connecting to the existing storm drainage
system on the site, which ultimately discharges to the municipal system located in Edmonds
Way.
Section 3 - Developed Conditions
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 9
Upstream Analysis
A portion of the hillside to the southwest of the proposed development drains toward the site,
although the vegetated/forested cover of the hillside will result in minimal runoff. It is
anticipated that runoff generated from this area will be captured in the retaining wall drainage
prior to being deposited on the site. No other offsite areas are anticipated to flow toward the
site.
Downstream Analysis
Drainage from the site is directed toward an existing municipal stormwater system in Edmonds
Way. Edmonds Way flows to the southwest, where the stormwater system eventually discharges
into Puget Sound approximately 2 miles downstream of the site. A map of the downstream
analysis can be seen in Figure 4.
Figure 4 - Downstream Area (Edmonds GIS Maps)
Section 4 - Off-Site Analysis
PUGET
SOUND
SUB-BASIN POINT OF
ENTRY TO CITY OF
EDMONDS STORM
SYSTEM
SUB-BASIN
DISCHARGE
POINT
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 10
Figure 5 - Immediate Downstream Area (Edmonds GIS Maps)
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 11
As required by the SWMMWW, this project is subject to a full drainage review. Therefore, the
storm drainage design for this project is required to comply with all nine (9) Minimum
Requirements. The requirements have been met as follows:
Minimum Requirement #1: Preparation of Stormwater Site Plans
This project proposes to replace greater than 2,000 square feet of impervious surface; thus,
Minimum requirement #1 applies and a Stormwater Site Plan must be prepared for review by
the local jurisdiction.
Response: This Storm Drainage Report has been prepared to address this requirement.
Minimum Requirement #2: Construction Stormwater Pollution Prevention
All erosion and sediment control measures shall be governed by the requirements of
Department of Ecology’s SWMMWW and the General Permit for Construction Stormwater. The
twelve elements as identified in the manual and provided below will be incorporated into the
TESC plans:
Element 1: Preserve Vegetation/Mark Clearing Limits
Element 2: Establish Construction Access
Element 3: Control Flow Rates
Element 4: Install Sediment Controls
Element 5: Stabilize Soils
Element 6: Protect Slopes
Element 7: Protect Drain Inlets
Element 8: Stabilize Channels and Outlets
Element 9: Control Pollutants
Element 10: Control De-watering
Element 11: Maintain BMPs
Element 12: Manage the Project
Element 13: Protect Low Impact Development BMPs
Response: Temporary Erosion and Sediment control plans will be prepared to address the
elements identified. Since the project will result in greater than 1 acre of soil disturbance, a
Stormwater Pollution Prevention Plan (SWPPP) will be prepared and coverage under the NPDES
Construction Stormwater General Permit will be obtained prior to construction. See Section 8 of
this report for a complete description of the construction and erosion control strategies being
implemented.
Section 5 - Conditions and Requirements
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 12
Minimum Requirement #3: Source Control of Pollution
All known, available, and reasonable source control BMPs must be applied to all projects. Source
control BMPs must be selected, designed, and maintained according to the SWMMWW.
Response: Source control BMPs will be implemented according to the standards outlined in the
SWMMWW.
Minimum Requirement #4: Preservation of Natural Drainage Systems and Outfalls
Natural drainage patterns shall be maintained, and discharges from the project site shall occur
at the natural location, to the maximum extent practicable. The manner by which runoff is
discharged from the project site must not cause a significant adverse impact to downstream
receiving waters and downgradient properties. All outfalls require energy dissipation.
Response: The proposed drainage design is consistent with the existing site. Basin A will continue
to drain to the north side of the site and connect to the municipal system along Edmonds Way.
Basin B will continue to be collected in catch basins and routed to the on-site stormwater system
prior to detention, treatment, and discharge into the municipal system. Refer to Section 6 for more
detail about the existing and proposed drainage hydrology.
Minimum Requirement #5: On-site Stormwater Management
Projects shall employ On-site Stormwater Management BMPs in accordance with the following
projects thresholds, standards, and lists to infiltrate, disperse, and retain stormwater runoff on-
site to the extent feasible without causing flooding or erosion impacts.
Response: Per ECC stormwater guidelines, the project qualifies for direct discharge due to the
projects discharge to the Edmonds Way stormwater system that discharges to Puget Sound. As
such, the project is not required to achieve the LID Performance Standard, nor consider
bioretention, rain gardens, permeable pavement, or full dispersion. However, some of these
elements will still be incorporated, as they are listed as desired features in the Westgate Mixed Use
zone.
Minimum Requirement #6: Runoff Treatment
Projects in which the total of effective, pollution-generating impervious surface (PGIS) is 5,000
SF or more in a threshold discharge area of the project require construction of stormwater
treatment facilities. Projects in which the total of pollution-generating pervious surfaces (PGPS)
is three-quarters (3/4) of an acre or more in a threshold discharge area, and from which there is
a surface discharge in a natural or man-made conveyance system from the site also require
treatment facilities.
Response: Basin A will utilize a bioretention cell for treatment of runoff generated from pollutant-
generating surfaces. Basin B will utilize permeable pavement for treatment of runoff generated
from pollutant-generating surfaces.
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 13
Minimum Requirement #7: Flow Control
Projects that discharge directly to Puget Sound through the City’s MS4 (in accordance with the
restrictions applicable to direct discharges to Puget Sound presented in Section 2.5.7 of Volume
I of the SWMMWW) must meet the following: for all project site runoff (e.g., from unmanaged
surfaces, or from overflow from on-site BMPs), the post-development 10-, and 100-year
recurrence interval peak flows shall not exceed, 0.25 and 0.45 cubic feet per second per acre of
impervious surface area, respectively.
Response: LID elements included for the requirements of the WMU zone will provide flow control
for runoff from pollutant-generating surfaces. Additionally, subsurface detention chambers and
flow control structures will be installed to receive runoff from non-pollutant-generating surfaces,
detain stormwater, and slowly release stormwater per the requirements stated above.
Minimum Requirement #8: Wetlands Protection
The thresholds identified in Minimum Requirement #6 and Minimum Requirement #7 shall also
be applied for any discharge to wetlands.
Response: There are no wetlands within the development area of the site.
Minimum Requirement #9: Basin/Watershed Planning
Projects may be subject to equivalent or more stringent minimum requirements for erosion
control, source control, treatment, and operation and maintenance, and alternative requirements
for flow control and wetlands hydrologic control as identified in Basin/Watershed Plans.
Response: No known alternative requirements for flow control have been identified for this site.
Minimum Requirement #10: Operation and Maintenance
An operation and maintenance manual that is consistent with the provisions in the SWMMWW
shall be provided for proposed stormwater facilities and BMPs, and the parties responsible for
maintenance and operation shall be identified.
Response: The Operations and Maintenance of the storm water management system is detailed in
the Operations and Maintenance section of this report.
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 14
Existing Site Hydrology
The site can be generally defined by two separate drainage basins, with Basin A consisting of the
existing residential property in the northwest corner of the site and Basin B consisting of a
portion of the commercial property on the east side of the site. The two basins function
independently, with drainage from Basin A flowing directly to the municipal system in Edmonds
Way and drainage from Basin B being collected in an existing on-site stormwater system prior to
discharging to the municipal system. The pre-developed conditions are shown in Table 3. An
exhibit showing the existing conditions is provided in Appendix A.
Table 3
Existing Conditions
Land Cover Area (Acres)
Basin A Basin B
Pavement 0.18 0.41
Landscape 0.13 0.43
Roof 0.08 0.00
Forest 0.14 0.06
Total 0.53 0.90
Developed Site Hydrology
The developed site will preserve the general configuration of the existing drainage sub-basins.
The project will be separated into two basins, with Basin A continuing to connect directly to the
municipal stormwater system in Edmonds Way and Basin B continuing to connect to the existing
on-site stormwater system prior to connecting to the municipal system. The post-developed
conditions are shown in Table 4. An exhibit showing the developed conditions is provided in
Appendix A. The proposed grading and drainage plan is included in Appendix B.
Table 4
Developed Conditions
Land Cover Area (Acres)
Basin A Basin B
Pavement (PGIS) 0.10 0.16
Pavement (Permeable) 0.00 0.16
Sidewalk 0.04 0.12
Landscape 0.04 0.08
Roof 0.21 0.32
Forest 0.14 0.06
Total 0.53 0.90
Section 6 - Hydrologic Modeling
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 15
Flow Control Analysis
As described in Section 3 above, the project will utilize a combination of LID BMPs and
conventional detention to control flows from the developed site. Because the project discharges
to Edmonds Way, the developed flows are subject to the requirements of Edmonds Way-specific
direct discharge requirements outlined in ECC 18.30. As identified in Table 1, release flow rates
shall not exceed 0.25 cfs per acre of impervious area for the 10-year storm event, and 0.45 cfs
per acre of impervious area for the 100-year storm event. Based on the areas identified in Table
4, Basin A release rates shall not exceed 0.09 cfs for the 10-year storm event or 0.16 cfs for the
100-year storm event, and Basin B release rates shall not exceed 0.15 cfs for the 10-year storm
event or 0.27 cfs for the 100-year storm event.
Flow control calculations, performed using WWHM, can be found in Appendix E.
Table 4
Existing Flow Rates
Storm Event Flow Rate (cfs)
Basin A Basin B
2-year 0.0876 0.1431
5-year 0.1215 0.2022
10-year 0.1468 0.2468
25-year 0.1822 0.3099
50-year 0.2110 0.3620
100-year 0.2422 0.4187
Table 6
Developed Flow Rates
Storm Event Flow Rate (cfs)
Basin A Basin B
2-year 0.0559 0.0984
5-year 0.0717 0.1257
10-year 0.0833 0.1459
25-year 0.0992 0.1741
50-year 0.1119 0.1970
100-year 0.1256 0.2216
As shown, in Table 6, developed flow rates for the 10 and 100-year storm events do not exceed
the maximum allowable rates. Furthermore, since the existing basins are currently partially
developed, by providing detention for each basin, developed flow rates are reduced from
existing conditions, despite the increase in site impervious areas due to development.
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 16
Water Quality System
Runoff from pollutant-generating surfaces will be managed using LID BMPs, including
bioretention and permeable pavements. Per the SWMMWW, water quality treatment will be
provided such that 91% of the runoff generated from pollutant-generating surfaces. In order to
minimize the size of treatment BMPs, runoff generated from non-pollutant-generating surfaces
has been routed directly to conventional detention systems.
Information regarding sizing of water quality BMPs, performed using WWHM, can be found in
Appendix E.
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 17
All erosion and sediment control measures shall be governed by the requirements Department
of Ecology’s SWMMWW and the General Permit for Construction Storm Water. A National
Pollutant Discharge Elimination System (NPDES) Permit will be obtained and a Stormwater
Pollution Prevention Plan (SWPPP) will be prepared for this project.
A temporary erosion and sedimentation control plan will be prepared to assist the contractor in
complying with these requirements. The Erosion and Sediment Control (ESC) plan will be
included with the construction plans.
1. Construction Sequence and Procedure
The proposed development will include an erosion/sedimentation control plan designed to
prevent sediment-laden run-off from leaving the site during construction. The erosion potential
of the site is influenced by four major factors: soil characteristics, vegetative cover, topography,
and climate. Erosion/sedimentation control is achieved by a combination of structural
measures, cover measures, and construction practices that are tailored to fit the specific site.
The contractor will be responsible for implementing the following erosion control and storm
water management control measures. The contractor may designate these tasks to certain
subcontractors as they see fit, but the ultimate responsibility for implementing these controls
and ensuring their proper functioning remains with the contractor. The order of activities will be
as follows.
Phase 1
1. Prior to any construction work on the site, representatives from the City of Edmonds
must approve the storm water pollution prevention plan.
2. Mark clearing limits.
3. Install inlet protection to all existing catch basins.
4. Install temporary stabilized construction entrance.
5. Install perimeter silt fences, interceptor swales, etc. Protect existing and proposed
infiltration areas.
6. Construct temporary sediment and infiltration pond.
7. Remove any existing structures that may be on site.
8. Protect and stabilize slopes.
9. Begin clearing and grubbing operations. Clearing and grubbing done from October 1st
through April 30th is authorized as long as there are erosion and sediment control
measures.
10. Commence site grading.
Section 7 - CSWPPP Analysis and Design
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 18
Phase 2
1. Disturbed areas of the site where construction activity has ceased for more than 7 days
between May 1 and September 30 or 2 days between October 1 and April 30 shall be
temporarily seeded and watered.
2. Install haul road.
3. Construct building pad and install concrete wash out area.
4. Construct permanent storm water facilities. Remove temporary sedimentation ponds.
5. Install utilities, underdrains, storm sewers, curbs and gutters.
6. Install inlet/outlet protection at the locations of all grate inlets, curb inlets, and at the
ends of all exposed storm sewer pipes.
7. Install rip rap around outlet structures.
8. Prepare site for paving. Finalize pavement subgrade preparation.
9. Remove inlet protection around inlets and manholes no more than 48 hours prior to
placing stabilized base course.
10. Install base material as required for pavement. Pave site. Do not pave over catch basins.
11. Complete final grading in non-parking areas and install permanent seeding and planting.
12. Remove silt fencing only after all paving is complete and exposed surfaces are stabilized.
13. Remove temporary construction exits only prior to pavement construction in these areas
(These areas are to be paved last).
The degree of erosion risk on the proposed project site is minimal. Slope across the site is
minimal. Runoff will not travel at high velocities across the site and, therefore, will not cause
noticeable erosion impacts.
2. Temporary Soil Stabilization
Temporary stabilization practices for this project include:
• Temporary seeding and planting of all unpaved areas using the hydro-mulching grass
seeding technique.
• Mulching exposed areas.
• Installation of rolled erosion control products.
Structural practices for this project include the following. Refer to the Erosion Control plans for
specific locations and details:
• Inlet protection using fiber fabric.
• Outlet protection (velocity dissipation) using rip rap.
• Perimeter protection using silt fences.
• Stabilized construction entrance/exit points and staging area.
• Temporary sediment basin.
• Silt fence.
• Interceptor swales.
• Temporary storm drain riser.
Daily inspection of the erosion control measures will be required during construction. Any
sediment buildup shall be removed and disposed offsite at an appropriate disposal facility.
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 19
Vehicle tracking of mud off-site shall be avoided. A gravel construction entrance/exit will be
installed at a location to enter the site. The construction entrance/exit is a minimum
requirement and may be supplemented if tracking of mud onto public streets becomes
excessive. In the event that mud is tracked off site, it shall be swept and disposed of offsite on a
daily basis.
Because vegetative cover is the most important form of erosion control, construction practices
must adhere to stringent cover requirements. More specifically, the contractor will not be
allowed to leave soils open for more than 7 days between May 1st and September 30th and 2
days between October 1st and April 30th. Soils shall be stabilized at the end of the shift before a
holiday or weekend if needed based on the weather forecast. Applicable practices include, but
are not limited to, temporary and permanent seeding, sodding, mulching, plastic covering, and
soil application of polyacrylamide.
Soil stockpiles must be stabilized from erosion, protected with sediment trapping measures, and,
where possible, be located away from storm drain inlets, waterways and drainage channels.
3. Temporary Sediment Control Pond
Storm water shall be detained on-site during construction in a temporary sediment control pond
and two temporary sediment ponds located in the southwest corner of the site. The contractor
shall construct interceptor swales to keep storm water from leaving the site. The sediment
ponds will need to adhere to all NPDES and SWPPP requirements. The DOE requires that
temporary sediment ponds need to be designed to contain the 10-year developed event. The
sediment ponds are sized to approximately 30% of the volume of the infiltration pond to
provide for pre-settling.
4. Permanent Erosion Control and Site Restoration
Upon completion of the project, areas of the site that are not stabilized with paving, rooftops, or
landscaping as shown on the site plans will be protected with either grass, ground
cover/plantings or existing vegetation as shown on the Landscape Plans. In general, storm
runoff from the site will be collected by catch basins connected to a storm water quality
structure and then flows into detention system.
5. Inspection Sequence
The construction site operator will periodically inspect its sites. Because our project disturbs
more than one acre, a certified erosion and sediment control lead will be identified within the
SWPPP. This individual will be present on site or on call at all times.
6. Control of Pollutants Other than Sediments
Pollutants shall be controlled on the work site through the utilization of a centralized area for
equipment, a concrete truck washout, and an area designated for temporary storage of debris
and stockpiled materials.
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 20
The following Special Reports and Studies were used or have been completed for this project:
• Geotechnical Report, prepared by PanGEO, Inc., dated September 17, 2016
• FEMA FIRM Panel 1315 of 1575 Map number 53061C1315 E dated November 8, 1999
Section 8 - Special Reports and Studies
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 21
The following governmental approvals or permits will likely be required for this project:
• City of Edmonds SEPA Determination
• City of Edmonds Building Permit
• City of Edmonds Construction Permit
• City of Edmonds Demolition Permit
• Department of Ecology Construction Stormwater General Permit
• National Pollutant Discharge Elimination System Permit
• Stormwater Pollution Prevention Plan These permits will require approval by the City of Edmonds or the Department of Ecology.
Section 9 - Other Permits
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 22
The owner or operator of the project shall be responsible for maintaining the stormwater
facilities in accordance with local requirements. A manual detailing the operations and
maintenance for all privately-maintained drainage facilities has been prepared and can be found
in Appendix C.
Section 10 - Operations and Maintenance Manual
Westgate Mixed Use Storm Drainage Report Edmonds, Washington
PACLAND Project #10-382-001 Page 23
Appendix A – Exhibits
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Appendix B – Plans
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Appendix C – Operations and Maintenance Manual
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If you are unsure whether a problem exists, please contact a Professional Engineer.
Comments:
A = Annual (March or April, preferred)
M = Monthly (see schedule)
S = After major storms (use 1-inch in 24 hours as a guideline)
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Appendix D – Geotechnical Report
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Appendix E - Hydrologic Modeling Calculations
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