Edmonds CA Senior Living_Stormwater Site Plan_190516
May 16, 2019
Prepared for:
City of Edmonds
7110 210th St SW
Edmonds, WA 98026
Reviewed by:
Jenelle Taflin, P.E., LEED AP
Principal
jtaflin@navixeng.com
STORMWATER
SITE PLAN
CA Senior Living
Edmonds, Washington
PRE
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STORMWATER SITE PLAN
NAVIX CA Senior Living – Edmonds, WA Page 2
Table of Contents
PROJECT SITE MAP ................................................................................................................................. 3
PROJECT OVERVIEW ............................................................................................................................... 4
SITE LOCATION ................................................................................................................................... 4
DESIGN CRITERIA ................................................................................................................................ 5
EXISTING CONDITIONS AND HYDROLOGY ............................................................................................. 6
EXISTING CONDITIONS ....................................................................................................................... 6
EXISTING SITE HYDROLOGY ................................................................................................................ 7
DEVELOPED CONDITIONS AND HYDROLOGY ......................................................................................... 9
DEVELOPED CONDITIONS ................................................................................................................... 9
DEVELOPED SITE HYDROLOGY ........................................................................................................... 9
MINIMUM REQUIREMENTS ................................................................................................................. 10
OFFSITE ANALYSIS REPORT .................................................................................................................. 14
OVERVIEW ........................................................................................................................................ 14
DOWNSTREAM ANALYSIS ................................................................................................................ 14
EXISTING IMPACTS AND CRITICAL AREAS ........................................................................................ 21
POTENTIAL OFFSITE IMPACTS .......................................................................................................... 22
100-YEAR FLOOD/OVERFLOW CONDITION ...................................................................................... 22
PERMANENT STORMWATER CONTROL PLAN...................................................................................... 23
ON-SITE STORMWATER BMPs .......................................................................................................... 23
HYDROLOGIC MODELING ................................................................................................................. 25
FLOW CONTROL SYSTEM ................................................................................................................. 27
CONVEYANCE SYSTEM ANALYSIS AND DESIGN ............................................................................... 27
OPERATION AND MAINTAINANCE ................................................................................................... 28
CSWPPP ANALYSIS AND DESIGN .......................................................................................................... 30
SPECIAL REPORTS AND STUDIES .......................................................................................................... 40
PERMITS ............................................................................................................................................... 40
APPENDIX: A – WWHM DATA AND OUTPUT .......................................................................... 41
B – OPERATION AND MAINTENANCE MANUAL ................................................... 60
C – GEOTECHNICAL ENGINEERING REPORT .......................................................... 66
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PROJECT SITE MAP
Figure 1: Project Site Map
Total Project Area: 36,781 SF (0.844 acres)
Total Proposed Hard Surfaces: 30,770 (0.706 acres)
Proposed Pollution Generating Hard/Impervious Surface: 3,359 SF (0.077 acres)
Proposed Pollution Generating Pervious Surface: 6,011 SF (0.138 acres)
Existing Unmanaged Hard Surfaces: 0 SF (0.00 acres)
Total Disturbed Area: 36,781 SF (0.844 acres)
Average Slope: 8-25%
Estimated Ultimate Infiltration Rate: ¼ inch/hour to ½ inch/hour
NRCS Soil Group: Alderwood-Urban Land Complex Soils (glacial till)
For conveyance system details, please see the stormwater plans. For infiltration feasibility
information, please see the geotechnical report (Appendix C).
Stormwater detention
vault in basement
Pump from stormwater
detention vault to
connection with existing
storm system
Intercept off-site
storm conveyance
system
Re-route off-site
conveyance to
existing storm
system
Project
Boundary
Line
Connection
with existing
storm system
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PROJECT OVERVIEW
The site is currently developed as a single-story building and parking lot (see Existing Conditions
section below). Onsite stormwater outfalls to an existing storm sewer main in 72nd Avenue W, which
ultimately outfalls to Halls Creek.
The proposed development will consist of an approximately 163,100 GSF, 6-story assisted living
facility with approximately 92 assisted living units and 34 memory care units on a 0.78-acre parcel
located at 21200 72nd Avenue W in Edmonds, Washington. The project includes associated driveway
access, trash enclosure pad, landscaping areas, sidewalks, and utility connections. No existing
unmanaged hard surfaces will remain.
SITE LOCATION
Figure 2: Project Location
Location: 21200 72nd Ave W Edmonds, WA 98026 (Snohomish County)
Section, Township, Range: NW 1/4, NW 1/4, SEC. 29, TOWNSHIP 27N, RANGE 04E, W.M.
Parcel/Tax Lot: 00580700000401
Parcel Size: 0.78 acres
Total Area of Disturbance: 0.844 acres
City, County, State: Edmonds, Snohomish County, Washington
Zoning: CG, General Commercial
PROJECT
SITE
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DESIGN CRITERIA
The stormwater management facilities have been designed in accordance with the 2017 City of
Edmond’s Stormwater Management Code, which utilizes the 2014 Stormwater Management Manual
for Western Washington and the City of Edmonds Stormwater Addendum. This project will result in
greater than 5,000 SF of new plus replaced hard surface, therefore the site is classified as a Category
2 project site and shall comply with Minimum Requirements 1-9 of the City of Edmonds Stormwater
Management Code.
Table 1 below summarizes the City of Edmonds stormwater requirements.
JURISDICTIONAL REQUIREMENTS
Duration Analysis
2-year: Reduce to ½ pre-developed duration
50-year: Match pre-developed
Water Quality Volume: N/A
Water Quality Flow Rate: Full 2-year release rate from the detention
facility*
Downstream Analysis: ¼ mile
*All continuous modelling to be performed using the “Puget East 36”
precipitation time series, available in WWHM2012 by using WS-DOT data
TABLE 1: Jurisdictional Requirements
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EXISTING CONDITIONS AND HYDROLOGY
EXISTING CONDITIONS
The site is currently comprised of an existing office building with an associated parking lot area and
landscaping. It is bound to the north by 212th Street SW, to the east by 72nd Avenue W, to the south
by an apartment complex, and to the west by a townhome development. There is existing storm
drainage infrastructure on-site that runs west to east and conveys off-site stormwater from the Hyde
Park Townhomes to the municipal conveyance system in 72nd Avenue West. See Figure 3 for the
Existing Conditions Map.
Figure 3: Existing Conditions Map
The property slopes from an approximate elevation of 382 feet at the northern property line up to
about 376 feet at the southern property line (Terracon). The NRCS soils map lists the site as having
slopes in the range of approximately 8-15%. Per the City of Edmonds GIS critical areas map the site
contains areas of landslide and erosion hazards (Figure 7).
A Geotechnical Engineering Report was prepared for this project by Terracon Consultants, Inc on
December 20, 2018 (Appendix C). Per the report on-site soils are comprised of asphalt and concrete
pavement with a gravel base course at the surface; silty sand with variable gravel content with
organics at 3.5’-7.5’ below grade; sand with silt content and trace gravel (recessional glacial outwash)
Active storm pipe
conveys runoff from
Hyde Park Townhomes
Off-site storm
conveyance from
Hyde Park
Townhomes to west
Existing pipe outfall
connection to
existing storm main
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at 10’-13’ below grade; and, silty sand and sandy silt with variable gravel content (glacial till) up to
26’ below grade. Groundwater was encountered at 20.5 feet deep in one of four borings and was not
encountered during or after drilling in the other three borings.
There are no known surface water bodies in the immediate vicinity of the site including wetlands. A
high-pressure gas line is located to the east of the project site within the 72nd Avenue West right-of-
way, outside of the proposed development area. The site is zoned CG, General Commercial, and is
within the Highway 99 Subarea Plan.
EXISTING SITE HYDROLOGY
Surface runoff from the existing site is collected in two catch basins located in the parking lot in the
northern half of the site and is conveyed east to the existing municipal conveyance system in 72nd
Avenue West. The site is in the Halls Creek Drainage Basin (Figure 4).
The existing conditions survey prepared by Axis Survey and Mapping on January 19, 2018 indicates a
stormwater drainage easement on the property that runs from the western edge of the site at its
boundary with the Hyde Park Townhomes to the east. The easement encompasses a 12-inch storm
conveyance pipes that originates from the Hyde Park Townhomes to the west and connects to the
existing municipal conveyance system in 72nd Avenue West. This off-site storm conveyance system
will be intercepted and rerouted around the proposed development.
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Figure 4: Watershed Map
PROJECT
SITE
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DEVELOPED CONDITIONS AND HYDROLOGY
DEVELOPED CONDITIONS
The proposed project will consist of an approximately 163,100 GSF 6-story assisted living facility with
approximately 92 assisted living units and 34 memory care units on a 0.78-acre parcel located at
21200 72nd Avenue W in Edmonds, Washington. All existing on-site structures will be demolished.
DEVELOPED SITE HYDROLOGY
Stormwater runoff from newly created pervious and impervious surfaces will be collected and routed
to an on-site stormwater management system consisting of an approximately 21,000 CF underground
detention vault for flow control. Flow from the vault will be restricted to pre-development forested
conditions in accordance with City of Edmonds stormwater requirements. The controlled-released
runoff will discharge to the municipal conveyance system in 72nd Avenue West by means of a
stormwater pump system. All stormwater runoff from hard surfaces on-site will be managed and no
existing, unmanaged hard surfaces will remain.
As the project is not creating 5,000 SF or more of pollution-generating hard surface (PGHS) or 0.75
acres or more of pollution-generating pervious surface (PGPS), water quality treatment of not
required.
The total proposed non-pollution-generating impervious surface is approximately 0.629 acres, the
proposed pervious areas is approximately 0.138 acres, and the proposed pollution-generating
impervious area is 0.077 acres.
Table 2, below, summarizes the threshold discharge areas, including off-site areas.
Threshold Discharge Area Area (AC)
Current Hard Surfaces 0.616
Proposed Hard Surfaces 0.706
New + Replaced PGPS 0.138
New + Replaced PGIS 0.077
New + Replaced PGHS 0.000
Effective Impervious Surface 0.706
Existing, Unmanaged Hard
Surface to Remain
0.000
Disturbed Pervious Surface 0.228
Total Additional Hard Surfaces 0.090
Table 2: Threshold Discharge Areas
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MINIMUM REQUIREMENTS
Figure 5- Minimum Requirements
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Minimum Requirement #1: Preparation of Stormwater Site Plans
The City shall require a Stormwater Site Plan from all projects meeting the thresholds in ECDC
18.30.060.C. Stormwater Site Plans shall use site-appropriate development principles to retain native
vegetation and minimize impervious surfaces to the extent feasible. Stormwater Site Plans shall be
prepared in accordance with Chapter 3 of Volume 1 of the SWMMWW and the requirements in the
Edmonds Stormwater Addendum.
Response: A stormwater site plan that includes the design drawings and this report has been
prepared for this development. The stormwater site plan has been designed using site-appropriate
development principles to meet City of Edmonds guidelines to the maximum extent feasible.
Minimum Requirement #2: Construction Stormwater Pollution Prevention Plan (SWPPP)
All development projects are responsible for preventing erosion and discharge of sediment and other
pollutants into receiving waters. Compliance with this minimum requirement can be achieved for an
individual site if the site is covered under Ecology’s General NPDES Permit for Stormwater Discharges
Associated with Construction Activities and fully implementing the requirements of that permit.
A Construction SWPPP is required for all projects which a) result in 2,000 square feet or more of new
plus replaced hard surface area, b) where a structure with an exterior hard surface area of at least
2,000 square feet is being demolished, c) which disturb 7,000 square feet or more of land, or d) when
the site falls within the Earth Subsidence Landslide Hazard Area, Landslide Hazard Area or steep slope
critical area.
Response: A preliminary Construction SWPPP has been prepared and is included in the “CSWPP
Analysis and Design” section of this report, below. A full Construction SWPPP report to be utilized by
the contractor will be submitted with this permit submittal package as a separate document.
Minimum Requirement #3: Source Control of Pollution
All known, available and reasonable source control BMPs must be required for all projects approved
by the City. Source control BMPs must be selected, designed, and maintained in accordance with
Volume IV of the SWMMWW.
Response: All known, available, and reasonable source control BMPs have been evaluated for
applicability to this project. This includes, but is not limited to, Dust Control at Disturbed Land Areas,
Landscaping and Lawn/Vegetation Management, and Maintenance of Stormwater Drainage and
Treatment Systems.
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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
down gradient properties. The discharge must have an identified overflow route that is safe and
certain and leads to the ultimate outfall location (such as a receiving water or municipal drainage
system). All outfalls require energy dissipation. To demonstrate compliance with this core
requirement, all projects shall submit an off-site qualitative analysis. If an existing problem (or
potential future problem after development) is identified, mitigation will be required to prevent
worsening of that problem. A quantitative analysis may be required for any project deemed to need
additional information or where the project proponent or the City determines that a quantitative
analysis is necessary to evaluate the off-site impacts or the capacity of the conveyance system. Per
the Edmond’s Stormwater Addendum, existing upstream flows must be accommodated without
causing erosion or flooding impacts. Upstream flows shall not be routed through the project’s
conveyance, treatment, or retention/detention systems, unless those systems are sized to control
those flows. Upstream flows that are collected and routed through or around the site in a separate
conveyance shall be dispersed at the downgradient property line, if feasible, or discharged at a
project outfall (or outfalls) in a manner that does not violate the criteria below or cause the capacity
of a conveyance system to be exceeded.
Response: Natural drainage systems and outfalls will be maintained to the maximum extent feasible
for the development of this site. Onsite stormwater will be collected in a new underground detention
vault in the basement of the proposed building and routed to the existing municipal conveyance
system in 72nd Ave W. In the proposed condition the existing drainage path and connection to the
existing system in 72nd Ave W will be maintained.
Minimum Requirement #5: On-Site Stormwater Management
On-site Stormwater Management BMPs are required in accordance with the following project
thresholds, standards, and lists to infiltrate, disperse, and retain stormwater runoff on-site to the
extent feasible without causing flooding or erosion impacts. Category 2 project sites that discharge
directly or indirectly to the City’s MS4 shall use On-Site Stormwater BMPs from List No.2 for all new
plus replaced hard surfaces and land disturbed.
Response: On-site stormwater management BMP’s have been evaluated in accordance with the City
of Edmonds stormwater code requirements and the project is electing to use the List No. 2 option for
on-site stormwater BMPs selection. Please see the Permanent Stormwater Control Plan – On-Site
Stormwater Management BMP’s section of this report for more information.
Minimum Requirement #6: Runoff Treatment
The following require construction of stormwater treatment facilities:
· Projects in which the total of pollution-generating hard surface (PGHS) is 5,000 square feet or
more in a threshold discharge area of the project, or
· Projects in which the total of pollution-generating pervious surfaces (PGPS) – not including
permeable pavements – is 0.75 acres or more in a threshold discharge area, and from which
there will be a surface discharge in a natural or man-made conveyance system from the site.
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Response: The total proposed pollution-generating hard surface (PGHS) is 2,580 SF and the pollution-
generating pervious surface (PGPS) consisting of lawn and landscaping area is 5,811 SF. Therefore,
water quality treatment facilities are not required.
Minimum Requirement #7: Flow Control
Flow control is required on projects meeting the thresholds summarized below to reduce the impacts
of stormwater runoff from hard surfaces and land cover conversions. Standard Flow Control
Requirement (applies to discharges directly or indirectly to the City’s MS4, except for projects that
meet the direct discharge requirements outlined in “a” above): Stormwater discharges shall match
developed discharge durations to pre-developed durations for the range of pre-developed discharge
rates from 50 percent of the 2-year peak flow up to the full 50-year peak flow. The pre-developed
condition to be matched shall be a forested land cover.
Response: Flow control will be provided by utilizing a detention vault located in the basement level.
On-site roof drainage will be collected and routed to the detention vault prior to discharge to the
storm drainage system located in 72nd Avenue W. The detention vault is sized to meet discharge
requirements for the entire redevelopment area in accordance with the 2014 Stormwater
Management Manual for Western Washington requirements utilizing continuous modelling with
WWHM2012.
Minimum Requirement #8: Wetlands Protection
The requirements below apply only to projects whose stormwater discharges into a wetland, either
directly or indirectly through a conveyance system.
Response: This project does not discharge into a wetland.
Minimum Requirement #9: Operation and Maintenance
An operation and maintenance manual that is consistent with the provisions in Volume I and Volume
V of the SWMMWW is required for proposed Stormwater Treatment and Flow Control
BMPs/facilities. The party (or parties) responsible for maintenance and operation shall be identified
in the operation and maintenance manual. For private facilities approved by the City, a copy of the
operation and maintenance manual shall be retained on-site or within reasonable access to the site
and shall be transferred with the property to the new owner. For public facilities, a copy of the
operation and maintenance manual shall be retained in the appropriate department. A log of
maintenance activity that indicates what actions were taken shall be kept and be available for
inspection.
Response: An Operation and Maintenance Manual is included in Appendix B of this report.
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OFFSITE ANALYSIS REPORT
OVERVIEW
The project site consists of one drainage sub basin which outfalls to Halls Creek. Approximately 0.25
miles downstream is the intersection of 216th Street SW and 72nd Avenue W. See Figure 6, below. No
emergency services are located along the flow path.
Figure 6: Downstream Analysis Path
DOWNSTREAM ANALYSIS
A formal downstream analysis was completed in accordance with the City of Edmond’s 2017
Stormwater Addendum. This analysis was conducted on March 7, 2019. The temperature was
approximately 40 degrees and overcast.
¼ mile
downstream
location
Project Site
Downstream
Flow Path
Outfall to Halls
Creek
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# Picture Description
1
View from the northwest side of the site
looking east.
Drainage is conveyed east across the site to
an open-grate catch basin on the northeast
side of the site.
2
View from the northeast side of the site
looking east.
Drainage is conveyed east offsite where it
turns south to a manhole in the western-
most drive lane of 72nd Ave W north of
213th Pl SW.
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3
View from the west side of 72nd Ave W
looking south.
Drainage is conveyed south through a 36”
concrete pipe to a manhole in the western-
most drive lane of 72nd Ave W.
4
View from the west side of 72nd Ave W
looking south.
Drainage is conveyed south through a 36”
concrete pipe to a manhole in the western-
most drive lane of 72nd Ave W near the
southeast corner of the Edmonds Park and
Ride.
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5
View from the west side of 72nd Ave W
looking south.
Drainage is conveyed south through a 36”
concrete pipe to a manhole on the west
side of 72nd Ave W.
6
View from the sidewalk on the west side of
72nd Ave W looking east.
Drainage is conveyed south through a 36”
concrete pipe to a manhole on 216th St SW.
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7
View from the southwest corner of the
intersection of 72nd Ave W and 216th St SW.
Drainage is conveyed east through a 36”
pipe to a manhole east of the intersection
of Highway 99 and 216th St SW.
Image Credit: Google Maps
8
View from the intersection of Highway 99
and 216th St SW looking northwest.
Drainage is conveyed to the southeast
corner of the intersection of Highway 99
and 212th St SW.
Image Credit: Google Maps
Catch Basin
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9
View from southeast corner of Highway 99
and 216th St SW looking east.
Drainage is conveyed east along 216th St
SW through a 36” concrete pipe to a
manhole on the south side of 212th St SW.
10
View from the south side of 216th St SW
looking east.
Drainage is conveyed east along 216th St
SW through a 36” concrete pipe to a
manhole on the south side of 212th St SW.
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11
View from the south side of 212th St SW
looking southeast.
Drainage is conveyed southeast to an
outfall along Halls Creek
12
View from the south side of the Interurban
Trail looking south.
Drainage outfalls to Halls Creek.
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EXISTING IMPACTS AND CRITICAL AREAS
Per the City’s website, there is an existing erosion hazard area along the northern and western
perimeters of the site, most likely having to do with the existing sloped/retaining wall areas in those
locations. The existing retaining wall along the western property boundary will be maintained in the
developed condition. The shorter, existing retaining wall in the southwest corner of the site will be
removed and replaced by retaining via the building. The downstream storm drainage system within
0.25 miles of the project is adjacent to existing erosion hazards, but there are no known impacts or
problems associated with the drainage system.
See Figure 7 below for the critical areas map.
Figure 7- Critical Areas Map
¼ mile
downstream
location
Project Site
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POTENTIAL OFFSITE IMPACTS
The proposed on-site stormwater management system will be designed according to the 2017 City
of Edmonds Stormwater Management Code. No impacts to groundwater, surface waters, or drainage
patterns are anticipated.
100-YEAR FLOOD/OVERFLOW CONDITION
The stormwater conveyance system for this project has been designed to address all storm events,
including the 100-year, 24-hour storm, in accordance with common industry practices. Please see
FIRM map shown in Figure 8 below. The project site is in Zone X, which is outside the 500-year
floodplain
Figure 8: FIRM Map
Project Site
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PERMANENT STORMWATER CONTROL PLAN
ON-SITE STORMWATER BMPs
This project is electing to use the List No. 2 option for selection of on-site stormwater BMPs. After
evaluating the stormwater BMPs provided in the City of Edmond’s Stormwater Addendum for
applicability to this project site, it has been determined that a stormwater detention vault is required
for on-site stormwater flow control. A vault has been sized using the Western Washington Hydrology
Model (WWHM) per City of Edmonds requirements (using WSDOT data) and a report has been
generated through the WWHM software (Appendix A).
Based on the Geotechnical Engineering Report by Terracon Consultants (Appendix C), the on-site soils
were evaluated for infiltration potential and sieve analyses were performed to estimate an infiltration
rate. The glacial till soils observed exhibited an appreciable fines content and the soils were dense to
very dense. The geotechnical engineer concluded that the site is not suitable for infiltration and,
therefore, stormwater management via infiltration is not recommended.
The following tables list the evaluated on-site BMPs and reasons for infeasibility based on the 2017
City of Edmonds Stormwater Management Code, Appendix A.
Lawn and Landscape Areas
BMP Feasible
(Yes/No)
Explanation
Post-Construction
Soil Quality and
Depth
Yes On-site disturbed areas that result in lawn or landscaping will be amended
with compost or replaced with topsoil meeting Post-Construction Soil
Quality and Depth requirements.
Roofs
BMP Feasible
(Yes/No)
Explanation
Full Dispersion
No A minimum forested or native vegetation flow path length of 100 feet (25 feet
for sheet flow from a non-native pervious surface) cannot be achieved due to
the nature of the zero-lot line development.
Downspout Full
Infiltration
Systems
No The site does not have outwash or loam soils.
Geotechnical investigation does not recommend infiltration on this site.
Bioretention or
Rain Gardens
No Geotechnical investigation does not recommend infiltration on this site.
Downspout
Dispersion
Systems
No For splash blocks, a vegetated flow path of at least 50 feet in length from the
downspout to the downstream property line, structure, stream, wetland,
slope over 15 percent, or other impervious surface is not feasible due to the
nature of the zero-lot line development.
For trenches, a vegetated flow path of at least 25 feet between the outlet of
the trench and any property line, structure, stream, wetland, or impervious
surface is not feasible due to the nature of the zero-lot line development.
Perforated Stub-
Out Connections
No There is not at least 1 foot of permeable soil from the proposed bottom of the
perforated stub-out connection trench to the highest estimated groundwater
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table or other impermeable layer. Geotechnical investigation shows no
permeable soils on site.
Detention Vaults
and Pipes
Yes A detention vault has been sized using WWHM 2012 (Appendix A).
Other Hard Surfaces
BMP Feasible
(Yes/No)
Explanation
Full Dispersion
No A minimum forested or native vegetation flow path length of 100 feet (25
feet for sheet flow from a nonnative pervious surface) cannot be achieved
due to the nature of the zero-lot line development.
Permeable
Pavement
No Not feasible at multi-level parking garages such as this.
Geotechnical investigation does not recommend infiltration on this site.
Bioretention or
Rain Gardens
No Geotechnical investigation does not recommend infiltration on this site.
Sheet Flow
Dispersion
No For flat to moderately sloped areas, at least a 10-foot-wide vegetation buffer
for dispersion of the adjacent 20 feet of contributing surface cannot be
achieved.
Concentrated Flow
Dispersion
No A minimum 3-foot length of rock pad and 50-foot flow path OR a dispersion
trench and 25-foot flow path for every 700 sq. ft. of drainage area (within
applicable setbacks) cannot be achieved.
Detention Vaults
and Pipes
Yes A detention vault has been sized using WWHM 2012 (Appendix A).
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HYDROLOGIC MODELING
The hydrologic analysis for this project was performed using Western Washington Hydrology Model
(WWHM2012), based on matching flow durations. MGS rainfall data was used to model the site
runoff and size the stormwater facility.
The following table summarizes the surface area of the existing site conditions as inputted into
WWHM.
Basin Area (AC) Description Grade
0.844 Soil Type C, Forest Flat
Table 3: Pre-Developed Condition
The following tables summarize the tributary areas to the proposed detention vault as well as the
bypass area of the developed conditions as inputted to WWHM.
Tributary Basin Area (AC) Description Grade
0.133 Soil Type C, Lawn Flat
0.629 Rooftops Flat
0.059 Sidewalks/Driveways Flat
Table 4: Tributary Area to Vault in Developed Condition
Bypass Basin Area (AC) Description Grade
0.005 Soil Type C, Lawn Flat
0.018 Sidewalks/Driveways Flat
Table 5: Bypass Area in Developed Condition
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Figure 9, below, shows the predeveloped forested condition as inputted in WWHM.
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Figure 9: Predeveloped Forested Condition
Figure 10, below, shows the on-site, non-pollution generating impervious surfaces (NPGIS), pollution
generating impervious surfaces (PGIS), and pervious surfaces as inputted into WWHM.
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Figure 10: WWHM Tributary and Bypass Areas
FLOW CONTROL SYSTEM
The proposed flow control facility was designed in accordance with the 2014 Amendment to the 2012
Stormwater Manual for Western Washington and the 2017 City of Edmonds Stormwater Code. Pre-
developed conditions are modeled as forested for all areas within the limits of disturbance. WWHM
2012 was used for flow control calculations.
The proposed detention vault will have a live storage depth of 10 feet, a width of 30 feet, and a length
of 70 feet. Basin data and results from the WWHM 2012 model are provided in Appendix A.
CONVEYANCE SYSTEM ANALYSIS AND DESIGN
The site conveys stormwater runoff through 6-inch, 8-inch, and 12-inch diameter conveyance pipes
at a minimum slope of 1.0%. As shown in the Manning’s Calculations in Figure 11 below, the max.
flow rate for a 6-inch storm drainage pipe at a min. slope of 1.0% is 0.5626 cfs. The max. flow rate
for an 8-inch storm drainage pipe at a min. slope of 1.0% is 1.2117 cfs. The max. flow rate for a 12-
inch storm drainage pipe at a min. slope of 1.0% is 3.5724 cfs. All of which are greater than the 100-
year peak flow event (0.037 cfs).
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Figure 11: Manning’s Calculations for 6-Inch, 8-Inch, and 12-Inch Pipes
OPERATION AND MAINTAINANCE
The proposed development will include an approximately 21,000 CF detention vault located at the
basement level. The basement level will be accessible via a ramp leading down from the ground level.
It is not anticipated that the detention vault will accumulate appreciable amounts of solids because
the majority of collected runoff will be from clean roof surfaces. However, periodic maintenance is
still required, including cleaning with a vactor truck. ProVac Clean Service has verbally confirmed that
a typical vactor truck will need a height clearance of approximately 14 feet. Therefore, the truck will
be able to access the vault through the vehicle and loading entry off 72nd Avenue W to park in the
loading area and run the hose down the parking ramp to the detention vault at the basement level.
See Figure 12 for the proposed vactor truck parking configuration.
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Figure 12: Proposed Vactor Truck Configuration
A complete on-site BMP operation and maintenance manual is included in Appendix B.
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CSWPPP ANALYSIS AND DESIGN
All erosion and sediment control measures shall be governed by the requirements of the City of
Edmonds. 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.
Element 1: Preserve Vegetation/Mark Clearing Limits
· Before beginning land disturbing activities, including clearing and grading, clearly mark all
clearing limits, sensitive areas and their buffers, and trees that are to be preserved within
the construction area.
· Retain the duff layer, native top soil, and natural vegetation in an undisturbed state to the
maximum degree practical.
· Plastic, metal, or fabric fence may be used to mark the clearing limits. [Note: the difference
between the practical use and proper installation of silt fencing and the proper use of
clearing boundary fencing.]
· If it is not practical to retain the duff layer in place, then stockpile it on-site, cover it to
prevent erosion, and replace it immediately when you finish disturbing the site.
Element 2: Establish Construction Access
· Limit construction vehicle access and exit to one route, if possible.
· Stabilize access points with a pad of quarry spalls, crushed rock, or other equivalent BMPs, to
minimize tracking sediment onto roads.
· Locate wheel wash or tire baths on site, if the stabilized construction entrance is not effective
in preventing tracking sediment onto roads.
· If sediment is tracked off site, clean the affected roadway thoroughly at the end of each day,
or more frequently as necessary (for example, during wet weather). Remove sediment from
roads by shoveling, sweeping, or pick up and transport the sediment to a controlled sediment
disposal area.
· Conduct street washing only after sediment is removed in accordance with the above bullet.
· Control street wash wastewater by pumping back on site or otherwise preventing it from
discharging into systems tributary to waters of the State.
· Minimize construction site access points along linear projects, such as roadways. Street
washing may require local jurisdiction approval.
Element 3: Control Flow Rates
· Protect properties and waterways downstream of development sites from erosion and the
associated discharge of turbid waters due to increases in the velocity and peak volumetric
flow rate of stormwater runoff from the project site, as required by local plan approval
authority.
· Where necessary to comply with the bullet above, construct stormwater retention or
detention facilities as one of the first steps in grading. Assure that detention facilities
function properly before constructing site improvements (e.g. impervious surfaces).
· If permanent infiltration ponds are used for flow control during construction, protect these
facilities from siltation during the construction phase.
· Conduct downstream analysis if changes in off-site flows could impair or alter conveyance
systems, streambanks, bed sediment, or aquatic habitat.
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· Even gently sloped areas need flow controls such as straw wattles or other energy
dissipation / filtration structures. Place dissipation facilities closer together on steeper
slopes. These methods prevent water from building higher velocities as it flows downstream
within the construction site.
· Outlet structures designed for permanent detention ponds are not appropriate for use
during construction without modification. If used during construction, install an outlet
structure that will allow for long-term storage of runoff and enable sediment to settle.
Verify that the pond is sized appropriately for this purpose. Restore ponds to their original
design dimensions, remove sediment, and install a final outlet structure at completion of
the project.
· Erosion has the potential to occur because of increases in the volume, velocity, and peak
flow rate of stormwater runoff from the project site. The local permitting agency may
require pond designs that provide additional or different stormwater flow control. These
requirements may be necessary to address local conditions or to protect properties and
waterways downstream.
· Sites that must implement flow control for the developed site condition must also control
stormwater release rates during construction. Construction site stormwater discharges shall
not exceed the discharge durations of the pre-developed condition for the range of pre-
developed discharge rates from ½ of the 2-year flow through the 10-year flow as predicted
by an approved continuous runoff model. The pre-developed condition to be matched shall
be the land cover condition immediately prior to the development project. This restriction
on release rates can affect the size of the storage pond and treatment cells
Element 4: Install Sediment Controls
· The Permittee must design, install and maintain effective erosion controls and sediment
controls to minimize the discharge of pollutants. At a minimum, the Permittee must design,
install and maintain such controls to:
o Construct sediment control BMPs (sediment ponds, traps, filters, etc.) as one of the
first steps in grading. These BMPs shall be functional before other land disturbing
activities take place.
o Minimize sediment discharges from the site. The design, installation and
maintenance of erosion and sediment controls must address factors such as the
amount, frequency, intensity and duration of precipitation, the nature of resulting
stormwater runoff, and soil characteristics, including the range of soil particle sizes
expected to be present on the site.
o Direct stormwater runoff from disturbed areas through a sediment pond or other
appropriate sediment removal BMP before the runoff leaves a construction site or
before discharge to an infiltration facility. Runoff from fully stabilized areas may be
discharged without a sediment removal BMP but must meet the flow control
performance standard in Element #3, bullet #1.
o Locate BMPs intended to trap sediment on site in a manner to avoid interference
with the movement of juvenile salmonids attempting to enter off-channel areas or
drainages.
o Provide and maintain natural buffers around surface waters, direct stormwater to
vegetated areas to increase sediment removal, and maximize stormwater
infiltration, unless infeasible.
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o Where feasible, design outlet structures that withdraw impounded stormwater
from the surface to avoid discharging sediment that is still suspended lower in the
water column.
· Outlet structures that withdraw impounded stormwater from the surface to avoid
discharging sediment that is still suspended lower in the water column are for the
construction period only. If the pond using the construction outlet control is used for
permanent stormwater controls, the appropriate outlet structure must be installed after the
soil disturbance has ended.
· Seed and mulch earthen structures such as dams, dikes, and diversions according to the
timing indicated in Element #5.
· Full stabilization includes concrete or asphalt paving; quarry spalls used as ditch lining; or
the use of rolled erosion products, a bonded fiber matrix product, or vegetative cover in a
manner that will fully prevent soil erosion.
· The Local Permitting Authority may inspect and approve areas fully stabilized by means
other than pavement or quarry spalls.
· If installing a floating pump structure, include a stopper to prevent the pump basket from
hitting the bottom of the pond.
Element 5: Stabilize Soils
· Stabilize exposed and unworked soils by application of effective BMPs that prevent erosion.
Applicable BMPs include but are not limited to: temporary and permanent seeding, sodding,
mulching, plastic covering, erosion control fabrics and matting, soil application of
polyacrylamide (PAM), the early application of gravel base early on areas to be paved, and
dust control. Control stormwater volume and velocity within the site to minimize soil
erosion.
· Control stormwater discharges, including both peak flow rates and total stormwater
volume, to minimize erosion at outlets and to minimize downstream channel and stream
bank erosion.
· Soils must not remain exposed and unworked for more than the time periods set forth
below to prevent erosion.
o During the dry season (May 1 - Sept. 30): 7 days.
o During the wet season (October 1 - April 30): 2 days.
· Stabilize soils at the end of the shift before a holiday or weekend if needed based on the
weather forecast.
· Stabilize soil stockpiles from erosion, protect with sediment trapping measures, and where
possible, be located away from storm drain inlets, waterways, and drainage channels.
· Minimize the amount of soil exposed during construction activity.
· Minimize the disturbance of steep slopes.
· Minimize soil compaction and, unless infeasible, preserve topsoil.
· Soils must not remain exposed and unworked for more than the time periods set forth
above to prevent erosion for linear projects.
· Soil stabilization measures should be appropriate for the time of year, site conditions,
estimated duration of use, and potential water quality impacts that stabilization agents may
have on downstream waters or ground water.
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· Ensure that gravel base used for stabilization is clean and does not contain fines or
sediment.
Element 6: Protect Slopes
· Design and construct cut-and-fill slopes in a manner to minimize erosion. Applicable
practices include, but are not limited to, reducing continuous length of slope with terracing
and diversions, reducing slope steepness, and roughening slope surfaces (for example, track
walking).
· Divert off-site stormwater (run-on) or ground water away from slopes and disturbed areas
with interceptor dikes, pipes, and/or swales. Off-site stormwater should be managed
separately from stormwater generated on the site.
· At the top of slopes, collect drainage in pipe slope drains or protected channels to prevent
erosion.
o Temporary pipe slope drains must handle the peak volumetric flow rate calculated
using a 10-minute time step from a Type 1A, 10-year, 24-hour frequency storm for
the developed condition. Alternatively, the 10-year, 1-hour flow rate predicted by an
approved continuous runoff model, increased by a factor of 1.6, may be used. The
hydrologic analysis must use the existing land cover condition for predicting flow
rates from tributary areas outside the project limits. For tributary areas on the
project site, the analysis must use the temporary or permanent project land cover
condition, whichever will produce the highest flow rates. If using the Western
Washington Hydrology Model (WWHM) to predict flows, bare soil areas should be
modeled as "landscaped" area.
· Place excavated material on the uphill side of trenches, consistent with safety and space
considerations.
· Place check dams at regular intervals within constructed channels that are cut down a slope.
· Where 15-minute time steps are available in an approved continuous runoff model, they
may be used directly without a correction factor.
· Consider soil type and its potential for erosion.
· Stabilize soils on slopes, as specified in Element #5.
· BMP combinations are the most effective method of protecting slopes with disturbed soils.
For example, use both mulching and straw erosion control blankets in combination.
Element 7: Protect Permanent Drain Inlets
· Protect all storm drain inlets made operable during construction so that stormwater runoff
does not enter the conveyance system without first being filtered or treated to remove
sediment.
· Clean or remove and replace inlet protection devices when sediment has filled one-third of
the available storage (unless a different standard is specified by the product manufacturer).
· Where possible, protect all existing storm drain inlets so that stormwater runoff does not
enter the conveyance system without first being filtered or treated to remove sediment.
· Keep all approach roads clean. Do not allow sediment and street wash water to enter storm
drains without prior and adequate treatment unless treatment is provided before the storm
drain discharges to waters of the State.
· Inlets should be inspected weekly at a minimum and daily during storm events.
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Element 8: Stabilize Channels and Outlets
· Design, construct, and stabilize all on-site conveyance channels to prevent erosion from the
following expected peak flows:
o Channels must handle the peak volumetric flow rate calculated using a 10- minute
time step from a Type 1A, 10-year, 24-hour frequency storm for the developed
condition. Alternatively, the 10-year, 1-hour flow rate indicated by an approved
continuous runoff model, increased by a factor of 1.6, may be used. The hydrologic
analysis must use the existing land cover condition for predicting flow rates from
tributary areas outside the project limits. For tributary areas on the project site, the
analysis must use the temporary or permanent project land cover condition,
whichever will produce the highest flow rates. If using the Western Washington
Hydrology Model (WWHM) to predict flows, bare soil areas should be modeled as
"landscaped area.”
· Provide stabilization, including armoring material, adequate to prevent erosion of outlets,
adjacent streambanks, slopes, and downstream reaches at the outlets of all conveyance
systems.
· The best method for stabilizing channels is to completely line the channel with a blanket
product first, then add check dams as necessary to function as an anchor and to slow the flow
of water.
Element 9: Control Pollutants
· Design, install, implement and maintain effective pollution prevention measures to
minimize the discharge of pollutants.
· Handle and dispose of all pollutants, including waste materials and demolition debris that
occur on-site in a manner that does not cause contamination of stormwater.
· Provide cover, containment, and protection from vandalism for all chemicals, liquid
products, petroleum products, and other materials that have the potential to pose a threat
to human health or the environment. On-site fueling tanks must include secondary
containment. Secondary containment means placing tanks or containers within an
impervious structure capable of containing 110% of the volume contained in the largest
tank within the containment structure. Double-walled tanks do not require additional
secondary containment.
· Conduct maintenance, fueling, and repair of heavy equipment and vehicles using spill
prevention and control measures. Clean contaminated surfaces immediately following any
spill incident.
· Discharge wheel wash or tire bath wastewater to a separate on-site treatment system that
prevents discharge to surface water, such as closed-loop recirculation or upland land
application, or to the sanitary sewer, with local sewer district approval.
· Apply fertilizers and pesticides in a manner and at application rates that will not result in
loss of chemical to stormwater runoff. Follow manufacturers’ label requirements for
application rates and procedures.
· Use BMPs to prevent contamination of stormwater runoff by pH-modifying sources. The
sources for this contamination include, but are not limited to: bulk cement, cement kiln
dust, fly ash, new concrete washing and curing waters, waste streams generated from
concrete grinding and sawing, exposed aggregate processes, dewatering concrete vaults,
concrete pumping and mixer washout waters.
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· Adjust the pH of stormwater if necessary to prevent violations of the water quality
standards.
· Assure that washout of concrete trucks is performed off-site or in designated concrete
washout areas only. Do not wash out concrete trucks onto the ground, or into storm drains,
open ditches, streets, or streams. Do not dump excess concrete on site, except in
designated concrete washout areas. Concrete spillage or concrete discharge to surface
waters of the State is prohibited.
· Obtain written approval from Ecology before using chemical treatment other than CO2 or
dry ice to adjust pH.
Element 10: Control De-Watering
· Discharge foundation, vault, and trench dewatering water, which have characteristics
similar to stormwater runoff at the site, into a controlled conveyance system before
discharge to a sediment trap or sediment pond.
· Discharge clean, non-turbid de-watering water, such as well-point ground water, to systems
tributary to, or directly into surface waters of the State, as specified in Element #8, provided
the de-watering flow does not cause erosion or flooding of receiving waters or interfere
with the operation of the system. Do not route clean dewatering water through stormwater
sediment ponds. Note that “surface waters of the State” may exist on a construction site as
well as off site; for example, a creek running through a site.
· Handle highly turbid or contaminated dewatering water separately from stormwater.
· Other treatment or disposal options may include:
o Infiltration.
o Transport off-site in a vehicle, such as a vacuum flush truck, for legal disposal in a
manner that does not pollute state waters.
o Ecology-approved on-site chemical treatment or other suitable treatment
technologies.
o Sanitary or combined sewer discharge with local sewer district approval if there is
no other option.
o Use of a sedimentation bag with outfall to a ditch or swale for small volumes of
localized dewatering.
· Channels must be stabilized, as specified in Element #8.
· Construction equipment operation, clamshell digging, concrete tremie pour, or work inside
a cofferdam can create highly turbid or contaminated dewatering water.
· Discharging sediment-laden (muddy) water into waters of the State likely constitutes
violation of water quality standards for turbidity. The easiest way to avoid discharging
muddy water is through infiltration and preserving vegetation.
Element 11: Maintain BMPs
· Maintain and repair all temporary and permanent erosion and sediment control BMPs as
needed to assure continued performance of their intended function in accordance with
BMP specifications.
· Remove all temporary erosion and sediment control BMPs within 30 days after achieving
final site stabilization or after the temporary BMPs are no longer needed.
· Note: Some temporary erosion and sediment control BMPs are bio-degradable and
designed to remain in place following construction such as compost socks.
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· Provide protection to all BMPs installed for the permanent control of stormwater from
sediment and compaction. All BMPs that are to remain in place following completion of
construction shall be examined and placed in full operating conditions. If sediment enters
the BMPs during construction, it shall be removed, and the facility shall be returned to the
conditions specified in the construction documents.
· Remove or stabilize trapped sediment on site. Permanently stabilize disturbed soil resulting
from removal of BMPs or vegetation.
Element 12: Manage the Project
· Phase development projects to the maximum degree practicable and consider seasonal
work limits.
· Inspection and monitoring – Inspect, maintain, and repair all BMPs as needed to assure
continued performance of their intended function. Conduct site inspections and monitoring
in accordance with the Construction Stormwater General Permit or local plan approval
authority.
· Maintaining an updated construction SWPPP – Maintain, update, and implement the
SWPPP in accordance with the Construction Stormwater General Permit.
· Projects that disturb one or more acres must have, site inspections conducted by a Certified
Erosion and Sediment Control Lead (CESCL). Project sites less than one acre (not part of a
larger common plan of development or sale) may have a person without CESCL certification
conduct inspections. By the initiation of construction, the SWPPP must identify the CESCL or
inspector, who shall be present on-site or on-call at all times.
· The CESCL or inspector (project sites less than one acre) must have the skills to assess the:
o Site conditions and construction activities that could impact the quality of
stormwater.
o Effectiveness of erosion and sediment control measures used to control the quality
of stormwater discharges.
· The CESCL or inspector must examine stormwater visually for the presence of suspended
sediment, turbidity, discoloration, and oil sheen. They must evaluate the effectiveness of
BMPs and determine if it is necessary to install, maintain, or repair BMPs to improve the
quality of stormwater discharges. Based on the results of the inspection, construction site
operators must correct the problems identified by:
· Reviewing the SWPPP for compliance with the 13 construction SWPPP elements and making
appropriate revisions within 7 days of the inspection.
· Immediately beginning the process of fully implementing and maintaining appropriate
source control and/or treatment BMPs as soon as possible, addressing the problems no
later than within 10 days of the inspection. If installation of necessary treatment BMPs is
not feasible within 10 days, the construction site operator may request an extension within
the initial 10-day response period.
· Documenting BMP implementation and maintenance in the site log book (applies only to
sites that have coverage under the Construction Stormwater General Permit).
· The CESCL or inspector must inspect all areas disturbed by construction activities, all BMPs,
and all stormwater discharge points at least once every calendar week and within 24 hours
of any discharge from the site. (For purposes of this condition, individual discharge events
that last more than one day do not require daily inspections. For example, if a stormwater
pond discharges continuously over the course of a week, only one inspection is required
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that week.) The CESCL or inspector may reduce the inspection frequency for temporary
stabilized, inactive sites to once every calendar month
· Phasing of Construction.
o Phase development projects where feasible in order to prevent soil erosion and, to
the maximum extent practical, and prevent transporting sediment from the site
during construction. Revegetate exposed areas and maintain that vegetation as an
integral part of the clearing activities for any phase.
o Clearing and grading activities for developments shall be permitted only if
conducted using an approved site development plan (e.g., subdivision approval) that
establishes permitted areas of clearing, grading, cutting, and filling. Minimize
removing trees and disturbing or compacting native soils when establishing
permitted clearing and grading areas. Show on the site plans and the development
site permitted clearing and grading areas and any other areas required to preserve
critical or sensitive areas, buffers, native growth protection easements, or tree
retention areas as may be required by local jurisdictions.
· Seasonal Work Limitations
o From October 1 through April 30, clearing, grading, and other soil disturbing
activities is permitted only if shown to the satisfaction of the local permitting
authority that the site operator will prevent silt-laden runoff from leaving the site
through a combination of the following:
Site conditions including existing vegetative coverage, slope, soil type, and
proximity to receiving waters.
Limit activities and the extent of disturbed areas.
Proposed erosion and sediment control measures.
o Based on the information provided and/or local weather conditions, the local
permitting authority may expand or restrict the seasonal limitation on site
disturbance. The local permitting authority has the authority to take enforcement
action –such as a notice of violation, administrative order, penalty, or stop-work
order under the following circumstances:
· If, during the course of any construction activity or soil disturbance during the seasonal
limitation period, sediment leaves the construction site causing a violation of the surface
water quality standard; or
· If clearing and grading limits or erosion and sediment control measures shown in the
approved plan are not maintained.
· The following activities are exempt from the seasonal clearing and grading limitations:
o Routine maintenance and necessary repair of erosion and sediment control BMPs;
o Routine maintenance of public facilities or existing utility structures that do not
expose the soil or result in the removal of the vegetative cover to soil.
o Activities where there is one hundred percent infiltration of surface water runoff
within the site in approved and installed erosion and sediment control facilities.
· Coordination with Utilities and Other Contractors
o The primary project proponent shall evaluate, with input from utilities and other
contractors, the stormwater management requirements for the entire project,
including the utilities, when preparing the Construction SWPPP.
· Inspection and Monitoring
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o All BMPs must be inspected, maintained, and repaired as needed to assure
continued performance of their intended function. Site inspections must be
conducted by a person knowledgeable in the principles and practices of erosion and
sediment control. The person must have the skills to 1) assess the site conditions
and construction activities that could impact the quality of stormwater, and 2)
assess the effectiveness of erosion and sediment control measures used to control
the quality of stormwater discharges.
o For construction sites one acre or larger that discharge stormwater to surface
waters of the state, a CESCL must be identified in the construction SWPPP; this
person must be on-site or on-call at all times. Certification must be obtained
through an approved training program that meets the erosion and sediment control
training standards established by Ecology.
o Appropriate BMPs or design changes shall be implemented as soon as possible
whenever inspection and/or monitoring reveals that the BMPs identified in the
Construction SWPPP are inadequate, due to the actual discharge of /or potential to
discharge a significant amount of any pollutant.
· Maintaining an Updated Construction SWPPP
o Retain the Construction SWPPP on-site or within reasonable access to the site.
o Modify the SWPPP whenever there is a change in the design, construction,
operation, or maintenance at the construction site that has, or could have, a
significant effect on the discharge of pollutants to waters of the state.
· The SWPPP must be modified if, during inspections or investigations conducted by the
owner/operator, or the applicable local or state regulatory authority, it is determined that
the SWPPP is ineffective in eliminating or significantly minimizing pollutants in stormwater
discharges from the site. Modify the SWPPP as necessary to include additional or modified
BMPs designed to correct problems identified. Complete revisions to the SWPPP within
seven (7) days following the inspection.
Element 13: Protect Low Impact Development BMPs
· Protect all Bioretention and Rain Garden BMPs from sedimentation through installation and
maintenance of erosion and sediment control BMPs on portions of the site that drain into
the Bioretention and/or Rain Garden BMPs. Restore the BMPs to their fully functioning
condition if they accumulate sediment during construction. Restoring the BMP must include
removal of sediment and any sediment-laden Bioretention/rain garden soils, and replacing
the removed soils with soils meeting the design specification.
· Prevent compacting Bioretention and rain garden BMPs by excluding construction
equipment and foot traffic. Protect completed lawn and landscaped areas from compaction
due to construction equipment.
· Control erosion and avoid introducing sediment from surrounding land uses onto
permeable pavements. Do not allow muddy construction equipment on the base material or
pavement. Do not allow sediment-laden runoff onto permeable pavements.
· Pavements fouled with sediments or no longer passing an initial infiltration test must be
cleaned using procedures from the local stormwater manual or the manufacturer’s
procedures.
· Keep all heavy equipment off existing soils under LID facilities that have been excavated to
final grade to retain the infiltration rate of the soils.
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· See Chapter 5: Precision Site Preparation, Construction & Inspection of LID Facilities in the
LID Technical Guidance Manual for Puget Sound (2012) for more detail on protecting LID
integrated management practices.
· Note that the LID Technical Guidance Manual for Puget Sound (2012) is for additional
informational purposes only. You must follow the guidance within this manual if there are
any discrepancies between this manual and the LID Technical Guidance Manual for Puget
Sound (2012).
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SPECIAL REPORTS AND STUDIES
Special reports and studies prepared for this project include:
· Geotechnical Engineering Report, prepared by Terracon Consultants, Inc., dated December
20, 2018
· Operation and Maintenance Manual
· CSWPP Narrative and Plans
PERMITS
The following permits are anticipated to be needed for the project:
· City of Edmonds Design Review and SEPA Environmental Review
· City of Edmonds Building Permit Review with Mechanical, Electrical, Plumbing, and Fire
Code Review
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APPENDIX A
WWHM DATA AND OUTPUT
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APPENDIX B
OPERATIONS AND MAINTENANCE MANUAL
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APPENDIX C
GEOTECHNICAL REPORT