Edmonds Shops - Stormwater Report.pdf
STORMWATER REPORT
Shops @ Aurora Village
8431 244 th Street SW
Edmonds, WA 98020
Prepared By:
April 2017
Shawn Nguy, P.E.
Table of Contents
Section Page
Project OverviewÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ3
IntroductionÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ...3
Existing ConditionsÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ..3
Redeveloped ConditionsÈ.ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ..3
Edmonds Stormwater Addendum - Project Minimum RequirementsÈÈÈÈÈÈÈÈÈÈÈÈÈ..4
Minimum Requirement #1: Preparation of Stormwater Site Plans ÈÈÈÈÈÈÈÈÈÈÈÈÈ.5
Minimum Requirement #2: Construction Stormwater Pollution Prevention Plan (SWPPP) È..5
Minimum Requirement #3: Source Control of Pollution ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ..5
Minimum Requirement #4: Preservation of Natural Drainage Systems & Outfalls ÈÈÈÈÈ..5
Minimum Requirement #5: On-site Stormwater Management ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ6
Minimum Requirement #6: Runoff Treatment ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ..6
Minimum Requirement #7: Flow Control ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ..7
Minimum Requirement #8: Wetlands Protection ÈÈÈÈÈÈÈÈÈÈ.ÈÈÈÈÈÈÈÈÈÈÈ7
Minimum Requirement #9: Operation and Maintenance ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ.7
Other PermitsÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ..È.7
AppendixÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ..È8
Appendix A:
A-1: Web Soil Survey
A-2: FEMA Flood Insurance Rate Map Number 53061C1315E
A-3 Geotech Report
Appendix B:
B-1: Existing Site Topographic Survey
B-2: Grading and Drainage Plan and Details
B-3: Erosion Control Plan
B-4: Existing and Post Development Exhibits
Appendix C: Analysis
C-1: WWHM Continuous Runoff Model Report
C-2: Operation and Maintenance Manual
Appendix D: Construction SWPPP
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This stormwater report is prepared to discuss the proposed stormwater management system and to
provide the documentation and the analysis showing that the proposed system is feasible and
designed to address the jurisdictional requirements. All components of the stormwater
management system were designed in accordance with the 2014 DOE Stormwater Manual as
modified by Edmonds Community Development Code (ECDC) 18.30 Stormwater Management and
the Stormwater Addendum.
St SW in Edmonds, WA. A
The project site is located at the existing DennyÔs site at 8431 244 th
vicinity map is provided below:
According to the FEMA Flood Insurance Rate Map Number 53061C1315E, dated November 8,
1999, the site is located within Zone X, described as areas determined to be outside the 500-year
floodplain. See Appendix A.
The site is approximately 1.08 acres and is currently developed with an existing DennyÔs restaurant
and associated asphalt parking lot and limited vegetation. The site is north of the existing ArbyÔs
restaurant on the corner of 244 th St. SW & Hwy 99. According to the Natural Resources
Conservation Services (NCRS), the native soil underlying the site consists of Alderwood-Urban
Land Complex (2-15% slopes). The Geotechnical Engineering Investigation report by GeoTest,
dated 3/24/2017, found that soils below the existing pavement are composed of fill consisting of
medium dense, silty, fine to medium grained sand and medium dense to very dense Vashon till.
The fill and Vashon till soils are categorized within the Soil Conservation Service (SCS) hydrologic
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soil group C. Due to the presence of fill and Vashon till below the site, infiltration is not a feasible
option for the subject property. The Geotech Report is included in the Appendix.
The project proposes to demolish the existing DennyÔs restaurant and redevelop/re-grade most of
the site to accommodate an approximate 10,000 square foot retail pad building, with associated
parking, landscaping, lighting, stormwater and utility improvements. The project will consist of
approximately 0.96 acres of total land disturbance.
The project proposes to convey stormwater runoff from the redeveloped site to a stormwater filter
manhole and underground detention vault via curb and gutter and a system of catch basins and
underground pipes. Stormwater treatment will be accomplished via a FloGard Perk Filter manhole
meeting DOE General Use Level Designation for Basic and Phosphorus Treatment for the
new/replaced asphalt within the project area as described below. The underground detention
facility will be constructed along the east portion of the property downstream of the Perk Filter
manhole.
The stormwater system for this project has been designed in accordance with regulatory criteria
described above and is consistent with sound engineering practice. This systemÔs design has
incorporated stormwater treatment BMPÔs for the mitigation of stormwater runoff quality impacts.
Per ECDC 18.30.060, since the proposed project results in greater than 5,000 sf of new and
replaced hard surface area, the project is classified as a category 2 project that needs to be
designed to comply with minimum requirements No. 1 through No. 9.
The proposed site improvements will increase the amount of impervious surface in the project area
by approximately 98 sf. The existing project area consists of approximately 36,351 SF of
impervious area, with 10,606 SF of pervious area. All existing site improvements will be
demolished as needed to accommodate the proposed development. The re-developed project area
will consist of approximately 36,449 SF of new/replaced impervious area and 10,508 SF of
landscape area.
Table 1 below summarizes each land disturbing activity for the proposed development and
provides a Total Effective Impervious Surface for the Threshold Drainage Area (TDA).
Existing Conditions (1.08 acres±) Area (SF)
Existing Impervious Surface 36,351
Existing Landscaped Area 10,606
Proposed Conditions (1.08 acres±) Area (SF)
Site Impervious Area 36,449
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Site Pervious Area 10,508
Total Site Area 46,957
Area of site that wonÔt be disturbed by project 5,275
Total Land Disturbing Activity 41,682
Based on coordination with City Engineering, the project drains to Lake Ballinger and is subject to
flow control. Additionally, the onsite stormwater management BMPÔs shall be determined
following the DOE List 2 or LID Performance Standards. Both runoff treatment and flow control
are required with the project.
The Western Washington Hydrologic Model (WWHM2012) was used to determine treatment
flows proposed for the mitigated developed conditions. The required treatment flow for the
Effective Impervious Area is 0.1353 CFS or 60.7 GPM. This treatment rate is achieved with the
FloGard Perk Filter manhole. For these systems, WSDOE has approved treatment rates of 10.2
GPM/cartridge for the 18-inch cartridges. Six (6) 18-inch cartridges are required for treatment.
Results of the continuous runoff simulation are provided as part of the WWHM2012 Project Report
in the appendix (See Appendix C).
The Stormwater Site Plan has been prepared in accordance with Section 5.1 Ï Preparation of
Stormwater Site Plan in the Edmonds Stormwater Addendum. This Stormwater Site Plan
summarizes the methods and analysis for design of the stormwater system for during and after
construction.
The Construction Stormwater Pollution Prevention Plan (SWPPP), along with erosion and sediment
control plans, is included with this report. See Appendix F.
Available and reasonable source control BMPs have been selected, designed, and maintained in
accordance with the 2016 Edmonds Stormwater Addendum and SWMMWW Volume IV.
A variety of stormwater pollutant controls are recommended for this project. Some controls are
intended to function temporarily and will be used as needed for pollutant control during the
construction period. These include temporary sediment fence, catch basin insert, biofilter bags, and
gravel construction exit as noted on the SWPPP plans. Permanent stabilization will be
accomplished in all disturbed areas by covering the soil with pavement, building foundation,
vegetation, or other forms of soil stabilization (mulch, pea gravel, etcÈ).
Additional pollutant controls include dust control during construction, appropriate solid waste
disposal and above ground portable concrete washout containers.
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Long-term pollutant controls include various site maintenance and, including garbage containment
and parking lot sweeping. All potential pollutant sources within the building will be routed to the
sanitary sewer system. Landscape areas will be maintained by the property owner. Lynch style
catch basins with sumps are proposed throughout the site to capture sediment, debris, and
associated pollutants in stormwater runoff prior to discharging downstream. BMPS selected for this
site also include a Perk Filter manhole for Phosphorous treatment. Maintenance of these facilities
will be the ongoing responsibility of the building owner / operator.
There are no natural drainage systems or outfalls on the subject property. The site is located within
the Lake Ballinger Watershed according to the CityÔs watershed online map.
Stormwater runoff on the site and surrounding streets are managed through a system of catch
basins and pipes. There are currently no stormwater treatment or flow control facilities on site.
Within the subject site, runoff from the existing parking lot either sheet flows directly into an
existing grassy area within the High 99E right of way or flows to catch basins throughout the
subject site and discharges directly into the cityÔs public stormwater system within 244 th Street and
Highway 99E. There are no known problems with the cityÔs existing storm system on 244 th which
currently received runoff from the project area. The projectÔs inclusion of storm water treatment
and flow control facilities will mitigate any potential impacts to the cityÔs storm water system
downstream of the site.
The proposed project will employ on-site stormwater management BMPs to treat stormwater runoff
in accordance with the requirements of 2016 Edmonds Stormwater Addendum and ECDC
18.30.060.D.5 without causing flooding and erosion. While the proposed project increases the
impervious area by 98 SF over the existing conditions, impervious areas from the new/replaced
pavement and building areas are mitigated using an underground detention facility that is sized
using WWHM 2012 and does not negatively affect the watershed basin.
New lawn and landscape areas will be installed in accordance with BMP T5.13 in Chapter 5 BMP
of Volume V of the SWMMWW.
Based on the Trip Generation Memo prepared by the Transpo Group dated March 8, 2017, this site
is not classified as a Ñhigh use siteÒ as it is not expected to generate more than 100 vehicles per
1,000 sf of gross building area. An oil control facility will not be required.
Since the project runoff ultimately drains to Lake Ballinger, per ECDC 18.30.060.D.6, phosphorus
treatment will be required
The project proposes approximately 36,449 SF of total new/replaced impervious area and
approximately 10,508 of new/replaced pervious area. As noted earlier, the Western Washington
Hydrologic Model (WWHM2012) was used to determine treatment flows proposed for the
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developed conditions. Since the proposed water quality manhole is located upstream of the flow
control facility, the water quality analysis of the 701 inflow to POC 1 Mitigated flow data set is
evaluated. As shown in the results on the WWHM2012 report, the required treatment flow for the
impervious area is 0.1353 CFS or 60.7 GPM.
This treatment option proposed for the project is the DOE Approved for GULD FloGard Perk Filter
((using ZPC Filter Media for Enhanced Treatment) manhole. For this system, DOE has approved
treatment rates of 10.2 GPM/cartridge for the 18-inch cartridges. Six (6) 18-inch cartridges are
required for treatment. Results of the continuous runoff simulation are provided as part of the
WWHM2012 Project Report in the appendix (See Appendix C).
An 84Ò Dia, 6-filter FloGard Perk Filter manhole will meet and exceed the water quality
requirements.
Per ECDC 18.30.060.D.7, since the project has a total effective impervious area of greater than
10,000 sf and discharges to the CityÔs MS4, flow control is required. An underground detention
facility will provide mitigation of storm water runoff from the roof of the building and pavement
areas. The detention vault has been designed and sized such that stormwater discharges match the
developed discharge durations to pre-developed durations for the range of pre-developed discharge
rates from 50% of the 2-year peak flow up to the full 50-year peak flow. A minimum 32.8Ô x 65.6Ô
x 7Ô depth stormwater vault is required to meet the noted requirements. Flow control will be
accomplished via a 3-orifice riser pipe configuration.
A summary of the flows and detention facility stage level is provided below:
Storm Event Pre-Developed Post-Developed Post-Developed Detention Facility
Q (cfs) Mitigated Q (cfs) Stage (ft) from bottom
Q (cfs)
2-yr 0.0357 0.4000 0.0211 415.88
5-yr 0.0534 0.5489 0.0335 416.72
10-yr 0.0665 0.6590 0.0444 417.26
25-yr 0.0847 0.8118 0.0618 417.93
50-yr 0.0993 0.9359 0.0779 418.42
100-yr 0.1149 1.0692 0.0972 418.92
Note: Bottom of detention vault is at elevation 413.00
This requirement does not apply since the proposed project will not discharge to any known
wetlands directly or indirectly through a conveyance system.
An operation and maintenance manual (O&M Manual) is included per provisions outlined in
Volume V of the DOE Stormwater Manual, for all proposed stormwater facilities and BMPs (See
Appendix D) necessary for the project. The property owner will be responsible for the cost of
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installation, as well as operation and maintenance of the proposed onsite stormwater facilities. See
the appendix for the O&M manual.
Based on coordination with Ecology, an NPDES permit will not be required for this project since
the project limit of land disturbance is expected to be less than 1 acre.
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Appendix
Appendix A: Existing Conditions Info
A-1: Web Soil Survey
A-2: FEMA Flood Insurance Rate Map Number 53061C1315E
A-3 Geotech Report
A-4: Topographic Survey
A-5: Existing Site Conditions Exhibit
Appendix B: Proposed Conditions Info
B-1: Post Development Exhibit
B-2: Grading and Drainage Plan and Details
B-3: Erosion Control Plan
Appendix C: Analysis
C-1: WWHM Continuous Runoff Model Report
C-2: Operation and Maintenance Manual
Appendix D: Construction SWPPP
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A-1
APPENDIX
A-2
APPENDIX