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STORMWATER/ EROSION CONTROL:
• Refer to City Standards, Section 200 for Stormwater System Requirements. [PMC 17.42]
• Stormwater design shall be in accordance with PMC Chapter 21.10 and the current stormwater management manual as adopted by the City Council at the time of project application. The City is currently using the 2019 Department of Ecology (Ecology) Stormwater Management Manual for Western Washington (aka “Ecology Manual”).
• The applicant shall complete the stormwater flowchart, Figure 1-3.1 and/or Figure 1-3.2, contained in the Ecology Manual. The completed flowchart shall be submitted at the time of formal application to the City, i.e., Landuse application, Building Permit application, and Civil Permit application.
• If the thresholds triggering Minimum Requirement (MR) 1-5 or MR 1-9 are met, the applicant is responsible for submitting a preliminary stormwater management site plan which meets the design requirements provided by PMC Section 21.10 and the Ecology Manual. The preliminary stormwater site plan (PSSP) shall be submitted with the landuse application to ensure that adequate stormwater facilities are anticipated prior to execution of the proposed project. The preliminary stormwater site plan shall reasonably estimate the quantity of stormwater runoff and the application of On-site Stormwater Management BMPs for the proposed development.
• The written technical report shall clearly delineate any offsite basins tributary to the project site and include the following information: [PMC 21.10.060]
- the quantity of the offsite runoff;
- the location(s) where the offsite runoff enters the project site;
- how the offsite runoff will be routed through the project site.
- the location of proposed retention/detention facilities
- and, the location of proposed treatment facilities
- For offsite basin inflow: At time of civil application, document compliance with 2019 Ecology Manual, Vol III, Sec III-2.4 (2014 Manual, Vol. III, Appendix III-B, Section 6) for the Offsite Basin inflow.
• Development and redevelopment projects are required to employ, wherever feasible, Low Impact Development (LID) Best Management Practices (BMPs) to meet the design criteria set forth in PMC 21.10.190, the Ecology Manual Volume I, Minimum Requirement 5; and Volume V.
• Preliminary feasibility/infeasibility testing for infiltration facilities/BMPs shall be in accordance with the site analysis requirements of the Ecology Manual, Volume III, Chapter 3, specifically:
- Groundwater evaluation, either instantaneous (MR1-5), or continuous monitoring (MR1-9), during the wet weather months (December 1 through April 30).
- Hydraulic conductivity testing:
i. If the development meets the threshold to require implementation of Minimum Requirement #7 (flow control); or, if the site soils are consolidated; or, if the property is encumbered by a critical area, then Small Scale Pilot Infiltration Testing (PIT) during the wet weather months (October 1 through April 1) is required.
ii. If the development does not meet the threshold to require implementation of Minimum Requirement #7; or, is not encumbered by a critical area; and is located on soils unconsolidated by glacial advance, grain size analyses may be substituted for the Small Scale PIT test at the discretion of the review engineer.
- Testing to determine the hydraulic restriction layer.
- Mounding analysis may be required in accordance with Ecology Volume V Section V-5.4.
• Upon submission of any geotechnical infiltration testing, appropriate long-term correction factors shall be noted for areas utilizing infiltration into the underlying native soils in accordance with the Ecology Manual, Volume III, Chapter 3. Provide the long-term infiltration rate calculation in the stormwater report.
• The proposed project is part of a larger, common plan of development, and may include the use of existing stormwater facilities. The Technical Information Report (TIR) or Stormwater Site Plan (SSP), shall provide supporting documentation and engineering calculations which substantiate any affect the proposed project may have on the original design assumptions of the existing stormwater facilities. [PMC 21.10.060]
• If Minimum Requirement 1 (MR1) is triggered, at the time of civil permit application, the applicant is responsible for submitting a permanent storm water management plan which meets the design requirements provided by PMC Section 21.10. [PMC 21.10.190, 21.10.060]
- When using WWHM for analysis, provide the following WWHM project files with the civil permit application:
- Binary project file (WHM file extension)
- ASCII project file (WH2 file extension)
- WDM file (WDM file extension)
- WWHM report text (Word file)
• Stormwater R/D facilities shall be a minimum of 20-feet from any public right-of-way, tract, vegetative buffer, and/or property line measured from the toe of the exterior slope/embankment of the facility. The setback area shall not exceed 5% maximum cross-slope. Facilities with retaining walls, the setback area shall be measured from the facility’s emergency overflow elevation to the face of the wall. [PMC 21.10 & DOE Manual, Vol. V]
• The project is immediately adjacent to two separate designated wetlands, Wetland D and Wetland E. Based on the information provided in the December 2020 Wetland and Fish and Wildlife Habitat Assessment Report created by Soundview Consultants, LLC, both wetlands are Category IV, with a Habitat Score less than 4 and both do not provide habitat for “rare, threatened, endangered, or sensitive species”. As a result, the applicant is not required to apply the Wetland Hydroperiod Protection(s) described in the Ecology Manual, Appendix I-C.4. Other regulatory wetland protections shall apply as outlined in the Ecology Manual, Minimum Requirement 8 and Appendix I-C.
• Water quality treatment of stormwater shall be in accordance with the Ecology Manual, Volume 1, Minimum Requirement 6; and Volume 5, Runoff Treatment.
• If the applicant proposes to use bioretention cells for water quality treatment, the following notes shall be added to the civil design plans:
- “At the completion of the bioretention cells construction, the engineer-of-record shall provide a written statement to the City of Puyallup that the bioretention cells were built per the approved design.”
- “The bioretention soil media (BSM) supplier shall certify in writing that the bioretention soil media meets the guidelines for Ecology-approved BSM including mineral aggregate gradation, compost guidelines, and mix standards as specified in the 2012 Low Impact Development Technical Guidance Manual for Puget Sound. And, if so verified, no laboratory infiltration testing, cation exchange, or organic content testing is required.”
• Applicant will be required to delineate, dimension, field stake, and flag all limits of clearing, wetland buffers, and other sensitive areas PRIOR to work commencing.
• A Construction Stormwater General Permit shall be obtained from the Department of Ecology if any land disturbing activities such as clearing, grading, excavating and/or demolition will disturb one or more acres of land, or are part of larger common plan of development or sale that will ultimately disturb one or more acres of land.
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