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R Y <br /> 6. ON-SITE STORMWATER MANAGEMENT <br /> List #1 in the SWMM volume 1 section 2.5.5 provides multiple BMP options for runoff <br /> management, conveyance BMPs and Discharge BMPs. Each option was reviewed for this <br /> project and the most suitable was selected. <br /> Standard Project Runoff Manage Options <br /> Lawn and Landscaped Areas: <br /> 1. Post-Construction Soil Quality and Depth: Required <br /> Roofs: <br /> 1. Full Dispersion: Not feasible due to not having 65% preserved vegetation. <br /> 2. Downspout infiltration: Not feasible due to the steep slopes on-site. The infiltration <br /> feasibility map provided by the City of Everett shows the majority of the parcel as infeasible <br /> for infiltration. There is only a small area downhill of the SFR that is shown as feasible for <br /> infiltration, and this area is not large enough to fit the required amount of trench, while also <br /> maintaining the required setbacks. <br /> 3. Bioretention/Rain Gardens: Not feasible for the same reason as infiltration. <br /> 4. Downspout Dispersion: All requirements for downspout dispersion can be met. <br /> Therefore roof runoff will be dispersed via a dispersion trench. Though there are steep <br /> slopes on the property, there is a relatively level spot on the northern side of the parcel that <br /> allows for the required 25 foot vegetated flow path before leaving the property. No portion <br /> of this flow path will be on slopes greater than 15 percent and runoff will not be directed at <br /> any steep slopes. <br /> 5. Perforated Stub-Out Connections: Higher priority option selected. <br /> The roof shall drain to a dispersion trench. The amount of dispersion trench required was <br /> determined using section 3.1.2 of the SMMWW, which states "For roof areas larger than <br /> 700 square feet, a dispersion trench with notched grade board as shown in Figure III-3.1.6 <br /> Standard Dispersion Trench with Notched Grade Board or alternative material approved by <br /> the 2014 Stormwater Management Manual for Western Washington Volume Ill - Chapter 3 <br /> - Local Plan Approval Authority may be used. The total length of this design must not <br /> exceed 50 feet and must provide at least 10 feet of trench per 700 square feet of roof area" <br /> Therefore the dispersion trench shall be 40 feet long. <br /> Ocean Ave SFR 17-0802 <br /> Revised:October 2017 Page 9 <br />