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SOUNDVIEW TECHNOLOGY CENTER EVERETT, WA <br />allow a significant amount of runoff to infiltrate behind the 10'-20' on -site walls, which is <br />not an acceptable design. <br />• Downspout Dispersion Systems: Determined infeasible, as there is insufficient area to <br />provide the required vegetated flowpath on -site with the required vegetated flowpath <br />setbacks from the proposed walls, steep slopes, and property line. <br />• Perforated Stub -out Connections: Infiltration on this site is not recommended due to <br />reasonable concerns regarding erosion and down gradient flooding. <br />Other Hard Surfaces: <br />• Full Dispersion: Determined infeasible, as there is insufficient area to provide the <br />required vegetated flowpath on -site with the required vegetated flowpath setbacks from <br />the proposed walls and property line. <br />• Permeable pavement: Infiltration on this site is not recommended due to reasonable <br />concerns regarding erosion and down gradient flooding. <br />• Bioretention BMPs: Infiltration on this site is not recommended due to reasonable concerns <br />regarding erosion and down gradient flooding. <br />• Sheet Flow Dispersion: Determined infeasible, as there is insufficient area to provide the <br />required vegetated flowpath on -site with the required vegetated flowpath setbacks from <br />the proposed walls and property line. <br />All finished landscape areas on -site will contain compost amended soils. Refer to Earth Solutions <br />NW Infiltration and Low Impact Development Evaluation in Appendix C for further analysis on the <br />storm water design. <br />Minimum Requirement #6: Runoff Treatment <br />Thresholds <br />The following require construction of stormwater treatment facilities (see Table 2.1): <br />• Projects in which the total of effective, pollution -generating impervious surface (PGIS) is 5,000 square <br />feet or more in a threshold discharge area of the project, or <br />• Projects in which the total of pollution -generating pervious surfaces (PGPS) is three-quarters (3/4) of <br />an acre or more in a threshold discharge area, and from which there is a surface discharge in a natural <br />or man-made conveyance system from the site. <br />Response: Stormwater from detention systems 1, 2, and 5 only collect roof runoff and do not require <br />treatment. A Modular Wetland water quality system will be located downstream of vaults (#3 & 4). Each <br />Modular Wetland will be sized for the full two year release rate. <br />Minimum Requirement #7: Flow Control <br />Projects must provide flow control to reduce the impacts of stormwater runoff from impervious surfaces <br />and land cover conversions. The requirement below applies to projects that discharge stormwater directly, <br />or indirectly through a conveyance system, into a fresh water. <br />Response: Flow control is provided by means of on -site detention vaults or pipes that are sized to release <br />stormwater at the existing basins release rate. Each vault is sized using the WWHM2012 program. <br />Minimum Requirement #8: Wetlands Protection <br />The requirements below apply only to projects whose stormwater discharges into a wetland, either <br />directly or indirectly through a conveyance system. These requirements must be met in addition to <br />meeting Minimum Requirement #6, Runoff Treatment. <br />MAY 23, 2017 STORMWATER DRAINAGE REPORT PAGE 6 1 <br />