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MINIMUM REQUIREMENT #1: STORMWATER SITE PLAN <br />Existing Conditions Summary <br />The project site is currently a playfield on the east side of the existing Silver Lake Elementary <br />School in Everett, Washington. The existing site ground cover is a grass playfield east of the <br />existing portable buildings. The grass playfield slopes gently north-northeast to an existing <br />bioswale that runs along the north side of the playfield. <br />EXISTING PROJECT LIMITS LAND COVER <br />Lawn: 3,254 Square Feet <br />EXISTING RUNOFF <br />Existing flow frequency rates from WWHM 2012 (Appendix B): <br />2 year 0.006207 cfs <br />10 year 0.016529 cfs <br />50 year 0.031035 cfs <br />100 year 0.039048 cfs <br />Downstream Analysis <br />This project is designed to infiltrate stormwater. Any stormwater that overflows the infiltration <br />system will flow overland across the grass playfield to an existing bioswale along the north edge <br />of the playfield. The bioswale drains north through a 12-inch HDPE pipe to 2 1 " Avenue SE. <br />The 12-inch HDPE pipe drains to a detention pond at the north end of 21" Avenue SE. The <br />detention pond outfalls to Sitka Creek (Silver Lake Creek). A description of the downstream <br />system is included in the 2015 Stormwater Technical Memorandum in Appendix C. <br />Site Soils <br />A geotechnical boring, done by Associated Earth Sciences in 2006 near the portables site, shows <br />site soils to be 3 feet of silty fine to medium sand with gravel underlain by glacial till. An <br />excerpt of the geotechnical report is included in Appendix C. <br />Proposed Conditions Summary <br />A Stormwater Management Plan, including Temporary Erosion and Sedimentation Control Best <br />Management Practices (BMPs), is included in Appendix A. <br />This project will construct two 887-square-foot portable buildings adjacent to the existing <br />portables in the playfield on the east side of the school. The portable buildings will be connected <br />with an elevated metal grate walkway connecting to an existing elevated walkway serving the <br />existing portables. Site features will include 437 square feet of asphalt walkway and a gravel <br />building pad underneath the portable buildings that will serve as an infiltration facility. Runoff <br />from the gravel walkway will sheet flow across lawn to the gravel infiltration facility. <br />Stormwater Site Plan, Including SWPPP 4 <br />Silver Lake Elementary School Portables <br />