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7. - ON -SITE STORMWATER MANAGEMENT <br />As documented in Section 1 of this report this project is required to meet Minimum <br />Requirements 1-5. As a result, this project is required to use the On -Site Stormwater <br />Management BMP's from the 2019 DOE SWMMWW. <br />Per the DOE SWMMWW the BMP's must be evaluated in the order they are listed. <br />BMP's for roofs: <br />1. Full Dispersion: Not feasible due to the 65:35 requirement. <br />2. Downspout Infiltration Systems: <br />Per Section 8.1.5 of the geotechnical report (see Appendix A), "the infiltration rate was <br />determined to be 0.24 inches per hour. This rate is lower than what is considered to be <br />feasible for widespread infiltration." Therefore, infiltration is infeasible. <br />3. Downspout Dispersion Systems: <br />Minimum flow, paths cannot be achieved, therefore downspout dispersion is infeasible. <br />4. Perforated .Stub. -out Connections <br />Feasible and selected. The roof runoff shall be directed to a perforated stubout <br />connection before connecting to the existing stormwater stub on -site that drains to the <br />detention vault. <br />BMP's for other hard surfaces <br />1. Full Dispersion: Not feasible due to not having 65% preservedvegetation. <br />2. -'Permeable -Pavement- The geotechnical engineer s#atesthat=the design infiltration <br />rate is 0.24 inches per hour. Section 8.1.5 of the geotechnical report also states <br />"Depending on the final site layout and grading, it may be possible''to utilize <br />permeable pavements for new driveways. Thus, permeable pavement is feasible <br />and is being proposed for the new driveway. <br />The 2012 Western Washington Hydrology Model wasusedto size the permeable <br />pavement sublayers. With a pavement thickness -of 6 inches, leveling coarse <br />thickness (optional) of 1 inch and a reservoir coarse layer thickness of 6 inches, <br />..the permeable pavement- is'shown Ito, infritrate ,100; percent of runoff (see Appendix <br />A for WWHM printout). <br />3. Bioretention/Rain Gardens: Higher priority BMP selected. <br />4. Sheet Flow Dispersion: Higher priority BMP selected. <br />5. Concentrated Flow Dispersion: Higher priority BMP selected. <br />Moir SP Lot 4 20-0230 <br />February 2020 <br />Revised: October 2020 Page 9 <br />