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5. SOURCE CONTROL <br />This project proposes to construct a SFR and driveway. Per Volume 4 of the 2019 DOE <br />SWMMWW, residential development does not require source control. <br />6 PRESE-R ATfOWOf NATURAUDRAINAG E SYSTEM <br />By not grading or, compacting the pervious and amended surfaces to remain after <br />construction in the area of disturbance a maximum amount of site runoff from the developed <br />project will ,be absorbed and infiltrated into the existing` on -site soils. This will .mimic the <br />existing conditions by maintaining the'existing direction and condition of flow from this site. <br />7._` ON -SITE STORMWATER MANAGEMENT <br />As documented in Section 1 of this report this project is required :to meet .Minimum <br />Requirements 1-9. 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 />Lawn and Landscaped -Areas: <br />1. Post -Construction Soil Quality and Depth: Required <br />Roofs: <br />1. BMP T5.30 - Full Dispersion: Not feasible due to not having 65% preserved <br />vegetation. <br />2. BMP T5.10A — The geotechnical report advises against stormwater infiltration in the <br />existing conditions. However, to avoid Flow Control for this project, Phil Haberman <br />at Cobalt Geosciences, LLC was contacted directly for permission to use a shallow <br />infiltration facility with an engineered overflowthat connects to the stormwater <br />system within Baker Drive. ` He approved this ;request and ' asked that the final <br />stormwater site plan besent to him so that upon approval. The final stormwater site <br />plan. was recently sent, to. him, 'and he prepared an addendum approving the <br />proposed design (see Appendix A) Therefore, shallow infiltration is feasible and is <br />the selected BM for this project. <br />Downspout infiltration shall take place in a series of infiltration trenches. Due to poor <br />soil infiltration characteristics and shallow water table, no more than 2,700 sf of the <br />proposed roof runoff shall be directed to the infiltration trenches. In this way,.this <br />project will result in a total effective impervious surface area of less than 5,000 <br />square feet, and Minimum Requirement #7 (Flow Control) can be avoided. <br />The series of infiltration trenches was sized using the`2012' Western Washington <br />Hydrology Model and an infiltration rate of 0.21..inches/hour provided by Cobalt <br />Geosciences to infiltrate 100 percent of collected runoff (see Appendix A for WWHM <br />printout). <br />Dhaliwal SFR 19-1208 <br />December 2020 <br />Reiiseti:'January�202 i Page 9 <br />