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Cooley Smiles Dental Clinic 17318.20 October 18,2017 <br /> Drainage Report Section IV,Page 1 <br /> Section IV — Permanent Stormwater Control Plan <br /> Section IV Summary <br /> Narrative <br /> LID Performance Standard <br /> Runoff Treatment <br /> Flow Control <br /> WWHM Report <br /> Narrative <br /> The project must address MR#5: On-Site Stormwater Management BMPs, MR#6: Runoff Treatment, <br /> and MR#7: Flow Control per Section II of this report. <br /> The proposed land coverage is as follows: <br /> Proposed Land Coverage <br /> Landscape: 6,309 sf(0.145 ac) <br /> Roof: 4,787 sf(0.110 ac) <br /> Sidewalk: 936 sf(0.021 ac) <br /> North Parking (Asphalt), Flat: 2,049 sf(0.047 ac) <br /> South Parking(Permeable), Flat: 7,139 sf(0.164 ac) <br /> Total: 21,220 sf(0.487 ac) <br /> On-Site Storm water BMPs <br /> Per Table 1-2.5.1 from the SWMMWW, redevelopment projects on any parcel inside the Urban Growth <br /> Area that trigger Minimum Requirements#1 through#9 must either: <br /> 1. Use On-Site Stormwater Management BMPs from List#2 for all surfaces within each type of <br /> surface in List#2; or <br /> 2. Demonstrate compliance with the LID Performance Standard and BMP T5.13. <br /> The project proposes to demonstrate compliance with the LID Performance Standard.To meet this <br /> standard, developed discharge durations must match pre-developed durations for the range of pre- <br /> developed discharge rates from the 8%of the 2-year peak flow to 50%of the 2-year peak flow. <br /> The underlying soils consist of glacial till (medium dense to very dense silty fine to medium sand with <br /> gravel) per the geotechnical report.A shallow infiltration system with an overflow is feasible per the <br /> geotechnical engineer, and a design long-term infiltration rate of 0.236 in/hr was recommended.A <br /> permeable pavement facility with a storage layer and overflow is proposed for the south parking lot.The <br /> facility is designed to meet the LID Performance Standard as well as the Flow Control Duration Standard. <br /> See the WWHM2012 report at the end of this section and civil plans for the sizing and design. <br /> 250 4th Avenue South,Suite 200 <br /> ® Edmonds,WA 98020 <br /> ENGINEERING ph.425.778.8500 I f.425.778.5536 <br /> www.cgengineering.com <br />