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r i <br /> Cooley Smiles Dental Clinic 17318.20 October 18,2017 , <br /> Drainage Report Section IV,Page 2 <br /> Runoff Treatment <br /> The project requires runoff treatment because it adds and replaces more than 5,000 sf of pollution- <br /> generating hard surface (PGHS). <br /> When using an approved continuous runoff model,the water quality facility must be sized for 91%of <br /> the total runoff volume.The project proposes a permeable pavement facility for flow control which will <br /> infiltrate more than 91%of the total runoff volume before overflowing to the public stormwater main. <br /> The native soil may be used for runoff treatment if it meets SSC-6: Soil Physical and Chemical Suitability <br /> for Treatment per Volume III-3.3.7 of the 2014 SWMMWW. If the existing soils do not meet the criteria, <br /> a 6-inch layer of media meeting the criteria, or a sand filter may be used. <br /> Testing will be done to determine if the soil meets the criteria for runoff treatment prior to construction. <br /> Flow Control <br /> The project must meet the Flow Control Duration Standard because it adds and replaces more than <br /> 10,000 sf of impervious surfaces. Developed discharge durations must match pre-developed durations <br /> for the range of pre-developed discharge rates from 50%of the 2-year peak flow up to the full 50-year <br /> peak flow.The pre-developed condition to be matched shall be forested.The permeable pavement <br /> facility described above for the LID Performance Standard will also be used to meet this Flow Control <br /> 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 />