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Part 3 Stormwater Management Plan <br />Conceptual Drainage Plan <br />Stormwater runoff from the Phase 3 Divisions 15 &16 proposed roadways impervious surface <br />will be collected in catch basins or roadside water quality swales and directed to water quality <br />and infiltration facilities. <br />Stormwater runoff from the lots will be handled separately on each lot using dispersion. <br />The majority of the project area consists of glacial outwash with some glacial moraine deposits. <br />For additional information regarding the existing geology and soils information,see the EIS <br />Technical Report for Geology,Groundwater and Soils prepared by AESI and dated February 10, <br />2006. <br />Infiltration Facilities <br />Infiltration facilities will be designed based on infiltration rates recommended by the project <br />Geotechnical Engineer. <br />Water Quality Treatment <br />Water quality treatment will be provided for runoff from impervious road surfaces requiring <br />stormwatertreatment.Treatment will be provided in one of several Department of Ecology <br />recommended treatment facility types. Water quality treatment options available for Suncadia <br />are wetponds/lakes,stormfilter systems,biofiltration swales,bio-infiltration and sheet flow <br />dispersion.All water quality facilities are sized to treat the water quality storm.The water <br />quality storm is that storm for which all storms equal or smaller in size account for 90 percent <br />of the average annual runoff.For Phase 3,the water quality design storm is 1.96 inches in 24- <br />hours. Proposed water quality facilities are described below. <br />Sheet Flow Dispersion <br />Sheet flow dispersion is an approved Department of Ecology water quality and quantity <br />control method for areas that preserve the existing forest duff.D.O.E.allows this <br />treatment when the impervious area is less than 10 percent of the basin and 65 percent <br />or more of the forest duff is retained. <br />This method will be used for flow control for the 105 detached resort residential units. <br />Biofiltration Swales <br />Biofiltration swales are another approved DOE water quality treatment facility.This <br />facility is also designed to treat the water quality storm. The design criterion for a <br />biofiltration swale is residence time in the swale.For a given flow rate,the required <br />February 2019 Phase 3 Divisions 15 & 16 Engineering Summary <br />ESM Consulting Engineers, L.L.C 3 -2 <br />\\Esm8\engr\ESM-JOBS\998\718\016\document\P3Dl5&l6 Eng Summ_Part 3.doc <br />Index #11