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from the end of a storm event for surface features, and will utilize 12 inches of composite sand and <br /> organic materials beneath the floor grade of the basins, as outlined by the Watershed District Rules 9.c. <br /> The composite materials are classified as Hydrologic Soil Group "A", which has an infiltration rate of <br /> 0.80 in/hour. The required surface area of the finished floor grade is determined by the required water <br /> quality treatment volume, composite materials' filtration rate, and drawdown time. Refer to Sheet C 1.1 <br /> for cross-section detail of the filtration basin system. <br /> Required water quality treatment volume for filtration basin, per Watershed District Rules 6.c: <br /> Water Quality = Area of All Impervious x 1.1 (in) ± TP Removal ± 12(in/ft) <br /> Treatment Volume(ft3) Impervious Surface(ft2) Factor From <br /> Table Cl <br /> Total Volume Required <br /> Tota I Impervious Area (A) = 159643 sf <br /> Rainfall Depth (P) = 1.1 n <br /> Filtration Factor (F) = 0.5 <br /> Volume Required = (P)*(A) <br /> F <br /> Vol,req = 29268 cf <br /> Two filtration basins are proposed, one on the east of the site and another on the north. The eastern <br /> filtration basin is sized to handle the runoff from Drainage Areas (6, 7, 8, and 13). The rest of the <br /> impervious area will be treated by the norther basin. <br />