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<br /> <br /> <br /> <br />Proposed peak runoff rates (in cubic feet per second): <br /> 2-year 10-year 100-year <br />Off Site to the East 1.32 2.23 4.41 <br />Wetland 20.27 39.16 92.51 <br />Total Site 21.05 40.55 95.00 <br /> <br /> <br />Water Quality Volume: <br />Per Ramsey-Washington Metro Watershed District requirements, the water quality volume shall <br />be retained on site. Infiltration on site has been deemed infeasible due to the existing clay <br />soils, see attached geotechnical report. Therefore, a filtration basin was implemented as the <br />BMP. The water quality treatment volume requirement is 1.82” of rainfall over all <br />new/reconstructed impervious surfaces. <br /> <br />Stormwater entering the filtration basin will be first pretreated by a forebay. The forebay was <br />designed based off of standards set in the Minnesota Stormwater Manual. <br /> <br />Forebay Pretreatment: <br />Required Dead Storage = WQV x 10%= 3367 CF <br />Provided Dead Storage = 4083 CF <br />Required Surface Area = 0.1% x Total Drainage Area= 1096 SF <br />Provided Surface Area = 3593 SF <br /> <br />Forebay Summary <br />Bottom Elev. 875.00 <br />Normal Water Elev. 878.00 <br /> <br />Water Quality Volume: <br />New/Reconstructed Impervious Area = 222,001 SF <br />Required Water Quality Volume = 222,001 SF x 1.82” = 33670 CF <br />Provided filtration Volume = 52,631 CF <br /> <br />Filtration Basin Summary <br />Bottom Elev. 879.00 <br />100-year HWL Elev. 882.97 <br />WQV 52,631 CF <br /> <br />Allowable Drawdown Time: <br />Filtration Basin Allowable Drawdown = 1.6 in/hr x 48 hrs = 6.4’ <br />Filtration Basin Bottom = 879.00’ <br />Filtration Basin Outlet = 880.50’ <br />