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• Introduction <br />when topsoil stockpiles are not associated with structures identified in Rule 4.05.2(4), <br />where routing topsoil stockpile runoff to a sedimentation pond for treatment is not <br />feasible, and where the topsoil stockpile is not within a drainage controlled by a <br />sedimentation pond, a small area exemption (SAE) will be utilized. Catchment sumps will <br />be constructed to completely contain the runoff from topsoil stockpiles. Where no runoff <br />volume is calculated, a catchment sump will not be constructed. The location of the <br />existing small area exemption stockpiles and associated catchment sumps are shown on <br />Exhibits 13-2 and 13-3. <br />Design AnalYSis <br />General. Catchment basins and ditches are designed to completely contain the runoff from <br />the 10-year, 24-hour storm event withouC experiencing any discharge. This will <br />demonstrate that the disturbed area of the topsoil stockpiles meet the requirements of <br />Rule 4.05.2(3)(a)- <br />• xvdrologv. The hydrology of the watershed of the topsoil pile was analyzed utilizing Che <br />SEDCAD computer program. lab 13, Attachment 13-1 contains a description of the design <br />t <br />methods and a listing of the SEDCAD input variables. <br />Input consists of precipitation amount (inches), watershed area (acres), time of <br />concentration (hours), and runoff curve number (CN). <br />The precipitation depth for a 10-year, 24-hour storm event was 1.6 inches. The time of <br />concentration was assumed to be between 0.02 and 0.04 hours. Since this tc is less than <br />0.125 hours, SEDCAD assumes an instantaneous hyd rograph. <br />The curve number ranged from 86 for new topsoil piles to 61 for a well vegetated <br />Stockpile. For the TAHR topsoil stockpiles, a curve number of 61 was assumed, based on <br />vegetation cover date. <br />The watershed area of the topsoil piles was assumed to be the largest area which could <br />• contribute to runoff volume. The SEDCAD+ computer program was run to obtain runoff <br />volumes for the topsoil stockpile watershed areas. <br />13-8-1 Revised 11/05/93 <br />