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Introduction <br />When topsoil stockpiles are not associated with structures identified in Rule 4.05.2(4), where <br />routing topsoil stockpile runoff to a sedimentation pond for treatment is not feasible, and where <br />the topsoil stockpile is not within a drainage controlled by a sedimentation pond, a small area <br />exemption (SAE) will be utilized. Catchment sumps will be constructed to completely contain the <br />runoff from topsoil stockpiles. Where no runoff volume is calculated, a catchment sump will not <br />be constructed. The location of the existing small area exemption stockpiles and associated <br />catchment sumps are shown on Exhibits 13 -2 and 13 -2A. <br />Stockpile area J, TT, XX and AAA will have ditches and sumps constructed to contain potential <br />runoff sediment once topsoil is removed from the stockpile area and will be maintained until the <br />areas are revegetated. The SEDCAD results for the four (4) areas are included in Addendum 3. <br />Table 13 -8 -1, 2 and 3 contain summaries of the SEDCAD results. <br />Design Analysis <br />General. Catchment basins and ditches are designed to completely contain the runoff from the 10- <br />year, 24 -hour storm event without experiencing any discharge. This will demonstrate that the <br />disturbed area of the topsoil stockpiles meet the requirements of Rule 4.05.2(3)(a). <br />Hydrology. The hydrology of the watershed of the topsoil pile was analyzed utilizing the <br />SEDCAD computer program. Tab 13, Attachment 13 -1 contains a description of the design <br />methods and a listing of the SEDCAD input variables. <br />Input consists of precipitation amount (inches), watershed area (acres), time of concentration <br />(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 0.125 <br />hours, SEDCAD assumes an instantaneous hydrograph. <br />The curve number ranged from 86 for new topsoil piles to 61 for a well vegetated stockpile. For <br />the TAHR topsoil stockpiles, a curve number of 61 was assumed, based on vegetation cover data. <br />The watershed area of the topsoil piles was assumed to be the largest area, which would contribute <br />to runoff volume. The SEDCAD computer program was run to obtain runoff volumes for the <br />topsoil stockpile watershed areas. <br />PR -06 13 -8 -1 Revised 10 /11 <br />