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allowing much of the rain/all to infiltrate rather than runoff, The coat mine waste in its <br />compacted in-place condition, has a C„ of 90 based on the data presented in Stover (1995), <br />• indicating that most rainfall will run off. These parameters were chosen assuming an <br />antecedent moisture condition (AMC) of II. Under this condition the soil is assumed to be <br />slightly moist, such that its ability to allow water to infiltrate is less than dry soil conditions. <br />Time to concentration was detemtirted based on average sub-watershed lengths, and average <br />roughnesses of the terrain. Sub-watershed areas were delineated by natural and constructed <br />divides. Fguro 4.1 shows the sub-watersheds delineated artd the ditches used in routing the <br />runoff water. <br />For worst case conditions, the coal mine waste pile was modeled in its unreclaimed, <br />completely constructed geometry. This condition is worst ease because it allows for maximum <br />C~ =90 coal mine waste area for maximum runoff. This sitwtion is not expected to occur, <br />since the coal mine waste is scheduled to be reclaimed concurtently with construction. <br />The result of this analyses, using the 10-year, 24hour storm of 2.0 incites, is a volume of <br />~ 12,700 cubic feet of water, or 0,29 acre-feet. The sedenent pond is required to completely <br />• contain this volume. The computer output for the evalwtion is presented in Appendix B. <br />4.2.2 Sedxnent Yield <br />Sediment yield into the constrict pond was predicted on a stone event basis for the Sanborn <br />Creek coal mine waste bank drainage. The predicted yield response for the basin was <br />evaluated so that pond sae would account for stone nnoff and sediment Toads. <br />The sedirt-ent yield was evaluated using the program SECAD ICivil Software Design, 1992). <br />SEDCAD offers three separate erosion sediment modeling wbroutines for predicting sediment <br />loading. The Revised Universal Soil Loss Equation (RUSI.E) wu judged the most appropriate <br />sediment generation subroutine for the steep slopes in rite study area. RUSLE incorporates <br />USDA/BLM study resuks for measured sediment production rates on steep soil slopes to <br />modify the Universal Soi Loss Equation IUSLE). SEDCAD irteorporatas runoff estimates using <br />SCS procedures in combination with sediment production estimates to predict total runoff, <br />peak discharge, and total sediment yield at designated bcationa in a drainage. <br />9 5 25 6/1 424.pVT CoU Mires Wut~ e~nk Owign fa tM S~nbarn Crest M6v <br />ocrowr ~eae wEStEe, lee. t4 <br />