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allowing much of the rainfall to infiltrate rather than runoff. The coal mine waste in its S compacted in-place condition, has a CN of 90 based on the data presented in Stover (1995-, indicating that most rainfall will run off. These parameters were chosen assuming an antecedent moisture condition (AMC) of Il. Under this condition the soil is assumed to be slightly moist, such that its ability to allow water to infiltrate is less than dry soil conditions. Time to concentration was determined based on average sub-watershed lengths, and average roughnesses of the terrain. Sub-watershed areas were delineated by natural and constructed divides. Figure 4.1 shows the sub-watersheds delineated and the ditches used in routing the runoff water. For worst case conditions, the coal mine waste pile was modeled in its unreclaimed, completely constructed geometry. This condition is worst case because it allows for maximum CN = 90 coal mine waste area for maximum runoff. This situation is not expected to occur, since the coal mine waste is scheduled to be reclaimed concurrently with construction. The result of this analyses, using the 10-year, 24-hour storm of 2.0 inches, is a volume of • 12,700 cubic feet of water, or 0.29 acre-feet. The sediment pond is required to completely contain this volume. The computer output for the evaluation is presented in Appendix B. 4.2.2 Sediment Yield Sediment yield into the construct pond was predicted on a storm event basis for the Sanborn Creek coal mine waste bank drainage. The predicted yield response for the basin was evaluated so that pond size would account for storm runoff and sediment loads. The sediment yield was evaluated using the program SECAD (Civil Software Design, 1992). SEDCAD offers three separate erosion sediment modeling subroutines for predicting sediment loading. The Revised Universal Soil Loss Equation (RUBLE) was judged the most appropriate sediment generation subroutine for the steep slopes in the study area. RUSLE incorporates USDA/BLM study results for measured sediment production rates on steep soil slopes to modify the Universal Soil Loss Equation (LISLE). SEDCAD incorporates runoff estimates using SCS procedures in combination with sediment production estimates to predict total runoff, peak discharge, and total sediment yield at designated locations in a drainage. • 95255/1424.RPT Coal Mina Waste Benk Design for the Sanborn Creek Mine October 1998 WESTEC, Inc. 14