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C„ = 90 coal waste area for maximum runoff. This situation is expected to occur near the end <br />• of the coal waste bank lifetime, and affords the maximum waste exposure. <br />The result of this analysis, using the 10-year, 24-hour storm of 1.8 inches, is a volume of <br />19,057 cubic feet of water, or 0.44 acre-feet. The sediment pond is required to completely <br />hold this volume. Computer modelling output is located in Appendix B. <br />4.2.2 Sedimentology <br />Sediment yield was predicted on a storm event basis for the Bowie No. 2 Mine coal mine <br />waste bank drainage. The predicted yield response for the basin was evaluated so that pond <br />volumes would account for storm runoff and sediment loads. This analysis was conducted <br />using the computer program SEDCAD (Civil Software Design, 1992). <br />SEDCAD offers three separate erosion sediment modeling subroutines for predicting sediment <br />generation. The Revised Universal Soil Loss Equation (RUBLE) was judged the most <br />appropriate sediment generation subroutine for the steep slopes in the study area. RUSLE <br />incorporates USDA/BLM study results for measured sediment production rates on steep soil <br />• slopes to modify the Universal Soil Loss Equation (LISLE). SEDCAD incorporates runoff <br />estimates using SCS procedures in combination with sediment production estimates from one <br />of the three sediment generation models to predict total runoff, peak discharge, and total <br />sediment yield at designated locations in a drainage. <br />To conduct the study, the basin was divided into sub-watersheds. Runoff hydrographs for <br />the individual sub-watersheds were developed and combined using the SEDCAD Program. <br />The program employs the SCS Curve Number Method to estimate the runoff hydrograph from <br />a drainage area generated during a given storm event. The SCS method calculates the <br />cumulative rainfall excess in inches as a function of the curve number and the depth of <br />precipitation. <br />SEDCAD routes the overland flow of the rainfall excess to the sub-watershed outlet using the <br />Unit Hydrograph Method. SEDCAD offers three unit hydrograph shapes to simulate rapid, <br />moderate, and slow runoff from the sub-watersheds. The worst case situation for sediment <br />yield was determined to be an unreclaimed, fully developed waste bank and surrounding <br />• natural ground. Reclamation is planned to take place as the waste bank develops including <br />96266/1424.RPT Coal Mine Waste Bank Final Design for the Bowie No. 2 Mine <br />Juno 1996 WESTEC <br />16 <br />ADLpUACY-01 <br />