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To estimate sediment yield, the drainage basin was divided into sub-watersheds. Runoff <br />hydrographs for the individual subwatersheds were developed and combined using the <br />SEDCAD Program. SEDCAD routes the overland flow of the rainfall excess to the <br />subwatershed outlet using the Unit Hydrograph Method. SEDCAD offers three unit hydrograph <br />shapes to simulate rapid, moderate, and slow runoff from the subwatersheds. The worst case <br />situation for sediment yield was determined to be an unreclaimed, fully developed waste bank <br />and surrounding natural ground. The rapid unit hydrograph shape was selected for the coal <br />mine waste bank due to its steep slopes and lack of vegetative cover. The moderate <br />hydrograph was selected for the natural ground subwatershed assuming flatter slopes and <br />vegetative cover. <br />The time of concentration is determined from the hydraulically longest flow path to the sub- <br />watershed outlet. The SCS curve number of a watershed represents the runoff potential for <br />the area. It is based on land use and treatment conditions, and the four hydrologic soil groups: <br />A,B,C and D. In conversations with OXBOW representatives (Stover, 19951, suggestions for <br />curve numbers were given based on previous analyses. The coal mine waste bank curve <br />number was assumed to be 90, which resulted in an average time of concentration of 2.22 <br />. minutes. For the natural ground, a curve number of 83 was used which resulted in an average <br />time of concentration of 2.62 minutes. <br />An estimate of erosional potential for the drainage was based on the peak runoff resulting from <br />the 10-year, 24-hour duration precipitation event. Precipitation for this design event was <br />estimated by OXBOW representatives to be 2.00 inches (Stover, 19951. This precipitation <br />was distributed according to the SCS Type II Starm. <br />Required soil input data for the sediment yield evaluation consists of an estimate of the <br />specific gravity of the soils, the submerged bulk specific gravity, and the soil gradation. The <br />specific gravity of the deposited sediment was estimated as 2.85 corresponding to sand <br />material. The submerged bulk specific gravity, based on the soil gradation analysis, was 1.35 <br />and 1.40 for the natural ground and the coal mine waste, respectively (Civil Software Design, <br />1992). <br /> <br />96266/1424.RPT Coel Mine Waste Benk Design for the Senborn Creek Mine <br />October 1998 WESTEC, Inc. 16 <br />