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• covering and revegetation, so this situation is not expected to occur. The rapid unit <br />hydrograph shape was selected for the coal waste bank due to its steep slopes and lack of <br />vegetative cover. The moderate hydrograph was selected for the natural ground sub- <br />watershed assuming flatter slopes and 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. Runoff potential is based on land use and treatment conditions, and the four <br />hydrologic soil groups: A, B, C, and D. In conversations with Bowie Resources <br />representatives (Stover, 1995) and DMG suggestions (DMG, 1996), curve numbers were <br />given based on previous analyses. The coal waste bank curve number was assumed to be <br />90, with an average time of concentration of 2.00 minutes. For the natural ground, a curve <br />number of 83 and an average time of concentration of 2.00 minutes was used. The reclaimed <br />waste bank used a curve number of 80, with an average time to concentration of 1.00 <br />minutes. <br />An estimate of erosional potential for the drainage was based on the peak runoff resulting <br />• from the 10-year, 24-hour duration precipitation event. Precipitation for this design event was <br />estimated by Bowie Resources representatives (Stover, 1995) to be 1.8 inches. This <br />precipitation was distributed according to the SCS Type II Storm. <br />Required soil input data for the sedimentologic 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 to be 2.65. The submerged bulk <br />specific gravity, based on the soil gradation analysis, was 1.35 and 1.40 for the natural <br />ground and the coal waste, respectively. <br />The soil erodibility factor (K) is a numeric representation of the ability of the soil to resist the <br />erosive energy of rainfall. The factor is dependent on particle size and distribution, structure, <br />void space and pore size, and organic matter content. For the study, 0.37 and 0.15 best <br />described the erodibility of the natural ground and the coal waste, respectively. <br />The control practice factor (CP) accounts for the effects of erosion control measures. It is <br />• defined as the ratio of sediment Toss from an area with a given cover and conservation <br />96266/1424.RPT Coal Mine Waste Bank Finel Design for the Bowie No. 2 Mine <br />June 1996 WESTEC 18 <br />ADCQVACY-el <br />