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• practice to that of a field left in continuous fallow for one year. Since the coal waste will be <br />mechanically manipulated, a practice factor of 1.2 was used. Assuming 60 percent of the <br />natural ground is covered by vegetation, a corresponding practice factor of 0.09 was used for <br />the study. <br />SEDCAD then routes the concentrated flow at the sub-watershed outlet to the basin outlet <br />as channelized flow using Muskingum's routing method. Finally, SEDCAD calculates the <br />volume of the deposited sediment at the basin outlet using these parameters. <br />Based on the SEDCAD model, a prediction of the amount of sediment that would be deposited <br />in the pond was estimated. The storage that is required is approximately 134.9 tons of coal <br />waste sediment and 19.6 tons of natural ground and cover fill sediment. If the coal material <br />is assumed to have a unit weight of 73 pounds per cubic foot as determined by laboratory <br />analyses (see Appendix Al, and 120 pounds per cubic foot as an average value for the natural <br />ground, the resulting volume is 4,022 cf. This volume of sediment was designated as the <br />maximum sediment transported to the pond for the 10-year design storm event. <br />Improvements in the sediment load are largely dependent on the degree of remediation <br />• activities performed on the coal waste bank as it is constructed. Computer output generated <br />during modelling of the sediment transportation is located in Appendix B. <br />4.2.3 Estimated Sediment Pond Volumes <br />Based on the hydrologic analysis and the sediment analyses, the sediment pond must be able <br />to hold 4,022 cf + 19,057 cf = 23,079 cf, or 0.53 acre feet. The design as shown on <br />Drawing 95266-04 will hold this volume up to the emergency spillway elevation. Figure 4.2 <br />shows the sediment pond height-volume curve and the calculated sediment and water <br />volumes. <br />4.2.4 Sediment Pond Embankment <br />The sediment pond will be partly excavated. Excavation will be conducted from EI. 5,977 ft <br />to approximately EI. 5,972 ft. An embankment will be constructed above EI. 5,977 ft to the <br />crest elevation of EI. 5,982 ft. The total depth of the pond is then 10 feet, upstream toe to <br />crest, and embankment height will be 5 feet, with a 9-foot crest width (110+351/5=9.0 ft <br />per Rule 4.05.6 (11) Ih)). Side slopes will be constructed to a 2:1 slope and vegetated after <br />• construction to minimize erosion. Because these combined slopes are steeper than the 5:1 <br />96266/1424.RPT Coel Mine Weste Benk Finel DesiOn for the Bowie No. 2 Mine <br />June 1996 W E S T E C aoepuxcr-or t~ <br />