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Mountain Coal Company <br />West Elk Mine <br />Exhibit 51 <br />Lower Refuse Pile <br />The factors for the equation were obtained from Soil Erosion by Kirkby & Morgan, <br />1980, the original lower refuse permit, and the projected slopes as shown on the <br />various phase diagrams. The sediment yield for each phase, the sediment yield for <br />vegetated areas, the sediment yield for unvegetated slopes, the expected maximum <br />sediment level for each phase, the 50 percent sediment level for each phase, the <br />maximum sediment level allowed for adequate storage for the 10-year, 24-hour storm <br />without discharge, and the maximum water elevation obtained for each phase is <br />shown on Table 11. For all phase, sediment will not be allowed to be greater than <br />the maximum allowable sediment level. It should be noted that the pond will be <br />pumped as soon as possible after a storm event following treatment described in <br />Section 6.4. The water or sediment level will not remain above the maximum <br />sediment elevation shown on Table 11 for an extended period of time because a <br />discharge from a 10-year, 24-hour storm event may occur. <br />c <br />Sediment level and water level will be monitored by Mountain Coal Company. The <br />information will be periodically reviewed during the various phases to verify the <br />original design assumptions. <br />The sediment yield analysis assumes a very active revegetation program. This will be <br />an ongoing process during the construction of each phase. Table 11 shows the effects <br />of the subsoil and topsoil stockpiles on the sedimentation basin for all four phases. <br />The CMLRB requires that the outlet structure for the sedimentation pond be able <br />to handle the peak discharge from a 25-year, 24-hour storm. The 25-year peak flow <br />for the lower refuse pile is based on the following parameters: Curve Number = 82,E <br />Total Runoff = .48 inches, the Time of Concentration = 0.15. Table 12 gives the <br />peak 25-year, 24-hour discharge for each phase. <br />26 <br />