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Mountain Coal Company, L.L.C. Exhibit 51 <br />West Elk Mine Lower Refuse Pile <br />• The sediment yield analysis for Phases 1 through 111 assumes a very active revegetation program. This will <br />• be an ongoing process during the construction of each phase. Table 1 I shows the effects of the subsoil and <br />topsoil stockpileson the sedimentationbasin for all four phases. <br />The CDMG requires that the outlet structure for the sedimentation pond be able to handle the peak <br />discharge from a 25-year, 24-hour storm. The 25-year peak flow for the lower refuse pile is based on the <br />following parameters: Curve Number = 88, Total Runoff= .89 inches. Table 12 gives the peak 25-year, 24- <br />hourdischarge for each phase, with the exception of Phase V which has been modeled to report to MB-2R <br />and includes drainage areas in addition to the LRP. This information is presented in Exhibit 66. <br />MB-2R has an 8-foot wide trapezoidal channel with 2H:1 V sideslopes for an emergency spillway. The <br />capacity for the spillway was ca{culated. using :he Chery-Manning equation (see Appendix L), to be 582- <br />cfs : `_s::'::i^c a 10% evade and a Mannings roughness constant (n) of 0.03. The pond and the emergency <br />outlet structure have adequate capacity to allow a 25-yeaz, 24-hour peak flow to pass. <br /> <br />. ~. <br />29 <br />