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Mountain Coal Company, L.L.C. Exhibit 51 <br />West Elk Mine Lower Refuse Pile <br />• 6.3.2.1 10-Year, 23-Hour Event Runoff Volume and Sediment Yield during Phase V <br />The runoff volume resulting from a 10-yr, ?4-hr event on the Phase V modification will remain the same <br />as the Phase IV design because the total surface area does not change. The sediment yield from such a <br />precipitation even[ will increase slightly. The surface water runoff estimates were derived using an SCS <br />CN of 88 and a 10-yr, 24-hr Type II precipitation event of I.8-inches. The sediment yield estimates are <br />based upon the universal soil loss equation computations in SedCAD+ Version 3. Results are provided <br />in Exhibit 66. The pond will be able to accommodate the 10-year, '?4-hour event runoff, in addition to <br />containing a 3-year quantity of sediment storage. <br />6.4 Clear Water Ditch Design <br />• <br />• <br />The clear water ditch design was performed using the methodology as used for the design of the sediment <br />control ditches as described in Section 6.3. The summary of the design for each phase is shown on Table <br />13 and states the phase, ditch location, peak discharge, velocity, normal depth, and lining requirements. <br />A clear water diversion berm (Berm D?-R'_) west of the southwest ditch (see Figure 3.0 and Map 54) is <br />capable of diverting flows from a 100-year. ?4-hour precipitation event. This design is verified in <br />Appendix L and Exhibit 66. <br />Table 13 -Clear Water Ditch Design', 100-Year Storm <br />Phase Ditch 100-year,24-Hour Velocity (fps) Normal RiprapSizeZ <br /> Location Peak Discharge Depth (ft) (inches) <br /> (cfs) <br />[ East 5.3 4.4 .63 unlined <br /> South 4.6 5.3 .~4 unlined <br />Il East 5.3 4.4 .63 unlined <br /> South 4.6 5.3 .54 unlined <br />111 South 4.3 4.0 .60 unlined <br /> Southwest 5.1 ~.6 .55 unlined <br />IV DCW-3 4.33 9.4'_ 0.98 shotcrete <br />V DCW-2 6.66 ---- 0.96 shotcrete <br />Ditch triangularshaped.0.7 to 0.98 feet deep, 3H:1 V slopes (DCW-2) <br />Riprapsizerepresents~Opercen[size <br />The LRP Phase V modification will not affect the existing clear water ditch and diversion berm designs <br />already in place. <br />6.5 Treatment of Water in Sediment Pond <br />MCC will treat the water collected in the sediment pond to meet all applicabledischargerequirements. <br />32 <br />