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Mountain Coal Company, L.L.C. Exhibit 51 <br />West Elk Mine ~ Lower Refuse Pile <br />introduction o(tltis section. The diversion ditches were designed using nvo computer programs. Phases 1- <br />• Ill were designed with a program called MANCRIT2, and Phase IV was designed with SEDCAD+ 3.0. <br />Both packages are based upon Manning's equation. The ditches were designed based upon a Manning's "n" <br />value of 0.03. a required minimum freeboard of 0.~ feet, and ditch side slopes of 3:1. Recommended ditch <br />velocities for unlined channels were estimated to be from 5 to 6 feet per second for the alluvial silts, clays. <br />and sands located at the site. The recommended velocities were obtained from Table 3 found of Page 54 of <br />The Design of Roadside Drainage Channels, published by the Bureau of Public Roads, 196. If velocities <br />exceed 6 feet per second. a lined channel will need [o be constructed. Table 9 states the various phases, <br />ditch locations, design peak discharge. velocities, normal depths. and channel lining requirements for the <br />sediment control ditches associated with Phases 1 - f V. <br />The riprap size required for the sediment control and clear water ditches are shown on Tables 9 and 13. The <br />size shown on these tables is ~0 percent gradation. Based upon recommended gradation requirements from <br />the U.S. Bureau of Reclamation, the maximum size of the riprap will be three times the 50 percent size. <br />Sandstone available in the area will be utilized for the riprap. <br />6.2.2 Sediment Control Ditches for Phase V <br />Surface runoff was computed for the worst case scenario which wou{d occur at the end of the Phase V <br />construction, when the north and east faces will be completed with ?H:I V slopes to a peak elevation of <br />6330-ft, and the surface will be covered with 18-inches of silty-clayey-loamy soil. The peak runoff <br />discharge was derived using an SCS runoff curve number (CN) of 88 and a 100-yr. ?4-hr Type II <br />precipitation event of 2.8-inches. Peak runoff discharge computations for each slope face are presented <br />in Exhibit 66: corresponding ditch flow volumes are listed in Table 44E, also in Exhibit 66. Ditch <br />capacities were computed using the SedCAD+ Version 3 Modeling Program developed by Civil <br />Software Design. <br />Surface runoff water from the lower refuse pile will flow into MB-2R primarily through the inlet culvert <br />located at the northeast comer of MB-2R. The culvert and ditch designs were checked to ensure <br />adequate capacity; results are provided in Exhibit 6ti. <br />All existing sediment control ditches and the MB-?R inlet structure have adequate capacity to convey the <br />100-yr, ?4-hr precipitation event. Phase V worst case, peak flow. <br />6.3 Design of Sediment Control Pond <br />6.3.1 Sediment Control Pond for Phases I through V <br />The sedimentation pond was designed to collect the sediment runoff from each phase of the lower refuse <br />pile and to meet [he discharge requirements of MCC's CDMG and CDPS Permits. In 1993, MCC <br />combined the Refuse Pile Pond (MB-~ with MB-'? [o establish MB-?R, which is on the site of MB-6. This <br />enlarged pond receives water Flows From approximately 13.3 acres rather than the original'_8.~ acres that <br />contributed to fv1B-6. The following discussion on specific hydrologic contributions to Iv1B-'_R during <br />Phases I V and V reflect contributions from the refuse disposal area exclusively. <br /> <br />36 <br />