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Mountain Coal Company, L.L.C. <br />West Elk Mine <br />1 <br />6.0 SURFACE HYDROLOGY <br />6.1 Introduction <br />Exhibit 51 <br />A surface hydrology analysis was performed to size tfie sediment and runoff ditches located around the <br />lower refuse pile for each of the five phases and determine if the existins sedimentation pond is capable of <br />handlinetheadditionalrunoffassocia[edwitheachphase. <br />The peak discharges and maximum runoff used in the design and size verification of all ditches and <br />sedimentation ponds for Phases 1 through IV were based upon procedures developed by the Soil <br />Conservation Service (SCS) and the U.S. Department of Agriculture. (Urban Hvdroloev for Small <br />Watersheds, Technical Release No. 5~. 197 and from Procedures for DetermininePeak Flows in Colorado, <br />Soil Conservation Service and the U.S. Department of Agriculture, March 1980). All ditches and diversions <br />were designed to accommodate the peak discharge from an SCS Type II 100-year. 2d-hour precipitation <br />event. A triangular channel design was used for all ditches. The sedimentation pond was designed to <br />contain the Type II 10-year. 24-hour storm event without discharge plus sediment storaee for each phase. <br />Also, the outlet structure for the sediment pond was analyzed to insure that the system could discharge the <br />peak flow from a 2~-year. 24-hour precipitation even[ for each phase without overtopping. The following <br />sections describe the design of the sediment control ditches. the clear water ditches, and the sedimentation <br />basin for each phase of the lower refuse pile. <br />Positive drainace is currentlyestablishedand will be maintained on the pile while under constructior>j and is <br />directed along the south side of the lower refuse pile. Surface runoff from the refuse pile is then collected in <br />r the designed perimeter sediment ditches and conveyed to sediment pond MB-?R. The drainace from the <br />w undisturbed area to the south is collected in a clear water ditch (DCW-2) and is diverted into pond MB-2Rj <br />At no time will surface runoff be allowed to collect at the head of the pile (Note [hat the pile surface is <br />roueh and some puddling in wheel ruts or other, small surface irreeularities may temporarily exist). The <br />final configuration sediment ditch locations around the entire lower refuse pile will remain as built for <br />Phase V and are shown on Map ~4 and in Exhibit 66. <br />Sediment yield was derived using the universal soil loss equation method as presented in Applied <br />Hvdroloev and Sedimentoloev for Disturbed Areas. B.J. Barfield, Oklahoma Technical Press. 1981. <br />Sedimentation pond MB-?R was examined [o ensure adequate capacity' to contain 3 years of sediment <br />volume plus a 10-yr. ?4-hr, Type II precipitation event without discharge. The sediment cleanout <br />schedule for MB-?R was also examined. <br />6.2 Sediment Control Ditches <br />6.2.1 Sediment Control Ditches for Phases 1 through IV <br />Sediment control ditches were constructed alone the perimeter of each phase of the proposed lower refuse <br />pile as shown on Figures LW P-00^_ through LWP-OOd and Figure'?.0. The sediment control ditches for the <br />first three phases were designed based on a 100-year. ?4-hour precipitation of event ?.6 inches, and a <br />waste pile curve number of 7~. Assumptions for Phase lV changed to accommodate a 100-year, '?4-hour <br />precipitation event of'_.8 inches and a waste pile curve number of 88 for non-topsoiled areas of the waste <br />pile and sub+vatershed-specifictimes of concentration(See Exhibit 66). The peak discharge for each phase <br />was calculated based upon procedures developed by the Soil Conservation Service mentioned in the <br />i~ <br /> <br />