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Mountain Coal Company, L.L.C. Exhibit 51 <br />West Elk Mine ~ Lower Refuse Pile <br />• 6.6 I00-Year Flood Events in Sylvester Gulch <br />6.6.1 100-Year Flood Event Determination <br />As illustrated on the various phase diaerams the realignment of Sylvester Gulch Road located the road <br />within .0 feet of Sylvester Gulch. To insure that the subsoil stockpile and the lower refuse pile would not <br />be Flooded, a 100-year Flood study was performed to determine the maximum elevation of the 100-year <br />flood. <br />The 100-year floodplain in the vicinity of the proposed topsoil stockpile for Sylvester Gulch is shown on <br />Figure LWP-002. The floodplain was determined based upon the peak discharge for the 100-year, 34-hour <br />storm. The peak discharge was computed by using a Type II distribution storm. The precipitation from a <br />100-year, 24-hour storm event was determined to be 2.6 inches based upon Figure S-7 of the manual <br />Procedures for Determining Peak Flows in Coloradq Soil Conservation Service, U.S. Department of <br />Agriculture, March 1980. The peak flow for the 100-year, 24-hour precipitation event for the 4.33 square <br />mile watershed upstream of the subsoil stockpile and lower refuse pile was determined using a method <br />found in the March 1980 SCS publication and based upon the following parameters: <br />100-year,24-hour Rainfall= 2.6 inches <br />Curve Number= 70 <br />Time ofConcentration= .56 hours <br />Runoff Depth = .50 inches <br />Peak Discharge= 930 <br />- i, Based upon the peak discharge. the flood elevation for various parts of SytvesterGulch were analyzed using <br />MANCRIT2. Different cross-sections were analyzed alone the subsoil and former topsoil stockpiles. <br />These cross sections. as shown on Map LWP-002, indicate that flooding of the subsoil and former topsoil <br />stockpiles will not occur. <br />The subsoil stockpile will not impact any riparian habitat. Sylvester Gulch, as it traverses the edge of the <br />site, is very deeply incised and contains only a few scattered areas of riparian vegetation in the channel <br />bottom and along the east bank. All areas outside the channel contain only oak brush and other dry, upland <br />type vegetation. <br />6.6.2 100-fear Flood Event Assuming Blockage o[ Sylvester Gulch <br />Since the lower refuse pile is located on an old debris flow, the 100-year flood event was studied assuming <br />that a flow event may fill the Sylvester Gulch Channel with debris to the top of its banks. Using this <br />assumption, the 100-year, 24-hour storm even[ was routed through [he gulch and onto the debris to <br />determine the maximum elevation of the gulch during a 100-year storm event. This study indicated that the <br />Sylvester Gulch Road in the vicinity of the subsoil stockpile would need to be raised to insure [hat the road <br />has a minimum of one foot clearance between the top of the road and the maximum water level event which <br />could occur. The minimum grade of the Sylvester Gulch realignment which would be acceptable and <br />maintain a minimum one footVfreeboard along the road would be six percent. However, in order [o meet <br />grade requirements further up Sylvester Gulch Road, the new alignment would need to be placed a[ <br />approximately a seven to ten percent grade. In no case, would the Flood Flows, assuming the Sylvester <br />~~ Gulch channel becomes filled, endanger flooding the road or the subsoil stockpile. <br />33 <br />