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Mountain Coal Company <br />West Elk Mine <br />Exhibit S1 <br />Lower Refuse Pile <br />the 4.33 square mile watershed upstream of the topsoil stockpile was <br />determined using a method found in the march 1980 SCS publication and <br />based upon the following parameters: <br />100-year, 24-hour Rainfall = 2.6 inches ~~~ <br />Curve Number = 70 <br />Time of Concentration = 0.56 hours <br />Runoff Depth = .50 inches 0 <br />Peak Discharge = 930 <br />Based upon the peak discharge, the flood elevation for various parts of <br />Sylvester Gulch were analyzed using MANCRIT2. Different cross-sections <br />were analyzed along the proposed subsoil and topsoil stockpiles. These cross <br />sections, as shown on Map LWP-002, indicate that flooding of the subsoil and <br />topsoil stockpiles will not occur. <br />The topsoil and subsoil stockpiles will not impact any riparian habitat. <br />Sylvester Gulch, as it traverses the edge of the site, is very deeply incised and <br />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 <br />Oak Brush and other dry, upland type vegetation. <br />6.6.2 100-Year Flood Event Assuming Blockage of Sylvester Gulch <br />Since the lower refuse pile is located on an old debris flow, the 100-year flood <br />event was studied assuming that a flow event may fill the Sylvester Gulch <br />Channel with debris to the top of its banks. Using this assumption, the 100- <br />year, 24-hour storm event was routed through the gulch and onto the debris <br />to determine the maximum elevation of the gulch during a 100-year storm <br />event. This study indicated that the Sylvester Gulch Road in the vicinity of <br />the subsoil stockpile will need to be raised to insure that the road has a <br />minimum of one foot clearance between the top of the road and the <br />maximum water level event which could occur. The minimum grade of the <br />Sylvester Gulch realignment which would be acceptable and maintain a <br />minimum one foot freeboard along the road would be six percent. However, <br />in order to meet grade requirements further up <br />I~l,~n~~ ~~~- ~~~~~ ~~ he <br />zn <br />