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the Elk Creek Topsoil Stockpilel, or a borrow source not associated with the proposed waste <br />bank. The total amount of topsoil required to be placed on the proposed waste bank will be <br />approximately 2,595 cy. Cover fill requirements will be approximately 20,750 cy, total. Using <br />this method of reclamation, only initially excavated materials will have to be stockpiled. <br />4.2 SEDIMENT POND <br />The sediment pond was designed to capture runoff water and its transported sediment, and <br />hold it until the suspended sediments have settled and the water is suitable for discharge <br />under the mine's water discharge permit. The quantity of water and sediment the pond must <br />hold is that produced by the 10-year, 24-hour storm. This storm frequency corresponds to <br />2.0 inches (J.E. Stover & Associates, 1995). <br />The sediment pond will be located at the toe of the coat mine waste bank at the bottom of the <br />draw, immediately north of the Denver & Rio Grange Western Railroad alignment. A small <br />basin with this draw was created by erosion into the Rollins Member sandstone. This basin <br />is essentially crock-lined basin that is open on the southern end. The sediment pond will be <br />constructed by building a concrete wall to close off the basin as shown in Drawing 95255-03. <br />Spreadsheets used to calculate the sediment pond volume are located in Appendix C. <br />4.2.1 Hydrology <br />Surface water hydrology was evaluated using the computer program StormSHED by Boss <br />International (Boss, 1994) StorrnSHED was used to calculate the total runoff produced by the <br />10-year, 24-hour storm event that would have to be contained in the sediment pond. The <br />program also was used to model the 100-year, 24-hour rainfall event for ditch sizing, and the <br />25-year, 24-hour event for sediment pond spillway sizing, using identical parameters for <br />consistency. <br />The computer program analyzes rainfall events using Soil Conservation Service (SCS) <br />methodology. Typical rainfall in the Western United States is patterned closely to the SCS <br />Type II rainfall distribution (SCS, 1986). SCS curve numbers IC„) describe the soil's ability <br />to allow water to runoff during a storm. An impermeable surface such as a house roof has <br />a CN of 100. A natural watershed in an undisturbed hillside in the area of the proposed coal <br />mine waste bank has a C„ of 63 (Stover, 1995), indicating the soil has the capability of <br />IJ <br />96266l1424.RPT Coel Mine Waete Bank Design for the Sanborn Creek Mina <br />October 1998 WESTEC, Inc. t3 <br />