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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 wiN be <br />approximately 2.595 cy. Cover fill requirements will be approxirtlately 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 end 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, 24hour storm. This storm frequency corresponds to <br />2.0 inches IJ.E. Stover ~ Associates, 19951. <br />The sediment pond wlll be bated at the toe of the coal 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 a rock-tined basin that is open on the southern end. Ths sediment pond wit be <br />constnrcted by building a concrete wall to cbss off the basin as shown in Drawing 95255-03. <br />~ Spreadsheets used to calculate the sediment pond volume are bated in Appendix C. <br />• <br />4.2.1 Hydrology <br />Surface water hydrology was evaluated using the computer program StortnSHED by Boss <br />Intematbnal (boss, 1994) StortnSHED 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, 24hour rainfall event for ditch sizing, and the <br />25-year, 24hour event for sediment pond spillway sizing, using identical parameters for <br />consistency. <br />The computer program analyzes rainfall events using SoU ConssrvaUon Service ISCS) <br />methodology. Typical rainfall in the Western United States is patterned closely to the SCS <br />Type II rainfa0 distribution (SCS, 19881. SCS curve numbers (C„1 describe the soil's ability <br />to allow water to runoff during a storm. An impertneabk surface such as a house roof has <br />a C„ of 100. A natural watershed in an undisturbed hillside in the area of the proposed coal <br />mine waste bank has a C„ of 83 (Stover, 19951, indicating the soil has the capabil-RY of <br />~, <br />96266/1424.RPr Cod MiM W~~t~ a~nk Design fa tM 9~nbom GNk Mlty <br />Oetotw taae WESTEC, Ino. t~ <br />