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• volume, embankment volume, and excavation volume are located in Appendix C. Output from <br />computer modelling of hydrology and sedimentology are located in Appendix B. <br />4.2.1 Hydrology <br />Hydrology was modeled by computer program, breaking the watershed down into several <br />components such as sub-watersheds and ditches. Results of peak outflows and total runoff <br />volumes were obtained for pond sizing. The computer program StormSHED by Boss <br />International (Boss, 1994) was used to calculate the total runoff produced by the 10-year, 24- <br />hour storm event that would have to be accommodated in the sediment pond. The program <br />also was used to model the 100-year, 24-hour rainfall event for ditch sizing, and the 25-year, <br />24-hour event for sediment pond spillway sizing using identical parameters for consistency. <br />The computer program modeled the rainfall 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 (CN) describe the soils ability to <br />allow water to runoff during a storm. An impermeable surface such as a house roof has a C„ <br />of 100. A natural watershed in an undisturbed hillside in the area of the proposed coal mine <br />• waste bank has a CN of 83 (DMG, 1996), indicating the soil has the capability of allowing <br />much of the rainfall to infiltrate rather than runoff. The coal waste in its compacted in-place <br />condition was modeled with a C„ of 90 (Stover, 1995), indicating that most rainfall will run <br />off. Reclaimed portions of the waste bank were modeled with a C„ of 80. These parameters <br />were chosen assuming an antecedent moisture condition (AMC) of II. Under this condition <br />the soil is assumed to be slightly moist, such that its ability to allow water to infiltrate is less <br />than dry soil conditions. <br />Time to concentration was determined based on average sub-watershed lengths, and average <br />roughnesses of the terrain. Sub-watershed areas were delineated by natural and constructed <br />divides. Figure 4.1 shows the sub-watersheds delineated and the ditches used in routing the <br />runoff water. <br />For worst case conditions, the coal waste pile was modeled in its partially reclaimed, <br />completely constructed geometry since reclamation will be performed with construction, as <br />contemporaneously as possible. This condition is worst case because it allows for some <br />u <br />96286/1424.RPT Coel Mine Weste Bank Finel Design for the Bowie No. 2 Mine <br />June 1996 WESTEC 14 <br />weQU~cr-o1 <br />