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velocity (2.9 cfs). Table 4.6-9 shows the calculations upon which the <br /> emergency spillway for the pond was based. <br />4.6.1.4 Simulation of Sediment Pond Performance <br />The computer model was used to check the design and performance of the <br />sediment pond. The results of the simulation for the sediment pond can be <br />found in Appendix 4.6-8 for the year of highest runoff (Year 20) and sediment <br />yield (Year 5). The appendix contains the runoff calculations for each <br />subwatershed, the combined runoff for the drainage area, and the performance <br />of the sediment pond. <br />Exhibit 4.6-1 shows the subwatershed conditions and location of the pond <br />during the years of highest sediment yield. Exhibit 4.6-2 shows the <br />subwatershed conditions of the highest runoff year. <br />To test pond performance, control variables were determined for input <br />into the computer model. The control variables consisted of pond dimensions <br />and discharge rates at stage elevations, particle size distribution of <br />sediment inflow, and a dead storage level. The pond dimensions and discharge <br />rates at stage levels can be seen in Appendix 4.6-B - "Basin Geometry" for the <br />pond. The particle size distribution of sediment inflow was derived from <br />Section 2.3, by weight averaging the particle distributions of the layers to <br />be removed for each soil series according to acres throughout the area. The <br />calculation of these values can be seen in Table 4.6-10. Based on <br />length/width ratios and the requirements of the SEDIMOT iI program, dead <br />storage value for the pond was set at 30 percent. <br />4.6.1.5 Diversion Ditches <br />Two diversion ditches were designed to carry water from the downslope <br />edge of the mining area to the sediment pond. The south diversion ditch will <br />be constructed during the first year. The north diversion ditch will be <br />constructed over time extending further north as mining progresses in that <br />direction. The computer model, SEDTMOT II was used to generate the peak flow <br /> rates of the drainage area above the diversion ditches for a 10-year, 24-hr <br />• storm event. To determine the maximum peak flow which would occur along any <br />4.6-32 <br />November 1,1989 <br />