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• pool pond with ten years of sediment storage (12.5917 acre-feet), a peak runoff <br />volume of 18.24 acre-feel, pumping storage from the pits of 1.6131 acre-feet for a <br />total of 32.4448 acre-feet. The actual constructed pond was built to a capacity of <br />33.23 acre-feet. The pond was designed with a principle spillway to handle The peak <br />flow from a 10 year 24 hour storm event and pumping from The pits of 45.0 cfs. A <br />36 inch steel corrugated mitered culvert was installed and will handle a flow of 45.5 <br />cfs before discharging into the emergency spillway. The emergency spillway for this <br />pond was designed for a peak flow of 138.4 cfs from a 100 year - 24 hour storm event. <br />The actual constructed spillway is a riprapped trapezoidal channel with I:I side slopes <br />and a 30 foot bottom width and will handle a flow of 508.0 cfs. <br />Since the submittal, some drainage patterns have been changed from natural <br />conditions existing at the time. The grading of Center Ridge has caused an addition <br />of 11.78 acres near the pond and a reduction of 45.72 acres along The ridge top. This <br />change will not affect the pond signficontly. These areas are shown on Exhibit 4.6- <br />17. <br />Experience has shown That the permanent pool on The West Ridge sediment pond <br />remains full since it receives pit pumpage as well as storm runoff. Thus, when runoff <br />• does occur, the pond discharges a volu me of water equivalent to the runoff and the <br />water balance (i.e. flow contribution) from the mined land is maintained. <br />Sediment volume was calculated using the Universal Soils Loss Equation. Runoff <br />volume and peak runoff was determined by the SCS Rainfall-Runoff prediction <br />method. <br />4.6.4.1 Sediment Yield Calculations for West Ridge Pond <br />Sediment yield for West Ridge mining and reclamation sequence was determined <br />from the publication, "Preliminary Guidance for Estimating Erosion on Areas <br />Disturbed by Surface Mining Activities in The Interior Western United States," July, <br />1977. The following paragraphs clarify the factors that were used in the universal <br />soils loss equation. <br />The "R" (Rainfall Factor) was determined from the "Average Annual Value of <br />Factor R" map found at the end of the publication. The "K" factor (Soil Erodibility <br />Factor) was determined for the different soil classifications in the drainage area from <br />Appendix A, with the exception of the mine spoil factor. The mine spoil "K" factor <br />• was determined from Figure 3, by estimating that the spoil consists of 509o silt and <br />very fine sand, 30°h (0.10 - 2.0 mm), and 09'0 organic material. The "LS" factor <br />Revised 7-81 4.6-63 <br />