2008-05-05_PERMIT FILE - C1980001A

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provided the greatest sediment yield for a 10-year, 24-hour storm event. Therefore, it was assumed that Year 5 would also provide the greatest annual sediment yield. The "R", "K" and "CP" factors were determined using the publication, "Preliminary Guidance for Estimating Erosion of Areas Disturbed by Surface Mining Activities in the Interior Western United States", July 1977. Two of the factors ("A" and "K") were set at a constant value for the subwatersheds. The ^A" for the Edna Mine area was determined to be 30. In order to enable the MUSLE subroutine to calculate the annual sediment yield, a storm of sufficient magnitude (2.3185 inches of precipitation in 29 hours) was used which resulted in a "R" factor of 30.03. The "K" factor of 0.36 was used as this value reflects the highest "K" factor of the soil types and the final soil composition is unknown. The "LS" factor was calculated by the computer program and varied with the slope length and percent of slope throughout the subwatersheds while the ^CP" factor varied with the type of disturbance occurring within the subwatersheds. Table 4.6-1 shows the subwatershed time of concentration and hydrograph routing parameters for each subwatershed. Table 9.6-2 shows the parameters for routing the subwatershed hydrographs between junctions and/or structures. Table 4.6-3 gives a description of the subwatersheds with corresponding "CP^ factors. Exhibit 4.6-1 indicates the watershed dissection used for the sediment yield calculations and Appendix 4.6-A contains the computer model results. Total sediment yield as calculated using the computer model was 3403.7 tons for the entire watershed. The yield in terms of acre-feet (assume 1960.41 tons/ac -ft) was 1.7. The three year sediment storage of 5.1 ac-ft reflects the yield from the highest year (year 5) times three. .~i 4.6-2 xovember 1,1989