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• Getty Mining Company March 26, 1985 Page 4 sediment storage requirement of 2.15 acre-feet. Because this is less than the 2.24 acre-feet available to store the 3-year sediment yield and runoff from the 10-year, 24-hour precipitation, we conclude that Sedimentation Pond F is adequate to control runoff and sediment Erom the 60-acre contributing drainage area which includes the proposed new facilities. Appendix 2 contains the computer printout of the 10-year, 24-hour storm inflow hydrograph to Sedimentation Pond F for the 60-acre watershed. This inflow hydrograph was routed through Sedimentation Pond F, assuming that the 2-inch diameter dewatering hole was located at elevation 98.0 feet. Results of this routing are shown in the computer printout in Appendix 3 and indicate that the maximum water surface elevation during the 10-year, 24-hour precipitation event would be at about elevation 99.2 feet, which is below elevation 99.5 feet (the crest of the 18-inch diameter decant riser). However, the 1.2 acre-Feet of sediment that could accumulate over a 3-year period would accumulate to an elevation of about 98.3 feet at the end of 3-years unless this sediment was removed. Therefore, we routed the inflow hydrograph Erom the 10-year, 24-hour precipitation through Sedimentation Pond F with the 2-inch diameter dewatering hole at elevation 98.3 feet. Computer printout of this routing also is attached in Appendix 3. Results of this analyses indicated that the maximum water-surface elevation was about 99.4 feet, again below elevation 99.5 feet. As part of the routing studies we estimated the time required to withdraw 90 percent of the inflow volume from the 10-year, 24-hour precipitation event. For both the dewatering hole elevation options, 90 percent of the inflow volume (approximately 0.86 acre-feet) is evacuated from Sedimentation Pond F in about 11 days. The centroid of the outflow hydrograph occurs approximately 76 hours after inflow begins. This indicates a theoretical detention time of about 75 hours. This detention time should be sufficient to reduce sediment to levels acceptable Eor discharge. We recommend that the sediment accumulations in Sedimentation Pond F be monitored so that the 2-inch diameter dewatering hole is not covered by sediment. We also analyzed the impact of the proposed facilities and 60 acres of contributing drainage area on the existing emergency • spillway. For this analysis, the spillway crest was assumed