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-3- <br />Table 1 including computational procedure, are discussed in the Utah State <br />• (1976) handbook. Rand K were determined from maps in the handbook. <br />As seen in Table 1, the potential erosion from Orchard Valley Mine is <br />approxim~[ely 300 tons per acre disturbed -approximately 0.14 acre-foot/acre <br />(assuming a unit weight of 100 lb/ft3), well below the 0.2 acre-foot in the <br />Regulations. It should be emphasized that this quantity refers to on-site <br />erosion. Assuming a sediment delivery ratio of 0.42 (Table 7, Appendix 1), <br />the estimated unit sediment yield from disturbed areas on the 85-acre <br />watershed would be 0.06 acre-foot/acre disturbed-just 30 ercent of the value <br />in the regulations, assuming no erosion control upstream from the settling <br />~_ - - - <br />It is estimated that CWI's current reclamation program has reduced onsite <br />erosion by 80 percent (VM = 0.2), based on published data on the effectiveness <br />of these practices (Utah State, 1976). Accordingly, potential on-site erosion <br />is 60.1 tons/acre (7.03 A-F/acre). Because extensive use of buffer areas is <br />proposed (Plate I}, this value can be reduced further. Leaf (1978, Appendix <br />VI) has presented calculations developed by Packer (1967) which show that buffer <br />• strips 256 feet wide will control virtually all sediment from culvert outfalls. <br />Thus, it appears that an assumption of 50 percent average effectiveness of <br />buffer areas is reasonable. <br />The analysis showing the conbined effectiveness of reclamation and buffer <br />areas proposed for Orchard Valley Mine suggests that approximately 0.015 <br />A-F/acre disturbed is a conservative estimate for dead storage. Considering <br />that approximately 32 acres are disturbed (Table 2), 0.48 acre-foot would be <br />ample dead storage capacity. The 2.5 acre-feet of combined capacity oricinally <br />proposed for 4 or the ponds (Plate I) is reasonable even if stormflow generated <br />from 'disturbed areas upstream are routed through the ponds. <br />The temporary pond to be constructed as part of the starter embankment <br />will provide an additional margin of safety in water quality control. <br />CONCLUSIONS <br />~4R <br />Alternative 5 (Table 2} would route all stormflows generated from disturbed <br />areas at Orchard Valley !•:ine through sediment t[~+• `~~~ S~cient <br />• capacity would be available to store runoff frbm'6$th d sT ~Pb~~nd u?sdisturbed <br />0~lY <br />