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According to a recently published atlas for the State of Colorado <br />• (Miller et al., 1973), a 10-year, 24-hour storm will generate 2.2 inches <br />of precipitation a1; Paonia. Using this value and a recharge requirement <br />of 2.75 inches in E°quation [1], results in direct runoff of approximately <br />0.46 inch (0.04 acre-foot/acre). This compares with approximately 0.15 <br />inch (0.01 acre-foot/acre) for lJestmoreland Canyon prior to disturbance-- <br />a four-fold increase in storm runoff potential. As seen in Tables 1 and <br />2 (column 3), the recharge requirements assumed for this analysis repre- <br />sent antecedent conditions that might reasonably be expected during the <br />summer ~•rhen a 10-year event is most likely to occur. <br />Sediment Control: Phase I <br />Initially, four dams are proposed for construction downstream from <br />the TLS Bench. The old Cowan Mine Bridge will not be disturbed. The dams <br />• are designed to detain water, acting to dissipate energy and reduce the <br />flow rate, allowing the suspended sediment to settle out. <br />As previously discussed, EPA effluent limitation guidelines for coal <br />mines require that retention structures for sediment control be designed <br />r•:ith a capacity sufficient to contain a 10-year, 24-hour storm event. <br />Each sediment retention pond has been designed with sur"ficient capacity to <br />meet these requirements .• <br />Figure 3 shovrs recommended locations for the check dams. Storm runoff <br />diverted out of tJestmcreland Canyon would not be impounded; however, drainage <br />at these points is generated from portions of the ~•ratershed that are <br />currently being revegetated, or from undisturbed areas. Accordingly, it <br />appears reasonable to exempt these flows from the regulations. All other <br />• drainage from the mine area proper mould be controlled by the four ponds <br />