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• The run-off calculations presented on the following Table Nos. <br />2.05.3(4)(d)-1 thru 6 present the calculated run-off, peak flow and <br />ditch sizing for the mine site. These calculations are applicable <br />to the reclaimed configuration of the mine site. The storm events <br />utilized in table 2.05.3(4)(d)-4 are conservative. A 2.2" and 2.6" <br />storm was utilized for the 10-year and 25-year event respectively. <br />U <br />Based on Figure S-7, from the manual Procedures for Determining <br />Peak Flows in Colorado, SCS, March 1984, a 2.0" and 2.3" storm may <br />be more appropriate for the 10-year and 25-year event respectively. <br />Table Nos. 2.05.3(4)(d)-1 thru 6 should not require revision since <br />conservative storm events were utilized in the calculations. <br />On page 128a, the 100-year peak flow of 46.7 cfs for the major <br />drainage channel is calculated based upon: 1) a 75 acre watershed; <br />2) average slope of 208; 3) a curve number of 74 and; 4) a 100-year <br />storm event of 2.5". <br />The 75 acre watershed for the major drainage channel includes the <br />reclaimed mine site and the drainage area immediately north of the <br />reclaimed mine site. The basis for the peak flow calculation is <br />for the permanent drainage channel to handle all run-off from the <br />reclaimed mine site including the watershed above the mine site. <br />Therefore, if the flow from the major drainage channel is routed to <br />and through the sediment pond, the peak flow based on the 100-year <br />event would be 46.7 cfs. <br />The capacity of the sediment pond emergency spillway is presented <br />in Table 2.05.3(4)(d)-6. The spillway is capable of handling 49.94 <br />cfs. Since the sediment pond will not be reclaimed as explained on <br />page 129, the sediment pond emergency spillway must be able to <br />handle the peak flow generated by the 25 year 24 hour event which <br />is calculated as follows: <br />The curve number selected for the reclaimed mine site (74) is <br />justified as follows: <br />Table 2-2d - Run-off curve numbers for arid and semiarid <br />rangelands. The cover type was chosen as sagebrush with grass <br />understory in fair condition, (308 to 708), soil type C, <br />yields a curve number of 74. The 1992 vegetation survey <br />determined that the ground cover of perennial vegetation was <br />34.98 and total vegetative cover was 47.88. <br />• 106 (Rev. 8-21-96) <br />