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In some Snstances, mining~will cause the drainage area to change during the life of the <br />sedimentation structure. When the pit moves Into [he watershed, runoff is Sntercepted by <br />the pit. In these cases, a conservative procedure was used; the structure xas analyzed <br />for the largest anticipated disturbed drainage area that will contribute runoff to the <br />structure. It was estimated thac this condition wl ll occur if all of the proposed mining <br />area is disturbed and the area ie in different phases of mining and reclamation. In <br />addition, [he structure was conservatively designed [o Snclude adequate capacity for <br />sediment storage and to treat the runoff from a 10-year, 24-hour precipitarion event. <br />FS nail y, the design conservatively assumed the pond Ss 100 percent full to the emergency <br />spillway during [he 25-year, 24-hour and 100-year, 24-hour precipitation events and <br />provides for maintenance of adequate freeboard ro prevent overtopping of the embankment. <br />SCS Curve Number and "k^ Factot. The curve numbers area based on Snformation presented 1n <br />Attachment 13-1. The "k" factors were determined with the aid of the USDA-ARS's revised <br />USLE software package and are shown Sn Attachment 13-1. The curve numbers and "k" factors <br />were then "weighted" for each subwatershed ae shown on the summary table (Table 13-3A-2). <br />A complete listing is shown Sn Addendum 13-3-E. <br />TABLE 13-3A-2 <br />Weighted CN, k-Factors and CP <br />Pond 010 <br />to <br /> 2 91.00 0.16 0.486 123.49 <br /> 52.00 0.24 0.113 402.? <br /> 3 80.66 0.233 0.365 83.93 <br /> 9 91.00 0.240 0.600 2.81 <br /> 5 76.98 0.226 0.259 24.14 <br /> 6 91.00 0.240 0.600 6.54 <br /> 7 91.00 0.240 0.600 9.63 <br /> 8 59.57 0.226 0.126 79.72 <br /> 9 66.09 0.234 0.155 152.51 <br /> 10 79.21 0.234 0.373 44.19 <br /> 11 90.72 0.240 0.595 14.31 <br /> 12 85.42 0.240 0.454 9.79 <br />• 13-3A-3 Revised 08/30/96 <br /> <br />