2001-12-11_PERMIT FILE - C1982056A (4)

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4. -5- ' ~ It shows the relation between the curve numbers for AMC's I, TI, and II1. As an example, for an AMC-II curve number of 68, the AMC-III curve number is 84, which would produce a runoff volume of 0.61 inches for the 10-year 24-hour precipitation event. This is equivalent to a snowmelt of approx- imately 1.85 inches for [he same period of time. i Peak Flow Rates Peak rates of runoff for each pond tributary area were calculated for the 10, 25, and 100-year 24-hour duration precipitation events in accordance with Procedures for Determining Peak Flows in Colorado, a publication of the Soil Conservation Service dated March, 1977. The flow rates are con- sidered conservative (on the high side). The tributary area to Sedimenta- tion Pond A is shown on Eckman Park Hydrology (EFC Map 14). The tributary areas for the other ponds (B through T) are shown on Energy Mines No. 1 and 2 Hydrology (EFC Map 13). Ditches have been utilized to modify the natural tributary area to some of the ponds in order to take better advantage of more • desirable pond sites. Hydrographs Triangular hydrographs were computed for the ponds by using the peak rate of runoff and adjusting the time base to obtain the previously computed volume. The time base of the hydrograph was divided by 2.67 to obtain the time of the rising limb. As an example, the inflow hydrograph to Pond D is shown on the LRCWE Plate II Sedimentation Ponds, Hydrology and Hydraulics. That same drawing summarizes the tributary area, APSC-III curve number, and the volume and peak rate of runoff for the 10-year, 25-year, and 100-year floods for each pond. OUTLET WORKS Primary Outlet 1Jorks Three different riser type outlets were designed for [he ponds to achieve [he theoretical detention time required by 30CFR816.46 (c). For Sedimentation • Permit Renewal No. 3 Exhibit 8q 1/IS/00 Page 7 of 22