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• r~ U 1. Provide storage for the runoff from the 10-year, 24-hour storm event without experiencing any discharge through the emereency spillway (5.74 ac-ft.) 2. Provide an emergency spillway capable of safely passing runoff from the 25-year, 24-hour storm event (11.05 ac-f t.) • 3. Provide are embankment with sufficient freeboard to contain the 100-year, 24-hour storm event without overtopping the embankment (14.93 ac-ft.) 4. Provide adequate storage for sediment generated from soil erosion. The Universal Soil Loss Equation was used to estimate soil loss on an annual basis (provides approximately 4.5 years of sediment storage). 5. Provide a nonclogging dewatering device and principal spillway that will provide adequate detention time for the 10-year, 24-hour storm event and still allow a method to dewater the pond down to the 100 percent sediment storage level in a reasonable amount of time. Hydrology The hydrology of the watershed was analyzed utilizing the SEDIMOT II computer program. In performing this analysis, the characteristics of the minesite and reclamation procedures along with on-site sediment control procedures were considered. Addendum A, B, and C contain complete listings of the SEDIMOT II inputs and outputs for the 10-year, 25-year, and 100-year design storms. Table 1 is a summary of these results. TABLE 1 As-Built Hydrology and Sedimentology Summary Structure 008 • DESCRIPTION lOvr-24hr Precipitation Depth 1.6 in. Inflow Runoff Volume 5.74 ac-ft Pond Volume at Peak Stage 12.73 ac-ft Peak Stage 11.84 ft Peak Inflow Rate 39.79 cfs Peak Discharge Rate 1.42 cfs Basin Trap Efficiency 100.00 ~ Detention of Flow (sediment) 49.85 hr Detention of Hydrograph Centers 14.59 hr r-24hr 100vr-24hr 2.09 in. 11.05 ac-ft 19.12 ac-ft 14.66 ft 87.50 cfs 18.76 cfs 2.4 in. 14.93 ac-ft 21.08 ac-ft 15.43 ft 123.27 cfs 23.06 cfs 2