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1. Provide storage for the runoff from the 10-year, 24-hour storm event without experiencing any discharge through the emergency spillway (5.74 ac-f t.) 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 an embankment with sufficient freeboard to contain the lOC-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 ZO-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. Hvdrology The hydrology of the watershed was analyzed utilizing the SEGIMOT 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 _'= _s ai DESCRIPTION lOyr-24hr 25yr-24hr 100.yr-24hr Precipitation Depth Inflow Runoff Volume Pond Volume at Peak Stage Peak Stage Peak Inflow Rate Peak Discharge Rate Basin Trap Efficiency Detention of Flow (sediment) Detention of Hydrograph Centers 1,6 in. 2.09 in. 2.4 in. 5.74 ac-ft 11.05 ac-ft 14.S3 ac-ft 12.73 ac-ft 19.12 ac-ft 21.08 ac-ft 11.84 ft 14.66 ft 15.43 ft 39.79 cfs 87.50 cfs 123.27 cfs 1.42 cfs 18.76 cfs 23.06 cfs 100.00 ~ --- -- - 49,85 hr --- -- - 14.59 hr --- -- - 7-6f.1-4