1988-04-18_PERMIT FILE - C1981008A (2)

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2. Provide an emergency spillway capable of passing runoff from the 25-year, 24-hour storm event (0.33 ac-ft.). 3. Provide adequate storage for the sediment generated from the 10-year, 24-hour (MUSLE) storm event (2.9, 10-year, 24-hour MUSLE storm events or 0.07 ac-ft. ). 4. 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 to the 100 percent sediment storage level in a reasonable amount of time. HYDROLOGY The hydrology of the watershed was analyzed by utilizing the SEDIMOT II computer modeling program. In performing this analysis, the characteristics of the minesite and reclamation procedures along with onsite sediment control procedures were considered. Addendums 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, respectively. Table 1 is a summary of these results. TABLE 1 As-Built Hydrology and Sedimentology Summary Structure 002 DESCRIPTION 10yr-24hr 25yr-24hr 100yr-24hr Precipitation Depth 1.60-in. 1.80-in. 2.40-in. Inflow Runoff Volume 0.257-ac-ft 0.334-ac-ft 0.593-ac-ft Pond Volume at Peak Stage 0.276-ac-ft 0.44-ac-ft 0.525-ac-ft Peak Stage 5.2 -ft 6.55-ft 7.15-ft Peak Inflow Rate 3.45-cfs 4.45-cfs 7.71-cfs Peak Discharge Rate 0.010-cfs 1.65-cfs 3.19-cfs Basin Trap Efficiency 99.1% --- --- Detention of Flow (Sediment) 37.87-hrs --- Detention of Hydrograph Centers 16.52-hrs --- --- Input consists of precipitation amount (inches) , watershed area (acres) , time of concentration (hours) , runoff curve number (CN) , representative particle-size distributions of soils in the area, sedimentology information, spillway information, etc. The drainage area for Sediment Pond 002 was determined from a 1" = 400' topographic map (see Exhibit 13-4) . 2