1988-04-18_PERMIT FILE - C1981008A (4)

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method to dewater the pond down to the 100 percent sediment storage level in a reasonable amount of time. Hydrology The hydrology of each watershed was analyzed utilizing the SEDIMOT II computer program. In performing this analysis, the characteristics of the mine site 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 Design Hydrology and Sedimentology Summary Structure 007 DESCRIPTION 10 yr-24 hr 25 yr-24 hr 100 yr-24 hr Precipitation Depth 1.60 in. 1.80 in. 2.40 in. Inflow Runoff Volume 16.67 ac-ft. 21.42 ac-ft. 37.36 ac-ft. Pond Volume at Peak Stage 26.23 ac-ft. 40.12 ac-ft. 48.33 ac-ft. Peak Stage 11.22 ft. 14.40 ft. 15.97 ft. Peak Inflow Rate 81.53 cfs 108.72 cfs 201.19 cfs Peak Discharge Rate 2.62 cfs 28.35 cfs 43.54 cfs Basin Trap Efficiency 100.00 % - - Detention of Flow (Sediment) 49.85 hr - - Detention of Hydrograph Centers 15.72 hr - Input includes 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, and spillway information, etc. The drainage area for this sediment pond was measured on a 1" = 400' and 1" = 2000' topographic map (see Exhibits 13-5 and 13-6). In some instances, the mining will cause the drainage area to change during the life of the sediment structure. When the pit moves into the watershed, runoff is intercepted by the pit and diverted away from the structure. In these cases, a conservative procedure was used; the structure was analyzed for the largest anticipated drainage area that will contribute runoff to the structure. This condition usually occurs at the start of mining 2