1988-04-20_PERMIT FILE - C1981008A

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TABLE 1 Design Hydrology and Sedimentology Summary Structure 005 DESCRIPTION 10yr-24hr 25yr-24hr 100yr-24hr Precipitation Depth 1.60-in. 1.80-in. 2.40-in. Inflow Runoff Volume 1.13-ac-ft 1.45-ac-ft 2.53-ac-ft Pond Volume at Peak Stage 1.50-ac-ft 1.82-ac-ft 2.94-ac-ft Peak Stage 9.04 -ft 10.06-ft 12.54-ft Peak Inflow Rate 12.30-cfs 16.00-cfs 28.49-cfs Peak Discharge Rate 0.06-cfs 0.07-cfs 10.41-cfs Basin Trap Efficiency 100% --- --- Detention of Flow (Sediment) 49.85-hrs --- --- Detention of Hydrograph Centers 16.57-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 005 was determined from a 1" = 400' topographic map (see Exhibit 13-4). 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 will be 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 and again during the reclamation period. SCS Curve Numbers (CN) and "k" Factor SCS curve numbers were estimated for each subwatershed based on the cover type, percent vegetation cover, hydrologic conditions, and hydrologic soil type. Several sources were used to obtain this data: 1. Cover Type - Aerial photographs of the mine site were used to identify the existing cover type. Maps delineating the proposed mining plan were used to identify areas that will be disturbed by mining. Three general categories of cover type were used: undisturbed, disturbed, and reclaimed. Further 3