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

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TABLE 1 Design 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 0.315-ac-ft 0.39-ac-ft 0.551-ac-ft Peak Stage 3.44 -ft 4.03-ft 5.09-ft Peak Inflow Rate 3.45-cfs 4.45-cfs 7.71-cfs Peak Discharge Rate 0.010-cfs 0.011-cfs 3.72-cfs Basin Trap Efficiency 100% --- --- Detention of Flow (Sediment) 49.85-hrs --- Detention of Hydrograph Centers 16.69-hrs --- 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 'Y' 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 sub-classifications were made in each category as shown in Table 2. The cover type (and the tributary drainage area) for some structures will vary throughout the life of the structure as mining and subsequent reclamation occurs. For these cases, the anticipated worst condition is assumed for the hydrologic analysis. 3