1988-04-18_PERMIT FILE - C1981008A (3)

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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 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. Usually the worst condition is the maximum disturbed area at the end of the mining activity and just propr to the start of land reclamation. 2. Percent Vegetation Cover - The percent of ground surface covered by vegetation in undisturbed areas was estimated from field inspections and aerial photographs. 3. Hydrologic Condition - The hydrologic condition was directly related to the percent vegetation cover as shown in Table 2. 4. Hydrologic Soil Type - Soil survey maps, as contained in Tab 9 of the current Mine Plan, provides the basis of determining the hydrologic soil type and soil erodibility factor, "k" factor (see Table 3) . 3