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In some instances, the mining will cause the drainage area to change <br /> during the life of the sediment structure. When the pit moves into the <br /> watershed, runoff will be intercepted by the pit and diverted away from <br /> the structure. In these cases, a conservative procedure was used; the <br /> structure was analyzed for the largest anticipated drainage area that <br /> will contribute runoff to the structure. This condition usually occurs <br /> at the start of mining and again during the reclamation period. <br /> SCS Curve Numbers <br /> SCS curve numbers were estimated for each subwatershed based on the <br /> cover type, percent vegetation cover, hydrologic conditions, and <br /> hydrologic soil type. Several sources were used to obtain this data: <br /> 1. Cover Type - Aerial photographs of the mine site were used to <br /> identify the existing cover type. Maps delineating the <br /> proposed mining plan were used to identify areas that will be <br /> disturbed by mining. Three general categories of cover type <br /> were used: undisturbed, disturbed, and reclaimed. Further <br /> sub-classifications were made in each category as shown in <br /> Table 2. <br /> The cover type (and the tributary drainage area) for some <br /> structures will vary throughout the life of the structure as <br /> mining and subsequent reclamation occurs. For these cases, <br /> the anticipated worst condition is assumed for the hydrologic <br /> analysis. Usually the worst condition is the maximum <br /> disturbed area at the end of the mining activity and just <br /> propr to the start of land reclamation. <br /> 2. Percent Vegetation Cover - The percent of ground surface <br /> covered by vegetation in undisturbed areas was estimated from <br /> field inspections and aerial photographs. <br /> 3. Hydrologic Condition - The hydrologic condition was directly <br /> related to the percent vegetation cover as shown in Table 2. <br /> 4. Hydrologic Soil Type - Soil survey maps, as contained in Tab 9 <br /> of the current Mine Plan, provides the basis of determining <br /> the hydrologic soil type and soil erodibility factor, "k" <br /> factor (see Table 3) . <br /> 3 <br />