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• Input includes precipitation amount (inches), watershed area (acres), <br />time of concentration (hours), runoff curve number (CN), representative <br />particle-size distributions of soils in the area, sedimentology <br />information, and spillway information. The drainage area for this <br />sediment pond was measured on a I" = 400' topographic map (see Exhibit <br />7-7). <br />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 is intercepted by the pit and diverted away from the <br />structure. In these cases, a conservative procedure was used; the <br />structure was analyzed for the largest anticipated drainage area that will <br />contribute runoff to the structure. This condition usually occurs at the <br />start of mining and again during the reclamation period. <br />SCS Curve Numbers and "k" Factor <br />SCS curve numbers were estimated for the watershed based on the cover <br />• type, percent vegetation cover, hydrologic conditions, and hydrologic soil <br />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 proposed <br />mining plan were used to identify areas that will be disturbed <br />by mining. Three general categories of cover type were used: <br />undisturbed, disturbed, and reclaimed. Further <br />sub-c]assifications were made in each category as shown in Table <br />7-6e-2. <br />The cover type for some structures will vary throughout the life <br />of the structure as mining and subsequent reclamation occurs. <br />For these cases, the anticipated worst conditions are assumed <br />for the hydrologic analysis. <br />2. Percent Vegetation Cover - The percent of ground surface covered <br />by vegetation in undisturbed areas was estimated from field <br />inspections and aerial photographs. <br />• 3. Hydrologic Condition - The hydrologic condition was directly <br />related to the percent vegetation cover as shown in Table <br />7-6e-2. <br />7-6e-3 <br />