1988-04-18_PERMIT FILE - C1981008A (4)

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and again during the reclamation period. The pond was conservatively designed with the assumption the pond was 100 percent full to the principal spillway during the 10-year, 24-hour precipitation event and no discharge was allowed through the emergency spillway. The principal spillway was sized to allow adequate detention time to allow 100 percent basin trap efficiency. In addition, the design conservatively assumed the pond was 100 percent full to the emergency spillway during the 25-year and 100-year, 24-hour precipitation events and adequate freeboard will be maintained to prevent overtopping of the embankment. SCS Curve Numbers and "k" Factor. SCS curve numbers were estimated for the watershed 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 subclassifications were made in each category as shown in Table 2. The cover type and 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 conditions are assumed for the hydrologic analysis. 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 permit, provides the basis of determining the hydrologic soil type and the soil erodibility factor, "k" factor (see Tables 2 and 3). 3