2004-05-19_REVISION - M1979205 (2)

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Runoff Curve Numbers(CN) for Watershed Area Description Hydrologic Soil Group Area (acres) CN Pasture or Rangeland: Good (50-75% ground Group C Soils 6.00 74 cover; not heavily grazed) Exposed Gravel Group A Soils b.00 44 Total Area and Average CN 12.00 59.00 Hydrologic Soil Group Group A Soils: High infiltration (low runoff. Sand, loamy sand, or sandy loam. Infiltmtion m[e> 0.3 inchPor when wet. Group B Soils: Moderate infiltration (moderate mnof~. Silt loam or loam. 1nlifUation rate 0.15 to 0.3 inchPor wfien wet. Group C Soils: Low infiltration (moderate to high mnof0. Sandy clay loam. Infiltration rate 0.05 to 0. ]5 inch/hr when we Group D Soils: Very low infiltration (high runoff). Clay loam, silty clay loam, sandy clay, silty clay, or clay. Infiltration rate 0 to 0.05 inch/hr when we[. The storage volume of the pond is calculated as the extraction azea above the watez table. The water table lies approximately 4 feet below the surrounding surface. This allows for a more than sufficient storage volume for all mining areas. Stormwater/Sediment Pond Design Event 100-yr 24-hr Event Design Precipitation (inches) 2.60 Watershed Area (acre) ]2.00 Watershed Average CN 59.00 Calculated S 6.95 Calculated Runoff Volume (Q) 0.18 Calculated Runoff Volume (acre-ft) 0.18 Ponding Percentage 5.0% Actual Runoff (acted[) 0.17 Since very little area lies outside the pit and with the ability to store 4 feet of inflow per acre, it is easily seen that the ponds will have sufficient storage. All water trapped from the storm event will be allowed to evaporate or seep out of the pond within a 24-hour period. Topsoil and Overburden Stripping Topsoil will be stripped and stockpiled prior to the commencement of mining in a particulaz mining azea. After stripping the topsoil stockpile will be seeded promptly in order to minimize soil loss. There will be an abundance of topsoil due to the reduction in surface area caused by the formation of a lake within the permit area. This excess topsoil will be used for extra growth media as well as slope backfill material during reclamation. 4126!04 Page D-3