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Runoff Curve Numbers(CN) for Watershed Area <br />l;lescriptiou Hydrologic Soil Group Area (acres) CN <br />Pasture or Rangeland: Good (50-75% ground Group C Soils 6.00 74 <br />cover; not heavily grazed) <br />Exposed Gravel <br />Group A Soils 6.00 44 <br />Total Area and Average CN <br />12.00 59.00 <br />Hydrologic Soil Group <br />Group A Soils: High infiltration (low runoff. Sand, loamy sand, or sandy loam. Infiltration rate> 0.3 inch/hr when wet. <br />Group B Soils: Moderate infiltration (moderate runofrJ. Silt loam or loam. Infiltration rate 0.15 to 0.3 inch/hr when wet. <br />Group C Soila: Low infiltration (moderate to high runotl). Sandy clay loam. infiltation rate 0.05 to 0.15 inch/hr when wet <br />Group D Soils: Very low infiltration (high runoff). Clay loam, silty clay loam, sandy clay, silty clay, or clay. Infiltration <br />rate 0 to 0.05 inch/ly when wet. <br />The storage volume of the pond is calculated as the extraction area above the water table. The <br />water table lies approximately 4 feet below the surrounding surface. This allows for a more <br />than sufficient storage volume for all mining areas. <br />Stormwater/Sediment Pond Design <br />Desigo Eveut 100-yr 24-hr Event <br />I~sigD Precipitation (inches) 2.60 <br />Watershed Area (acre) 12.00 <br />Watershed Average CN 59.00 <br />Calculated S 6.95 <br />Calculated Runoff Volume (l~ 0.18 <br />Calculated Ruuoff Volume (acr~ft) 0.18 <br />Poudiug Percentage 5.0% <br />Actual Ruuoff (acre-B) 0.17 <br />Since very little area lies outside the pit and with the ability to store 4 feet of inflow per acre, it <br />is easily seen that the ponds will have sufficient storage. All water trapped from the storm <br />event will be allowed to evaporate or seep out of the pond within a 24-hour period. <br />Topsoil and Overburden Stripping <br />Topsoil will be stripped and stockpiled prior to the commencement of mining in a particular <br />mining area. After stripping the topsoil stockpile will be seeded promptly in order to minimize <br />soil loss. There will be an abundance of topsoil due to the reduction in surface area caused by <br />the formation of a lake within the permit area. This excess topsoil will be used for extra <br />growth media as well as slope backfill material during reclamation. <br />7lL2/04 Page D-3 <br />