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Mining, Area #2 Stormwater Calculations <br />The area of 49.5 acres is the maximum that could drain to the pit, assuming no berms, ditches or <br />stockpiles. <br />Runoff Curve Numbers(CN) for Watershed Area <br />Description Hydrologic Soil Group Area (acres) CN <br />Mined land disturbed Group C Soils 13.9 87 <br />Pasture or Rangeland: Good (50-75% ground. Group C Soils 35.6 74 <br />cover; not heavily grazed) <br />Total Area and Average CN 49.5 78 <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 runoff). Silt loam or loam. Infiltration rate 0.15 to 0.3 inch/hr when <br />Group C Soils: Low infiltration (moderate to high runoff). Sandy clay loam. Infiltration rate 0.05 to 0.15 inch/hr when <br />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/hr 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 than <br />sufficient storage volume for all mining areas. <br />Stormwater/Sediment Pond Design <br />Design Event 100-yr 24-hr Event <br />Design Precipitation (inches) 2.60 <br />Watershed Area (acre) 49.5 <br />Watershed Average CN 78 <br />Calculated Runoff Volume (acre-ft) 3.4481 <br />Actual Runoff (acre-ft) 3.4481 <br />Even at the start of the excavation, the pit will be large enough to contain this volume. <br />Scott Pit March 10 D-4