Laserfiche WebLink
Release No. 55 "Urban Hydrology for Small Watersheds." Prior to discharge, sediment will be <br />controlled by keeping the pit pump lower than the working area and using gravel filter check dams <br />around the pump. <br />Mining Area # 1 Stormwater Calculations <br />The area of 13.7 acres shown below includes some area outside the pit that will drain to it. <br />Runoff Curve Numbers(CN) for Watershed Area <br />Description Hydrologic Soil Group Area (acres) CN <br />Mined land disturbed Group C Soils 6.6 87 <br />Pasture or Rangeland: Good (50-75% ground Group C Soils 7.1 74 <br />cover; not heavily grazed) <br />Total Area and Average CN 13.7 81 <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) 13.7 <br />Watershed Average CN 81 <br />Calculated Runoff Volume (acre-ft) 1.411 <br />Actual Runoff (acre-ft) 1.411 <br />Even at the start of the excavation, the pit will be large enough to contain this volume. <br />Scott Pit March 10 D-3