2002-11-26_PERMIT FILE - C1981022 (3)

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!i" • HYDROLOGIC DOCUMENTATION SMALL AREA EXEMPTION D-SEAM SOIL STOCKPILE - SOUTH OUTSLOPE The following calculations demonstrate the amount of sediment fence or straw bales surface area required to adequately handle the runoff from the south side outslope of the soil stockpile located at the D-Seam portal. Runoff volume and peak discharge calculations were performed utilizing the SEDCAD4 computer model consistent with the methodology previously discussed in this Exhibit. The sediment control structure was designed to handle the peak discharge from a 10-year, 24-hour storm event. 1 Hydrology Inputs All hydrology inputs are shown on the SEDCAD 4 computer printouts. The CN numbers used for the SAE watershed were obtained from Procedures for Determining Peak Flows in Colorado and Technical Release No. 55 (USDA-SCS, 1980)and consistent with previously approved hydrology reports. Based on soil type and vegetation cover, curve numbers were determined using Table 2-2a and 2-2 d, USDA-SCS TR-55, for the following conditions: Reclaimed (< 2 years) • Soil Type: C Ground Cover: <50~, Interim Vegetation Cover Type: Herbaceous CN: 87 Area: 0.84 ac 2 Hydrology Results Peak Discharge In 10-year, 24-hour event 0.82 cfs Peak Discharge Out - Silt Fence 10-year, 24-hour event 0.80 cfs Peak Discharge Out - Straw Bales 10-year, 24-hour event 0.81 cfs 3. Sediment Fence Requirements Sediment Fence Permissivity: 10 gal/min/ftZ or 0.022 cfs/ft2 Required Surface Area: 0.80 cfs / 0.022 cfs/ftz = 36 ftZ The silt fence should be installed in accordance with the manufactures recommendations. Design assumes regular inspections and maintenance. 4. Straw Bale Requirements Straw bale Permissivity: 14 gal/min/ftz or 0.031 cfs/f t2 Required Surface Area: 0.81 cfs / 0.031 cfs/ft2 = 26 ft2 Ma-t~9 Noy ?~• ~'