2009-01-07_PERMIT FILE - C1981022 (5)

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HYDROLOGIC DOCUMENTATION SM11LL AREA EXEMPTION BEAR CREEK FAN SI I'E -PAD The following calculations demonstrate the amount of silt fence or straw bales surface area required to adequately handle the runoff from the Bear Creek Fan Site. Runoff volume and peak discharge calculations were performed using the SEDCAD 4 computer model consistent with the methodology previously discussed in this Exhibit. The sediment control structure was designed using the pond routine in SEDCAD 4 to handle the peak discharge from a 10-year, 24-hour storm event. Length of the silt fence and straw bales was determined from the topography of the site and Erom the necessary sediment storage to comply with a settable solids limit of 0.5 ml/l. Input discharge rates for the Sedcad model were calculated from a rate of 5.6 gpm/ft2 for straw bales and a rate of 10.0 gpm/ft- for a silt fence. Printouts of the Sedcad designs for a silt fence and a straw bale boundary are provided below. 1. Hydrology Inputs All hydrology inputs are shown on the SEDCAD 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, 1986) and consistent with previously approved hydrology reports. Based on the soil type and vegetation cover, curve numbers were determined using Table 2-2a and 202d, USDA-SCS TR-55, for the following conditions: Disturbed: Soil Type: C-D Ground Cover: Bare Cover Type: Gravel Road CN: 90 Area: 0.8 acres Undisturbed: Soil Type: C-D Ground Cover: Fair to Good Cover Type: Oak-Aspen CN: 57 Area: 53.0 acres 2. H,, d~gy Results Peak Discharge -Silt Fence or Straw Bales 1.50 cfs 10-year, 24-hour event 3. Hydraulic Stnicture Inputs Assumed Length of Silt Fence or Straw Bales 80.0 ft Assumed Height of Silt Fence or Straw Bales 1.5 ft Assumed Slope behind Silt Fence or Straw Bales 8.0