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<br />~ <br />[ <br /> <br />2. Mud Gulch's Hydrology: TR-20 computer program model was used in <br />calculating frequency flows for the Mud Gulch basin. <br /> <br />* Precipitation P - The historical station precipitation data was <br />used as input to the TR-20 model. No areal adjustments were made because <br />of the small drainage areas involved. The Natural Resources Conservation <br />Service (NRCS) 24-hour Type IIA rainfall distribution was selected for <br />use in the model to generate frequency peak discharges. <br /> <br />* Runoff Curve Numbers CN's - Hydrologic runoff curve numbers (CN's) <br />were developed from the NRCS soil map, land use information, and field <br />observations. CN values for the TR-20 watersheds reflect weighted <br />average CN's for a variety of land uses and soils within each watershed. <br /> <br />* Time of Concentration Tc - Time of concentration values were <br />computed for the Mud Gulch sub-basins from an empirical equation in the <br />Natural Resources Conservation Service's Engineering Field Manual - <br />Chapter Two, intended for small rural watersheds. <br /> <br /> To = (Lo.8x [(lOOO/CN)-9]o.7) I (1140 x yo.s) <br />Where, <br /> Tc = Time of concentration (in hours) <br /> L = Flow length (in feet) <br /> CN = Run off curve number <br />and y = 100CI/A <br />Where, <br /> y = Average watershed slope (in percent) <br /> C = Total contour length (in feet) <br /> I = Contour interval (in feet) <br /> A Drainage area (in sq. feet) <br /> <br />* Base flow - A Base flow value of 25 cfs was obtained from the local <br />irrigation annual record. This base flow accounts for the inflow from <br />the oil Creek Ditch which enters Mud Gulch at a point 6,700 feet upstream <br />from the Arkansas River. <br /> <br />The TR-20 peak discharge values for Mud Gulch and the Four-Mile Creek <br />frequency flows which were developed by regional analysis, were used in <br />conjunction with U.S. Army Corps of Engineers' HEC-2 model in hydraulic <br />analyses to develop water surface profiles and flood plain maps. <br /> <br />C. HYDRAULICS <br />Hydraulic analyses conducted in this study were done using the U.S. Army <br />Corps of Engineers computer model HEC-2 Water Surface Profiles. All <br />flows were set up as subcritical flows in compliance with FEMA's 1985 <br />guidelines and specifications. <br /> <br />Discharge values were discussed in the hydrology section of this report. <br />Most of the cross-section data were hand-picked from topographic maps <br />with a scale of 1 inch = 200 ft with 2-foot contour intervals. <br />Dimensions of bridges and hydraulic roughness coefficients (n values) <br />were determined from field investigations. Tables 2a and 2b show the <br />estimated n values for HEC-2 cross-sections of Mud Gulch and Four-Mile <br />Creek. <br /> <br />9 <br />