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<br />. <br /> <br />specific function of these reservoirs, some residual flood control <br /> <br /> <br />effects are provided downstream. Two canals, Boomerang Ditch and the <br /> <br /> <br />North Boomerang Extension, are siphoned under Sheep Draw. These <br /> <br /> <br />crossings have no effect on discharges along Sheep Draw. The canals <br /> <br /> <br />are capable of intercepting some overland runoff in the basin, but it <br /> <br /> <br />is difficult to predict where this will occur. Equally difficult to <br /> <br /> <br />predict are points of failure along the canals. A good possibility <br /> <br /> <br />exists that these canals would be full at the beginning of a major <br /> <br /> <br />flood event. For these reasons, the canals were not included in the <br /> <br /> <br />detailed hydrologic analysis of Sheep Draw. All pertinent highway and <br /> <br /> <br />county road structures were evaluated for their flood storage effects. <br /> <br /> <br />Reservoir routings utilizing storage-discharge curves were made for 13 <br /> <br /> <br />of these structures. <br /> <br />MODELING STUDIES <br /> <br />GENERAL <br /> <br /> <br />The SWMM computer program was used to model rainfall-runoff char- <br /> <br /> <br />acteristics of the Sheep Draw basin. Storms varying in frequency from <br /> <br /> <br />10 to 500 years were centered over the basin to obtain the flood dis- <br /> <br /> <br />charges presented in this report. The SWMM model of the Sheep Draw <br /> <br /> <br />basin contains 107 catchments, 80 streams, 13 road structures, and 4 <br /> <br /> <br />dams. Basin characteristics for the model were taken from USGS 7.5- <br /> <br /> <br />minute quadrangle mapping having a scale of 1:24,000 and a contour <br /> <br />interval of 10 feet. Since the reservoirs are used for irrigation, it <br /> <br /> <br />is not known if they will remain intact as the basin urbanizes. There- <br /> <br /> <br />fore, hydrologic studies were made with and without the dams in place. <br /> <br /> <br />Unless otherwise stated, the road structures were assumed in place with <br /> <br /> <br />culverts open. <br /> <br />RAINFALL <br /> <br /> <br />Rainfall values for the 10-, 50-, and 100-year, I-hour events were <br /> <br />obtained from the Precipitation-Frequency Atlas of the Western United <br /> <br />. <br /> <br />41 <br />