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<br /> <br />" <br /> <br />C:-. <br />-". <br /> <br />Co,,, <br />--,.'~: . <br />',"';~ <br />.,' <br /> <br />the North Boomerang Extension, are siphoned und~r Sheep Draw. These <br /> <br />crossings have no jffect on discharges along Sheep Draw. The canals are <br /> <br />capable of intercepting some overland, runoff in the basin, but it is <br /> <br />difficult to predict where this will occur. Equally difficult to predict <br /> <br />are points of failure along the canals. There is also a good possibility <br /> <br />~ <br /> <br />that the canals would be full at the beginning of a major flood event. <br /> <br /> <br />For these reasons, the canals were not included in the detailed hydrologic <br /> <br />analysis of Sheep Draw. All pertinent highway and county road structures <br /> <br />were evaluated for their flood storage effects. Reservoir routings utilizing <br /> <br />storage-discharge curves were made for 13 of these structures. <br /> <br />~I <br />~ <br />...'C",,. <br />~~ <br />- <br /> <br />4. MODELING STUDIES. <br /> <br />a.General. The EPA Storm Water Management Model (SWMM) was used to <br /> <br />model rainfall-runoff characteristics of the Sheep Draw basin. Storms varying <br /> <br />in frequency from 2- to 500-years were centered over the basin to obtain the <br /> <br />;;'.,:';"-\";f~ <br />-.,..,.--,.. <br /> <br />discharges presented below. A printout of the Sheep Draw model is shown on <br />, <br /> <br />attachment 2. <br /> <br />~ <br />>-..-,.;.< <br /> <br />b. Rainfall. Rainfall values for the 2-, 5-, 10-, 50-, and 100-year <br /> <br />I-hour events were obtained from the Precipitation-Frequency Atlas 6fthe Western <br /> <br />;:',":"?,"" <br /> <br />United States, Atlas 2, Volume III, Colorado, published by the National Oceanic <br /> <br />and Atmospheric Administration in 1973. The 500-year value was estimated by <br /> <br />extrapolating the selected I-hour values on semi-logarithmic paper. Adjust- <br /> <br />" <br /> <br />ments for depth-area were based on the depth-area relationship presented in <br /> <br />'~.-:',,:"'l:'! <br />:~:~?:;:{ <br />--~-~.. <br />,,-:",';-A;~ <br /> <br />the NOAA publication. Adjustments for an average length of record of 40 years <br /> <br />were made on logarithmic probability paper using Beard's table of expected <br /> <br />probability adjustments. The I-hour rainfall distribution developed from NOAA <br /> <br />criteria is given in Table I below. <br /> <br />.;'.- <br /><,; <br /> <br />'It-"";'.- <br />7i~'; <br />~};y;' <br />~...," <br />~. <br />.--"""> <br /> <br />2 <br />