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<br />r- <br />i <br /> <br />Discharge at that point represents only 56 percent (36.6 sq. miles) of the <br />modeled watershed, and historic records indicate that larger flood magnitudes <br />usually occur from Fourmile Creek. Therefore, these flood estimat,es should <br />not be used as a representative measure of the flood magnitude at the mouth of <br />the canyon. <br /> <br />Flood frequency analyses have been performed in several other studies. <br />The final hydrology study by the U.S. Army Corps of Engineers that is accepted <br />by both UDFCD and the City of Boulder ill referenced in Boulder CreE'JI._rlood <br />Hazard Delineation, January 1983, by Muller Engineering Company, Inc. The <br />flood frequency magnitudes that are accepted tor Boulder Creek at the canyon <br /> <br />mouth are: <br />5()(I-year = 21,200 cts <br />10(t-year = 11,600 cts <br />5C-year = 8,000 cts <br />Ie-year = 2,000 cfs <br /> <br />COllPARISON OF 1I0DEL TO FUXlD FREQUENCY R~;SULTS <br /> <br />The model was run for both the 10-year and the 100-year rainfall and for <br />antecedent soil moisture at field capacity (DTHETA =0.25). That soil moisture <br />would represent the condition to be expected after the watershed had been <br />thoroughly wetted and then allowed to dry somewhat due to gravity drainage and <br />evapotranspiration. This 'wuld be representative of the water!lhed in early <br />summer. The flood peak estlmates using the model under those conditions are: <br />100-year = 9,735 cfs <br />lO-year = 1,570 cfs <br />These compare favorably with the accepted values that are listed above. <br /> <br />The model was run for the 100-year rainfall under four conditions of <br />antecedent soil moisture. These four conditions and resulting peak discharges <br />are summarized below: <br /> <br />5 <br />