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<br />1II-7 <br /> <br />I <br /> <br />peak flows downstream of the lake are more dependent upon the local peak runoff <br />rates than the outflow of the lake, and hence, independent of the storm pattern <br />used for Hidden Lake. <br />The infiltration rates used for this analysis were based on the criteria <br />of References 5 and 11. The Drainage Criteria Manual (Reference 5) suggests a <br />range of infiltration rates with a maximum infiltration of one-half inches <br />per hour for the 10 and 100 year recurrence interval storms. The Soil Conser- <br />vation Service (S.C.S.) (Reference 11), recommends that an average five-day <br />antecedent moisture condition (AMC) be used to determine the design runoff. <br />This average condition is referred to as AMC-II and is defined as the average <br />of the soil moisture conditions which have preceeded the occurrence of the <br />maximum annual flood (Reference 10). The soils in the study area are the <br />Nunn-Fort Collins and P1atner-U1m-Renohi11 associations (Reference 8) which <br />are the loam and.10amy clay type soils. The infiltration rates of these soil <br />groups vary from 0.2 to 2.0 in/hr (Reference 8) depending upon soil moisture <br />conditions and soil cover complexes. Since the antecedent moisture condition <br />discussed above implies reasonably saturated soil conditions, then the design <br />infiltration rates should be at the lower end of the infiltration spectrum <br />for the type of soils encountered. Therefore, a maximum value of one-half <br />inch per hour infiltration rate was used for this analysis. <br />The rainfall-runoff model used for this analysis is the Colorado Urban <br />Hydrograph Procedure (CUHP) and the parameters were obtained from Reference 5. <br />A computer program, based on the CUHP method, was used to generate the <br />hydrographs for the twenty-three sub-basins. The input to the program is <br />as follows: <br />