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I' I I I: II Ii I' I; I: I I I I: I: I' I] Ii Ii Ii The Corps hydrograph has a longer tail which is probably attributed to the fact that the Corps included the upper part of the basin in the watershed model. The times to peak do not coincide, but this is probably due to the differences in the rainfall distributions that were used in the two studies. As a result of these favorable comparisons, the model as presented, is judged to be an acceptable representation of the rainfall-runoff process in the watershed. The model input is summarized below: Rainfall - as per UDFCD Drainage Manual Rainfall Losses - Green and Ampt infiltration equation with a surface retention loss Unit hydrograph - Buckhorn Creek S-graph Muskingum Routing - mean velocity of 5 to 10 ft/sec. FLASH FLOOD PREDICTION GRAPH The final model configuration was used to generate flood peak discharges for various rainfalls. This was performed by running the model with the 100-year rainfall mUltiplied by ratios from 50 percent to 200 percent. All four assumptions of antecedent soil moisture were used. The results are shown in Table 5. The maximum rainfalls for 6-hours, 1-hour, and 20-minutes are also shown in Table 5. The results from Table 5 are plotted in ,Figure 4. Peak discharge at the mouth of the canyon can be predicted as a function of the maximum 1-hour rainfall, averaged over a 10 square mile portion of the watershed, by using Figure 4. An estimate of the antecedent soil moisture is required. Season of the year can be used if an estimate of the antecedent soil moisture is not available. v "" <'o..,~ Vi""'" HISTORICAL STORM RECONSTITUTION AND TRANsPOSITION The historical rainfall derived for this study is based on the reconsti- tution and transposition of three significant storm episodes into the Boulder Creek watershed: the Big Thompson flash flood of 31 July 1976; the Cheyenne, Wyoming flash flood of I August 1985; and the Masonville, Colorado flash flood 7