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I I I I I I I I I I I I I I I I I I I III-1 SECT ION I I I HYDROLOGICAL SUMMARY "Di scharge records for the Clear Creek stream gauges at Golden and Derby were analyzed using methods presented in Bulletin No. 17 published by the Water Resources Council ( Reference 29). The results of these analyses were used to cal ibrate the MITCAT (Reference 17) and SWMM (Reference 28) runoff model s. MITCAT was used to model the 400 square mile mountainous area upstream from Golden, and SWMM was used to model the lower 175 square mile plains basin. Rainfall values used by the U.S.A.C.E. in the models were obtained from the Precipitation Frequency Atlas of the Western United States, VolllTle III, Colorado, publ ished by the National Oceanic and Atmospheric Administration in 1973 (Reference 27)."1 GENERAL COMMfNTS The hydrological investigations for Clear Creek involved the determination of the magnitude, frequency and volllTle of storm water runoff. The frequency of a flood is indicated by its return period, and functionally, is inversely proportional to the return period. The return period is defined as the average interval of time within which a given event will be equalled or ex- ceeded once. Thus, a flood having a return period of 5 years has a 20 per- cent probabil ity of being equalled or exceeded in any given years. The mag- nitude of a flood can be judged by its maximllTl rate of flow, or peak dis- charge. The extent of flooding and the sizing and character of major drain- age works and other urban storm drainage facilities are determined by the magnitude of runoff. The 10, 50 and 100-year flood events were used in thi s master pl an. Thi s provides the dec i si on-makers wi th a range of flooding conditions thereby optimizing the evaluation and selection of a favorable solution to the flooding problems along Clear Creek. Table V-I lists the peak flows for the 10, 50 and 100-year events at various 1 ocati ons along Cl ear Creek. HYDROLOGY FOR CLEAR CREEK The hydrol og ical information and correspond ing flood di scharges needed to determine the extent of flooding on Clear Creek were provided by the CJnaha District of the U.S. Army Corps of Engineers (U.S.A.C.E.). Peak discharge frequency relationships were determined at many locations for flood events of 10, 50, 100, and 500-year return periods. These discharges are based upon 1977 developllent and future developllent conditions projected for the Clear Creek basin. T ABLE II I-I PEAK RUNOFF FLOWS FUTURE DEVELOPMENT CONDITIONS (all val ues in cubic feet per second) Location lO-year Peak Flows 50- year 100-year Youngfield Street to Coors Diversion Dam 3,500 8,500 13,500 Coors Diversion Dam to Golden City Limit Upstream of U. S. 6 3,300 8,000 12,500 1Gingery Associates, Inc., "Flood Hazard Area Delineation, Clear Creek, Ad<llls County and Jefferson County," November 1979.