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I I I I I I I I I I I I I I I I I I I SECTION II I 111-1 HYDROLOGICAL SUMMARY "Discharge records for the Clear Creek streiJll gauges at Golden and Derby ~re analyzed by the U.S.A.C.E. using methods presented in Bulletin No. 17 pub- 1 ished by the Water Resources Council ( Reference 35). The results of these analyses ~re used to cal ibrate the MITCAT (Reference 23) and SWMM (Reference 34) runoff models. MITCAT was used to model the 400 square mile mountainous area upstream from Golden, and SWMM was used to model the lo~r 175 square mile plains basin. Rainfall values used by the U.S.A.C.E. in the models ~re obta ined from the Prec i pitat ion Frequency At 1 as of the We stern Un ited St ates, Volune III, Colorado, publiShed by the National Oceanic and Atmospheric Admi- nistration in 1973 (Reference 33) ."1 GENERAL COMMENTS The basic components necessary for the developnent of any drainageway pl an- ning study are the ma9nitude and frequency of storm water runoff. This in- cludes the volune of runoff, as ~ll as the peak rates of flow for the flood events under consideration. 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 exceeded once. Thus, a flood having a return period of 5 years has a 20 percent probabil ity of being equalled or exceeded in any given year. Likewise, a flood having a return period of JOO years has a 1 percent probabil ity of being equalled or exceeded in any given year. Due to the nature of flooding on lo~r Clear Creek it was determined to anal- yze the 10 and lOa-year flood events for master pl anning purposes. Table III-l lists the peak flows for the 10 and 100-year events at various locations along Clear Creek as developed in the FHAD Study) Discharge-probabil ity profil es for the la-year and lOa-year frequency floods are shol'Kl in Figure 111-1. The magnitude of a flood can be judged by its maximun rate of flow, or peak discharge. The extent of flooding and the sizing and character of major drainage I'oIJrks and other urban storm drainage facil ities are determined by the magnitude of runoff. Pny major drainageway pl an must be based on ade- quate and thorough h)drological studies if the plan is to have a significant value and a firm factual basis. TABLE 111-1 PEAK RUNOFF FLOWS (all values in cubic feet per second) HYDROLOGY FOR CLEAR CREEK 1977 Bas in Conditions Future Development Conditions Location 100- year 10- year 100- year Youngfield Street 13,100 3,300 13 , 500 Below Lena Gulch 14,100 3,300 14,500 Below Ralston Creek 14,600 8,500 20,500 Below Little Dry Creek 16,200 9,800 22,600 At South Pl atte River Confl uence 16,400 10,100 23,000 The h)drological information and corresponding flood discharges needed to de- termine the extent of flooding on Clear Creek was provided by the Oliaha Dis- trict of the U.S. Army Corps of Engineers (U.S.A.C.E.). Peak discharge- frequency relationships ~re determined at many locations for flood events of la, 50, 100, and 500-year return periods. These discharges are based upon 1977 developnent and fut ure developnent cond it ions proj ected for the Cl ear Creek basin. IGingery Associates, Inc., "Flood Hazard Area Del ineation, Clear Creek, Adans County and Jefferson County," (FHAD), Novrn1ber 1979. (Reference 14).