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<br />3.0 ENGINEERING METHODS <br /> <br />For flooding sources studied in detail In the community, standard <br />hydrologic and hydraulic study methods were used to determine the <br />flood hazard data required for this study. Floods having recurrence <br />intervals of 10, 50, 100, and 500 years have been selected as having <br />special significance for floodplain management and for flood insurance <br />premium rates. The analyses reported here reflect current conditions <br />in the watersheds of the streams. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak <br />discharge-frequency relationships for floods of the selected <br />recurrence intervals for each stream studied in detail in the <br />community. <br /> <br />The discharges used in this study were pro~ided by the U.S. Army <br />Gorps of Engineer~,-OmahaDistr.ictf-Jor .the.Sqllt!L!,1a,1t:.E!_ River~ <br />These discharge profiles were keyed to the 100-year flood val~es <br />computed from the Denver and Henderson gaging records of the <br />South Platte River, which were adjusted for the regulating <br />effects of Chatfield, Cherry Creek, and Bear Creek Reservoirs. <br />The Denver gage is located 5 miles upstream of Commerce City, and <br />the length of gaging records was from 1895 to 1973, excluding <br />1902, 1907, and 1908. The Henderson gage is located 9.5 miles <br />downstream of Commerce City, and the length of gaging station <br />records used for the analysis was from 1926 to 1973. <br /> <br />The U.S. Army Corps of Engineers also provided the discharges <br />used for Sand Creek. The hydrologic analysis of Sand Creek was <br />based on the development of a surface runoff model of the Sand <br />Creek basin using the runoff block of the Environmental <br />Protection Agency's Storm Water Management Program (Reference 1). <br />Discharge hydrographs developed from the model were then routed <br />along the main stem of Sand Creek by use of an unsteady flow, <br />routing procedure developed by James C. Harder and modified by <br />the Missouri River Division Office of the U.S. Army Corps of <br />Engineers. Discharge probability relationships were developed by <br />inserting precipitation frequency values (Reference 2). <br /> <br />Peak discharges for the 10-, 50-, 100-, and 500-year floods for <br />the South Platte River and Sand Creek in the study area are shown <br />on Table 1. <br /> <br />10 <br />