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<br />For Clear Creek, floodflow-frequency data were based on a regional <br />hydrologic analysis in Manual for Estimatinq Flood Characteristics <br />of Natural-Flow Streams in Colorado (Reference 3). Values of the <br />10-, 50-, 100-, and 500-year peak discharges were obtained by using <br />a series of regression equations from which flood characteristics <br />were estimated from measurable physical and climatic parameters of <br />drainage basins. <br /> <br />Drainage area and average annual precipitation over that area are <br />the two factors required to estimate floodflows. <br /> <br />Results of the regional hydrologic analysis were compared with <br />statistical analyses of historical discharge records at two gaging <br />stations in Lawson and Golden, Clear Creek County, operated by the <br />U.S. Geological Survey (References 4,5,6,7 and 8). The Lawson gage, <br />which provided the major portion of the data, had a 25-year length <br />of record. <br /> <br />Analyses of these two stations followed the standard log-Pearson <br />Type III method as outlined by the U.S. Water Resources Council <br />(Reference 9) . <br /> <br />Peak discharge-drainage area relationships for Clear Creek are shown <br />in Table 1. <br /> <br />Table 1. Summary of Discharges <br /> <br />Flooding Source <br />and Location <br /> <br />Drainage Area <br />(Square Miles) <br /> <br />Peak Discharges <br />(Cubic Feet per Second) <br />10-Year 50-Year 100-Year 500-Year <br /> <br />Clear Creek <br />At Silver Plume <br /> <br />44.4 <br /> <br />650 <br /> <br />860 <br /> <br />9~ <br /> <br />1100 <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the commu- <br />nity were carried out to provide estimates of the elevations of <br />floods of the selected recurrence intervals along each stream <br />studied in the community. <br /> <br />Water-surface elevations of floods of the selected recurrence inter- <br />vals were computed through use of the U.S. Army Corps of Engineers <br />HEC-2 step-backwater computer program (Reference 10) and supplemented <br />by manual calculations. M~nuai calculations were primarily used to <br />determine the division of floodflows at bridge sections and to <br />verify water-surface elevations near critical depth. <br /> <br />5 <br /> <br />, <br />, <br />