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<br />2.4 Flood Protection Measures <br /> <br />No flood protection measures are in effect in Idaho Springs. <br />Floodflows abOve Idaho Springs must pass through Georgetown Lake, <br />Cabin Creek Reservoir, and several other small irrigation reservoirs <br />and natural lakes. No credit was given for flood attenuation due <br />to any of the impoundments because of their relatively small capacities <br />to store floodwaters, their operating criteria, and the fact that <br />they are almost always full. Structural analyses of the dams in <br />question were beyond the scope of this study. <br /> <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 flood <br />hazard data required for this study. Floods having recurrence intervals <br />of 10, 50, 100, and 500 years have been selected as having special sig- <br />nificance for flood plain management and for flood insurance premium <br />rates. The analyses reported here reflect current conditions in the <br />watersheds of the flooding sources. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br />frequency relationships for floods of the selected recurrence <br />intervals for each stream studied in detail in the community. <br /> <br />There are only two stream gaging stations within Clear Creek County <br />which are operated by the u.S. Geological Survey. One, located on <br />the Fall River, was not used in the hydrologic analysis for this <br />study due to shortness of record (9 years, 1930-1938). The other <br />gage is located on Clear Creek near Lawson, Colorado (Station No. <br />06716500), upstream of Idaho Springs. It has a drainage area of <br />147 square miles and ~9 years of record. <br /> <br />For all streams, flow-flow frequency data were based on a regional <br />hydrologic analysis set forth in Manual for Estimatinq Flood Charac- <br />teristics of Natural-Flow Streams in Colorado (Reference 2). <br /> <br />The method used to obtain the 10-, 50-, 100-, and 500-year flood- <br />flows for Clear Creek was taken from "Sites Near Gaged Sites on the <br />Same Stream" (Reference 2). This method was used because the drainage <br />area of Clear Creek within the corporate limits of Idaho Springs <br />varies from 192 to 256 square miles, and the ratio of these areas to <br />the Lawson gage area is 2.00 or less. <br /> <br />5 <br />