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<br />Cross sections for the backwater analyses were field surveyed and <br />were located at close intervals above and below bridges and culverts <br />in order to compute the significant backwater effects of these <br />structures in urban areas. <br /> <br />Locations of selected crossl sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). For stream segments <br />for which a flood~ay,is computed (section 4.2), selected cross <br />section locations are also shown on the Flood Boundary and Floodway <br />Map (Exhibit 2). <br /> <br />starting elevations for Clear Creek were assumed to be critical <br />depth at the eastern corporate limits below the Interstate Highway <br />70 culvert. <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Exhibit 1). <br /> <br />Channel roughness factors (Manning's "n") for these computations <br />were assigned on the basis of field inspection of flood plain areas, <br />comparison with streams of known roughness (Reference 11), and <br />computations relating to the U.S. Geological Survey stream gage at <br />Lawson. Channel roughness factors (Manning's "n") for Clear Creek <br />at Silver Plume range from 0.041 to 0.055 for the channel (0.020 <br />through concrete box culvert) and from 0.060 to 0.120 for the over- <br />banks. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks used in'the study <br />are shown on the maps. <br /> <br />The hydraulic analyses for this study are based only on the effects <br />of unobstructed flow. Flood elevations shown on the profiles are <br />thus considered valid only if hydraulic structures, in general, <br />remain unobstructed, and dams and other flood control structures <br />operate properly and do not fail. <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />A prime purpose of the National Flood Insurance Program is to encourage <br />State and local governments to adopt sound flood plain management pro- <br />grams. Each Flood Insurance Study, therefore, includes a flood boundary <br />map designed to assist communities in developing sound flood plain manage- <br />ment measures. <br /> <br />6 <br />