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<br />Cross section data for streams in the area were obtained from field <br />survey measurements taken, on the average, at approximately BOO-foot <br />intervals for the Yampa River analyses and approximately 30Q-foot <br />intervals for Dry Creek. Cross sections were located at close <br />intervals above and below bridges and culverts in order to compute <br />the backwater effects of these structures. At various locations <br />along Dry Creek, the original cross sections were manipulated and/or <br />extended by interpolation to better define the physical and natural <br />conditions that exist. All bridges and culverts were surveyed to <br />obtain elevation data and structural geometry. Locations of selected <br />cross sections are shown on the Flood Profiles (Exhibit 1). For <br />stream segments for which a floodway has been computed (Section 4.2), <br />selected cross section locations are also shown on the Flood Boundary <br />(Exhibit 2) and Floodway (Exhibit 3) Maps. <br /> <br />Roughness coefficients (Manning's "n") for water-surface computation::; <br />were determined from field inspection, aerial photographs, and a <br />u.s. Geologi.cal Survey Water Supply Paper (Reference 7) for all <br />flood plain areas. A roughness value of 0.030 was used for the main <br />channel of Dry Creek; roughness values for the flood plain ranged <br />from 0.035 to 0.200. Roughness values for the Yarnpa River, in <br />general, agreed closely with those used for Dry Creek. <br /> <br />Water-surface profiles were developed through use of the u.s. Bureau <br />of Reclamation computer program WSPPSH (Reference 8). Starting <br />flood elevations for Dry Creek were extracted from a rating curve <br />developed from the railroad crossing, which is constructed of four <br />large culverts. Profiles were determined for the 10-, 50-, 100-, <br />and SOO-year floods. <br /> <br />Flood profiles for Dry Creek were drawn showing computed water- <br />surface elevations to an accuracy of 0.5 foot for floods of the <br />selected recurrence intervals (Exhibit 1). Computations show that <br />the Yampa River would not present a flood threat to the incorporated <br />area of Hayden. Therefore, the flood profiles are not included in <br />this report. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929. The elevation reference mark used in this study is <br />shown on the maps. <br /> <br />4.0 PLOOD PLAIN MANAGEMENT APPLICATIONS <br />--'-'---'--.--,.~--. <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 <br />management measures. <br /> <br />7 <br />