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Cross section data for Little Dry, Comanche, Little Comanche, North- field, Niver, and Box Elder Creeks were field surveyed and were located at close intervals above and below culverts, bridges, and drop structures in order to compute the effects of backwater. For Big Dry Creek, cross section data was taken from previously published detailed topographic mapping (Reference 4). For the South Platte River, field-surveyed croSs sections included 49 field-surveyed cross sections and 30 supplemental valley cross sections. measured on the topographic maps used for the South Platte delineation (Refer- ences 10 and 23). When necessary, U.S. Geological Survey topographic maps at a scale of 1:24,000, with a contour interval of 10 feet (Reference 24) were used to supplement the field-surveyed data. Locations of selected cross sections are shown on the Flood Boundary and Floodway Maps. . Channel roughness factors (Manning's "n") for these computations were assigned on the basis of field inspection of the flood plain areas and engineering judgment. Bridge geometry and elevation information was obtained from the Colorado State Highway Department, when available, or measured in the field. .' flood elevations in the city are often raised by debris jams at bridges and culverts; the hydraulic analyses for this study, however, are based only on the effects of unobstructed flow. The flood elevations, as shown on the profiles, are thus considered valid only if hydraulic structures, in general, remain unobstructed and dams operate properly and do not fail. 4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS A prime purpose of the National Flood Insurance Program is to encourage state and local governments to adopt sound flood plain management pro- grams. Each Flood Insurance Study, therefore, includes a flood boundary map designed to assist communities in developing sound flood plain management measures. 4.1 Flood Boundaries In order to provide a national standard without regional discrimina- tion, the 100-year flood has been adopted by the Federal Insurance . Administration as the base flood for purposes of flood plain manage- ment measures. The SOO-year flood is employed to indicate additional areas of flood risk in the community, For each stream studied in detail, the boundaries of the 100- and SOO-year floods were delineated using the flood elevations determined at each crosG section; between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:24,000, with a contour interval of 10 feet (Reference 24). In cases where the 100- and SOO-year flood boundaries are close together, only the 100-year boundary has been shown. On the South Platte River, boundaries were interpolated using topograp- hic maps at a scale of 1:2400, with a contour interval of 2 feet (Reference 23). 17