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studied in detaiL Cross section data for the South Platte River and Sand Creek were obtained by field surveys and aerial contour mapping (References 12, 13, 14, and 15), All bridges and culverts were surveyed or the information was provided by the Colorado Department of Highways to obtain elevation data and structural geo- metry. Locations of selected cross sections used in the hydraulic analyses are shown, on the Flood Profiles (Exhibit 1). For stream segments for which a floodway is computed (Section 4.2), selected cross section locations are also shown on the Flood Insurance Rate Map (Exhibit 2). Rougness coefficients (Manning's "n") were estimated by field in- spection. The range of "n" values used for Sand Creek were from 0.025 to 0.045 and those for the South Platte River were from 0.035 to 0,070. Water~surface profiles were developed using the HEC-2 step-backwater computer program (Reference 16). Profi les were determined for the 10-, 50-, 100-, and 500-year floods, Flood profi les were drawn showing computed water-surface elevations to an accuracy of 0.5 foot. All starting water-surface elevations for the South Platte Ri vel' were taken from the Adams County, Colorado, Flood Insurance Study (Reference 17). The starting water-surface elevations for Sand Creek were those computed for the South Platte River at the confluence. The reach of Sand Creek between Vasquez and Brighton Boulevards was revised to reflect a revised' hydraulic analysis carried out by Simons, Li and Associates, Inc., based on new topographic informa- tion (Reference 18). This reanalysis resulted in a change in the base (lOO-year) flood elevations and floodplain and floodway boundaries for this reach and was reflected on the September 30, 1988 Flood Insurance Rate Map and Flood Boundary and Floodway Map. All elevations are referenced to the National Ceodetic Vertical Datum .of 1929 (NGVD), The location of elevation reference marks used in the study are shown in Exhibit 3. 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS A prime purpose of the NFIP is to encourage state and local governments to adopt sound floodplain management programs. Each Flood Insurance Study, therefore, includes a flood boundary map designed to assist communities in developing sound floodplain management measures. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 100-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of floodplain 13 II