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~ater-surface profiles for the floods studied in this report were developed using the COE HEC-2 step-backwater computer program (Reference 8). The computations were based on channel and floodplain conditions represented by survey data gathered in December 1976 and supplemented by later field investigations. The detailed topographic maps that were used to assist in delineating the 100- and SOO-year floodplain boundaries covering the South Platte River had a scale of 1:2,400 with a 2-foot contour interval (Reference 6), and maps covering a portion of the Pawnee Creek Overflow area southwest of Sterling had a scale of 1: 1,200 wi th a contour interval of 2 feet. These maps were prepared in 1977 by Blevins Mapping Company, under contract with the Colorado Water Conservation Board, in cooperation with Logan County (Reference 9). Locations of selected cross sections used in the analyses are shown on the Flood Profiles (Exhibit 1) shown on the Flood Insurance Rate Map (Exhibit 2). hydraulic and al so Channel roughness factors along the South Platte River ranged from 0.03 to 0.035. The starting water-surface elevation for the South Platte River was determined using normal-depth computations and 10-year contiguous peaks for Sand Creek and Pawnee Creek Overflow. The hydraulic analyses for this study are based on unobstructed flow. The flood elevations shown on the profile are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. All elevations are referenced to the National Geodetic Vertical Datum (NGVD) of 1929. Elevation reference marks used in this study are shown on the maps; the descriptions of the marks are presented in Elevation Reference Marks (Exhibit 3). 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP floodplain includes a developing encourages State and local governments to adopt sound management programs. Therefore, each Flood Insurance Study floodplain boundary map designed to assist communities in sound floodplain management measures. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 1 percent annual chance (lOO-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2 percent annual chance (SOO-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied by detailed methods, the 100- and SOn-year floodplain boundaries have been delineated using the flood elevations determined at each cross section. 7