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. The starting water-surface elevations for the South Platte River, Pawnee Creek Overflow, and Sand Creek were determined by the slope area method, 10-year coincident peak, and 10-year coincident peak, respectively. Flood profiles were drawn showing the computed water-surface elevations for floods of the selected recurrence intervals. The hydraulic analyses for this study are based on unobstructed flow. The flood elevations shown on the profiles 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 of 1929 (NGVD). 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 encourages State and local governments to adopt sound floodplain management programs. Therefore, each Flood Insurance Study includes a floodplain 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 I-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 boundaries of the 100- and SOO- year floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries along the South Platte River were interpolated using detailed topographic maps prepared by Blevins Mapping Company of Denver, Colorado in 1977. The maps were at a scale of 1 inch equal to 200 feet wi th a 2-foot contour interval. For Sand Creek and Pawnee Creek Overflow, Blevins Mapping Company and Landmark Mapping, Lakewood, Colorado, produced maps in 1976 and 1981, respectively. These maps were at a scale of 1 inch equal to 200 feet with a 2-foot contour interval. . The 100- and SOO-year floodplain boundaries are shown on the Flood Insurance Rate Map (Exhibit 2). On this map, the 100-year floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zone AE and AOJ, and the SOO-year floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 100- and SOO-year floodplain boundaries are close together, only the, 100-year floodplain 7