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The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus con- sidered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. All elevations are referenced to the NGVD. Elevation reference marks used in this study are shown on the maps; the description of the marks are presented in Elevation Reference Marks (Exhibit 3). 4.0 FLOOD PLAIN.MANAGEMENTAPPLICATIONS The NFIP encourages State and local governments to adopt sound flood plain management programs. Therefore, each Flood Insurance Study provides lOO-year flood elevations and delineations of the 100- and SOO-year flood plain boundaries and lOa-year floodway to assist communities in developing flood plain management measures. 4.1 Flood Plain Boundaries To provide a national standard without regional discrimination, the 1 percent annual chance (laO-year) flood has been adopted by FEMA as the base flood for flood plain 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 in detail, the 100- and SOa-year flood plain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were inter- polated using topographic maps at scales of 1:1,200 and 1:2,400, with a contour interval of 2 feet (References 1,2,4, and 5). The 100- and SOO-year flood plain boundaries are shown on the Flood Insurance Rate Map (Exhibit 2). On this map, the lOa-year flood plain boundary corresponds to the boundary of the areas of special flood hazards (Zones A, AE, and AH)i and the SOO-year flood plain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 100- and SOO-year flood plain boundaries are close together, only the laO-year flood plain boundary has been shown. Small areas within the flood plain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. Approximate lOa-year flood boundaries in the Chalk Cliffs debris fan area were delineated using aerial photographs and field recon- naissance (Reference 4). No effort was made to determine depth or direction of flow in the areas. Approximate lOO-year flood boundaries in some portions of the study area were taken directly from the Flood Hazard Boundary Map for Chaffee County (Reference 10). 9