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The hydraulic analyses for this flow. The flood elevations shown dered valid only if hydraulic operate properly, and do not fail. study are based on on the profiles are structures remain unobstructed thus consi- unobstructed, 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 encourages State and local governments plain management programs. Therefore, each includes a flood boundary map designed to developing sound floodplain management measures. to adopt sound flood- Flood Insurance Study assist communities in 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 (500-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 SOD-year floodplain boundaries have been delineated using the flood elevations determined at each cross section. Floodplain boundaries were interpolated between cross sections using maps at a scale of 1: 200, with a contour interval of 5 feet. These maps have the following tolerances: 10 percent of contours may have tolerances of approximately 5 feet, and 90 percent may have tolerances of approximately 2.5 feet. Field surveys have an allowable tolerance of 0.5 feet. Whenever differences existed between base map elevations and field surveys, the field surveys were used. Areas included in this study which are shown as approximate flooding have been taken from the flood hazard boundary maps for Hinsdale County (Reference 4). The 100- and SOD-year floodplain boundaries are shown on the Flood Insurance Rate Map (Exhibit 2). On this map, the 100-year flood- plain boundary corresponds to the boundary of the areas of special flood hazards (Zones A and AE) , and the 500-year floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 100- and 500-year floodplain boundaries are close together, only the 100-year floodplain boundary has been shown. Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topo- graphic data. 10