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All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in this study are shown on the mapSl the descriptions of the marks are presented in Elevation Reference Marks (Exhibit 3). 4. 0 FLOOD PLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound flood plain management programs. Therefore, each Flood Insurance Study pro- vides 100-year flood elevations and delineations of the 100- and SOO- year flood plain boundaries and 100-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 (lOO-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 SOO-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 1:2,400 and 1:4,800, with a contour interval of 2 feet (References 3, 8, and 9). The 100- and SOO-year flood plain 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 SOO-year flood plain bound- ary 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 100-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. For the streams studied by approximate methods, only the 100-year flood plain boundary is shown on the Flood Insurance Rate Map (Exhibi t 2). Approximate 100-year flood plain boundaries in some portions of the study area were taken directly from the Flood Hazard Boundary Map for Rio Grande County (Reference 11). 4.2 Floodways Encroachment on flood plains, such as structures and fill, reduces flOOd-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. 10