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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 maps; the descriptions of the marks are presented in Elevation Reference Marks (Exhibit 3). 4.0 FLOODPLAIN MANAGEMENT APPLICATION The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each Flood Insurance Study provides IOO-year flood elevations and delineations of the 100- and SOO-year floodplain boundaries and lOO-year floodway to assist communities in developing 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. For each stream studied by detailed methods, the 100- and SOO-year floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross see t ions, the boundaries were interpolated us ing topographic maps at a scale of 1:24,000, with a contour interval of 10 feet (Reference 14). The 100- and SOO-year floodplain boundaries are shown on the Flood Insurance Rate Map (FIRM) (Exhibit 2). On this map, the lOO-year floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A, AE, and AO); and the SOa-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 lOO-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 topographic data. For the streams studied by approximate methods, only the lOO-year floodplain boundary is shown on the FIRM (Exhibit 2). On Slaughterhouse Gulch, the floodplain has been delineated to include Fox Street and Powers Avenue. Above this location, floodwater will drain along streets and through existing storm sewers. An indefinable, relatively shallow flood profile will exist along the many streets which drain the basin above this point. Therefore, the flood out 1 ine has not been mapped for the South Tributary to Slaughterhouse Gulch for the reaches above Datura Street. In this area, the streets serve as an adequate s~orm water drain. 13