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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 100-year flood elevations' and delineations of the 100- and SOO-year floodplain boundaries and 100-year floodway to assist communities in developing floodplain management measures. 4.1 Floodplain Boundaries To provide a national standard ~ithout regional discrimination, the 1 percent annual chance (lOO-year) flood has been adopted by FEMA as the base flood for floodp~ain management purposes. The 0.2 percent animal chance (SOG-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 floodplain boundaries have been delineated using, the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:2,400 feet, with a contour interval of 2 feet (Reference 10). . The 100- andSOO-year floodplain boundaries are shown on the Flood Insurance Rate Map (Exhibit 2). On this map, the 100-year flood- plain boundary corresponds to ~he boundary of the areas of special flood hazards (Zones A, AE, and AO); 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 IpO-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 100-year floodplain. boundary is shown, on the Flood Insurance Rate Map (Exhibit 2.) 4.2 Floodways Encroachment on floodplains, $uch as structures and fill, reduces flood-carrying capacity" incrl!ases flood heights and velocities, and increases flood hazards ,in. areas. beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against. the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of 10 :