FLOOD10463 - Laserfiche WebLink

Home Browse Search

For more information on NAVD, see the FEMA publication entitled "Converting the National Flood Insurance Program to the North American Vertical Datum of 1988" (FEMA, June 1992), or contact the Vertical Network Branch, NGS, Coast and Geodetic Survey, National Oceanic and Atmospheric Administration, Rockville, Maryland 20910. Temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. Although these monuments are not shown on the FIRM, they may be found in the Technical Support Data Notebook associated with the FIS report and FIRM for this community. Interested individuals may contact FEMA to access these data. 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each FIS provides laO-year flood elevations and delineations of the 100- and SOa-year floodplain boundaries and 100-year floodway to assist communities in developing floodplain management measures. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the l-percent-annual-chance (lOO-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The O.2-percent-annual-chance (SOD-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the 100- and SOD-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 1:4,800, with contour intervals of 2 feet and 4 feet (Reference 7). The 100- and SOD-year floodplain boundaries are shown on the FIRM. On this map, the 100-year floodplain boundary corresponds to the boundary of the areas of special flood hazard (Zone AE), and the SOG-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 100-year floodplain boundary has been shown. Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown because of map scale limitations 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. 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases 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 7