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<br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound floodplain management programs. <br />To assist in this endeavor, each FIS report provides I-percent-annual-chance floodplain data, which <br />may include a combination of the following: 10-, 2-, 1-, and 0.2-percent-annual-chance flood <br />elevations; delineations of the 1- and O.2-percent-annual-chance floodplains; and a <br />I-percent-annual-chance floodway. This information is presented on the FIRM and in many <br />components of the FIS report, including Flood Profiles, Floodway Data tables, and Summary of <br />Stillwater Elevation tables. Users should reference the data presented in the FIS report, as well as <br />additional information that may be available at the local community map repository, before making <br />flood elevation and/or floodplain boundary determinations. <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the I-percent-annual-chance <br />flood has been adopted by FEMA as the base flood for floodplain management purposes. The <br />O.2-percent-annual-chance flood is employed to indicate additional areas of flood risk in the <br />community. For each stream studied by detailed methods, the 1- and <br />O.2-percent-annual-chance floodplain boundaries have been delineated using the flood <br />elevations determined at each cross section. Between cross sections, the boundaries were <br />interpolated using topographic maps at a scale of 1 :2,400, with a contour interval of 2 feet <br />(Reference 11). <br /> <br />The 1- and O.2-percent-annual-chance floodplain boundaries are shown on the FIRM. On this <br />map, the I-percent-annual-chance floodplain boundary corresponds to the boundary of the <br />areas of special flood hazard (Zones AE) and the O.2-percent-annual-chance floodplain <br />boundary corresponds to the boundary of areas of moderate flood hazard~ In cases where the <br />1- and O.2-percent-annual-chance floodplain boundaries are close together, only the <br />I-percent-annual-chance floodplain boundary has been shown. Small areas within the <br />floodplain boundaries may lie above the flood elevations, but cannot be shown because of <br />map scale limitations and/or lack of detailed topographic data. <br /> <br />F or the streams studied by approximate methods, only the I-percent-annual-chance floodplain <br />boundary is shown on the FIRM. <br /> <br />4~2 Floodways <br /> <br />Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, <br />increases flood heights and velocities, and increases flood hazards in areas beyond the <br />encroachment itself. One aspect of floodplain management involves balancing the economic <br />gain from floodplain development against the resulting increase in flood hazard. Forpurposes <br />of the NFIP, a floodway is used as a tool to assist local communities in this aspect of <br />floodplain management. Under this concept, the area of the I-percent-annual-chance <br />floodplain is divided into a floodwayand a floodway fringe. The floodway is the channel of a <br />stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the <br />base flood can be carried without substantial increases in flood heights. Minimum Federal <br />standards limit such increases to 1 foot, provided that hazardous velocities are not produced. <br />The floodways in this study are presented to local agencies as minimum standards that can be <br />adopted directly or that can be used as a basis for additional floodway studies. <br /> <br />6 <br />