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<br />Elevation reference marks used in this study are shown on the maps <br />to assist users in determining more accurately the boundaries of the <br />100- and SOD-year floods and to provide a means for determining the <br />first floor elevations of local structures in relation to the 100- <br />year flood. <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />A prime purpose of the National Flood Insurance Program is to encourage <br />State and local governments to adopt sound flood plain management pro- <br />grams. Each Flood Insurance Study, therefore, includes a flood boundary <br />map designed to assist communities in deve19ping sound flood plain <br />management measures. <br /> <br />4.1 Flood Boundaries <br /> <br />In order to provide a national standard without regional discrimina- <br />tion, the 100-year flood has been adopted by the Federal Insurance <br />Administration as the base flood for purposes of flood plain manage- <br />ment measures. The SOO-year flood is employed to indicate additional <br />areas of flood risk in the community. For each stream studied in <br />detail, the boundaries of the 100- and SOD-year floods have been <br />delineated using the flood elevations determined at each cross <br />section; between cross sections, the boundaries were interpolated <br />using topographic maps at a scale of 1:24,000, enlarged to 1:7200, <br />with a contour interval of 10 feet (Reference 8). <br /> <br />In cases where the 100- and SOO-year flood boundaries are close <br />together, only the 100-year flood boundary has been shown. <br /> <br />Flood boundaries for the 100- and SOO-year floods are shown on the <br />Flood Boundary and Floodway Map (Exhibit 2). <br /> <br />Approximate flood boundaries in some portions of the study area were <br />taken from the Federal Insurance Administration's Flood Hazard <br />Boundary Map (Reference 9). <br /> <br />Small areas within the flood boundaries may lie above the flood <br />elevations and, therefore, not be subject to flooding; owing to <br />limitations of the map scale, such areas are not shown. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as artificial fill, reduces the <br />flood-carrying capacity and increases flood heights, thus increasing <br />flood hazards in areas beyond the encroachment itself. One aspect <br />of flood plain management involves balancing the economic gain from <br />flood plain development again~t the resulting increase in flood <br /> <br />6 <br />