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<br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />A prime purpose of the National Flood Insurance Program <br />is to encourage state and local governments to adopt sound <br />flood plain management programs. Each Flood Insurance <br />Study, therefore, includes a flood boundary map designed <br />to assist communities in developing flood plain management <br />measures. <br /> <br />4.1 Flood Boundaries <br /> <br />In order to provide a national stanQard without regional <br />discrimination, the 100-year flood has been adopted <br />by the Federal Insurance Administration as the base <br />flood for purposes of flood plain management measures. <br />The SOD-year flood is employed to indicate additional <br />areas of flood risk in the community. For each stream <br />studied in detail, the boundaries of the 100-year <br />and the 500-year floods have been delineated using <br />the flood elevations determined at each cross section; <br />between cross sections, the boundaries were interpolated <br />using topographic maps at a scale of 1:24,000 with <br />a contour interval of 20 feet (Reference 8). In cases <br />where the 100-year and SOD-year boundaries are close <br />together, only the 100-year boundary has been shown. <br /> <br />The flood boundaries of all streams studied by <br />approximate methods were determined using historical <br />flood information, engineering judgment, approximated <br />flood elevations, and topographic maps (Reference 8). <br /> <br />Small areas within the flood boundaries may lie above <br />the flood elevations, and therefore, not be subject <br />to flooding; owing to limitations of the map scale, <br />such areas are not shown. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as artificial fill, <br />reduces the flood-carrying capacity and increases <br />flood heights, thus increasing flood hazards in areas <br />beyond the encroachment itself. One aspect of flood <br />plain management involves balancing the economic gain <br />from flood plain development against the resulting <br />increase in flood hazard. For purposes of the Flood <br />Insurance Program, the concept of a floodway is used <br />as a tool to assist local communities in this aspect <br />of flood plain management. Under this concept, the <br />area of the 100-year flood is divided into a floodway <br />and a floodway fringe. The floodway is the channel <br /> <br />22 <br />