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<br />4.0 FLOOD PLAIN MANAGE/1ENT 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 developing 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 IOO-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 SOO-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 enlarged to a scale of 1:7200, with a contour <br />interval of 10 feet (Reference 8), and aerial photographs at a scale <br />of 1:7200 (Reference 9). <br /> <br />In cases where the 100- and SOO-year f~ood boundaries are close <br />together, only the lOa-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 /1ap (Exhibit 2). <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 against the resulting increase in flood <br />hazard. For purposes of the Flood Insurance Program, the concept of <br />a floodway is used as a tool to assist local communities in this <br />aspect of flood plain management. Under this concept, the area of <br />the lOa-year flood is divided into a floodway and a floodway fringe. <br />The floodway is the channel of a stream, plus any adjacent flood <br />plain areas, that must be kept free of encroachment in order that <br />the lOa-year flood be carried without substantial increases in flood <br />heights. As minimum standards, the Federal Insurance Administration <br />limits such increases in flood heights to 1.0 foot, provided that <br />hazardous velocities are not produced. <br /> <br />9 <br />