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<br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Exhibit 1). All elevations are referenced to the National <br />Geodetic Vertical Datum of 1929. Elevation reference marks used in <br />the study are shown on the map. <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 developing sou~d 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 SOQ-year flood is employed to indicate additional <br />areas of flood risk in the community. For the stream studied in <br />detail, the boundaries of the IOO-year and the SOD-year floods have <br />been 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 1:4800, with a contour interval <br />of 2 feet (Reference 5). In cases where the 100-year and the 500- <br />year flood boundaries are close together, only the 100-year boundary <br />has been shown. The flood boundaries are shown on the Flood Bound- <br />ary Map (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 floadway 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 />?he 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 IOO-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 /> <br />10 <br />