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<br />4.1 Flood Boundaries <br /> <br />In order to provide a national standard without regional discrimina- <br />tion, the lOa-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 com~unity. 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 at a scale of 1:1200, with a contour interval <br />of 2 feet (Reference 12). <br /> <br />In cases where the 100- and SOa-year flood boundaries are close <br />together, only t.ne IO)-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 />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 hazard. <br />For purposes of the National 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 lOO-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 />The floodway presented for this study was computed on the basis of <br />equal conveyance reduction from each side of the flood plain or <br />encroachment to the streambanks. The results of these computations <br />are tabulated at selected cross sections for each stream segment for <br />which a floodway is computed (Table 2). <br /> <br />7 <br />