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<br />Flood boundaries for the 100- and SOD-year floods are shown on <br />the 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 of <br />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 the <br />IOO-year flood be carried without sub3tantial 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 velaei tie::; dre not produced. <br /> <br />The floadway for this study was computed on the basis of equal <br />conveyanc~ reduction from each side of the flood plain. The results <br />of these computations are tabulated at selected cross sections for <br />eac~ stream segment for which a floodway is computed (Table 2). <br /> <br />As shown on the Flood Boundary and Floodway Map (Exhibit 2), the <br />floodway boundaries were determined at cross sections; between <br />cross sections, the boundaries were interpolated. <br /> <br />The area between the floodway and the boundary of the 100-year flood <br />is termed the floadway fringe. The floodway fringe thus encompasses <br />the portion of the flood plain that could be completely obstructed <br />without increasing the water-surface elevation of the IOO-year flood <br />more than 1.0 foot at any point. Typical relationships between the <br />floodway and the floadway fringe and their significance to flood <br />plain development are shown in Figure 6. <br />