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<br />For the streams studied by approximate methods, only the 100-year <br />flood plain boundary is shown. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as structures and fill, reduces <br />flood-carrying capacity, increases flood heights and velocities, <br />and increases flood hazards in areas beyond the encroachment itself. <br />One aspect of flood plain management involves balancing the economic <br />gain from flood plain development against the resulting increase <br />in flood hazard. For purposes of the NFIP, a floodway is used as <br />a tool to assist local communities in this aspect of flood plain <br />management. Under this concept, the area of the IOO-year flood <br />plain is divided into a floodway and a floodway fringe. The <br />floodway is the channel of a stream, plus any adjacent flood plain <br />areas, that must be kept free of encroachment so that the IOO-year <br />flood can be carried without substantial increases in flood heights. <br />Minimum Federal standards limit such increases to 1.0 foot, provided <br />that hazardous velocities are not produced. The floodways in this <br />study are presented to local agencies as minimum standards that <br />can be adopteu directly or that can be used as a basis for addi- <br />tional floo0way studies. <br /> <br />'l'he floodway presented in this study was computed on the basis of <br />equal-conveyance reduction from each side of the flood plain. The <br />results of these computations are tabulated at selected cross <br />sections for the stream segment for which a floodway is computed <br />(Table 2). <br /> <br />^s shown on the Flood Boundary and Floodway Map (Exhibit 2), the <br />floodway boundaries were computed at cross sections. Between cross <br />sections, the boundaries were interpolated. In cases where the <br />floodway and 100-year flood plain boundaries are either close <br />together or collinear, only the floodway boundary has been shown. <br /> <br />The area between the floodway and 100-year flood plain boundaries <br />is termed the floodway fringe. The floodway fringe encompasses <br />the portion of the flood plain that could be completely obstructed <br />without increasing the water-surface elevation of the 100-year <br />flood by more than 1.0 foot at any point. Typical relationships <br />between the floodway and the floodway fringe and their significance <br />to flood plain development are shown in Figure 2. <br /> <br />10 <br />