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<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 <br />itself. One aspect of flood plain management involves balancing <br />the economic gain from flood plain development against the <br />resulting increase in flood hazard. For purposes of the NFIP, a <br />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 lOO-year flood plain is divided into a floodway and a floodway <br />fringe. The floodway is the channel of a stream, plus any <br />adjacent flood plain areas, that must be kept free of encroachment <br />so that the lOO-year flood can be carried without substantial <br />increases in flood heights. Minimum Federal standards limit such <br />increases to 1.0 foot, provided that hazardous velocities are not <br />produced. The floodways in this study are presented to local <br />agencies as minimum standards that can be adopted directly or that <br />can be used as a basis for additional floodway studies. <br /> <br />The floodways presented in this study were computed for certain <br />stream segments on the basis of equal conveyance reduction from <br />each side of the flood plain. Floodway widths were computed at <br />cross sections. Between cross sections, the floodway boundaries <br />were interpolated. The results of the floodway computations are <br />tabulated for selected cross sections (Table 2). The computed <br />floodways are shown on the Flood Insurance Rate Map (Exhibit 2). <br />In cases where the floodway and 100-year flood plain boundaries <br />are either close together or collinear, only the floodway boundary <br />is shown. <br /> <br />The area between the floodway and lOO-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 />