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<br />cannot be shown due to limitations of the map scale and/or lack of <br />detailed topographic data. <br /> <br />4. ~ 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 100-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 100-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 adopted directly or that can be used as a basis for addi- <br />tional floodway studies. <br /> <br />. <br /> <br />The floodways presented in this study were computed on the basis <br />of equal conveyance reduction from each side of the flood plain. <br />The results of these computations are tabulated at selected cross <br />sections for each stream segment for which a floodway is computed <br />(Table 2). <br /> <br />As 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 />. <br /> <br />8 <br />