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<br />For the streams studied <br />floodplain boundary is <br />(Exhibit 2.) <br /> <br />by approximate methods, only the lOa-year <br />shown on the Flood Insurance Rate Map <br /> <br />4.2 Floodways <br /> <br />Encroachment on floodplains, 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 floodplain management involves balancing the <br />economic gain from floodplain development against the resulting <br />lncrease in flood hazard. For purposes of the NFIP, a floodway is <br />used as a tool to assist local communities in this aspect of <br />floodplain management. Under this concept, the area of the 100- <br />year floodplain is divided into a floodway and a floodway fringe. <br />The floodway is the channel of a stream, plus any adjacent <br />floodplain areas, that must be kept free of encroachment so that <br />the lOa-year flood can be carried without substantial increases in <br />flood heights. Minimum Federal standards limit such increases to <br />1.0 foot, provided that hazardous velocities are not produced. The <br />floodways in this study are presented to local agencies as minimum <br />standards that can be adopted directly or that can be used as a <br />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 floodplain. Floodway widths were computed at cross <br />sections. Between cross sections, the floodway boundaries were <br />interpolated. The results of the floodway computations are <br />tabulated for selected cross sections (Table 3). In cases where <br />the floodway and lOa-year floodplain boundaries are ei ther close <br />together or collinear, only the floodway boundary is shown. <br /> <br />The area between the floodway and lOa-year floodplain boundaries is <br />termed the floodway fringe. The floodway fringe encompasses the <br />portion of the floodplain that could be completely obstructed <br />without increasing the water-surface elevation of the lOO-year <br />flood more than 1.0 foot at any point. Typical relationships <br />between the floodway and the floodway fringe and their significance <br />to floodplain development are shown in Figure 4. <br /> <br />5.0 INSURANCE APPLICATION <br /> <br />For flood insurance rating purposes, flood insurance zone designations <br />are assigned to a community based on the engineering analyses. These <br />zones are as follows: <br /> <br />Zone A <br /> <br />Zone A is the flood insurance rate that corresponds to the lOa-year <br />floodplains that are determined in the Flood Insurance Study by <br />approximated methods. Because detailed hydraul ic analyses are not <br /> <br />16 <br />