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<br />The 100- and SOD-year floodplain boundaries are shown on the Flood <br />Insurance Rate Map (Exhibit '2). On this map, the 100..year <br />floodplain boundary corresponds. to the boundary of the areas of <br />special flood hazards (Zones /\, AE, and AD); and the SOD-year <br />floodplain boundary correspond~ to the boundary of areas of <br />moderate flood hazards. In ca1ses where the 100- and SOD-year <br />floodplain boundaries are clo:se together, only the 100-year <br />floodplain boundary haa been: shown. Small areas within the <br />floodplain boundaries may lie abpve the flood elevations but cannot <br />be shown due to limitations 6f the map scale and/or lack of <br />detailed topographic data. <br /> <br />For the streams studied <br />floodplain boundary is <br />(Exhibit 2). <br /> <br />by apprpximate methods, only the 100-year <br />shown pn 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 veloci ties, <br />and increases flood hazards in areas beyond the encroachment <br />itself. One aspect of floodplaiti management involves balancing the <br />economic gain from floodplain idevelopment against the resulting <br />increase in flood hazard. For purposes of the NFIP, a floodway is <br />used as a tool to assist loc~l communities in this aspect of <br />floodplain management. Under t):1i s concept, the area of the 100- <br />year floodplain is divided intoia 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 100-year flood can be carried without substantial increases in <br />flood heights. Minimum Federal istandards limit such increases to <br />1.0 foot, provided that hazardol~s velocities are not produced. The <br />floodways in this study are pre~ented to local agencies as minimum <br />standards that can be adopted 4irectly or that can be used as a <br />basis for additional floodway strdies. <br /> <br />The floodways presented in this study were computed for certain <br />stream segments on the basis of equal c:onveyance reduction from <br />each side of the floodplain. Floodway widths were computed at cross <br />sections. Between cross sectiors, the floodway boundaries were <br />interpolated. The results ofi the floodway computations are <br />tabulated at selected cross sect~ons (Table 3). In cases where the <br />floodway and 100-year floodp14in boundaries are either close <br />together or collinear, only the floodway boundary has been shown. <br /> <br />The area between the fl.oodway and 100-year floodplain boundaries is <br />termed the floodway fringe. Th~ floodway fringe encompasses the <br />portion of the floodplain that could be completely obstructed <br />without increasing the water-s~rface elevation of the 100-year <br />flood more than 1.0 foot at iany point. Typical relationships <br />between the floodway and the fl09dway fringe and their significance <br />to floodplain development are sh9Wll in Figure 2. <br /> <br />14 <br />