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<br />studied in detail, the 100- and SOO-year floodplain boundaries have <br />been delineated using the flood elevations determined at each cross <br />section. Between cross sections, the boundaries were interpolated <br />using topographic maps at a scale of 1:4800, with a contour <br />interval of 2 and 4 feet (Reference 6). <br /> <br />The 100- and SOO-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 (Zone AE); and the SOO-year floodplain <br />boundary corresponds to the boundary of areas of moderate flood <br />hazards. In cases where the 100- and SOO-year floodplain <br />boundaries are close together, only the lOa-year floodplain <br />boundary has been shown. Small areas within the floodplain <br />boundaries may lie above the flood elevations but cannot be shown <br />due to limitations of the map scale and/or lack of detailed <br />topographic data. <br /> <br />For the streams studied <br />floodplain boundary is <br />(Exhibit 2). <br /> <br />by approximate methods, only the laO-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 />increase 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 laO-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 <br />cross sections. Between cross sections, the floodway boundaries <br />were interpolated. The resul ts of the floodway computations are <br />tabulated at selected cross sections (Table 2). In cases where the <br />floodway and 100-year floodplain boundaries are either close <br />together or collinear, only the floodway boundary has been shown. <br /> <br />The area between the floodway and laO-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 /> <br />8 <br />