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<br />boundaries have been delineated using the flood elevations <br />determined at each cross section. Between cross sections, the <br />boundaries were interpolated using topographic maps at a scale of <br />1:2,400, with a contour interval of 4 feet (Reference 7). <br /> <br />The 100- and SOO-year floodplain boundaries are shown on the Flood <br />Insurance Rate Map (Exhibit 2). On this map, the lOa-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 />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. Dne 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 thi s 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 k.ept 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 proportional conveyance reduction <br />from each side of the floodplain. Equal conveyance reduction <br />methods were initially used to compute the floodway on the Cache La <br />Poudre River. However, due to the winding nature of the river and <br />the many areas of divided flow along the study reach, equal <br />conveyance reduction did not provide an accurate representation of <br />the floodway. Therefore, proportional conveyance reduction methods <br />were used for this study. Fl.oodway 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 2). 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 laO-year floodplain boundaries is <br />termed the floodway fringe. The floodway fringe encompasses the <br /> <br />7 <br />