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<br />4.2 Floodways <br /> <br />Encroachment on floodplains, such as structures and fill, reduces flood-carrying <br />capacity, increases flood heights and velocities, and increases flood hazards in areas <br />beyond the encroachment itself. One aspect of floodplain management involves <br />balancing the economic gain from floodplain development against the resulting increase <br />in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local <br />communities in this aspect of floodplain management. Under this concept, the area of <br />the toO-year floodplain is divided into a floodway and a floodway fringe. The floodway <br />is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of <br />encroachment so that the 100-year flood can be carried without substantial increases in <br />flood heights. Minimum Federal standards limit such increases to I foot, provided that <br />hazardous velocities are not produced. The floodways in this study are presented to local <br />agencies as minimum standards that can be adopted directly or that can be used as a basis <br />for additional floodway studies. <br /> <br />The floodways presented in this study were computed for certain stream segments on the <br />basis of equal-conveyance reduction from each side of the floodplain. Floodway widths <br />were computed at cross sections. Between cross sections, the floodway boundaries were <br />interpolated. The results of the floodway computations are tabulated for selected cross <br />sections (see Table 2, "F100dway Data"). In cases where the floodway and lOO-year <br />floodplain boundaries are either close together or collinear, only the floodway boundary <br />is shown. <br /> <br />The floodway along the Colorado River and Eagle River detailed studied reach were <br />determined by specifYing the elevations and stations of the left and right encroachments <br />for individual cross sections as desired in the HEC-RAS backwater model. <br /> <br />The floodway along Brush Creek was computed using two methods; equal conveyance <br />method and specifYing the elevations and stations of the left and right encroachments of <br />individual cross sections as desired in the HEC-RAS backwater model. <br /> <br />Because of the supercritical flow on Reach 2 of Brush Creek, the floodway was <br />delineated to include all of the 100-year flood limits except for areas of ineffective flow. <br /> <br />The area between the floodway and 100-year floodplain boundaries is termed the <br />floodway fringe. The floodway fringe encompasses the portion of the floodplain that <br />could be completely obstructed without increasing the WSEL of the IOO-year flood more <br />than 1 foot at any point. Typical relationships between the floodway and the floodway <br />fringe and their significance to floodplain development are shown in Figure 1. <br /> <br />29 <br />