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<br />must be kept free of encroachment so that the 100-year flood can <br />be carried without substantial increases in flood heights. Minimum <br />Federal standards limit such increases to 1.0 foot, provided that <br />hazardous velocities are not produced. The floodways in this study <br />are presented to local agencies as minimum standards that can be <br />adopted or that can be used as a basis for additional floodway. <br /> <br />The floodways for this study, with the exception of portions of <br />Happy Canyon Creek, were computed based on equal-conveyance reduc- <br />tion from each side of the flood plain. For portions of Happy <br />Canyon Creek where flow diversions occur, floodways were calculated <br />based on conveyance reduction from one side of the flood plain. <br />The results of these computations were tabulated at selected cross <br />sections for each stream segment for which a floodway was computed <br />(Table 6). <br /> <br />For any channel modification or fill in the sheetflow areas, a <br />detailed review of the improvement should be undertaken to deter- <br />mine the effects downstream. <br /> <br />AS shown on the Flood Boundary and Floodway Map (Exhibit 2), the <br />floodway boundaries were computed at cross sections. Between cross <br />sections, the boundaries were interpolated. In cases where the <br />floodway and the 100-year flood plain boundaries are either close <br />together or collinear, only the floodway boundary has been shown. <br /> <br />The area between the floodway and 100-year flood plain boundaries <br />is termed the floodway fringe. The floodway fringe encompasses <br />the portion of the flood plain that could be completely obstructed <br />without increasing the water-surface elevation of the 100-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 flood plain development are shown in Figure 11. <br /> <br />24 <br />