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<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 />Approximate 100-year flood plain boundaries were delineated on <br />USGS topographic maps (References 29. 30, and 31) using elevations <br />estimated by the hydraulic analyses described previously. <br /> <br />For the streams studied by approximate methods, only the 100-year <br />flood plain boundary is shown on the Flood Insurance Rate Map <br />(Exhibit 2). <br /> <br />Approximate 100-year flood plain boundaries in some portions of <br />the study area were taken directly from the effective Insurance <br />Rate Map for Douglas County (Reference 32). <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, 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 flood plain management involves balancing <br />the economic gain from flood plain development against the resul- <br />ting increase in flood hazard. For purposes of the NFIP, a flood- <br />way is used as a tool to assist local communities in this aspect <br />of flood plain management. Under this concept, the area of the <br />100-year flood plain is divided into a floodway and a floodway <br />fringe. The floodway is the channel of a stream, plus any adja- <br />cent flood plain areas, that must be kept free of encroachment so <br />that the lOa-year flood can be carried without substantial in- <br />creases in flood heights. Minimum Federal standards limit such <br />increases to 1.0 foot, provided that hazardous velocities are not <br />produced. The floodways in this study are presented to local <br />agencies as minimum standards that can be adopted directly or that <br />can be used as a 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 flood plain. Floodway widths were computed at <br />cross sections. Between cross sections, the floodway boundaries <br />were interpolated. The results of the floodway computations are <br />tabulated for selected cross sections (Table 2). The computed <br />floodways are shown on the Flood Insurance Rate Map (Exhibit 2). <br />In cases where the floodway and 100-year flood plain boundaries <br />are either close together or collinear, only the floodway boundary <br />is shown. <br /> <br />SCS and COE computer programs (References 33 and 34. respectively) <br />were used to establish the floodway width at cross sections. <br /> <br />27 <br />