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<br />For streams studied by approximate methods, the boundary of the lOO- <br />year flood was established according to the professional judgment of <br />engineers familiar with the region, takin9 into account flood eleva- <br />tions from historical flood data, minor hydrologic and hydraulic <br />analyses, correlations with similar stream basins studied in detail, <br />and field observations. Sheet flow boundaries were determined using <br />topographic data (Reference 4), and confirmation by field inspection. <br /> <br />Small areas within the flood boundaries may lie above the level of <br />flooding and, therefore, not be subject to inundation. Due to <br />limitations of map scale, such areas are not shown. <br /> <br />c <br /> <br />.'S <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as artificial fill, reduces the <br />flood-carrying capacity and increases flood heights, thus increasing <br />flood hazards in areas beyond the encroachment itself. One aspect <br />of flood plain manasrement involves balancinsr the economic gain from <br />flood plain development against the resul'ting increase in flood <br />hazard. For purposes of the Flood Insurance Program, the concept of <br />a floodway is used as a tool to assist local interests in this <br />aspect of flood plain management. Under this concept, the area of <br />the lOa-year flood is divided into a floodway and a floodway fringe. <br />The floodway is the channel of the stream, plus any adjacent flood <br />plain areas, that must be kept free of encroachment in order that <br />the 100-year flood be carried without substantial increases in flood <br />heights. As minimum standards, the Federal Insurance Administration <br />limits such increases in flood heights to l.O foot, provided that <br />hazardous velocities are not produced. The floodways in this report <br />are presented to the county as minimum standards that can be adopted <br />or that can be used as a basis for further studies. <br /> <br />v <br /> <br />A floo,jway is generally not appropriate where the lOO-year flood <br />would be confined to the stream channel. <br /> <br />Floodways designated in this study were computed on the basis of <br />equal conveyance reduction from each side of the flood plain. <br />However, under no circumstance does the floodway encroach on the <br />stream channel itself. The results of these computations were <br />tabulated at selected cross sections for each stream se~ent studied <br />in detail where the lOa-year flood was no.t confined to the stream <br />channel (Table l). Floodway boundaries w,~re determined at cross <br />sections used in the analysis; between cross sections, the boundaries <br />were interpolated m:ing topographic data (Reference 5). <br /> <br />; ~t <br /> <br />.ft <br /> <br />Floodway boundaries are shown on the Floodway Map (Exhibit 3). <br /> <br />... <br /> <br />15 <br />