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<br />detail, the boundaries of the 100- and SOO-year floods have been <br />delineated using the flood elevations determined at each cross <br />section; between cross sections, the boundaries were interpolated <br />using topographic maps at a scale of 1:24,000, with a contour inter- <br />val of 40 feet (RGfercnce 7), <br /> <br />In cases where the 100- and SOO-year flood boundaries are close <br />together, only the IOO-year flood boundary has been shown. <br /> <br />The study contractor has determined that some areas shown on the <br />Federal Insurance Administration Flood Hazard Boundary Map (Refer- <br />ence 8) are areas of minimal flooding; therefore, they were not <br />delineated on the maps. <br /> <br />Flood boundaries for the 100- and SOO-year floods are shown on the <br />Flood Boundary and Floodway Map (Exhibit 2). <br /> <br />Small areas within the flood boundaries may lie above the flood <br />elevations and, therefore, not be subject to flooding; owing to <br />limitations of the map scale, such areas are not shown. <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 management involves balancing the economic gain from <br />flood plain development against the resulting increase in flood <br />hazard. For purposes of the National Flood Insurance Program, the <br />concept of a floodway is used as a tool to assist local communities <br />in t0is aspect of flood plain management. Under this concept, the <br />area of the lOO-year flood is divided into a floodway and a floodway <br />fringe. The floodway is the channel of a stream, plus any adjacent <br />flood plain areas, that must be kept free of encroachment in order <br />that the 100-year flood be carried without substantial increases in <br />flood heights. As minimum standards, the Federal Insurance Adminis- <br />Lration limits such increases in flood heights to 1.0 foot, provided <br />that hazardous velocities are not produced. <br /> <br />The floodways presented in this study were computed on the basis of <br />equal conveyance reduction from each side of the flood plain. The <br />results of these computations are tabulated at selected cross sections <br />for each stream segment for which a floodway is computed (Table 2). <br /> <br />8 <br />