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<br />using topographic maps at a scale of 1:24,000, with contour intervals <br />of 20 and 40 feet (Reference 14). <br /> <br />Approximate flood boundaries in some portions of the study area were <br />taken from the Federal Insurance Administration's Flood Hazard <br />Boundary Map (Reference 15). <br /> <br />Flood boundaries for the 100- and SOD-year floods are shown on the <br />Flood Boundary and Floodway Map (Exhibit 2). In cases where the <br />100- and 500-year flood boundaries are close together, only the <br />100-year flood boundary has been shown. <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 this aspect of flood plain management. Under this concept, the <br />area of the 100-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 Admin- <br />istration limits such increases in flood heights to 1.0 foot, <br />provided that hazardous velocities are not produced. <br /> <br />A floodway was not computed for Hermosa Creek. It was determined <br />that a calculated floodway would not be appropriate because most of <br />the flow is supercritical with high velocities. Encroachment of <br />any kind within the 100-year flood plain is not recommended. Con- <br />sequently, the 100-year boundary for this stream has been designated <br />as the floodway boundary, except in areas of nonconveyance. Data <br />shown in Table 2 are based on IOO-year, Ullencroached condition. <br /> <br />The floodways for the remaining streams in the study area were <br />computed on the basis of equal-conveyance reduction from each <br />side of the flood plain. The results of these computations <br />are tabulated at selected cross sections for each stream <br />segment for which a floodway is computed (Table 2). <br /> <br />13 <br />