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<br />using topographic maps at a soale <br />of 20 and 40 feet (Reference 14). <br /> <br />I <br />~f <br />I <br />i <br />! <br /> <br />1;24,000, with contour intervals <br /> <br />Approximate flood boundaries in so~e portions of the study area were <br />taken from the Federal Insurance Aqministration's Flood Hazard <br />Boundary Map (Reference 15). <br /> <br />Flood boundaries for the 100- and SOO-year floods are shown on the <br />Flood Boundary and Floodway Map (E~hibit 2). In cases where the <br />100- and SOO-year flood boundarieslare close together, only the <br />100-year flood boundary has been s~own. <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 />, <br /> <br />4.2 Floodways <br /> <br />I <br />Encroachment on flood plains, suchias artificial fill, reduces the <br />flood-carrying capacity and increa~es flood heights, thus increasing <br />flood hazards in areas beyond the ~ncroachment 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 diviqed into a floodway and a floodway <br />fringe. The floodway is the chann~l of a stream, plus any adjacent <br />flood plain areas, that must be ke~t free of encroachment in order <br />that the 100-year flood be carried]without substantial increases in <br />, <br />flood heights. As minimum standarqs, the Federal Insurance Admin- <br />istration limits such increases iniflood heights to 1.0 foot, <br />provided that hazardous velocities ,are not produced. <br /> <br />A floodway was not computed for He,mosa Creek. It was determined <br />that a calculated floodway would not be appropriate because most of <br />the flow is supercritical with hig~ velocities. Encroachment of <br />any kind within the 100-year floodlplain is not recommended, Con- <br />sequently, the lOO-year boundary f4r this stream has been designated <br />as the floodway boundary, except i~ areas of nonconveyance. Data <br />shown in Table 2 are based on 100-year, unencroached condition. <br /> <br />The floodways for the remaining streams in the study area were <br />computed on the basis of equal-conieyance reduction from each <br />side of the flood plain. The results of these computations <br />are tabulated at selected cross se6tions for each stream <br />segment for which a floodway is co~puted (Table 2). <br />I <br /> <br />13 <br />