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<br />For the San Juan River, North Fork McCabe Creek, West Fork McCabe <br />Creek, and McCabe Creek, the boundaries of the 100- and SOO-year <br />floods were adopted from the results of the flood hazard study by <br />the u.S. Soil Conservation Service (Reference 8). <br /> <br />Flood boundaries for the 100- and SOD-year floods are shoWn on the <br />Flood Boundary and Floodway Map (Exhibit 2). <br /> <br />In cases where the 100- and SOO-year flood boundaries are close <br />together, only the 100-year flood boundary has been shown. <br /> <br />Approximate boundaries were delineated according to hydraulic calcula- <br />tions and engineering judgment, using topographic maps at a scale of <br />1:24,000, with contour intervals of 20 (Reference 14) and 40 feet <br />(Reference 15), and at a scale of 1:62,500, with contour intervals <br />of 40 and 80 feet (References 16 and 17, respectively). <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 18). <br /> <br />~mall 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 invclves 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 lOa-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 laO-year flood be carried without substantial increases in <br />flood heights. As minimum standards, the Federal Insurance Adminis- <br />tration limits such increases in flood heights to 1.0 foot, provided <br />that hazardous velocities are not produced. <br /> <br />The floodways for the San Juan River, North Fork McCabe Creek, West <br />Fork McCabe Creek, McCabe Creek, and the Rio Blanco were computed on <br />the basis of equal conveyance reduction from each side of the flood <br />plain. The results of these computations are tabulated at selected <br />cross sections for eac~ stream segment for which a floodway is <br />computed (Table 3). <br /> <br />13 <br />