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<br />In cases where the 100- and SOQ-year flood boundaries are close <br />together, only the lOO-year flood boundary has been shown. <br /> <br />Flood boundaries for the 100- and SOQ-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 />Approximate boundaries were delineated on the basis of the determined <br />elevations. <br /> <br />4.2 Floodways <br /> <br />~ncroachment on flood plains, such as artificial fill, reduces the <br />flood-carrying capacity and increases flood heights, thus increasing <br />flood hazards i~ 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 />haza~d. For purposes of the Flood Insurance Program, the concept of <br />a floodway is used as a tool to assist local communities in this <br />aspect of flood plain management. Under this concept~ the area of <br />Ule lO~-year flooe is divided into a floodway and a floodway fringe. <br />T~e floodway is the channel of a stream, plus any adjacent flood <br />plai~ areas, L~at must be kept free 0= encroachment in order that <br />the lOO-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 1.0 foot, provided that <br />hazardous velocities are not produced. <br /> <br />The floodway for the Animas River, Mineral Creek~ and Cement Creek <br />was computeci on the basis of equal conveyance reduc~ion from each <br />side of the flood plain. The results of these comrutations are <br />tabulated at selected cross sections for each stream segment for <br />which a floodway is computed (Table 3). <br /> <br />As shown on the Flood Boundary and Floodway Map {Exhibit 2), the <br />floodway boundaries were determined at cross sectionSi between cross <br />sections, the boundaries were interpolated. In cases where the <br />floodway and IOO-year flood boundaries are close together, only the <br />floodway boundary has been shown. <br /> <br />The area between the floodway and the boundary of the IOO-year flood <br />is ~ermed the floodwav fri~ge. T~e =loodway fr~~ge thus encompasses <br />the portion of the flood plain that could be completely obstructed <br />without increasing the water-surface elevation of the lOO-year flood <br />more than 1.0 foot at any point. Typical relationships between the <br />floodway and the floodway fringe and their significance to flood <br />plain development are shown in Figure 3. <br /> <br />12 <br />