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<br />The approximate lOO-year flood boundary for French Gulch was taken <br />from a Flood Plain Information report for Breckenridge (Reference 8), <br />and studied in conjunction with the computed flood elevations, Lower <br />portions of French Gulch have been relocated since the Flood Plain <br />Information report was completed. The new stream location and flood <br />boundary were adjusted using photoquadrangle and topographic maps <br />(References 25 and 26) , <br /> <br />Flood boundaries in the shallow flooding areas were delineated using <br />topographic maps (References 12 and 14, 17 through 24, and 27), <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 (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 F100dways <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 of <br />flood plain management involves balancing the economic gain from flood <br />plain development against the resulting increase in flood hazard. For <br />purposes of the National Flood Insurance Program, the concept of a <br />floodway is used as a tool to assist communities in this aspect of <br />flood plain management. Under this concept, the area of the lOO-year <br />flood is divided into a floodway and a floodway fringe, The floodway <br />is the channel of a stream, plus any adjacent flood plain areas, that <br />must be kept free of encroachment in order that the IOO-year flood be <br />carried without substantial increases in flood heights. As minimum <br />standards, the Federal Insurance Administration limits such increases <br />in flood heights to 1.0 foot, provided that hazardous velocities are <br />not produced, <br /> <br />The scope of work of a limited detailed study is such that a floodway <br />determination is not made; therefore, a floodway has not been <br />delineated for Blue River. <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 />As shown on the Flood Boundary and Floodway Map (Exhibit 2), the <br />floodway boundaries were determined at cross sections; between cross <br />sections, the boundaries were interpolated. In cases where the <br />floodway and lOO-year flood boundaries are close together, only the <br />floodway boundary has been shown, <br /> <br />The area between the floodway and boundary of the 100-year flood is <br />termed the floodway fringe. The floodway fringe thus encompasses the <br />portion of the flood plain that could be completely obstructed without <br />increasing the water-surface elevation of the lOO-year flood more than <br />1.0 foot at any point. Typical relationships between the floodway and <br />the f100dway fringe and their significance to flood plain development <br />are shown in Figure 2. <br /> <br />15 <br />