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<br />floodplain boundaries are close together, only the 100-year <br />floodplain boundary has been shown. Small areas within the <br />floodplain boundaries may lie above the flood elevations but cannot <br />be shown due to limitations of the n,ap scale and/or lack of <br />detailed topographic data. <br /> <br />For the streams studied <br />floodplain boundary is <br />(Exhibit 2.) <br /> <br />by approximate <br />shown on the <br /> <br />methods, only the 100-year <br />Flood Insurance Rate Map <br /> <br />. <br /> <br />Floodplain boundaries in areas studied by approximate methods were <br />either taken from COE Flood Hazard Information reports for the area <br />(References 15 and 25) or established according to the professional <br />judgment of engineers familiar with the region, taking into account <br />flood elevations from historical flood data, minor hydrologic and <br />hydraulic analyses, correlations with similar stream basins studied <br />in detail, field observations, and available topographic data <br />(Reference 23). In addition, areas of approximate study along <br />portions of the Gunnison and Dolores Rivers were taken from the <br />Flood Hazard Boundary Map (Reference 26). <br /> <br />. <br /> <br />Some areas in Mesa County are subject to broad, shallow, overland <br />flooding that is generally less than 3 feet deep and characterized <br />by unpredictable flow paths. The water-surface elevations of <br />flooding in these areas are essentially independent of those along <br />adjacent streamways and are affected principally by barriers to <br />flow in the flooded areas. <br /> <br />4.2 <br /> <br />F100dways <br /> <br />. <br /> <br />Encroachment on floodplains, such as structures and fill, reduces <br />flood-carrying capacity, increases flood heights and velocities, <br />and increases flood hazards in areas beyond the encroachment <br />itself. One aspect of floodplain management involves balancing the <br />economic gain from floodplain development against the resulting <br />increase in flood hazard. For purposes of the NFIP, a f100dway is <br />used as a tool to assist local communities in this aspect of <br />floodplain management. Under this concept, the area of the 100- <br />year floodplain is divided into a floodway and a floodway fringe. <br />The floodway is the channel of a stream, plus any adjacent <br />floodplain areas, that must be kept free of encroachment so that <br />the 100-year flood c,n be carried without substantial increases in <br />flood heights. Minimum Federal standards limit such increases to <br />1.0 foot, provided that hazardous velocities are not produced. The <br />floodways in this study are presented to local agencies as minimum <br />standards that can be adopted directly or that can be used as a <br />basis for additional floodway studies. <br /> <br />25 <br />