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<br />Between cross sections, the boundaries were interpolated using <br />orthophoto contour maps at a scale of 1: 2,400, wi th a contour <br />interval of 2 feet (References 6 and 7) for Gunnison River and <br />Tomichi Creek, while topographic maps at a scale of 1:6,000 with a <br />S-foot contour interval were used for the North Fork Gunnison <br />River (Reference 1). <br /> <br />Approximate 100-year floodplain boundaries in some portions of the <br />study area were taken directly from the Flood Hazard Boundary Map <br />(FHBM) for the unincorporated areas of Gunnison County <br />(Reference 9). <br /> <br />The 100 and SOO-year floodplain boundaries are shown on the Flood <br />Insurance Rate Map (FIRM) (Exhibit 2). On this map, the lOO-year <br />floodplain boundary corresponds to the boundary of the areas of <br />special flood hazards (Zones A, AE), and the SOO-year floodplain <br />boundary corresponds to the boundary of areas of moderate flood <br />hazards. In cases where the 100- and SOO-year floodplain <br />boundaries are close together, only the 100-year floodplain <br />boundary has been shown. Small areas within the floodplain <br />boundaries may lie above the flood elevations, but cannot be shown <br />due to limitations of the map scale and/or lack of detailed <br />topographic data. <br /> <br />For the streams studied by approximate methods, only the 100-year <br />floodplain boundary is shown on the (FIRM). <br /> <br />4.2 Floodways <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. One <br />aspect of floodplain management involves balancing the economic <br />gain from floodplain development against the resulting increase in <br />flood hazard. For purposes of the NFIP, a floodway is used as a <br />tool to assist local communities in this aspect of floodplain <br />management. Under this concept, the area of the 100-year flood- <br />plain is divided into a floodway and a floodway fringe. The <br />floodway is the channel of a stream, plus any adjacent floodplain <br />areas that must be kept free of encroachment so that the 100-year <br />flood can be carried without substantial increases in flood <br />heights. Minimum federal standards limit such increases to <br />1.0 foot, provided that hazardous velocities are not produced. <br /> <br />Floodways were not calculated for the flooding sources studied. <br /> <br />10 <br />