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<br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the <br />1-percent annual chance (100-year) flood has been adopted by FEMA <br />as the base flood for floodplain management purposes. The <br />0.2 percent annual chance (SaG-year) flood is employed to indicate <br />additional areas of flood risk in the community. For each stream <br />studied by detailed methods, the 100- and 500-year floodplain <br />boundaries have been delineated using the flood elevations <br />determined at each cross section. <br /> <br />Between cross sections, the boundaries for the NE Canon Drainage <br />Area were interpolated using aerial photography and contour mapping <br />prepared by Ponderosa Engineering, Canon City, Colorado, in <br />partnership with Kucera and Associates, at a scale of 1:2400, with <br />a contour interval of 2 feet (Reference 2). For the Arkansas <br />River, the boundaries between cross sections were interpolated from <br />aerial photography and contour mapping prepared by the COE, <br />Albuquerque District, in conjunction with Sharf & Associates, at a <br />scale of 1:4800, with contour intervals of 2 feet (Reference 1). <br /> <br />For the streams studied by approximate methods, only the 100-year <br />floodplain boundary is shown. Approximate 100-year floodplain <br />boundaries in some portions of the study area were taken directly <br />from the Flood Hazard Boundary Map (Reference 14). <br /> <br />The laO-year floodplain boundaries are shown on the Flood Insurance <br />Rate Map (Exhibit 2). On this map, the 100-year floodplain <br />boundary corresponds to the boundary of the areas of special flood <br />hazards (Zones A and AE). 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 />4.2 F100dways <br /> <br />For the streams studied by approximate methods, only the 100-year <br />floodplain boundary is shown on the Flood Insurance Rate Map. <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 <br />floodplain 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 determined for this FIS. <br /> <br />9 <br />