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4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the l-percent annual chance (lOO-year) flood has been adopted by FEMA as the base flood for floodplain management, purposes. The 0.2-percent annual change (SOD-year) flood is employed to indicate additional areas of flood risk in the conununity. For each stream studied in detail, the 100- and SOD-year floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:1,200, with a contour interval of 2 feet (Reference 8). For the revision to Animas River from HP 63.000 to HP 63.251, the boundaries were interpolated using topographic maps with contour interval of 1 foot (Reference 9). The 100- and SOD-year floodplain boundaries are shown on the Flood Insurance Rate Map (Exhibit 2). On this map, the 100-year floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zone AE); and the SOD-year floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 100- and SOD-year floodplain boundaries are close together, only the 100-year floodplain boundary has been shown. Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. 4.2 Floodways Encroachment on floodplains, such as structures and fi 11, reduces flOOd-carrying capacity, increases flood heights and velocities. and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the lOO-year floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the lOO-year flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to 1.0 foot, provided that hazardous ve10ci ties are not produced. The floodways in thi s study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional f100dway studies. The floodway of the Animas River and Junction Creek was computed on the basis of equal conveyance reduction from each side of the floodplain. The results of these computations are tabulated at selected cross sections for each stream segment for which a floodway is computed (Table 2). 9