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4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the I-percent annual chance (100-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2-percent annual chance (500-year) flood is employed to indicate addi tional areas of flood risk in the community. For each stream studied by detailed methods, the boundaries of the 100- and 500-year floodplain have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topo- graphic maps at a scale of l:1,200 and 1:2,400 with a contour in- terval of 2 feet (Reference 10). Approximate areas of flooding were plotted using USGS Flood Prone Quadrangle Maps at a scale of l:24,000 with contour intervals of 40 feet (Reference ll). The 100- and 500-year floodplain boundaries are shown on the Flood Insurance Rate Map (Exhibit 2). On this map, the 100-year flood- plain boundary corresponds to the boundary of the areas of special flood hazards (Zones A and AE), and the 500-year floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the lOO- and 500-year floodplain boundaries are close together or collinear, only the 100-year floodplain boundary has been shown. Small areas wi thin 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. For the streams studied floodplain boundary lS (Exhibit 2). by approximate methods, only the 100-year shown on the Flood Insurance Rate Map 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment. 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 100-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 velocities are not produced. 10