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<br />4.1 Flood Plain Boundaries <br /> <br />To provide a national standard without regional discrimination, the 1 <br />percent annual chance (100-year) flood has been adopted by FEMA as <br />the base flood for flood plain management purposes. The 0.2 percent <br />annual chance (SOO-year) flood is employed to indicate additional <br />areas of flood risk in the community. For each stream studied in <br />detail, the 100- and SOO-year flood plain boundaries have been <br />delineated using the flood elevations determined at each cross <br />section. Between cross sections, the boundaries were interpolated <br />using aerial topographic maps at scales of 1:2,400 and 1:6,000 with <br />contour intervals of 2 and S feet (Reference 18, 19, 20, 21, and 22), <br />topographic maps at scales of 1:1,200 with contour intervals of 2 <br />feet (References 2, 24, and 2S) and "as constructed" channelization <br />plans at scales of 1:600 and 1:1,200 with contour" intervals of 2 feet <br />(References 23, 26, 27, 28, and 29). Some flood plain boundaries for <br />reaches studied by approximate methods were delineated using the <br />existing Flood Hazard Boundary Maps for the City of Colorado Springs <br />and El Paso County (References 30 and 31). <br /> <br />The 100- and SOO-year frood plain boundaries are shown on the Flood <br />Insurance Rate Map (Exhibit 2). On this map, the 100-year flood <br />plain boundary corresponds to the boundary of the areas of special <br />flood hazards (Zones A and AE); and the SOO-year flood plain boundary <br />corresponds to the boundary of areas of moderate flood hazards. In <br />cases where the 100- and SOO-year flood plain boundaries are close <br />together, only the 100-year flood boundary has been shown. Small <br />areas within the flood plain boundaries may lie above the flood <br />elevations but cannot be shown due to limitation of the map scale <br />and/or lack of detailed topographic data. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as structures and fill, reduces <br />flood-carrying capacity, increases flood heights and velocities, and <br />increases flood hazards in areas beyond the encroachment itself. One <br />aspect of flood plain management involves balancing the economic gain <br />from flood plain development against the resulting increase in flood <br />hazard. For purposes of the NFIP, a floodway is used as a tool to <br />assist local communities in this aspect of flood plain management. <br />Under this concept, the area of the 100-year flood plain is divided <br />into a floodway and a floodway fringe. The floodway is the channel <br />of a stream, plus any adjacent flood plain areas, that must be kept <br />free of encroachment so that the laO-year flood can be carried <br />without substantial increases in flood heights. Minimum Federal <br />standards 1 imit such increases to 1. 0 foot, provided that hazardous <br />velocities are not produced. The floodways in this study are <br />presented to local agencies as minimum standards that can be adopted <br />directly or that can be used as a basis for additional floodway <br />studies. <br /> <br />19 <br />