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4.1 Flood Plain Boundaries To provide a national standard without regional discrimination, the 1 percent annual chance (lOO-year) flood has been adopted by FEMA as the base flood for flood plain management purposes. The 0.2 percent annual chance (SOO-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the 100- and SOO-year flood plain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were inter- polated using aerial topographic maps at scales of 1:2,400 and 1:6,000 with contour intervals of 2 and S feet (Reference 18, 19, 20, 21, and 22), topographic maps at scales of 1:1,200 with contour intervals of 2 feet (References 2, 24, and 2S) and "as constructed" channelization plans at scales of 1:600 and 1:1,200 with contour intervals of 2 feet (References 23, 26, 27, 28, and 29). Some flood plain boundaries for reaches studied by approximate methods were delineated using the existing Flood Hazard Boundary Maps for the City of Colorado Springs and El Paso County (References 30 and 31) . The 100- and SOO-year flood plain 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 SOO-year flood plain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 100- and SOO-year flood plain boundaries are close together, only the 100-year flood boundary has been shown. Small areas within the flood plain boundaries may lie above the flood elevations but cannot be shown due to limitation of the map scale and/or lack of detailed topographic data. 4.2 Floodways Encroachment on flood plains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of flood plain management involves balancing the economic gain from flood plain 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 flood plain management. Under this concept, the area of the lOa-year flood plain is divided into a floodway and a floodway fringe. The flood- way is the channel of a stream, plus any adjacent flood plain 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. The floodways in this study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a b~sis for additional floodway studies. ' -- 22