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4.1 Floodplain 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 floodplain 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 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 1:4,800, wi th a contour interval of 2 feet and 10 feet NGVD for the Dolores River, West Dolores River, and Lost Canyon Creek (Reference 1). Topographic maps at a scale of 1:2,400, with a contour interval of 2 feet were used for Mancos River and Chicken Creek (Reference 2). Approximate 100-year floodplain boundaries the study area were taken directly from the Map (FHBM) (Reference 7). in some portions of Flood Hazard Boundary The 100-year floodplain boundaries are shown on the FIRM (Exhibit 2). On this map, the 100-year floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A, AE, AH, and AD). 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. For the streams studied by approximate methods, only the 100-year floodplain boundary is shown on the FIRM (Exhibit 2). 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 with the resulting potential 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 100-year flood can be carried without any substantial increase in flood height. Minimum federal standards limit such increases to 1.0 foot, provided that hazardous velocities are not produced. For this Flood Insurance Study, floodways were not computed for the flooding sources. 9