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flowlines, used to establish the respective profile distances, are delineated and labeled as Profile Base Lines. 4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound flood plain management programs. To assist in this endeavor, each FIS provides lOO-year flood plain data, which may include a combination of the following: 10-, 50-, 100-, and 500-year flood elevations; delineations of the lOa-year and SOO-year flood plains; and lOa-year floodway. This information is presented on the FIRM and in many components of the FIS, including Flood Profiles, Floodway Data tables and Surrunary of Stillwater Elevation tables. Users should reference the data presented in the FIS as well as additional information that may be available at the local community map repository before making flood elevation and/or flood plain boundary determinations. 4.1 Flood Boundaries In order to provide a national standard without regional discrimination, the lOa-year flood has been adopted by FEMA as the base flood for purposes of flood plain management measures. The SOO-year flood is employed to indicate additional areas of flood risk in the cormnunity. For each stream studied in detail, the boundaries for the 100- and SOO-year floods 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 1:2,400 with a contour interval of 2 feet (Reference 7). In cases where the lOa-year and SOQ-year flood boundaries are close together, only the lOa-year flood boundary has been shown. Small areas wi thin the flood boundaries lie above the flood elevations and, therefore, may not be subj ect to flooding; owing to limitations of the map scale, such areas are not shown. 4.2 Floodways Encroachment on flood plains, such as artificial fill, reduces the flood-carrying capacity and increases flood heights, thus increaseing 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 National Flood Insurance Program, the concept of 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 is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, -9-