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4,0 FLOOD PLAIN MANAGEMENT APPLICATIONS A prime purpose of the National Flood Insurance Program is to encourage State and local governments to adopt sound flood plain management pro- grams. Each Flood Insurance Study, therefore, includes a flood boundary map designed to assist communities in developing sound flood plain management measures. 4.1 Flood Boundaries In order to provide a national standard without regional discrimina- tion, the 100-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of flood plain manage- ment measures. The SOQ-year flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the boundaries of the 100- and 500-year floods have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated llsing topographic maps at a scale o~ 1:4800, with a contour interval of 4 feet (Reference 8). In cases where the 100- and SOO-year flood boundaries are close together, only the 100-year flood boundary has been shown. Flood boundaries for the 100- and 500-year floods are shown on the Flood Boundary and Floodway Map (Exhibit 2), Small areas within the flood boundaries may lie above the flood elevations and, therefore, not be subject 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 ca~acity and increases flood heights, thus increasing 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 100-year flood is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment in order that the lOa-year flood be carried without substantial increases in flood heights. As minimum standards, the Federal Insurance Adminis- tration limits such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. 8 ~ j ~ , J ~ , . . (~... I ':l 'I t ,I ,