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4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS The National Flood I nsurance Program encourages state and local govern- ments to adopt sound flood plain management programs. Therefore, each Flood Insurance Study incl udes a fl ood boundary map designed to assi st communities in developing sound flood plain management measures. 4.1 FlOod Boundaries In order to provide a national standard without regional discrimi- nation, the lOO-year flood has been adopted by FEMA as the base fl ood for purposes of fl ood p1 a i n management measures. The 500-year flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the boundaries for the 100- and 500-year floods have been delineated usi ng the fl ood e1 evati ons determi ned at each cross secti on; between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:2400, with a contour interval of 2 feet (Reference B). In cases where the IOO-and 500-year flood boundari es are close together, only the IOO-year fl ood boundary ha s been shown. Flood boundaries for the 100- and 500-year floods are shown on the Flood Boundary and Floodway Map. Small areas within the flood boundaries may lie above the flood elevations and, therefore, may not be sUbject to flooding. Owing to limitations of the map scale, such areas are not shown. 4.2 F100dways Encroachment on flood plains, such as artificial fill, reduces the flood-carrying capacity and increases flood heights, thus increasing flood hazards in areas beyond the encroachment itsel f. 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 I nsurance Program, the concept of a fl oodway is used as a tool to assist local conmunites in this aspect of flood plain management. Under thi s concept, the area of the 100-year fl ood is divided into a floodway and a f100dway fringe. The floodway is the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment so that the 100-year flood may be carried without substantial increase in flood heights. Minimum standards of the Federal Emergency Management Agency 1 imit such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The floodways in this report are presented to local agencies as minimum standards that can be adopted or that can be used as a basis for additional studies. The floodways presented in this study were computed on the basis of a maximum one- foot ri se in water surface e1 evation over the 100-year water surface, for the encroached fl oOdway. The results of 8