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<br />4.D FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />The National Flood I nsurance Program encourages state and local govern- <br />ments to adopt sound flood plain management programs. Therefore, each <br />Flood Insurance Study incl udes a fl ood boundary map designed to assi st <br />communities in developing sound flood plain management measures. <br /> <br />4.1 FlOod Boundaries <br /> <br />In order to provide a national standard without regional discrimi- <br />nation, the lOO-year flood has been adopted by FEMA as the base <br />f1 ood for purposes of fl ood pl a i n management measures. The <br />5DO-year flood is employed to indicate additional areas of flood <br />risk in the community, For each stream studied in detail, the <br />boundaries for the 10D- and 50D-year floods have been delineated <br />usi ng the fl ood el evati ons determi ned at each cross secti on; <br />between cross sections, the boundaries were interpolated using <br />topographic maps at a scale of 1:24DO, with a contour interval of <br />2 feet (Reference 8). In cases where the 100-and 500-year flood <br />boundari es are close together, only the 10D-year fl ood boundary ha s <br />been shown. <br /> <br />Flood boundaries for the 100- and 500-year floods are shown on the <br />Flood Boundary and Floodway Map. Small areas within the flood <br />boundaries may lie above the flood elevations and, therefore, may <br />not be sUbject to flooding. Owing to limitations of the map scale, <br />such areas are not shown. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as artificial fill, reduces the <br />flood-carrying capacity and increases flood heights, thus <br />increasing flood hazards in areas beyond the encroachment itsel f. <br />Dne aspect of flood plain management involves balancing the <br />economic gain from flood plain development against the resulting <br />increase in flood hazard. For purposes of the National Flood <br />I nsurance Program, the concept of a fl oodway is used as a tool to <br />assist local communites in this aspect of flood plain management. <br />Under thi s concept, the area of the 100-year fl ood is divided into <br />a floodway and a floodway fringe. The floodway is the channel of a <br />stream, plus any adjacent flood plain areas, that must be kept free <br />of encroachment so that the IDO-year flood may be carried without <br />substantial increase in flood heights. Minimum standards of the <br />Federal Emergency Management Agency 1 imit such increases in flood <br />heights to 1.0 foot, provided that hazardous velocities are not <br />produced. The f100dways in this report are presented to local <br />agencies as minimum standards that can be adopted or that can be <br />used as a basis for additional studies. <br /> <br />The floodways presented in this study were computed on the basis of <br />a maximum one- foot ri se in water surface el evation over the <br />100-year water surface, for the encroached fl oOdway. The results of <br /> <br />8 <br />