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4.2 Floodways 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 itself. One aspect of flood plain management involves balancing the econOOlic 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 communites in this aspect of flood plain management. Under this concept, the area of the IOO-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 so that the IOO-year flood may be carried without substantial increase in flood heights. Minimum standards of the Federal Emergency Management Agency limit 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 max imum one- foot ri se in water surface el evati on over the IOO-year wa ter surface, for the encroached fl oodway. The resul ts of these computations were tabulated at selected cross sections for each stream segment for which a floodway was computed (Table 2). As shown on the Flood Boundary and Floodway Map, the fl oodway boundaries were determined at cross sections, and between cross sections the boundaries were interpolated. In cases where the f100dway and IOO-year flood boundaries are close together or co1;n- ear, onl y the fl oodway boundary ha s been shown. The area between the floodway and the boundary of the IOO-year flood ;s tenned the floodway fringe. The floodway fringe thus encompasses the portion of the flood plain that could be cOOlpletely obstructed wi thout increasi n9 the water-surface el evati on of the IOO-year fl ood more than 1.0 feet at any point. Typical relationships between the floodway and the floodway fringe and their significance to flood plain development are shown in Figure 2. 5.0 INSURANCE APPLICATION In order to establish actuarial insurance rates, Federal Emergency Management Agency has developed a process to transfonn the data from the engineering study into fiood insurance criteria. This process includes the detennination of reaches, Flood Hazard Factors (FHF), and flood insurance zone designations for each flooding source studied in detail affecting the City of Sal ida. 9