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<br />4.1 Flood Boundaries <br /> <br />To provide a national standard without regional discrimination, <br />the 1 percent annual chance (lOO-year) flood has been adopted by <br />FEMA as the base flood for flood plain management purposes. The <br />0.2 percent annual chance (500-year) flood is employed to indicate <br />additional areas of flood risk in the community. For each stream <br />studied in detail, the 100- and 500-year flood plain boundaries <br />have been delineated using the flood elevations determined at each <br />cross section. Between cross sections, the boundaries were inter- <br />polated using topographic maps at a scale of 1:2,400, with a contour <br />interval of 2 feet (Reference 6). For streams studied by approximate <br />methods, the 100-year flood plain boundaries were delineated using <br />topographic maps at a scale of 1:24,000 with a contour interval of <br />40 feet (Reference 8). <br /> <br />The 100- and 500-year flood plain boundaries are shown on the Flood <br />Boundary and Floodway Map (Exhibit 2). In cases where the 100- <br />and 500-year flood plain boundaries are close together, only the <br />100-year flood plain boundary has been shown. Small areas within <br />the flood plain boundaries may lie above the flood elevations but <br />cannot be shown due to limitations of the map scale and/or lack of <br />detailed topographic data. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as structures and fill, reduces <br />flood-carrying capacity, increases flood heights and velocities,- <br />and increases flood hazards in areas beyond the encroachment itself. <br />One aspect of flood plain management involves balancing the economic <br />gain from flood plain development against the resulting increase <br />in flood hazard. For purpOses of the NFIP, a floodway is used as <br />a tool to assist local communities in this aspect of flood plain <br />management. Under this concept, the area of the 100-year flood <br />plain is divided into a floodway and a floodway fringe. The flood- <br />way is the channel of a stream, plus any adjacent flood plain areas, <br />that must be kept free of encroachment so that the 100-year flood <br />can be carried without substantial increases in flood heights. <br />Minimum Federal standards limit such increases to 1.0 foot, provided <br />that hazardous velocities are not produced. The floodways in this <br />study are presented to local agencies as minimum standards that <br />can be adopted directly or that can be used as a basis for addi- <br />tional floodway studies. <br /> <br />The floodways presented in this study were computed on the basis <br />of equal-conveyance reduction from each side of the flood plain. <br />The results of these computations are tabulated at selected cross <br />sections for each stream segment for which a floodway is computed <br />(Table 2). <br /> <br />9 <br /> <br />-'"'> <br />