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<br />information that my be available at the local community map repository before <br />making flood elevation and/or floodplain boundary determinations. <br /> <br />4.1 Flood Boundaries <br /> <br />In order to provide a national standard without regional <br />discrimination, the IOO-year flood has ~een adopted by the Federal <br />Insurance Administration as the base flood for purposes of flood plai~ <br />management measures. The SOD-year flood is employed to indicate <br />additional areas of flood risk in the community. For each stream <br />studied in deLail, the bouIldaries of the 100- arId SOD-year floods have <br />been delineated using the flood elevations determined at each cross <br />section; between cross sections, the boundaries were interpolated using <br />topographic maps at a scale of 1:1,200, with a contour interval of 2 <br />feet (Reference 8). The boundaries of the shallow flooding areas were <br />also determined using topographic maps (Reference 8). <br /> <br />In cases where the 100- and SOD-year flood boundaries are close <br />together, only the lOO-year flood boundary has been shown. <br /> <br />Flood boundaries for the 100- and SOD-year floods are shown on the <br />Flood Boundary and Floodway Map ~Exhibit 2). <br /> <br />Small areas within the flood boundaries may lie above the flood <br />elevations and, therefore, not be subject to f].ooding; owing to <br />limitations of the map scale and/or lack of detailed topographic data, <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 increasing <br />flood hazards in areas beyond the encroachment itself. One aspect of <br />flood plain management involves balancing the economic gain from flood <br />plain development against the resulting increase in flood hazard. For <br />purposes of the National Flood Insurance Program, the concept of a <br />floodway is used as a tool to assist local communities in this aspect <br />of flood plain management. Under this concept, the area of the 100- <br />year flood is divided into a floodway and a floodway fringe. The <br />floodway is the channel of a stream, plus any adjacent flood plain <br />areas, that must be kept free of encroachment in order that the 100- <br />year flood be carried without substantial increases in flood heights. <br />As minimum standards, the Federal Insurance Administration limits such <br />increases in flood heights to 1.0 foot, provided that hazardous <br />velocities are not produced. <br /> <br />The floodways presented in this ~tudy were computed on the basis of <br />equal conveyance reduction from each side of the flood plain. The <br />results of these computations are tabulated at selected cross sections <br />for each stream segment for which a floodway was computed (Table 2) . <br /> <br />As shown on the Flood Boundary and Floodway Map (Exhibit 2), the <br />floodway boundaries were determined at cross sections; between cross <br />sections, the boundaries were interpolated. In cases where the <br />floodway and 100-year flood boundaries are close together, only the <br />floodway boundary has been shown. <br /> <br />The area between the floodway and the boundary of the 100-year flood is <br />termed the floodway fringe. The floodway fringe thus encompasses the <br />portion of the flood plain that could be completely obstructed without <br />increasing the water-surface elevation of the 100-year flood more than <br />1.0 foot at any point. Typical relationships between the floodway and <br />the floodway fringe and their significance to flood plain development <br />are shown in Figure 2. <br /> <br />8 <br />