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<br />4.1 Flood Bounda~ies <br /> <br />In order to provide a national standard without regional discriminu- <br />tion, the 100-year flood has been adopted by the Federal Emergency <br />Management Agency as the base flood for purposes of flood plain <br />management measures. The SOO-year flood is employed to i~dicate <br />additional areas of flood risk in the cO~TIunity. For each stream <br />studied in detail, the boundaries of the 100- and 500-year floods <br />have been delineated using the flood elevations determined at <br />each cross section; between cross sections, the boundaries were <br />interpolated using topographic maps at a scale of 1:2400, with <br />a contour interval of 4 and 5 feet (References 13 and 14). <br /> <br />Flood boundaries for the 100- and SOQ-year floods are shown on <br />the Flood Boundary and Floodway Map (Exhibit 2). In cases where <br />the 100- and 500-year flood boundaries are close together, only <br />the 100-year flood boundary has been shown. Small areas within <br />the flood boundaries may lie above the flood elevations and, there- <br />fore, not be subject to flooding; owing to limitations of the <br />map scale, such areas are not shown. <br /> <br />4.2 Floodways <br /> <br />EncJ:oachment on flood plains, such as artificial fill, reduces <br />the flood-carrying capacity, increases the flood heights of str~ams, <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 National Flood Insurance <br />Program, the concept of a floodway is used as a tool to assist <br />local communities in this aspect of flood plain management. Under <br />this concept, the area of the lOO-year flood is divided into a <br />floodway and a floodway fringe. The floodway is the channel of <br />a stream plus any adjacent flood plain areas that must be kept <br />free of encroachment in order that the lOa-year flood may be carried <br />without substantial increases in flood heights. Minimum standards <br />of the Federal Emergency Management Agency limit such increases <br />in flood heights to 1.0 foot, provided that hazardous velocities <br />are no't produced. The floouways in this repor.t are presented <br />to local agencies as minimum standards that can be adopted or <br />that can be used as a basis for ~dditional Rtudies. <br /> <br />Floodways for this study were computed on the basis of equal- <br />conveyance reduction from each side of the flood plain. <br /> <br />The results of these computations 'rl'2rf?; tabulated at select~r1 cross <br />sections for each stream segmp-nt for \rlhich a floodway \.,a5 r::omplJ t0d <br />(Table 2). <br /> <br />As show:1 on the Flood Boundary and Flood,<ay Map (Exhibit 2), the <br />flood\'lay widths were determined at cross sections; betw21':~n cross <br /> <br />11 <br />