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<br />Small areas within the flood boundaries may lie above the flood <br />elevations and, therefore, not be subject to flooding; owing to <br />limitations of the map scale and/or lack of detailed topographic <br />dat~, 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 aspec~ <br />of flood plain management involves balancing the economic gain r <br />from flood plain development against the resulting increase in <br />flood hazard. For purposes of the National Flood Insurance Program, <br />the concept of a floadway is used as a tool to assist local communi- <br />ties in this aspect of flood plain management. Under this concept, <br />the area of the lOa-year flood is divided into a floodway and a <br />floadway fringe. The floadway is the channel of a stream, plus <br />any adjacent flood plain areas, that must be kept free of encroach- <br />ment in order that the lOa-year flood be carried without substantial <br />increases in flood heights. AS minimum standards, the Federal <br />Insurance Administration limits such increases in flood heights to <br />1.0 foot, provided that hazardous velocities are not produced. <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 was computed <br />(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 <br />cross sections, the boundaries were interpolated. In cases where <br />the floodway and laO-year flood boundaries are close together, <br />only the floodway boundary has been shown. <br /> <br />The area between the floodway and the boundary of the lOa-year <br />flood is termed the floodway fringe. The floodway fringe thus <br />encompasses the portion of the flood plain that could be completely <br />obstructed without increasing the water-surface elevation of the <br />lOa-year flood more than 1.0 foot at any point. Typical relation- <br />ships between the floodway and the floodway fringe and their <br />significance to flood plain development are shown in Figure 2. <br /> <br />10 <br />