Laserfiche WebLink
<br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as artificial fill, reduces <br />the flood-carrying capacity, increases the flood heights of streams, <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 100-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 100-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 not produced. The floodways in this report are presented <br />to local agencies as minimum standards that can be adopted or <br />that can be used as a basis for additional 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 were 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 widths were determined at cross sections, between cross <br />sections, the boundaries were interpolated. In cases where the <br />boundaries of the floodway and the 100-year flood are either close <br />together or collinear, only the floodway boundary has been shown. <br /> <br />The area between the floodway and the boundary of the lOO-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 />100-year flood more than 1.0 foot at any point. Typical relation- <br />ships between the floodway and the floodway fringe and their signif- <br />icance to flood plain development are shown in Figure 2. <br /> <br />10 <br />