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<br />4.1 Flood Boundaries <br /> <br />In order to provide a national standard without regional discrimina- <br />tion, the lOO-year flood has been adopted by the Federal Insurance <br />Administration as the base flood for purposes of flood plain <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 detail, the boundaries of the 100- and SOD-year floods <br />have been delineated using the flood elevations determined at each <br />cross section; between cross sections, the boundaries were interpo- <br />lated using topographic maps at scales of 1:1,200 and 1:24,000 <br />(enlarged to a scale of 1:6,000), with contour intervals of 2 and <br />40 feet (References 8, 9, and 10). <br /> <br />In cases where the 100- and SOO-year flood boundaries are close <br />together, only the 100-year flood boundary has been shown. <br /> <br />Shallow flooding boundaries were delineated on topographic maps at <br />a scale of 1:1200 (References 8 and 9). <br /> <br />For the stream studied by approximate methods, the boundary of the <br />100-year flood was taken from the Flood Hazard Boundary Map for the <br />Town of Frisco, produced in October 1976 (Reference 11). The <br />stream channel has been relocated south of Belford Street near <br />Eighth Avenue~ The neW stre~~ location was obtained from a school <br />site plan dated July 1978 (Reference 12). The Flood Hazard Boundary <br />Map flood boundary was revised accordingly. <br /> <br />Flood boundaries for the 100- and SOO--year floods are shown on the <br />Flood Boundary and Floodway Map (Exhibit 2)_ <br /> <br />Small areas within the flood bcundaries may lie above the flood <br />elevations and, therefore, not be subject to flooding; owing to <br />limitations of the map scale, 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 <br />of flood plain management involves balancing the economic gain <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 floodway is used as a tool to assist local communities <br />in this aspect of flood plain management. Under this concept, the <br />area of the 100-year flood is divided into a floodway and a floodway <br />fringe. The floodway is the channel of a stream, plus any adjacent <br />flood plain areas, that must be kept free of encroachment in order <br />that the lOO-year flood be carried without substantial increases <br />in flood heights. As minimum standards, the Federal Insurance <br />Administration limits such increases in flood heights to 1.0 foot, <br />provided that hazardous velocities are not produced. <br /> <br />7 <br />