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<br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />A prime purpose of the National Flood Insurance Program is to encourage <br />State and local governments to adopt sound flood plain management pro- <br />grams. Each Flood Insurance Study, therefore, includes a flood boundary <br />map designed to assist communities in developing sound flood plain <br />management measures. <br /> <br />4.1 Flood Boundaries <br /> <br />In order to provide a national standard without regional discrimina- <br />tion. the 100-year flood has been adopted by the Federal Insurance <br />Administration as the base flood for purposes of flood plain manage- <br />ment measures. The 500-year flood is employed to indicate additional <br />areas of flood risk in the community. For each stream studied in <br />detail, the boundaries of the 100- and 500-year floods have been <br />delineated using the flood elevations determined at each cross <br />section; between cross sections, the boundaries were interpolated <br />using topographic maps at scale of 1:6000, with a contour interval <br />of 4 feet for the Cache La Poudre River Reference 8) and a scale <br />of 1:4800 for Spring Creek and Dry Creek, with a contour interval <br />of 2 feet (Reference 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 />Flood boundaries for the 100- and 500-year floods are shown on the <br />Flood Boundary and Floodway Map (Exhibi~ 2) . <br /> <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, 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 from <br />flood plain development against the resulting increase in flood <br />hazard. For purposes of the National Flood Insurance Program, the <br />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 100-year flood be carried without substantial increases in <br />flood heights. As minimum standards, the Federal Insurance Adminis- <br />tration limits such increases in flood heights to 1.0 foot, providec <br />that hazardous velocities are not produced. <br /> <br />9 <br />