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<br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />The National Flood Insurance Program (NFIP) encourages State and local <br />governments to adopt sound flood plain management programs. Therefore, <br />each Flood Insurance Study produces maps designed to assist communities <br />in developing sound flood plain management measures. <br /> <br />4.1 Flood Boundaries <br /> <br />To provide a national standard without regional discrimination, <br />the 1 percent annual chance (100-year) flood has been adopted by <br />FEMA as the base flood for flood plain management purposes. The <br />0.2 percent annual chance (500-year) flood is employed to indicate <br />additional areas of flood risk in the community. For Uncompahgre <br />River and Cedar Creek, the 100- and 500-year flood plain boundaries <br />have been delineated using the flood elevations determined at each <br />cross section. Between cross sections, the boundaries were inter- <br />polated using a topographic map at a scale of 1:400, with a contour <br />interval of 2 feet (Reference 10). For Montrose Arroyo, boundaries <br />for the 100-year flood have been delineated based on elevations <br />determined at culverts; between culvert crossings, boundaries were <br />interpolated using topographic maps at a scale of 1:2,400, with a <br />contour interval of 2 feet (Reference 12). <br /> <br />Flood plain boundaries for the 100- and 500-year floods are shown <br />on the Flood Boundary and Floodway Map (Exhibit 2). In cases where <br />the 100- and 500-year flood plain boundaries are close together, <br />only the 100-year flood plain boundary has been shown. Small areas <br />within the flood plain boundaries may lie above the flood elevations <br />and, therefore, not be subject to flooding; owing to limitations <br />of the map scale, such areas are not shown. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as structures and fill, reduces <br />flood-carrying capacity, increases flood heights and velocities, <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 />fl00dway and a floodway fringe. The floodway is the channel of a <br />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 Federal <br />standards limit such increases in flood heights to 1.0 foot, pro- <br />vided that hazardous velocities are not produced. The fl00dways <br />in this study are presented to local agencies as minimum standards <br />that can be adopted directly or that can be used as a basis for <br />additional floodway studies. <br /> <br />15 <br />