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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 />the Federal Emergency Management Agency as the base flood for flood <br />plain management purposes. The 0.2 percent annual chance (500- <br />year) flood is employed to indicate additional areas of flood risk <br />in the county. For each stream studied in detail, the 100-and <br />SOO-year flood plain boundaries have been delineated using the <br />flood elevations determined at each cross section. Between cross <br />sections, the boundaries were interpolated using topographic maps <br />at scales of 1:2,400 and 1:400, with a contour interval of 2 feet <br />(References 11 and 12). <br /> <br />For the reaches of the Uncompahgre and San Miguel Rivers, Happy <br />Canyon Creek, and Hieroglyphics Canyon that were studied by approxi- <br />mate methods, the 'OO-year flood plain boundaries were delineated <br />using USGS topographic maps at a scale of 1:24,000, with a contour <br />interval of 20 feet (Reference 16). <br /> <br />Approximate 100-year flood plain boundaries in some portions of <br />the study area were taken from the Flood Hazard Boundary Map for <br />Montrose County (Reference 17). <br /> <br />The 100- and SOO-year flood plain boundaries are shown on the Flood <br />Boundary and Floodway Map (Exhibit 2). In cases where the 100- <br />and SOO-year flood plain boundaries are close together, only the <br />100-year flood plain boundary has been shown. Small areas within <br />the flood plain boundaries may lie above the flood elevations but <br />cannot be shown due to limitations of the map scale and/or lack of <br />detailed topographic data. <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 NFIP, a floodway is used as <br />a tool to assist local communities in this aspect of flood plain <br />management. Under this concept, the area of the 100-year flood is <br />divided into a floodway and a floodway fringe. The floodway is <br />the channel of a stream plus any adjacent flood plain areas, that <br /> <br />23 <br />