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<br />--' <br /> <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 lOa-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 SOO-year flood is employed to indicate additional <br />areas of flood risk in the community. For each stream studied in <br />detail, the boundaries ,o~ the lOa-year and the SOO-year floods have <br />been delineated using the flood elevations determined at each cross <br />section; between cross sections, the boundaries were interpolated <br />using topographic maps at a scale of 1:2400 feet, with a contour <br />interval of 2 feet (Reference 5), with adjustments made on the basis <br />of field observations and engineering judgment. In cases where the <br />lOa-year and the 500-year flood boundaries are close together, only <br />the lOa-year boundary has been shown. <br /> <br />Boundaries of areas studied by approximate methods were determined <br />using computed elevations in conjunction with topographic maps <br />(Reference 5), field inspection, and engineering judgment. <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 />Some areas in Rangely are subject.to broad, shallow, overland flood- <br />ing generally less than 1.0 foot deep and characterized by unpredict- <br />able flow paths. The water-surface elevations of flooding in these <br />areas are essentially independent of those along the adjacent stream- <br />way, and are affected principally by natural and manmade barriers to <br />flow in the flooded a.reas. Therefore, boundaries of these areas <br />were determined using[ detailed field investigation.in conjunction <br />with topographic maps: (Reference 5). <br /> <br />4.2 Floodways <br /> <br />Encroachment in 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 manag€!ment involves balancing the economic gain from <br />flood plain developm€!nt against the resulting increase in flood <br />hazard. For purposes of the Flood Insurance Program, the concept of <br />a floodway is used as a tool to assist local communities in this <br /> <br />- <br /> <br />12 <br />