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<br />Water-surface profiles of the selected recurrence intervals on the <br />Rio Blanco were developed using the U.s. Army Corps of Engineers <br />HEC-2 step-backwater computer model (Reference 11). Starting eleva- <br />tions were computed using the slope-area method. Water-surface <br />profiles of San Juan River, McCabe Creek, North Fork McCabe Creek, <br />and West Fork McCabe Creek were developed using the U.S. Soil <br />Conservation Service WSP2 computer program (Reference 12). The <br />results were reviewed and adopted for this Flood Insurance Study. <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Exhibit 1). The flood profiles shown in this report are <br />based On the effects of flow through unobstructed hydraulic struc- <br />tures and are thus considered valid only if these structures remain <br />unobstructed and do not fail. <br /> <br />Approximate methods were used to study portions of the San Juan <br />River, Rio Blanco, Mill Creek, Piedra River, Navaho River, Little <br />Navaho River, Navaho Reservoir, Rio Chama, Devils Creek, and Stoll- <br />steimer Creek. Because there is not any previous study in these <br />areas, an area prone to flooding for the recurrence interval of <br />100 years was delineated by hydraulic calculations. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks used in the study <br />are shown on the maps. <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />A prime purpose of the National Flood Insurance Program is to encouraqe <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 laO-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 of the 100- and SOG-year floods have been <br />delineated using the flood elevations determined at each cross sec- <br />tion; between cross sections, the boundaries were interpolated using <br />topographic maps at scale of 1:2400, with a contour interval of 2 <br />feet (Reference 13) . <br /> <br />12 <br />