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<br />from 0.045 to 0.085. The roughness factors were calibrated using <br />the rating curve (gage height versus flood) for U.S. Geological <br />Survey Gage No. 09175500, located on San Miguel River downstream <br />of the State Highway 97 bridge in Naturita. <br /> <br />Starting water-surface elevations were calculated using the slope- <br />area method. <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). <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the flood profiles are, thus, <br />considered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />For Tributary 1, the approximate 100-year flood elevations were <br />determined by normal depth computations. It was determined that <br />the 100-year flood will be contained in the channel. <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 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 manage- <br />ment 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 addi- <br />tional areas of flood risk in the community. For the stream studied <br />in detail, the boundaries of the 100- and 500-year floods have <br />been delineated using the flood elevations determined at each <br />cross section; between cross sections, the boundaries were inter- <br />polated using topographic maps at a scale of 1:2400, with a limited <br />contour interval (Reference 10). <br /> <br />The approximate 100-year boundaries for Tributary 1 were deline- <br />ated using topographic maps at a scale of 1:24,000, with a contour <br />interval of 20 feet (Reference 11). Flooding from Tributary 1 <br />was found to be contained in the channel: therefore, it was not <br />shown on the maps. <br /> <br />8 <br />