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<br />are, therefore, considered valid only if hydraulic structures, in general, <br />remain unobstructed and if channel and overbank conditions remain <br />essentially the same as ascertained during this study. <br /> <br />All elevations are referenced from National Geodetic Vertical Datum of <br />1929 (NGVD); elevation reference marks used in the study are shown on the <br />maps, <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />A prime purpose of the National Flood Insurance Program is to encourage state and <br />local governments to adopt sound flood plain management programs. Each Flood <br />Insurance Study, therefore, includes a flood boundary map designed to assist <br />communities in developing soul)d flood plain management measures. <br /> <br />~ <br /> <br />4.1 Flood Boundaries <br /> <br />In order to provide a national standard without regional discrimination, the <br />100-year flood has been adopted by the Federal Insurance Administration as <br />the base flood for purposes of flood plain management measures. The 500- <br />year flood is employed to indicate additional areas of flood risk in the <br />community. For each stream studied in detail, with the exception of Cornet <br />Creek, the boundaries of the 100-year and the 500-year floods have been <br />delineated using the elevations determined at each cross section; between <br />cross sections the boundaries were interpolated using topographic maps at a <br />scale of 1:2400, with a contour interval of two feet (Reference 14). <br /> <br />',. ... <br /> <br />For Cornet Creek, the boundaries of the 100-year shallow- flooding were <br />delineated using the hydraulic computational results as described in Section <br />3.2. The 100-year flood boundaries were so delineated to include the entire <br />area which is susceptible to the unpredictable flow in the alluvial fan. For <br />approximate areas, boundaries were delineated using USGS Quadrangle Maps <br />at a scale of 1:24000, with a contour interval of 40 feet (Reference 15). <br />Additional approximate study areas were taken directly from the Flood <br />Hazard Boundary Map (Reference 16). The following table lists the streams <br />and their tributaries obtained from the Flood Hazard Boundary Map: <br /> <br />Major Rivers <br /> <br />Tributaries <br /> <br />Dolores River <br /> <br />Mary Jane Draw <br />Big Gypsum Creek . <br />McIntyre Canyon Creek <br />Summit Canyon Creek <br />Nicholas Wash <br />Disappointment Creek <br />One unnamed tributary <br /> <br />. <br /> <br />"" <br /> <br />11 <br /> <br />