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<br />lOa-years was determined by hy~raulic calculations, which included <br />estimation of cross section artd discharges to obtain depths of <br />flow. <br /> <br />The hydraulic analyses for thi~ study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus <br />considered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevationi reference marks used in this study <br />are shown on the maps; the des~ription of the marks are presented <br />in Elevation Reference Marks (E*hibit 3). <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages state and local governments to adopt sound <br />floodplain management programs. The~efore, each Flood Insurance Study <br />provides lOa-year flood elevations i and delineations of the 100- and <br />500-year floodplain boundaries and lOa-year floodway to assist <br />communities in developing floodplainjmanagement measures. <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the <br />1 percent annual chance (lOa-year) flood has been adopted by FEMA <br />as the base flood for floodplain management purposes. The 0.2 <br />percent annual chance (500-year) flood is employed to indicate <br />additional areas of flood risk! in the community. For each stream <br />studied by detailed methods, w~th the exception of Cornet Creek, <br />the 100- and 500-year floodp1aiin boundaries have been delineated <br />using the flood elevations determined at each cross section. <br />Between cross sections, the boundaries were interpolated using <br />topographic maps at a scale of 1:2,400, with a contour interval of <br />2 feet (Reference 16). <br /> <br />For Cornet Creek, the boundaries of the lOa-year shallow flood area <br />were delineated using the hydraulic computational results, as <br />, <br />described in Section 3.2, alon:g with the results via geological <br />approach furnished by the engiineering consultant to the Town of <br />Telluride (Reference 17). The ilOO-year floodplain boundaries were <br />so delineated to include the entire area that is susceptible to the <br />unpredictable floodflows in the alluvial fan. The 500-year <br />floodplain boundaries, as devel~ped via geological approach by the <br />Town Engineering Consultant, we~e also shown on the Flood Insurance <br />Rate Map (Exhibit 2). <br /> <br />For approximate areas, bound~ries were delineated using USGS <br />topographic maps at a scale of 1:24,000, with a contour interval of <br />40 feet (Reference 18). Additiional approximate study areas were <br />taken directly from the Flood Hazard Boundary Map for San Miguel <br />County (Reference 19). The following tabulation lists the streams <br /> <br />12 <br />