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100-yeare was determined by hydraulic calculations, which included estilll8tion of cross section and discharges to obtain depths of flow. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference IIl8rks used in this study are -shown on the IIl8pS; the description of the IIl8rks are presented in Elevation Reference Marks (Exhibit 3). 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages state and local governments to adopt sound floodplain management programs. Therefore, each Flood Insurance Study provides lOa-year flood elevations and delineations of the 100- and SOO-year floodplain boundaries and lOa-year floodway to assist communities in developing floodplain IIl8nagement measures. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 1 percent annual chance (lOa-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2 percent annual chance (SOO-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied by detailed methods, with the exception of Cornet Creek, the 100- and SOO-year floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:2,400, with a contour interval of 2 feet (Reference 16). For Cornet Creek, the boundaries of the lOa-year shallow flood area were delineated using the hydraulic computational results, as described in Section 3.2, along with the results via geological approach furnished by the engineering consultant to the Town of Telluride (Reference 17). The lOa-year floodplain boundaries.were so delineated to include the entire area that is susceptible to the unpredictable floodflows in the alluvial fan. The SOO-year floodplain boundaries, as developed via geological approach by the Town Engineering Consultant, were also shown on the Flood Insurance Rate Map (Exhibit 2). For approximate areas, boundaries were delineated using USGS topographic maps at a scale of 1:24,000, with a contour interval of 40 feet (Reference 18). Additional approximate study areas were taken directly from the Flood Hazard Boundary Map for San Miguel County (Reference 19). The following tabulation lists the streams 12