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<br />Starting WSELs for the Crystal River and Castle Creek were taken at <br />their confluence with the Roaring Fork River. Starting WSELs for <br />Maroon, Snowmass, Coal, and Hunter Creeks were determined by the <br />critical depth method. The starting elevation for the Roaring Fork <br />River was obtained by the slope-area method. The starting elevation <br />for the Fryingpan River was taken from a 1978 Floodplain Information <br />Report (Reference 15). <br /> <br />The hydraulic analyses for this 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 />Hydraulic analyses of approximate-study areas were based on <br />normal-depth computations and information obtained from available <br />topographic maps and field reconnaissance. <br /> <br />An area of shallow flooding exists along the southern bank of Coal <br />Creek. This area is specified as a shallow flooding source in the <br />Floodplain Information Report for the Crystal River and Coal Creek <br />(Reference 16). <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation Reference Marks (ERMs) used in this <br />study are shown on the maps; the descriptions of the marks are <br />presented in ERMs (Exhibit 3). <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound floodplain <br />management programs. Therefore, each FIS report provides 100-year flood <br />elevations and delineations of the 100- and SOD-year floodplain boundaries <br />and 100-year floodway to assist communities in developing floodplain <br />management measures. <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the <br />1-percent-annual-chance (100-year) flood has been adopted by FEMA as <br />the base flood for floodplain management purposes. The <br />O.2-percent-annual-chance (SOO-year) flood is employed to indicate <br />additional areas of flood risk in the community. For each stre.arn <br />studied in detail, the 100- and SOD-year floodplain boundaries have <br />been delineated' using the flood elevations determined at each cross <br />section. Between cross sections, the boundaries were interpolated <br />using topographic maps at scales of 1:600 and 1:1,200, with a <br />contour interval of 2 feet (Reference 12); 1:2,400, with contour <br />intervals of 2 feet (References 13 and 16) and 5 feet (References 6 <br />and 21); 1:4,800, with contour intervals of 5 feet and 40 feet <br />(References 14 and 22, respectively); and 1:24,000, with a contour <br />interval of 2 feet (Reference 23). <br /> <br />15 <br />