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<br />relative distance shown on the profile for Rifle Creek Splitflow. <br />This profile base line is shown and identified on the maps. <br /> <br />The hydraulic analysis for the portion of Mitchell Creek downstream <br />of Interstate Highway 70 was performed by Claycomb Engineering <br />Associates, Inc. <br /> <br />Alluvial fan methodologies were applied to calculate flow depths <br />and velocities on Helmer and Ramsey Gulches (Reference 13). <br /> <br />Approximate flood boundaries were based on approximating 100-year <br />elevations by engineering judgment, giving consideration to <br />available data and field observation. <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the prOfiles are thus con- <br />sidered 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). Elevation reference marks used in this study <br />are shown on the maps. <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />The National Flood Insurance Program (NFIP) encourages State and local <br />governments to adopt sound flood plain management programs. Therefore, <br />each Flood Insurance Study produces maps designed to assist communities <br />in developing flood plain management measures. <br /> <br />4.1 Flood Boundaries <br /> <br />To provide a national standard without regional discrimination, <br />the 1 percent annual chance (lOO-year) flood has been adopted by <br />FEMA as the base flood for flood plain management purposes. The <br />0.2 percent annual chance (SOO-year) flood is employed to indicate <br />additional areas of flood risk in the community. For each stream <br />studied in detail, the 100- and SOO-year flood plain boundaries <br />have 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 1:2,400, 1:6,000, and <br />1:24,000, with contour intervals of 2, 5, 20 and 40 feet (Refer- <br />ences 14 through 17). <br /> <br />Flood boundaries for the Colorado River at Rifle were originally <br />delineated using topographic maps at a scale of 1:24,000, with <br />contour intervals of 20 and 40 feet (Reference 17). Those boundaries <br />have been revised for the Flood Insurance Study using topographic <br />maps at a scale of 1:6,000 with a contour interval of 5 feet <br />(Reference 16). <br /> <br />] 3 <br />