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<br />Peak discharge-drainage area relationships for the Colorado River <br />and Parachute Cre"k are shown in Table l. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding <br />sources studied were carried out to provide estimates <br />elevations of floods of the selected recurrence intervals. <br /> <br />from <br />of <br /> <br />the <br />the <br /> <br />Water-surface profiles for Parachute Creek and the Colorado Ri ver <br />were calculated 'Jith the use of the COE HEC-2 computer program <br />(Reference 2). The Sacramento District of the COE performed the <br />HEC-2 analysis of the Colorado River and the SCS performed the <br />original and revised calculations for Parachute Creek. J. E. <br />Langford and Associates performed the revised 500-year flood <br />profile calculations for the overbank area of Parachute Creek 1n <br />the town. Output listings for all runs are contained in a <br />Technical Addendum on file with the CWCB. <br /> <br />Cross sectional data were obtained from photogrammetric mapping of <br />the area provided by Analytical Surveys, Inc. (Reference 3). Field <br />investigations for the Parachute Creek analysis were originally <br />performed by the SCS in 1982 and 1983. These included measurements <br />of bridges and verification of channel and overbank roughness <br />coefficients (Manning's un"~ values). Field investigations <br />including measurement of bridges were performed by the COE for the <br />Colorado River in 1985. <br /> <br />Additional measures of the D&RG railroad bridge over Parachute <br />Creek were obtained in 1986 by J.E. Langford and Associates at the <br />request of the town. The cross sectional area at the railroad <br />bridge was found to be 367 square feet, some 24 percent greater <br />than the area used by the SCS in the original HEC-2 model for the <br />bridge. <br /> <br />Other factors that contribute to differences between the original <br />SCS hydraulic analysis and the revised analysis contained in this <br />report are revised modeling of overbank flows relative to the main <br />channel flows, and revi sed topographic data in the east overbank <br />upstream of 1-70 which is based on field measurement rather than <br />the aerial mapping. <br /> <br />Based on a field inspection in April, 1987, Un" values for the <br />channel used in the new HEC-2 model were lowered from 0.047 to <br />0.035 between the railroad bridge and the U.S. Highway 6 bridge. <br />This value reflects a fairly straight and uniform section which is <br />not obstructed by vegetation. Upstream of the highway bridge, the <br />channel "n" value was increased from 0.047 to 0.06 to reflect a <br />highly variable channel cross section with dense vegetation and <br />large trees posing possible obstructions to floodwaters. <br /> <br />The hydraul ic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus <br /> <br />7 <br />