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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />revised I1Ddeling of overbank flows relative to the nain channel flows, and <br />revised topographic data in the east overbank upstream of 1-70 which is based on <br />field measurement rather than the aerial mapping. <br /> <br />Modifications to the SCS Analysis of Parachute Creek: <br /> <br />In the SCS' original HEC2 model, not all of the 100-year discharge was <br />passed through the D&RG railroad bridge which resulted in the railroad <br />embankment being overtopped. 'llie larger opening allows the entire 100-year flow <br />to pass through the bridge opening, eliminating the overtopping of the railroad <br />embankment. <br /> <br />The SCS originally modelled Parachute Creek through the Town limits with <br />very long cross sections resulting in large overbank areas, as is illustrated in <br />Figure 3. 'llie original HEC2 model did not distinguish between effective and <br />non-effective flow areas in the overbanks and as a result, the percentage of the <br />total flood discharge carried in the channel was reduced considerably. The <br />revised HEC2 lIDdel restricted overbank flCM areas which resulted in substantial <br />increases in channel conveyance (and lower water surface elevations) from the <br />O&RG Railroad bridge to the north limit of town. <br /> <br />Field surveying by J. E. Langford and Assoc. in 1986 more precisely defined <br />the ridge of high ground east of Parachute Creek that was not reflected in the <br />cross-sections digitized from the aerial napping. As a result of the surveying, <br />cross-sections 240 through 244 (from upstream of 1-70 to between 2nd and 3rd <br />Streets) were revised, and a new cross-section (243.5) was added to the HEX::2 <br />llDdel. <br /> <br />Finally, based on a field inspection in April, 1987, 'n' values for the <br />channel used in the new HEC2 rrodel were lowered from 0.047 to 0.035 between the <br />railroad bridge and the U. S. Highway 6 bridge. This value reflects a fairly <br />straight and uniform section which is not obstructed by vegetation. Upstream of <br />the highway bridge, cross-sections 243 through 245, the channel 'n' value was <br />increased from 0.047 to 0.06 to reflect a highly variable channel cross-section <br />with dense vegetation and large trees posing possible obstructions to <br />floodwaters. <br /> <br />13 <br />