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revised rrodeling of overbank flows relative to the ll'I'.Iin channel flC'<ls, and revised topographic data in the east overbank upstream of 1-10 which is based on field measurement rather than the aedal mapping. Conclusions Reqardinq t>arachuteCreek: Modifications to the SCS Analysis of Parachute Creek: The boo hydraulic analyses ::;how two different types of flow for the IOO-year flood. The O[iginal SCS flow could be charactedzed as a lake with a significantly restricted outlet at the O&RG railroad bridge, The limited outlet caused floodwaters to spill over the railroad entlankrrent to the east of the br idge and flow through developed areas of Town. The flC'<l rrodeled in the revised HEe2 analysis is more characteristic of high water in the Parachute Creek channel. 'll1e Il'I'.Ijodty of the flood discharges remain within and are conveyed by the channel an::! a relatively small portion of the overbanks. Chaml(!l velocities calculated by the revised HElC2 model are significantly higher t.'lan t.".c original SCS rodel, an::! bridge capacities are not as serious a hydraulic constraint. There is no flow over t.'le railroad elTbankment in the 100-yearevent. In the SCS' original HeC2 model, not all of the 100-year discharge was passed through the O&RG railroad bddge which resulted in the railroad errbanl<ment being overtopped. The larger opening allows the entire 100-year flow to pass through the bridge opening, eliminating the overtopping of the railroad effbankment. The SCS originally lr(ldelled Parachute Creek throogh the Town limits with ve~ long cross sections resulting in large overbank areas, as is illustrated in Figure 3. '!11e original REe2 lTlOdel did not distinguish between effective and non~ffective flC'<l areas in the overbanks and as a result, the percentage of the total flood discharge carried in the channel waS reduced considerably. The revised HEC2 model restricted overbank flow areas which resulted in substantial increases in channel conveyance (and lower water surface elevations) fromt fl.'" D&R:> RailrOad bridge to the north limit of town. Cooq'larisonof listedin'I'ableJ. the results of theHOC2 analyses Containedin'I'ablc3areenergy of the t'NO types of flow are grade line elevations, water velocities at selected cross Parachute Creek. surface elevations. channel conveyancesilnd channel sections for the original and revised aeC2 m:xlel of Fi",ld sur""yin<;! hy J. E. L>>ongforei "nd 'Issoc. in 1""86 ~re prc,:i~'lly defined the ridge of high ground east of Parachute Creek that was not reflected in the cross-sections digitized from the aerial mapping. As a result of the surveying, crocs--~ctionc 240 through 244 (from upstream of 1-70 to between 2nd and 3cd Streets) were revised, andanfMcross-section(243.5)wasaddedtotheHOC2 model. For the IOO-year flow the cesult of the revised analysis ia a much smaller flC>0d9lain. Upstream of the railroad, 1-70 and 0. S. 6, the revised floodplain is narrower. DcooInstream of the railr03d embanlor.;nt, there is only one floodplain associated with Parachute Creek and no separate overflow flood~lain. Controls of bridge capacities and high grooOO in the overban,:;s ate only exceeded br flows greater than the 100-year. Once the capacities of the structures are exceeded, ~~e two analyses are consistent: i.e., the 500-year floodplain is basically unchanged from the original SCS analysis. ~lnally, based on a held inspection in April, 1987, 'n' values for the channel used in the new HEC2 rrodel were lowered from 0.047 to 0.035 between the railroad bridge and the U. S. Highway 6 bridge. This "alue reflects a fairly straight and uniform section whiCh is not obstructed by vegetation. Upstream of the highway bridge, cross-sections 243 through 245, the channel 'n' value was increased from 0.047 to 0.06 to reflect a highly variable chsnnel cross-section with dense "'.'getation and large trees p:lsin;: pos::;ible obstructions to flood....atcrs. 13 "