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I I I I I I I I I I I I I I I I I I I Conclusions Regarding Parachute Creek: The two hydraulic analyses show two different types of flow for the 100-year flood. The original SCS flow could be characterized as a lake with a significantly restricted outlet at the D&RG railroad bridge. The limited outlet caused floodwaters to spill over the railroad embanknent to the east of the bridge and flow through developed areas of Town. The flow llOdeled in the revised HEC2 analysis is rrore characteristic of high water in the Parachute Creek channel. The majority of the flood discharges remain within and are conveyed by the channel and a relatively small portion of the overbanks. Channel velocities calculated by the revised HEC2 model are significantly higher than the original SCS model, and bridge capacities are not as serious a hydraulic constraint. There is no flow over the railroad embanknent in the 100-year event. Comparison of the results of the HEC2 analyses of the two types of flow are listed in Table 3. Contained in Table 3 are energy grade line elevations, water surface elevations, channel conveyances and channel velocities at selected cross sections for the original and revised HEC2 model of Parachute Creek. For the IOO-year flow the result of the revised analysis ia a nuch smaller floodplain. Upstream of the railroad, 1-70 and U. S. 6, the revised floodplain is narrower. D<Mnstream of the railroad embanknent, there is only one floodplain associated with Parachute Creek and no separate overflow floodplain. Controls of bridge capacities and high ground in the over banks are only exceeded by flows greater than the IOO-year. Once the capacities of the structures are exceeded, the two analyses are consistent: i.e., the 500-year floodplain is basically unchanged from the original SCS analysis. 14