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TABLE TWO Existing Conditions M.anning "n" values "nil Value Land Use Description 0.100 South Platte main channel Old South Platte river chutes with light vegetation Farm Fields Farm Fields with roads and fences Farm Fields with occasional houses, trees, and roads. Heavily uJ::banized areas 0.035 0.040 0.050 0.060 - 0.065 0.070 - 0.080 3.1.1 Comparison with Flood Plain Information Study. For this analysis, contraction and expansion coefficients of 0.1 and 0.3 were used except at the bridges where 0.3 and 0.5 w..re used for the Normal Bridge model routines. This differs from the Flcl(Jd Plain Information Model where the bridges were rated by hand calCulations. In the Flood Plain Information Study, flow was allowed into the overbanks and hand "alculated rating curves were used to set the upstream water surfaCE! elevations. To model the U.S. Highway 6 bridge and the Burlington Northern Railroad bridge using the Normal Bridge routine, th.. effective flow areas had to be determined. If the overbanks were allowed as an effective flow area for the 100-year discharge, the water surfa"e I.levation upstream of the bridges remained below the railroad and road surface grades and the entire discharge flowed through the bridge openings. When the effective areas were set for all the flow to pass through the bridge ope!nings, the upstream water surface elevations were high enough to allow flow over the railroad and road profiles. The Flood Plain Information sttLdy, however, documents a flood in 1969 with a discharge greater than the lOO-year event that did not flow across the Highway 6 road grade. The st:udy explains that obstructions upstream of the road grade limit the amount of flow that could actually overtop the road grade. In the finalize~d existing conditions model, the 10-,50-, and 100-year flood eventE! confin,ed the flows through both bridges entirely to the channel to provid~ conservatiye water surface E~levations. For the 500-year event flow was allowed ov"r the road. Other differences include changing the friction loss calculati.on method from the geometric mean friction loss to t:he average conveyance method and different "n" value designations. The major differemce between the Flood Information study water surface profile!s and this study's profilee: is in the bridge loss calculation methods us~d. 5