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<br />'. <br /> <br />BRIDGE ANALYSIS <br /> <br />The three bridges within the Elm Creek study reach were field <br />surveyed by the Nebraska NRC, In each case, bridge losses in the <br />hydraulic model at all three bridges were determined using the HEC-2 <br />Normal Bridge Method, <br /> <br />HYDRAULIC ANALYSIS <br /> <br />The hydraulic model was developed based on the known high-water <br /> <br /> <br />marks and flood boundary from the June 1990 flood, Using the range of <br /> <br /> <br />Corps-predicted discharges, Manning's On" values ,tanged from 0.042 to <br /> <br /> <br />0.060 with an average of 0,044 in the c)lannel and':from 0,033 to 0.15, <br /> <br /> <br />with an average of 0,07 . along the overbanks, Full valley cross <br /> <br /> <br />sections were used with the excepcion of cross seccions 3400 through <br /> <br /> <br />5150. At these seccions, enc~oachments were established on the right <br /> <br /> <br />bank along the cencerline of t~e main UPRR track. See appendix A for <br /> <br /> <br />cross section locations, Because the lst Street bridge was replaced <br /> <br /> <br />as a result of the flood, the surveyed elevation of the bridge deck <br /> <br /> <br />was slightly higher than the original bridge. Based on information <br /> <br /> <br />provided by the Platte County superintendent, the deck elevation in <br /> <br /> <br />the hydraulic model was lowered 2 inches t~ r~flect 1990 condLtions, <br /> <br />0' <br /> <br />Yith this hydraulic model, a discharge of approximately 13,200 <br /> <br /> <br />cubic feet per second (c. f. s.) "as required to meet the high-water <br /> <br /> <br />marks, the 50-year frequency based on discharge estimates developed in <br /> <br /> <br />this study. Figure 2 shows a comparison of the water surface profile <br /> <br /> <br />developed with this hydraulic model and the high-water marks. <br /> <br />- <br /> <br />- <br /> <br />- <br />To determine the accuracy of the hydraulic model, the flood <br /> <br /> <br />boundary was p10cced based on che elevations projected at each cross <br /> <br />8 <br />