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<br />. <br /> <br />Hvdraulic Analvses <br /> <br />a, General <br /> <br />The main purpose of the hydraulic analyses was to determine proposed conditions <br />impacts to the 100-year floodplain, The hydraulic analyses completed for the Hamburg <br />Bend project rely on Corps provided topographic mapping and aerial photographs of <br />the site, The 1" = 400' topographic mapping has a contour interval of 4 feet and is <br />based on 1974 aerial photographs, Current aerial photographs of the site (3/23/94) <br />compare reasonably well with 1989 aerial photographs and with the 1974 mapping, <br />The historic chute channel flow paths appear to have essentially the same location <br />today as they did at the time of the mapping, It is unknown at this time exactly how <br />much scour and/or deposition took place in the point bar area as a result of the 1993 <br />flood flows, Although the site topography has obviously changed somewhat since the <br />time of mapping, and especially since the large flood event in 1993, it is assumed that <br />the mapping provides sufficient information and it was used for the purposes of this <br />analysis and design, <br /> <br />The HEC-2 models used in all analyses assume a starting energy slope of 0,000135 <br />ft/ft for the Missouri River main channel. This slope is consistent with the starting slope <br />obtained from calibrating the existing conditions HEC-2 model to the construction <br />reference plane (CRP) in the vicinity of the project area, Details of the calibration run <br />can be found in .Chute Channel Restoration Design for Hamburg Bend Near Nebraska <br />City, Nebraska" (Resource Consultants and Engineers, Inc" May, 1992), The channel <br />slopes between surveyed model cross-sections do not necessarily match the starting <br />slope assumption, but the inconsistencies are likely to be a result of local channel <br />anomalies, channel bed forms, and normal errors introduced by channel survey <br />techniques, <br /> <br />Several cross-sections in each of the HEG-2 models were encroached by the use of X3 <br />cards, The encroachments were used to restrict flow to the effective flow widths <br />between the Federal Levees, and to prevent low overbank areas from being used in <br />hydraulic computations where flow could not realistically be conveyed, <br /> <br />b, Flow-Through Chute (Constructed) <br /> <br />Hydraulic modeling for analysis of the pilot channels was performed using HEC-2 <br />backwater models, Three trapezoidal pilot channels (bottom widths of 10', 25', and 50') <br />with side slopes of 2H to 1 V were analyzed, Manning's .n" value for the pilot channels <br />was assumed to be 0,035 with a longitudinal slope of 0,0003 ft/ft, Existing HEC-2 <br />models prOvided by the Corps were used as the basis for this analysis, Chute <br />discharges for each pilot channel width at various Missouri River stages were not <br />specifically computed, However, a detailed discussion of discharges for ultimate width <br />chute channel conditions can be found in .Chute Channel Restoration Design for <br />Hamburg Bend Near Nebraska City, Nebraska" (RCE, May, 1992), For each channel <br />configuration, it was assumed that the excavated material would be placed adjacent to <br />the pilot channel as a berm with a top width of 2 times the channel bottom width and <br />side slopes of 2H to 1V, A 100-year floodplain run for each proposed configuration <br />was compared to the existing conditions 100-year floodplain run, The tabulated and <br />plotted results are included in this appendix, and generally show that the effects of the <br />chute channel on the Missouri River floodplain are insignificant. The HEC-2 output files <br /> <br />.. <br /> <br />. <br /> <br />, <br /> <br />,...# <br /> <br />, <br />