by B.A. Bryan
This report is available as a pdf below
An investigation was conducted at the Hatchie River near the U.S. Highway 51 crossing in West Tennessee to (1) describe the channel cross-section evolution near the bridge crossing, (2) describe the evolution of velocity and discharge distributions near the bridge crossing, and (3) define streamflow duration and flood frequencies at the bridge site and compare these statistics with flows prior to the bridge collapse. The project was designed to document the collapse on April 1, 1989, of three spans of the bridge. Discharge measurements at the site available from the U.S. Army Corps of Engineers indicate that the channel was widening at a rate of 0.8 foot per year from 1931 through about 1975. The channel bed was stable at an average elevation of about 235 feet above sea level by 1975. Construction of a southbound bridge in 1974 and 1975 reduced the effective flow width of the channel from about 4,000 to about 1,000 feet. A flood during March 1975 scoured the channel bed lowering its elevation about 6 feet. Data collected by the U.S. Army Corps of Engineers from 1975 to 1981 indicate that the channel bed degraded to an elevation of about 230 feet and the widening rate increased to about 4.5 feet per year. The channel bed returned to approximately the pre-construction level of 235 feet as channel width increased. The widening rate decreased to about 1.8 feet per year based on U.S. Army Corps of Engineers data collected from 1981 through 1989. Channel geometry data indicates that recent channel morphology changes have occurred at the toe of the right bank, and have resulted in continued bank undercutting and ban failure. Cross-section geometry and flow-velocity distributions determined from U.S. Geological Survey discharge measurements made between April 6 and 10, 1989, indicate that there is a high-flow meander pattern through this river reach. The bridges are located at or near the apparent meander inflection.
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