Bathymetric and velocimetric surveys at highway bridges crossing the Missouri and Mississippi Rivers near St. Louis, Missouri, August 3–10, 2020

Bathymetric and velocimetric data were collected by the U.S. Geological Survey, in cooperation with the Missouri Department of Transportation, near 15 bridges at 10 highway crossings of the Missouri and Mississippi Rivers near Washington, Louisiana, and St. Louis, Missouri, on August 3–10, 2020. A multibeam echosounder mapping system was used to obtain channel-bed elevations for river reaches about 1,640 to 1,970 feet longitudinally and generally extending laterally across the active channel from bank to bank during moderate flood-flow conditions. These surveys provided channel geometry and hydraulic conditions at the time of the surveys and provided characteristics of scour holes that may be useful in developing predictive guidelines or equations for computing potential scour depth. These data also may be useful to the Missouri Department of Transportation as a low to moderate flood-flow assessment of the bridges for stability and integrity issues with respect to bridge scour during floods.

Bathymetric data were collected around every in-channel pier. Scour holes were present at most piers for which bathymetry could be obtained, except those on banks or surrounded by riprap. All the bridge sites in this study were previously surveyed and documented in previous studies, including the two new bridge structures at Louisiana and Washington (structures A8141 and A8504, sites 22 and 32, respectively). Comparisons between bathymetric surfaces from the previous surveys and those of the current (2020) study do not indicate any consistent correlation in channel-bed elevations with streamflow conditions. The comparisons of the 2020 surveys to two previous surveys at the new bridge structure A8141 at Washington (site 22) resulted in net erosion of the channel bed in both comparisons, despite the 2020 streamflow being less than either previous survey. Alternatively, there was a net gain of sediment at new bridge structure A8504 at Louisiana (site 32) between 2014 and 2020, which was the most substantial increase in the surveys detailed in this report; substantially less flow in 2020 than in 2014 or changes to the channel and spur dikes near the bridge may have contributed to the observed sediment gain.

Pier size, nose shape, and skew to approach flow had a substantial effect on the size of the scour hole observed at a given pier. Larger and deeper scour holes were present at piers with wide or blunt noses caused by exposed footings, seal courses, or caissons. When a pier was skewed to primary approach flow, the scour hole was generally deeper and larger than at a similar pier without skew; however, the shape of the scour hole near skewed piers in this study generally was longer and deeper on the leeward side, contrary to the general shape of scour holes for skewed piers. However, this phenomenon has been observed historically at these sites, and likely is exacerbated by debris rafts or other turbulence-inducing features near the atypical scour holes. A substantial scour hole was observed near pier 11 of structure A6500 (site 33), which was deeper than in the 2016 survey. The scour holes observed at pier 17 of structure L0561 (site 25) and piers 3 and 4 of structure A1500 (site 34) also were slightly deeper and wider in 2020 than in 2016. At new bridge structures A8141 at Washington (site 22) and A8504 at Louisiana (site 32), the smaller cross-sectional area and configuration of the piers of the new bridges resulted in substantially less scour than with the wider old piers.