Bathymetric and Velocimetric Surveys at Highway Bridges Crossing the Missouri and Mississippi Rivers on the Periphery of Missouri, June 2014

Abstract

Bathymetric and velocimetric data were collected by the U.S. Geological Survey, in cooperation with the Missouri Department of Transportation, in the vicinity of 8 bridges at 7 highway crossings of the Missouri and Mississippi Rivers on the periphery of Missouri from June 3 to 11, 2014. A multibeam echosounder mapping system was used to obtain channel-bed elevations for river reaches ranging from 1,525 to 1,640 feet longitudinally, and extending laterally across the active channel from bank to bank during low- to moderate-flow conditions. These bathymetric surveys indicate the channel conditions at the time of the surveys and provide characteristics of scour holes that may be useful in the development of predictive guidelines or equations for scour holes. These data also may be useful to the Missouri Department of Transportation as a low- to moderate-flow comparison to help assess the bridges for stability and integrity issues with respect to bridge scour during floods.

Bathymetric data were collected around every pier that was in water, except those at the edge of water or in very shallow water (less than about 6 feet). Scour holes were observed at most piers for which bathymetry could be obtained, except at piers on channel banks, on exposed bedrock outcrops, or surrounded by riprap. Scour holes observed at the surveyed bridges were examined with respect to depth and shape, and the effects of riprap blankets or other rock near the piers. The presence of riprap blankets, depth of fluvial material on top of a riprap blanket, and alignment to flow had a substantial effect on the size of the scour hole observed for a given pier. Piers that were surrounded by riprap blankets had scour holes that were substantially smaller (to non-existent) compared to piers at which no rock or riprap was present. Although exposure of parts of foundational support elements was observed at several piers, at most sites the exposure likely can be considered minimal compared to the overall substructure that remains buried in channel-bed material; however, there were several notable exceptions where the bed material thickness between the bottom of the scour hole and bedrock was less than 6 feet. Such substantial exposure of usually buried substructural elements may warrant special observation in future flood events, even when designed to be exposed.

Previous bathymetric surveys had been done at both of the sites on the Missouri River and one of the sites on the Mississippi River examined in this study. Comparisons between bathymetric surfaces from the previous surveys during the 2011 flood and those of this study generally indicate that there was an increase in the elevation of the channel bed at these sites that likely was caused by a substantial decrease in discharge and water-surface elevation compared to the 2011 surveys. However, the scour holes observed at these sites were either the same size or larger in 2014 compared to the 2011 surveys, indicating that the flow condition is not the sole variable in the determination of the size of scour holes, and that local velocity and depth also are critical variables, as indicated by predictive pier scour equations.

First posted May 12, 2015