Bathymetric and velocimetric surveys at highway bridges crossing the Missouri River near Kansas City, Missouri, June 2–4, 2015

Bathymetric and velocimetric data were collected by the U.S. Geological Survey, in cooperation with the Missouri Department of Transportation, near 8 bridges at 7 highway crossings of the Missouri River in Kansas City, Missouri, from June 2 to 4, 2015. A multibeam echosounder mapping system was used to obtain channel-bed elevations for river reaches ranging from 1,640 to 1,660 feet longitudinally and extending laterally across the active channel from bank to bank during low to moderate flood 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 flood 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 surrounded by a debris raft, and scour holes were observed at most surveyed piers. The observed scour holes at the surveyed bridges were examined with respect to shape and depth. Although exposure of parts of substructural support elements was observed at several piers, the exposure likely can be considered minimal compared to the overall substructure that remains buried in bed material at these piers.

The frontal slope values determined for scour holes observed in the current (2015) study generally are similar to recommended values in the literature and values determined for scour holes in previous bathymetric surveys. Several of the structures had piers that were skewed to primary approach flow, and generally the scour hole was deeper and longer on the side of the pier with impinging flow, with some amount of deposition on the leeward side, typical of conditions observed at piers skewed to approach flow; however, at structure A7650 (site 10), the scour hole was deeper and longer on the leeward side of the pier, possibly because of a deflection and contraction of flow caused by a protrusion of the corresponding bank at the bridge.

Previous bathymetric surveys exist for all the sites examined in this study. Comparisons between bathymetric surfaces from the previous surveys (in March 2010 and during the 2011 flood) and those of this study do not indicate any consistent correlation in channel-bed elevations with flow conditions. A simplified assumption of equal to lesser magnitude scour for the lower discharge in the 2015 surveys did not consistently prove to be true, particularly in respect to the depth of observed scour near the piers when compared to results collected during the 2011 flood.

A local spatial minimum average channel-bed elevation at structure A7650 (site 10) compared to adjacent sites may indicate this site is at or near a local feature that controls sediment deposition and scour. The average channel-bed elevation values and the distribution of channel-bed elevations imply that sediment unable to deposit near structure A7650 is flushed downstream and deposits at the next downstream site, structure A5817 (site 11).