Abstract
Bathymetric and velocimetric surveys were conducted by
the U.S. Geological Survey, in cooperation with the Kansas
and Missouri Departments of Transportation, in the vicinity
of 36 bridges at 27 highway crossings of the Missouri River
between Brownville, Nebraska and St. Louis, Missouri, from
July 13 through August 3, 2011, during a summer flood. A
multibeam echo sounder mapping system was used to obtain
channel-bed elevations for river reaches ranging from 1,350
to 1,860 feet and extending across the active channel of the
Missouri River. These bathymetric scans provide a “snapshot”
of 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 used by the Kansas and
Missouri Departments of Transportation to 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, in extremely
shallow water, or surrounded by debris rafts. Scour holes were
present at most piers for which bathymetry could be obtained,
except at piers on channel banks, those near or embedded
in lateral or longitudinal spur dikes, and those on exposed
bedrock outcrops. Scour holes observed at the surveyed
bridges were examined with respect to depth and shape.
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 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.
Previous bathymetric surveys had been done at several of
the sites, and comparisons between bathymetric surfaces from
the previous surveys and those of this study indicate substantial
variability in the response of the channel bed to the 2011
summer flood conditions. At sites in Kansas City, there was no
consistent deepening of the channel or increase in the size of
scour holes, despite substantially more discharge and a higher
water-surface elevation in the 2011 surveys, which implies
the high-flow conditions during the 2011 surveys created a
similar scour scenario to the previous surveys. At Jefferson
City and the St. Louis sites, there was a consistent deepening
of the channel, and a slight to substantial increase in the
depth of scour holes in the 2011 surveys compared to previous
surveys, although the effects of the higher flow appeared to be
mitigated by the shape and alignment of the piers at most sites
in St. Louis. Construction activities related to a new bridge at
the Atchison, Kansas, site likely have contributed to the substantial
additional scour observed there in a previous survey
during the 2010 flooding, and the subsequent aggradation of
the channel bed observed in the 2011 survey. Pier size, nose
shape, and alignment to flow also had a profound effect on the
size of the scour hole observed for a given pier.
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First posted September 27, 2012
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