Purpose and Scope

The purpose of this report is to describe the equipment and methods used and to document results of bathymetric and velocimetric surveys completed on May 19–26, 2021, of the Missouri River channel near nine highway bridges at eight crossings between Kansas City and St. Louis, Mo. (fig. 1; table 2). The results obtained from the bathymetric and velocimetric surveys of the channels document the channel-bed geometry and velocity distribution at the time of the surveys and provide characteristics of scour holes that may be useful in developing or modifying predictive guidelines or equations for computing potential scour depth. These data also may be used by MoDOT as a low to moderate flood-flow comparison to help assess the bridges for stability and integrity issues with respect to bridge scour. Results are also compared to previous surveys at the sites (Huizinga, 2012, 2014, 2020a).

Description of Study Area

The study area for this report is the Missouri River between Kansas City and St. Louis, Mo., but excludes the new bridge at Washington, Mo., surveyed in 2020 (Huizinga, 2023; fig. 1). From Kansas City, the Missouri River flows generally eastward to the greater St. Louis area, joining the Mississippi River north of downtown St. Louis, Mo. (fig. 1). All the highway crossings on the Missouri River between Lexington and Hermann, Mo. (sites 14 through 21; nine bridges at eight crossings) were examined as part of this study. The site numbering sequence used in previous studies on the Missouri River (Huizinga, 2012, 2014) is used in this report for consistency and comparability.

Description of Streamflow Conditions

Data from the selected streamflow-gaging stations (hereinafter referred to as “streamgages”) on the Missouri River at Waverly, Mo. (USGS station 06895500; hereafter referred to as the “Waverly streamgage”); Missouri River at Glasgow, Mo. (USGS station 06906500); Missouri River at Boonville, Mo. (USGS station 06909000); Missouri River at Jefferson City, Mo. (USGS station 06910450); and Missouri River at Hermann, Mo. (USGS station 06934500; hereinafter referred to as the “Hermann streamgage;” U.S. Geological Survey [USGS], 2023a; fig. 1), indicated the Missouri River was on or between minor to moderate flood rises when the sites on the Missouri River were surveyed on May 19–26, 2021 (fig. 2A); furthermore, this rise happened during generally higher late-spring flows (fig. 2B).

Streamflow on the Missouri River as measured at the Waverly streamgage was about 74,500 cubic feet per second (ft³/s) during the survey at the U.S. Highway 24 crossing at Waverly on May 20, 2021 (the lowest measured value), whereas streamflow on the Missouri River as measured at the Hermann streamgage was about 148,000 ft³/s during the survey at the Missouri State Highway 19 crossing at Hermann on May 26, 2021 (the highest measured value). The streamflow of 74,500 ft³/s at the Waverly streamgage has a daily exceedance probability of about 24 percent (the probability that the indicated streamflow value would be equaled or exceeded on any given day at that streamgage; USGS, 2023b) and is less than the 90-percent annual exceedance probability (AEP; the probability that the indicated streamflow value would be equaled or exceeded within a period of one year; also known as the 1-year recurrence interval) flood discharge of 93,200 ft³/s (U.S. Army Corps of Engineers [USACE], 2003, plate E–20). The streamflow of 148,000 ft³/s at the Hermann streamgage has a daily exceedance probability of about 15 percent (USGS, 2023c) and is greater than the 90-percent AEP (the 1-year recurrence interval) flood discharge of 134,000 ft³/s but less than the 50-percent AEP (the 2-year recurrence interval) flood discharge of 248,000 ft³/s (USACE, 2003, plate E–20). Daily exceedance probabilities for the Waverly and Hermann streamgages were computed using daily streamflow from 1957 to 2022 (USGS, 2023b, c).

Streamflow conditions during the dates of each survey were in the low to moderate flood-flow regime. In an analysis of real-time scour monitoring data at Jefferson City, Mo., Huizinga (2014) noted that substantial pier scour generally begins soon after the onset of hydrograph rise (substantial rise of 8 feet [ft] or more), although the scour often does not reach maximum depth until the peak stage is reached or sometime thereafter (refer to fig. 35 in Huizinga, 2014). Several peaks were observed earlier in the year (fig. 2B), and streamflow was substantially higher than base flow based on the daily exceedance values despite the lack of peak streamflow conditions during the study. Although the scour conditions captured in this study may not represent the maximum scour potential at the sites, the cumulative information gathered during multiple surveys from 2011 to 2021 remains useful for determining scour for a variety of flow conditions, particularly when combined with, or compared to, a scour scenario captured at high flood-flow conditions.

Description of Equipment and Basic Processing

The bathymetry of the Missouri River at each of the bridges was determined using a high-resolution MBMS. The various components of the MBMS used for this study are described in previous studies on the Missouri and Mississippi Rivers in Missouri (refer to report references listed in table 1) and on the Missouri and Yellowstone Rivers in North Dakota (Densmore and others, 2013). MBMS survey methods and data quality checks were similar to these previous studies. A brief description of the equipment follows; a more-complete description of the various system components and methods used in this study are available in the previous reports by Huizinga (2010), Huizinga and others (2010), and Densmore and others (2013).

An MBMS is an integration of several individual components: the multibeam echosounder (MBES), a sound-velocity probe, an inertial navigation system (INS), and a data-collection and data-processing computer. The MBES used in this study is the Norbit iWBMSh (fig. 3), operated at a frequency of 400 kilohertz (kHz). The iWBMSh is similar in operation to the MBES systems used in previous studies in the mid-Missouri area, except that it has a curved piezoceramic receiver array, which enables bathymetric data to be collected throughout a swath range of 210 degrees. Optimum data usually are collected in a swath of less than 160 degrees (80 degrees on each side of nadir, or straight down below the MBES); nevertheless, the swath can be electronically rotated to either side of nadir, enabling data collection along sloping banks up to a depth just below the water surface. The sound-velocity probe provides real-time measurements of the speed of sound at the MBES to accurately determine the depth readings of the MBES (Hughes Clarke and others, 1996). The river environment tends to be well mixed, such that the sound velocity at the MBES is sufficiently representative of a sound-velocity profile of the full water column. The Norbit iWBMSh uses the Applanix Position Orientation Solution for Marine Vessels (POS MV) OceanMaster INS system, consisting of an inertial measurement unit (IMU) attached directly to the MBES mount in a “tightly coupled” configuration (fig. 3A) and two Global Navigation Satellite System (GNSS) antennae. The INS provides position in three-dimensional space and measures the attitude of the vessel (pitch, roll, yaw, and heading) to accurately position the data received by the MBES. Real-time kinematic (RTK) differential corrections for the INS came from cellular communication with the MoDOT GNSS real-time network for the navigation and tide solution during the 2021 surveys.

Similar to all previous surveys after 2010 (refer to report references listed in table 1), the navigation information from the 2021 surveys was postprocessed using the POSPac Mobile Mapping Suite (MMS) software (version 8.0; Applanix Corporation, 2021) to mitigate the effects of degraded positional accuracy of the vessel while near or under a bridge resulting from GNSS outages. The POSPac MMS software provides tools to identify and compensate for sensor and environmental errors to compute an optimally blended navigation solution from the GNSS and raw IMU data. The blended navigation solution (called a “smoothed best estimate of trajectory” or “SBET” file) generated by postprocessing the navigation data was applied to the survey at a given bridge to minimize the effects of the GNSS outages. Because of advances in technology, surveys before 2010 do not have an SBET file and are more likely to have artifacts resulting from minor positional variations between surveys.

Data from the MBES and INS components were processed and integrated into a cohesive dataset for cleanup and visualization. A computer onboard the survey vessel ran the HYPACK/HYSWEEP data acquisition software (version 2020; HYPACK, Inc., 2020) that was used to prepare for and do bathymetric surveys. After completing the surveys, the acquired depth data were further processed to remove data spikes and other spurious points in the multibeam swath trace, georeferenced using the navigation and position solution data from the SBET file and visualized in HYPACK/HYSWEEP as a gridded bathymetric surface or a point cloud.

Information about the velocity of the river at various points throughout each study reach was collected using an acoustic Doppler current profiler (ADCP), similar to previous studies since 2011 by Huizinga (for example, refer to Huizinga, 2012, 2022a). A Teledyne RD Instruments Rio Grande ADCP operating at 600 kHz was used to obtain velocities at 1.64-ft increments, or “bins,” throughout the water column. The Rio Grande ADCP operates in depths from 2.3 to 230 ft to determine the velocity of water by measuring the Doppler shift of an acoustic signal reflected from various particles suspended in the water (Mueller and others, 2013). By measuring the Doppler shift in four different beam directions, the velocity of the water in each bin can be determined in three dimensions. The depth-averaged velocities from the two traverses of a given section line were computed using averaging algorithms from the Velocity Mapping Toolbox (version 4.09; Parsons and others, 2013).

Basic Description of Methods

The methods used to acquire and ensure the collection of quality data were the same as those used in previous studies using the MBES (methods are detailed in Huizinga and others, 2010; Huizinga, 2010, 2012). A brief summary of—and any differences from—these methods are highlighted below.

Survey Quality-Assurance Measures

For the MBMS, the principal quality-assurance measures were assessed in real time during the survey. The MBMS operator assessed the quality of the collected data in real time by making visual observations of cross-track swath shape (such as convex, concave, or skewed bed returns in flat, smooth bottoms), noting data-quality flags and alarms from the MBES and the INS, and noting comparisons between adjacent overlapping swaths.

In addition to the real-time quality-assurance assessments, beam angle checks and a suite of patch tests were done to ensure quality data were acquired from the MBMS for the 2021 surveys. An initial patch test was completed on May 5, 2021, at Forest Lake near Kirksville, Mo. (fig. 1). The beam angle check and an additional patch test were completed at Lake of the Ozarks near Osage Beach, Mo. (fig. 1) on November 18, 2021. The results of the beam angle check (table 3) were within the recommended performance standards used by the USACE for hydrographic surveys for all the representative angles below 70 degrees (USACE, 2013). Points acquired outside of the central 100–110 degrees of the swath generally had overlap with adjacent swaths, which increases the quality of the survey in the overlapped areas because of duplication.

Table 3.    

Results of a beam angle check from two check lines over a reference surface at Lake of the Ozarks near Osage Beach, Missouri, on November 18, 2021.

[<, less than; --, no data]

Table 3.    Results of a beam angle check from two check lines over a reference surface at Lake of the Ozarks near Osage Beach, Missouri, on November 18, 2021.

Beam angle limit, in degrees	Maximum outlier, in feet	Mean difference, in feet	Standard deviation, in feet	95-percent confidence, in feet
0	0.24	−0.09	0.04	0.07
5	0.28	−0.09	0.03	0.07
10	0.23	−0.08	0.04	0.07
15	0.22	−0.08	0.04	0.07
20	0.23	−0.08	0.04	0.08
25	0.23	−0.07	0.04	0.08
30	0.27	−0.07	0.04	0.08
35	0.27	−0.07	0.05	0.09
40	0.23	−0.05	0.04	0.08
45	0.21	−0.04	0.04	0.09
50	0.22	−0.04	0.05	0.09
55	0.23	−0.03	0.05	0.10
60	0.21	−0.01	0.05	0.10
65	0.25	0.02	0.06	0.12
70	0.30	0.03	0.06	0.12
Performance standards1
Threshold	1.00	<0.20	--	<0.80
Result	Met	Met	--	Met

¹

Performance standard check values are from U.S. Army Corps of Engineers (2013, table 3-1) for soft sand/silt bottoms.

Patch tests are a series of dynamic calibration tests that are used to check for subtle variations in the orientation and timing of the MBES with respect to the INS and real-world coordinates (Huizinga, 2023), and primarily are used to determine angular offsets to roll, pitch, and yaw caused by the alignment of the transducer head relative to the INS (fig. 4). These offsets have been observed to be essentially constant for a given equipment configuration and survey season, barring an event that causes the mount to change such as striking a floating or submerged object (refer to report references listed in table 1; Huizinga and others, 2023; Rivers and others, 2023).

Although the MBES had several minor strikes of floating or submerged debris at various times during the 2021 survey season, no changes to the roll or pitch angles were apparent from the beginning to the end of the river surveys in May (table 4). The tightly coupled configuration of the Norbit iWBMSh, wherein the IMU of the INS is mounted on the same mounting bracket as the MBES (fig. 3A) results in no measured timing offset and no measured angular offset for pitch (table 4). The yaw is a measure of the alignment of the GNSS receivers relative to the IMU of the INS on the echosounder head, and the measured offset for yaw changed from –2.5 to –1.5 between the two patch tests (table 4). The variable angular offset for yaw observed in the 2021 surveys is believed to be the result of a combination of a loose connection on the T-pole on which the GNSS receivers were mounted (fig. 3B) and an incorrectly applied magnetic variation or grid convergence parameter in the post-processing of the survey data (Rivers and others, 2023). The measured angular offset for roll remained a constant −0.10 (table 4), which is different from previous results for this equipment configuration in other recent surveys (Huizinga, 2022a; Huizinga and others, 2023); however, the MBES had undergone calibration testing in January 2021, which is the likely source of the change. Furthermore, the roll offset of −0.10 is consistent with other data collected since the calibration testing in 2021 (Rivers and others, 2023). It was noted in the earliest work with the MBMS in Missouri (Huizinga, 2010) that a sensitivity analysis of the four offsets indicated that the ultimate position of surveyed points in three-dimensional space was least sensitive to the angular offset for yaw, whereas it was most sensitive to the angular offset for roll. Processing all the data for the bridge surveys detailed in this report with an angular offset of roll of –0.10 degree, no angular offset for pitch, and an angular offset for yaw of –2.0 degrees generally yielded good results with no noticeable artifacts caused by incorrect offsets.

Table 4.    

Patch test results at two locations on May 4 and November 18, 2021.

Table 4.    Patch test results at two locations on May 4 and November 18, 2021.

Date of test	Timing offset, in seconds	Angular offset for roll, in degrees	Angular offset for pitch, in degrees	Angular offset for yaw, in degrees	Location (fig. 1)
05/04/2021	0	−0.10	0.00	−2.50	Forest Lake near Kirksville, Missouri
11/18/2021	0	−0.10	0.00	−1.50	Lake of the Ozarks near Osage Beach, Missouri

Angular offsets were applied to the bathymetric data before removing data spikes and other spurious points in the multibeam swaths through the use of automatic filters and manual editing. The bathymetric data were then projected to a three-dimensional grid at a resolution of 1.64 ft using the Combined Uncertainty and Bathymetry Estimator (CUBE) method (Calder and Mayer, 2003), as implemented in the MBMax processing package of the HYPACK/HYSWEEP software (HYPACK, Inc., 2020) and used to generate a gridded raster surface of the channel bed (and associated uncertainty) near each bridge (hereinafter referred to as a “bathymetric surface”) using ArcMap (version 10.8.1; Esri, 2022).

A quality-assurance plan has been established for streamflow and velocity measurements using ADCPs that includes several instrument diagnostics checks and calibrations. These standard operating procedures were followed when acquiring the velocity profile data for these surveys. For a detailed description of these procedures, refer to Mueller and others (2013).

Uncertainty Estimation

Similar to recent bathymetry studies in Missouri (refer to report references listed in table 1; Huizinga and others, 2023; Rivers and others, 2023), uncertainty in each survey was estimated by computing the uncertainty for each survey-grid cell in the bathymetric surface of the survey area using the CUBE method (Calder and Mayer, 2003). The gridded uncertainty is a measure of the variability of the individual points in the cell used to determine the CUBE-derived elevation for the cell. Statistics of gridded uncertainty for each of the survey areas are shown in table 5. An example of the spatial distribution of gridded uncertainty typically observed in the survey data at structure G0069 on State Highway 240 is shown in figure 5. The uncertainty data were output and combined with the three-dimensional bathymetry data and are included with metadata in the USGS data release associated with this study (Huizinga and Rivers, 2023).

Table 5.    

Total gridded uncertainty results for bathymetric data at a 1.64-foot grid spacing from surveys on the Missouri River between Kansas City and St. Louis, Missouri, May 19–26, 2021.

[Data are summarized from Huizinga and Rivers (2023)]

Table 5.    Total gridded uncertainty results for bathymetric data at a 1.64-foot grid spacing from surveys on the Missouri River between Kansas City and St. Louis, Missouri, May 19–26, 2021.

Site number (fig. 1)	Structure number	Uncertainty, in feet				Percentage of bathymetry points with uncertainty value less than a given threshold
		Maximum	Mean	Median	Standard deviation	1.00 foot	0.50 foot	0.25 foot	0.10 foot
14	A5664	2.89	0.07	0.07	0.08	99.9	99.2	97.5	91.6
15	A5910	3.25	0.07	0.07	0.07	99.9	99.5	98.2	91.1
16	K0999	3.02	0.07	0.07	0.07	99.9	99.5	97.8	90.4
17	G0069	3.25	0.09	0.10	0.06	100.0	99.7	98.5	83.3
18	A4574	3.25	0.08	0.07	0.06	99.9	99.6	98.6	87.6
19	L0962	3.18	0.09	0.07	0.08	99.9	99.3	97.2	83.6
20	L0550/A4497	1.12	0.08	0.07	0.04	100.0	100.0	99.6	91.5
21	A6288	1.44	0.09	0.07	0.05	100.0	99.9	99.0	82.2

More than 99 percent of the uncertainty values at all the sites were less than 0.50 ft (table 5), which is within the specifications for a “Special Order” survey, the second-most-stringent survey standard of the International Hydrographic Organization (IHO; International Hydrographic Organization, 2020). More than 97 percent of the uncertainty values were less than 0.25 ft, and more than 82 percent of the uncertainty values were less than 0.10 ft at all the sites (table 5). The tops of bridge substructural elements (pier footings and seal courses) typically had uncertainty values of less than 0.10 ft. Similar to previous surveys with this type of equipment (refer to, for example, Huizinga, 2012, 2016), the uncertainty values were larger near moderate-relief features (banks, spur dikes, rock riprap and outcrops, and scour holes near piers). The largest uncertainty in this group of surveys was 3.25 ft (table 5); however, as noted in previous studies, uncertainty values of this magnitude typically happened near high-relief features, such as the front or side of a pier footing. Occasionally, the uncertainty values also were larger (1.00 ft or greater) in the outermost beams of the multibeam swath where overlap existed with an adjacent swath, particularly when the MBES head was electronically tilted for the survey lines along the banks or near the piers. Overlapping adjacent swaths in the channel thalweg (the line of maximum depth in the channel) also can display larger uncertainty values because substantial bed movement can happen between survey passes (fig. 5).

The uncertainty of the gridded data computed using the CUBE method has been decreasing with time compared to previous surveys (refer to table 4 in Huizinga, 2012, 2014, 2020a). The decrease in uncertainty primarily is the result of improvements in data-collection equipment and methods. The tightly coupled configuration of the Norbit iWBMSh used in these surveys decreases some of the uncertainty by substantially reducing the lever arm length (and therefore the potential movement) between the MBES and IMU. The ability to electronically tilt the swath substantially reduces the time between when unrotated, down-looking data and rotated, side-looking data are collected, which reduces the uncertainty of the data in the swath overlap zone that might otherwise experience more substantial bed movement.

The survey at structure G0069 on Missouri State Highway 240 at Glasgow had the highest maximum value (3.25 ft) and the highest median value (0.10 ft) of gridded uncertainty, as well as a lower percentage of bathymetry points with a gridded uncertainty of less than the various thresholds (table 5). The survey at this site was obtained with relatively smooth longitudinal swaths (fig. 5), which was the case at nearly all the sites surveyed in this study. The primary anomalies at this site were observed along the banks and near the piers, where the MBES was used in an electronically tilted configuration to extend the potential coverage in these areas, resulting in higher uncertainty values. Generally, the magnitude and distribution of uncertainty observed at this site are representative of those observed at all the sites.

Structure A5664 on State Highway 13 at Lexington, Missouri

Structure A5664 (site 14; table 2) on State Highway 13 crosses the Missouri River at river mile (RM) 314.9 at Lexington, Mo., east of and downstream from Kansas City, Mo. (fig. 1). The site was surveyed on May 19, 2021, and the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 681.3 ft (table 6; fig. 6). Streamflow on the Missouri River was about 82,200 ft³/s, as determined from the rated streamflow measured at Waverly (streamgage 06895500) 6 hours after the survey (table 6, especially note “e”). Velocity data were not collected at this site owing to a faulty ADCP unit.

The survey area was about 1,640 ft long and about 1,000 ft wide, extending across the active channel from bank to bank (fig. 6). The survey area extended about 670 ft upstream from the centerline of structure A5664, and piers 21 and 22 were in the water and away from the banks (fig. 6). The channel-bed elevations ranged from about 653 to 668 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; fig. 7), except near the spur dikes on the right (south) bank (fig. 6; table 6). The channel bed between the left (north) bank and the spur dikes was filled with medium dune features, and small dunes and ripples were present in the area between the spur dikes on the right (south) side of the channel (fig. 6). Localized scour holes were present downstream from and off the ends of the spur dikes on the right (south) side (fig. 6). As in previous surveys (Huizinga, 2012, 2014, 2022a), stone revetment was present on the left (north) bank throughout the reach, and spur dikes were present on the right (south) bank (fig. 6).

The minor scour holes near main channel piers 21 and 22 were difficult to discern from nearby dunes and ripples (figs. 6 and 1.1). Information from bridge plans indicates that piers 21 and 22 are founded on shafts drilled 28 to 50 ft into bedrock, having about 52 ft of bed material between the channel bottom and bedrock at pier 21 and about 27 ft of material at pier 22 (fig. 8; difference between “Approximate minimum elevation in scour hole near pier/bent” and “Approximate elevation of bedrock near pier/bent” in table 7). The surveyed bed generally was similar to the previous multibeam surveys in 2013 and 2017, except on the left side of pier 21 (fig. 8). The scour hole near pier 21 appears to penetrate to the top of the pier footing (figs. 6, 8, 1.1A, 1.1B), which might provide some mitigation to the scour hole at this pier as the downward flow of the horseshoe vortex at the pier column face is blunted by the footing (Arneson and others, 2012).

The computed difference between the survey DEMs with a probabilistic threshold mask of 95-percent confidence based on uncertainty (hereinafter referred to simply as “the difference”) from the survey on May 19, 2021, and the previous survey on May 23, 2017 (fig. 9), indicates about 92 percent of the joint area of interest had change greater than the 95-percent confidence interval of uncertainty (hereinafter referred to as “detectable change”), which means about 8 percent of the differences in the joint area of interest are equivocal and within the 95-percent confidence bounds of uncertainty of detectable change (hereinafter referred to as “within the bounds of uncertainty”; table 8). Bed variation seemed about equal between scour and deposition in most of the channel from 2017 to 2021 in the DoD (fig. 9); however, deposition is dominant in the thalweg along the left (north) bank throughout the reach, whereas substantial scour is apparent downstream from the spur dikes on the right (south) bank (fig. 9). The average difference between the bathymetric surfaces (the statistical mean value of the gridded raster surface [fig. 9] created from the thresholded difference between the 2021 and 2017 gridded raster bathymetric surfaces) was –0.52 ft (table 8), indicating minor to moderate channel degradation between the 2017 and 2021 surveys. The net volume of cut in the reach from 2017 to 2021 was about 89,700 cubic yards (yd³), and the net volume of fill was about 61,200 yd³, resulting in a net loss of about 28,500 yd³ of sediment between 2017 and 2021 (table 8). The cross sections from the two surveys along the upstream face of the bridge are not substantially different from one another for most of the section, except on the left side of pier 21 where the 2017 section is 5 to 10 ft lower than the 2021 section (fig. 8). The frequency distribution of bed elevations in 2021 also was similar to 2017 (fig. 7). The stone revetement on the left (north) bank showed areas of minor scour and deposition, and the spur dike on the right (south) bank downstream from the bridge showed minor erosion on one side and deposition on the other (fig. 9); however, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 19, 2021, and the survey on Aril 24, 2013, (fig. 10), indicates about 61 percent of the joint area of interest had detectable change, which means about 39 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Scour and deposition are roughly balanced throughout most of the reach between 2013 and 2021 in the DoD, except along the right (south) bank, particularly downstream from the spur dikes (fig. 10). The average difference between the bathymetric surfaces was –1.14 ft (table 8), indicating moderate channel degradation between the 2013 and 2021 surveys. However, the net loss of sediment between 2013 and 2021 was about 32,100 yd³, which is similar to the net loss of sediment between 2017 and 2021 (table 8). The cross-section from the 2021 survey along is very similar to the 2013 section (fig. 8), and the frequency distribution of bed elevations in 2021appears very similar to 2013 (fig. 7). The stone revetment on the left (north) bank is within the bounds of uncertainty (fig. 10). A substantial part of the channel on the right (south) side was not surveyed in 2013 because of generally lower flows (table 8) and shallow depths in that area. As with the previous DoD, deposition or scour apparent on opposing faces of a feature (such as a pier or spur dike) likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 19, 2021, and the earliest survey during flooding on July 20, 2011 (fig. 11), indicates about 88 percent of the joint area of interest had detectable change, which means about 12 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition is dominant throughout most of the reach between 2011 and 2021 in the DoD, except along the right (south) bank, and near the spur dikes near the right bank (fig. 11). The average difference between the bathymetric surfaces was +3.22 ft (table 8), indicating substantial channel aggradation between the 2011 and 2021 surveys. The net gain of sediment in the reach between 2011 and 2021 was about 167,600 yd³, which is the second-largest net gain shown in table 8. The cross section from the 2011 survey along the upstream face of the bridge varies from 5 to 15 ft below the other survey sections, except near the banks (fig. 8). The frequency distribution of bed elevations in 2011 is unique compared to the other distributions at this site, with a substantially higher percentage of cells at lower channel-bed elevations (fig. 7). The stone revetment on the left (north) bank showed signs of localized substantial deposition, whereas the spur dikes on the right (south) bank showed minor scour (fig. 11). As with the previous DoD, deposition or scour apparent on opposing faces of a feature (such as a pier or spur dike) likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

Structure A5910 on U.S. Highway 24 at Waverly, Missouri

Structure A5910 (site 15; table 2) on U.S. Highway 24 crosses the Missouri River at RM 293.2 at Waverly, Mo., east of Lexington and Kansas City, Mo. (fig. 1). The site was surveyed on May 20, 2021, when the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 661.7 ft (table 6; fig. 12) and streamflow was about 75,500 ft³/s during the survey (table 6).

The survey area was about 1,640 ft long and about 1,090 ft wide, extending from bank to bank in the main channel (fig. 12). The survey area extended about 650 ft upstream from the centerline of structure A5910 (fig. 12). The channel-bed elevations ranged from about 635 to 652 ft throughout the survey area (5th to 95th percentile range of the bathymetric data; fig. 13), except downstream from the spur dikes on the left (north) bank and in the channel thalweg along the toe of the right (south) bank (fig. 12; table 6). The thalweg was about 10 to 15 ft deeper than the channel bed in the middle of the channel (fig. 12). A line of small to medium dune features was observed in the middle of the channel through the survey reach, and numerous smaller dunes and ripples were present throughout the channel reach (fig. 12). A localized deep scour hole downstream from the downstream spur dike on the left bank had a minimum channel-bed elevation of about 618 ft (table 6; fig. 12). As in previous surveys (Huizinga, 2012, 2014, 2022a), a rock outcrop was evident on the right (south) bank at the upstream end of the reach, and the stone revetment and a longitudinal spur dike also were present on the right bank throughout the reach (fig. 12).

The minor scour holes near piers 10 and 11 were difficult to discern from nearby dunes and ripples (figs. 12, 1.2). Information from bridge plans indicates that piers 10 and 11 are founded on shafts drilled 30 to 42 ft into bedrock, and the thickness of bed material between the bottom of the scour hole and bedrock at pier 11 is 35 ft (table 7; fig. 14). Along the upstream bridge face, the surveyed bed from 2021 generally falls within the fluctuating range of elevations seen in previous multibeam surveys (fig. 14).

The difference between the survey on May 20, 2021, and the previous survey on May 23, 2017 (fig. 15), indicates about 91 percent of the joint area of interest had detectable change, which means about 9 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition appears dominant in the thalweg along the right (south) bank throughout the reach between 2017 and 2021 in the DoD, whereas scour is evident in the area along a line between the southern tips of the spur dikes (fig. 15). The average difference between the bathymetric surfaces was –0.33 ft (table 8), indicating minor channel degradation between the 2017 and 2021 surveys. The net volume of cut in the reach from 2017 to 2021 was about 95,900 yd³, and the net volume of fill was about 77,600 yd³, resulting in a net loss of about 18,300 yd³ of sediment between 2017 and 2021 (table 8). The cross section from the 2021 survey along the upstream face of the bridge is similar to the 2017 survey section between the left bank and pier 10, is 5 to 12 ft below the 2017 survey section between piers 10 and 11 and is 5 to 12 ft above the 2017 survey section between pier 11 and the right bank (fig. 14). The frequency distribution of bed elevations in 2021 is similar to the 2017 distribution above an elevation of 644 ft, but the 2017 distribution has a higher percentage of cells at a lower channel-bed elevation than 2021 (fig. 13). The rock outcrop, stone revetment, and longitudinal spur dike on the right (south) bank showed localized signs of minor scour and deposition, and the various spur dikes throughout the channel showed minor scour on one face and deposition on the other (fig. 15). As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 20, 2021, and the survey on April 25, 2013 (fig. 16), indicates about 64 percent of the joint area of interest had detectable change, which means about 36 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Minor erosion and deposition appear nearly balanced throughout the reach between 2013 and 2021 in the DoD, with scour slightly more dominant and with localized substantial erosion and deposition downstream from the spur dikes (fig. 16). However, the average difference between the bathymetric surfaces was –1.55 ft (table 8), indicating moderate channel degradation between the 2013 and 2021 surveys, and the net loss of sediment between 2013 and 2021 was about 43,000 yd³, which is more than double the net loss of sediment between 2017 and 2021 (table 8). The cross section from the 2021 survey along the upstream face of bridge is roughly similar to the 2013 survey section, varying 5 to 10 ft above and below the 2013 section (fig. 14). The frequency distribution of bed elevations in 2021 appears similar in shape to 2013, but with a lower percentage of cells in the middle range of channel-bed elevations (fig. 13). The rock outcrop, stone revetment, and longitudinal spur dike on the right (south) bank are within the bounds of uncertainty (fig. 16). A substantial part of the channel on the left (north) side was not surveyed in 2013 because of generally lower flows (table 8) and shallow depths in that area. As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 20, 2021, and the earliest survey during flooding on July 21, 2011 (fig. 17), indicates about 86 percent of the joint area of interest had detectable change, which means about 14 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition is dominant in the thalweg along the right (south) bank between 2011 and 2021 in the DoD, and substantial deposition is balanced with moderate to substantial erosion downstream from the upstream spur dike, whereas the scour hole downstream from the downstream spur dike is substantially larger in 2021 than 2011 (fig. 17). The average difference between the bathymetric surfaces was +0.44 ft (table 8), indicating minor to moderate deposition between the 2011 and 2021 surveys, with a net gain of sediment of about 23,000 yd³ (table 8). The frequency distribution of bed elevations in 2011 was wider and lower than 2021, with a lower percentage of cells at a wider range of channel-bed elevations (fig. 13). The rock outcrop, stone revetment, and longitudinal spur dike on the right (south) bank showed localized signs of minor scour and deposition, and the various spur dikes throughout the channel showed minor scour on one face and deposition on the other (fig. 17). As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The vertically averaged velocity vectors indicate mostly uniform flow in the middle of the channel, with velocities ranging from about 3 to 9 feet per second (ft/s; fig. 18). Local lower velocities and turbulence were observed downstream from the various spur dikes on the left (north) bank (fig. 18). The wake vortices downstream from the main channel piers were not pronounced and seemed to be no greater than the general nonuniformity of flow observed in the channel (fig. 18). Minor localized turbulence was present in all the sections (fig. 18).

Structure K0999 on State Highway 41 at Miami, Missouri

Structure K0999 (site 16; table 2) on State Highway 41 crosses the Missouri River at RM 262.6 at Miami, Mo., east of Waverly and Kansas City, Mo. (fig. 1). The site was surveyed on May 20, 2021, when the average water-surface elevation near the bridge, determined by the RTK GNSS tide solution, was 634.5 ft (table 6; fig. 19) and streamflow was measured at about 79,800 ft³/s during the survey (table 6).

The survey area was about 1,640 ft long and varied from about 1,380 ft wide at the upstream end to about 1,050 ft wide near the downstream end, extending from bank to bank in the main channel (fig. 19). The survey area extended about 625 ft upstream from the centerline of structure K0999 (fig. 19). The approximate channel-bed elevations ranged from about 607 to 626 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; table 6; fig. 20) except near pier 5, in the scour hole downstream from the spur dike on the left (north) bank, and in the troughs of the medium dune features in the thalweg along the toe of the rock outcrop along the right (south) side of the channel (fig. 19; table 6). The thalweg was along the outside of the river bend on the right (south) bank and was about 10 to 15 ft deeper than the bed in the middle of the channel (fig. 19). Numerous superimposed small to medium dunes and ripples were present throughout the channel (fig. 19). As in previous surveys (Huizinga, 2012, 2014, 2020a), a rock outcrop was present on the right (south) bank throughout the reach (fig. 19).

A substantial scour hole was observed near pier 5 of structure K0999 (figs. 19, 1.3C, 1.3D), and was about 20 ft below the average channel-bed elevation upstream from the scour hole (table 7). Information from bridge plans indicates that piers 5 and 6 are caissons founded on bedrock, with about 43 ft of bed material between the bottom of the scour hole and bedrock at pier 6 but only about 9 ft of bed material between the bottom of the scour hole and bedrock at pier 5 (table 7; fig. 21). Pier 4 is founded on a footing on bedrock according to bridge plans (fig. 21), and no scour hole was observed near the pier (fig. 19; table 7). The surveyed bed along the cross section generally was similar to previous multibeam surveys (fig. 21), but as much as 15 ft higher than previous surveys between the left bank and pier 6. The approximate top-of-bedrock line from bridge plans confirms that the fluvial material near the right bank has washed away, exposing the bedrock along the thalweg, as in previous surveys (fig. 21). In modern construction, bridge substructural elements usually are pinned or socketed to bedrock (Brown and others, 2018; American Association of State Highway Transportation Officials, 2020), but full exposure of usually buried substructural elements warrants special consideration and observation.

The difference between the survey on May 20, 2021, and the previous survey on May 24, 2017 (fig. 22), indicates about 85 percent of the joint area of interest had detectable change, which means about 15 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Scour appears dominant throughout most of the reach between 2017 and 2021 in the DoD, except downstream from the spur dike on the left (north) bank and in the troughs of the medium dunes near the thalweg (fig. 22). The average difference between the bathymetric surfaces was –0.95 ft (table 8), indicating moderate channel degradation between the 2017 and 2021 surveys. The net volume of cut in the reach from 2017 to 2021 was about 97,800 yd³, and the net volume of fill was about 42,000 yd³, resulting in a net loss of about 55,800 yd³ of sediment between 2017 and 2021 (table 8). The cross section from the 2021 survey along the upstream face of the bridge generally varies above and below the 2017 survey section (fig. 21). The frequency distribution of bed elevations in 2021 appears similar in shape to 2017, but with a slightly higher percentage of cells at a lower channel-bed elevations (fig. 20). The rock outcrop on the right (south) bank showed localized signs of minor deposition and scour, but most of the rock outcrop indicated changes within 1 ft of the 2017 elevation, or changes outside the 95-percent confidence interval (fig. 22).

The difference between the survey on May 20, 2021, and the survey on April 25, 2013 (fig. 23), indicates about 58 percent of the joint area of interest had detectable change, which means about 42 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). A substantial part of the channel on the left (north) side was not surveyed in 2013 because of generally lower flows (table 8) and shallow depths in that area. Erosion again appears dominant throughout most of the joint reach between 2013 and 2021 in the DoD, except localized minor deposition throughout the downstream channel thalweg (fig. 23). The average difference between the bathymetric surfaces was –0.70 ft (table 8), again indicating moderate channel degradation between the 2013 and 2021 surveys, but the net loss of sediment between 2013 and 2021 was about 16,900 yd³, which is less than the net loss of sediment between 2017 and 2021 (table 8). The cross section from the 2021 survey along the upstream face of bridge generally is similar to but 5 to 10 ft lower than the 2013 survey section (fig. 21). The frequency distribution of bed elevations in 2013 has a substantially higher percentage of cells at lower channel-bed elevations than 2021 (fig. 20). The difference in the distribution may be the result of the substantially narrower survey area in 2013, and general lack of bank information (fig. 23). The rock outcrop on the right (south) bank showed localized signs of minor scour (fig. 23); however, the differences of most of the channel near the rock outcrop were equivocal. As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 20, 2021, and the earliest survey during flooding on July 21, 2011 (fig. 24), indicates about 73 percent of the joint area of interest had detectable change, which means about 27 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Scour and deposition appear approximately balanced throughout the reach between 2011 and 2021 in the DoD, with substantial scour near the spur dike on the left (north) bank balanced with deposition downstream, and alternating scour and deposition in the thalweg (fig. 24). The average difference between the bathymetric surfaces was –0.09 ft (table 8), indicating a rough balance between aggradation and degradation between the 2011 and 2021 surveys. There was a net loss of sediment between 2011 and 2021 of only 4,400 yd³ (table 8). The frequency distribution of bed elevations in 2011 is remarkably similar to 2021 (fig. 20). The rock outcrop on the right (south) bank showed localized signs of minor scour, particularly higher on the bank (fig. 24); however, the differences of most of the channel near the rock outcrop were equivocal. As with the previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The vertically averaged velocity vectors indicate mostly uniform flow in the middle of the channel, with velocities ranging from about 2 to 8 ft/s (fig. 25). Lower velocities and turbulence were observed downstream from the spur dike on the left (north) side of the channel (fig. 25). The wake vortices downstream from piers 5 and 6 were not pronounced and seemed to be no greater than the general nonuniformity of flow observed in the channel (fig. 25). Minor localized turbulence again was present in all the sections (fig. 25).

Structure G0069 on State Highway 240 at Glasgow, Missouri

Structure G0069 (site 17; table 2) on State Highway 240 crosses the Missouri River at RM 226.3 at Glasgow, Mo., southeast of Miami and about halfway between Kansas City and St. Louis, Mo. (fig. 1). The site was surveyed on May 20, 2021, when the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 605.1 ft (table 6; fig. 26) and streamflow on the Missouri River was about 102,000 ft³/s during the survey (table 6).

The survey area was about 1,640 ft long and varied from about 940 ft wide at the upstream end to about 1,160 ft wide near the downstream end, extending from bank to bank in the main channel (fig. 26). The upstream end of the survey area was about 590 ft upstream from the centerline of structure G0069, and piers 2, 3, and 4 were in the water and away from the banks (fig. 26). The channel-bed elevations ranged from about 575 to 593 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; table 6; fig. 27), except near the piers and downstream from the spur dike on the downstream right (west) bank (fig. 26; table 6). A poorly defined thalweg was present along the left (east) bank and deepened in the downstream reach (fig. 26). The thalweg was about 13 to 15 ft deeper than the channel bed in the middle of the downstream reach (fig. 26). Two spur dikes, and a sunken barge observed during the previous surveys, were present on the right (west) bank (fig. 26). A series of medium to large dunes were present in the middle of the channel throughout the reach, and numerous small dunes and ripples were present throughout the rest of the channel (fig. 26).

A small scour hole was observed near pier 2 (fig. 26), which is founded on a footing on bedrock according to bridge plans (fig. 28; table 6). The ground line from the bathymetric survey indicated that the right side of the footing seemed to be undermined in previous surveys in 2011 and 2013 but was partially buried in bed sediments in the 2017 and 2021 surveys (figs. 28, 1.4B, 1.4C). The details of the footing of pier 2 in the bridge plans for structure G0069 are not sufficient to tell if it was entrenched in the bedrock in any way, particularly if the top of the bedrock was not level, as indicated by previous surveys (Huizinga, 2014; fig. 28). In modern construction, bridge substructural elements usually are pinned or socketed to bedrock (Brown and others, 2018; American Association of State Highway Transportation Officials, 2020), but full exposure of usually buried substructural elements warrants special consideration and observation, particularly at an older bridge such as structure G0069 (built in 1922).

A substantial scour hole was observed near main channel pier 3 (figs. 26, 1.4D, 1.4E), and a smaller scour hole was observed near pier 4 (fig. 26). Near pier 3, the scour hole had a minimum channel-bed elevation of about 566 ft (table 7), which is about 12 ft below the average channel bed immediately upstream from the pier (figs. 26, 28; table 7). Near pier 4, the scour hole had a minimum channel-bed elevation of about 579 ft (figs. 26, 28; table 7). Information from bridge plans indicates that piers 3 and 4 are caissons founded on bedrock, and there is about 4 ft of bed material between the bottom of the scour hole and bedrock at pier 3 and about 22 ft at pier 4 (fig. 28; table 7). The scour observed near all the highway bridge piers was substantially affected by the upstream railroad bridge piers (fig. 26). The 2021 surveyed bed generally was most similar to the 2011 multibeam survey (fig. 28). The approximate top of bedrock confirmed in the previous multibeam surveys seems to have a thin layer of fluvial material near the left bank in the 2021 survey (fig. 28), with further evidence from small dunes and ripples in that area (fig. 26).

The difference between the survey on May 20, 2021, and the previous survey on May 24, 2017 (fig. 29), indicates about 87 percent of the joint area of interest had detectable change, which means about 13 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Scour and deposition appear roughly balanced throughout the reach between 2017 and 2021 in the DoD, with scour and deposition alternating in the troughs of the medium to large dune features in the middle of the channel and lengthwise throughout the right (west) side of the reach (fig. 29). The average difference between the bathymetric surfaces was –0.09 ft (table 8), indicating a rough balance between aggradation and degradation between the 2017 and 2021 surveys. The net volume of cut in the reach from 2017 to 2021 was about 64,400 yd³, and the net volume of fill was about 59,900 yd³, resulting in a net loss of only about 4,500 yd³ of sediment between 2017 and 2021 (table 8). The scour holes near piers 3 and 4 were narrower and shallower in 2021 and in 2017 (fig. 28). The frequency distribution of bed elevations in 2021 appears similar in shape to 2017, with a slightly higher percentage of cells in the middle range of channel-bed elevations than 2017 (fig. 27). The rock outcrop and stone revetment along the left (east) bank showed localized signs of minor to moderate scour, but most of the rock outcrop indicated changes within 1 ft of the 2017 elevation, or changes outside the 95-percent confidence interval (fig. 29).

The difference between the survey on May 20, 2021, and the survey on April 26, 2013 (fig. 30), indicates about 71 percent of the joint area of interest had detectable change, which means about 29 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition appears dominant throughout most of the reach between 2013 and 2021 in the DoD, except for localized erosion in the troughs of the medium to large dunes in the middle of the channel, and erosion along the upstream right (west) bank and downstream left (east) bank (fig. 30). The average difference between the bathymetric surfaces was +0.66 ft (table 8), indicating moderate channel aggradation between the 2013 and 2021 surveys, and the net gain of sediment between 2013 and 2021 was about 26,100 yd³ (table 8). The scour holes near piers 3 and 4 are nearly identical in 2013 and 2021 (fig. 28). However, the frequency distribution of bed elevations in 2013 has a substantially higher percentage of cells at lower channel-bed elevations than 2021 (fig. 27). The stone revetment along the right (southeast) bank showed localized signs of scour and deposition downstream from the bridge and moderate deposition upstream from the bridges (fig. 30). The bedrock outcrop and stone revetment along the left (east) bank showed localized signs of minor to moderate scour (fig. 30); however, the differences of most of the channel near the rock outcrop were equivocal. As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 20, 2021, and the earliest survey during flooding on July 22, 2011 (fig. 31), indicates about 76 percent of the joint area of interest had detectable change, which means about 24 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition appears dominant throughout the reach between 2011 and 2021 in the DoD, with substantial localized erosion in the troughs of the medium to large dunes in the middle of the channel and downstream from the spur dike on the downstream right (west) bank (fig. 31). The average difference between the bathymetric surfaces was +1.84 ft, and a net gain of sediment between 2011 and 2021 of about 79,800 yd³ further indicates substantial deposition in the reach between 2011 and 2021 (table 8). Nevertheless, the frequency distribution of bed elevations in 2021 appears similar in shape to 2011 above 585 ft; however, below about 585 ft, the 2011 distribution has a substantially higher percentage of cells for a given elevation (fig. 27).

The vertically averaged velocity vectors indicate mostly uniform flow throughout the reach, ranging from about 4 to 9 ft/s, except downstream from the spur dikes on the right (west) bank and the boat ramp on the left (east) bank (fig. 32). The wake vortices downstream from piers 2 and 3 were not pronounced and seemed to be no greater than the general nonuniformity of flow observed in the channel (fig. 32). Minor localized turbulence again was present in all the sections (fig. 32).

Structure A4574 on State Highway 5 at Boonville, Missouri

Structure A4574 (site 18; table 2) on State Highway 5 crosses the Missouri River at RM 196.6 at Boonville, Mo., south of Glasgow and about halfway between Kansas City and St. Louis, Mo. (fig. 1). The site was surveyed on May 21, 2021, when the average water-surface elevation near the bridge, determined by the RTK GNSS tide solution, was 580.9 ft (table 6; fig. 33) and streamflow on the Missouri River was about 112,000 ft³/s during the survey (table 6).

The survey area was about 1,640 ft long and about 1,300 ft wide, extending from bank to bank in the main channel (fig. 33). The upstream end of the survey area was about 620 ft upstream from the centerline of structure A4574, and piers 5 through 8 were in the water, although piers 5 and 8 were on the extreme edges of the surveyed area (fig. 33). The approximate channel-bed elevations ranged from about 555 to 567 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; table 6; fig. 34) reaching a minimum elevation of 548 ft in the channel thalweg along the toe of the left (north) bank (fig. 33; table 6). The well-defined thalweg was present along the left (north) bank throughout the surveyed area and was about 8 to 10 ft deeper than the channel bed in the middle of the channel. Small to medium dune features with superimposed small dunes and ripples were present throughout the channel (fig. 33).

The minor scour holes near piers 6 and 7 were difficult to discern from nearby dunes and ripples (figs. 33, 1.5B, 1.5C, 1.5D, 1.5E). Near pier 7, the scour hole had a minimum channel-bed elevation of about 556 ft (table 7), about 23 ft above the elevation of the bottom of the pier seal course of 533.00 ft and about 43 ft above the level of bedrock near that pier (fig. 35; table 7). The minor scour hole near pier 6 had a minimum channel-bed elevation of about 563 ft (table 7). Information from bridge plans indicates that pier 7 is founded on shafts drilled 39 ft into bedrock (fig. 35; table 7), whereas pier 6 is founded on a footing on bedrock, having about 12 ft of bed material between the bottom of the scour hole and bedrock (fig. 35; table 7). The surveyed bed was similar to the previous multibeam surveys in 2011, 2013, and 2017, with the 2021 bed oscillating between the bed in the previous surveys except between piers 7 and 8 where the bed was 5 to 7 ft lower in 2021 than in the previous surveys (fig. 35). The scour hole near pier 7 appears to penetrate to the top of the pier footing (figs. 33 and 35), which might provide some mitigation to the scour hole at this pier as the downward flow of the horseshoe vortex at the pier column face is blunted by the footing (Arneson and others, 2012).

The difference between the survey on May 21, 2021, and the previous survey on May 25, 2017 (fig. 36), indicates about 85 percent of the joint area of interest had detectable change, which means about 15 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition and erosion appear roughly balanced throughout the reach between 2017 and 2021 in the DoD, except near the right (south) bank where erosion is more dominant (fig. 36). The average difference between the bathymetric surfaces was –0.44 ft (table 8), indicating minor channel degradation between the 2017 and 2021 surveys. The net volume of cut in the reach from 2017 to 2021 was about 77,500 yd³, and the net volume of fill was about 48,800 yd³, resulting in a net loss of about 28,700 yd³ of sediment between 2017 and 2021 (table 8). As indicated in the previous paragraph, the cross section from the 2021 survey along the upstream face of the bridge generally varies above and below the 2017 survey section (fig. 35). The frequency distribution of bed elevations in 2021 is remarkably similar in shape to all the previous surveys, with a slightly higher percentage of cells in the mid-range of channel-bed elevations (fig. 34). The stone revetment on the left (north) bank showed localized areas of erosion and deposition, but nearly all the indicated changes are within 1 ft of the 2017 elevation, or outside the 95-percent confidence interval (fig. 36).

The difference between the survey on May 21, 2021, and the survey on April 29, 2013 (fig. 37), indicates only 64 percent of the joint area of interest had detectable change, which means about 36 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Areas of moderate deposition are balanced with areas of moderate erosion throughout most of the reach between 2013 and 2021 in the DoD, with erosion dominant on the right (south) side and deposition dominant near the tip of the upstream spur dike and in the downstream left (north) side of the channel (fig. 37). The average difference between the bathymetric surfaces was –0.54 ft (table 8), again indicating minor to moderate channel degradation between the 2013 and 2021 surveys, and the net loss of sediment between 2013 and 2021 was about 23,800 yd³, which is less than but consistent with the net loss of sediment between 2017 and 2021 (table 8). As mentioned in the previous paragraph, the frequency distribution of bed elevations in 2021 is remarkably similar in shape to the other surveys, but the 2013 survey distribution is spread over a slightly larger range of channel-bed elevations (fig. 34). The stone revetment on the left (north) bank showed localized areas of erosion and deposition (fig. 37); however, most of the area was equivocal, and the moderate apparent deposition likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 21, 2021, and the earliest survey during flooding on July 25, 2011 (fig. 38), indicates about 76 percent of the joint area of interest had detectable change, which means about 24 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). As with the other comparisons, scour and deposition appear approximately balanced throughout the reach between 2011 and 2021 in the DoD, with more scour in the upstream part of the reach, and deposition in the downstream part of the reach (fig. 38). The average difference between the bathymetric surfaces was –0.53 ft (table 8), again indicating minor degradation between the 2011 and 2021 surveys. There was a net loss of sediment between 2011 and 2021 of about 29,700 yd³ (table 8). The stone revetment and longitudinal spur dike on the left (north) bank showed substantial deposition (fig. 38), which may be the result of these features being repaired or otherwise built up after the 2011 flood. As with several previous DoDs, some of this apparent deposition may result from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The vertically averaged velocity vectors indicate mostly uniform flow with localized moderate turbulence throughout most of the reach, ranging from about 3 to 9 ft/s (fig. 39). Other exceptions to uniform conditions include variable velocities and flow reversal on the right (south) bank downstream from the upstream spur dike (fig. 39). All the piers were aligned with flow, as indicated by little to no turbulence observed downstream that can be directly attributed to the piers (fig. 39).

Structure L0962 on Interstate 70 near Rocheport, Missouri

Structure L0962 (site 19; table 2) on Interstate 70 crosses the Missouri River at RM 185.1 near Rocheport, Mo., east of Boonville and west of St. Louis, Mo. (fig. 1). The site was surveyed on May 21, 2021, when the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 571.1 ft (table 6; fig. 40) and streamflow on the Missouri River was about 110,000 ft³/s during the survey (table 6).

The survey area was about 1,640 ft long and about 1,200 ft wide, extending essentially from bank to bank in the main channel (fig. 40). The upstream end of the survey area was about 580 ft upstream from the centerline of structure L0962 (fig. 40), and piers 13 and 14 as well as bent 16 were in the water. The channel-bed elevations ranged from about 541 to 559 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; fig. 41; table 6). A poorly defined thalweg was present along the left (northeast) bank, and a series of medium dunes were detected along the middle of the channel, with numerous small dunes and ripples throughout the rest of the channel (fig. 40). The persistent debris raft near bent 16 in previous surveys was not present in 2021, which permitted bathymetric data to be collected near that bent (fig. 40).

A moderate scour hole near pier 14 had a minimum channel-bed elevation of about 531 ft (fig. 40; table 7), which is about 17 ft below the average channel-bed elevation upstream from the bridge (figs. 42, 1.6E, 1.6F; table 7). Information from bridge plans indicates that pier 14 is founded on a caisson on bedrock, having about 33 ft of bed material between the bottom of the scour hole and bedrock (fig. 42; table 7). The surveyed bed in 2021 generally was similar to the previous multibeam surveys between pier 14 and the right (southwest) bank, but there was substantial erosion between the left (northeast) bank and pier 15, and substantial deposition of as much as 20 ft between piers 14 and 15 (fig. 42).

Pier 15 is embedded in the longitudinal spur dike, and the minimum channel elevation at the toe of the dike near the pier was about 538 ft (figs. 42, 1.6C, 1.6D; table 7). Pier 15 also is founded on a caisson on bedrock, having about 19 ft of material between the minimum channel-bed elevation and bedrock (fig. 42; table 7); however, the rock of the spur dike will limit or prevent a local scour hole near pier 15. Near bent 16, the minimum channel-bed elevation also was about 538 ft (figs. 42, 1.6B; table 7), about 5 ft below the average channel bed upstream from the scour hole, and only about 2 ft above the elevation of bedrock near that bent (fig. 42; table 7). Information from bridge plans indicates that bent 16 is founded on a footing on bedrock (fig. 42; table 7). In modern construction, bridge substructural elements usually are pinned or socketed to bedrock (Brown and others, 2018; American Association of State Highway Transportation Officials, 2020), but full exposure of usually buried substructural elements warrants special consideration and observation.

The difference between the survey on May 21, 2021, and the previous survey on May 25, 2017 (fig. 43), indicates about 91 percent of the joint area of interest had detectable change, which means only about 9 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition appears dominant throughout most of the reach between 2017 and 2021 in the DoD, except in localized troughs in the medium dune features in the upstream part of the reach and downstream from the spur dikes on both banks (fig. 43). Substantial erosion of as much as 45 ft was observed near pier 15 and bent 16 (fig. 43). The average difference between the bathymetric surfaces was +1.59 ft (table 8), indicating moderate channel aggradation between the 2017 and 2021 surveys. The net volume of cut in the reach from 2017 to 2021 was about 77,300 yd³, and the net volume of fill was about 177,500 yd³, resulting in a net gain of about 100,200 yd³ of sediment between 2017 and 2021 (table 8). The frequency distribution of bed elevations in 2021 is somewhat unique (fig. 41); similar in overall shape to 2017 (and the others), but with a 3–5-ft shift towards higher elevations at the centroid of the elevation distribution. The longitudinal spur dikes along the left (northeast) bank showed minor to moderate scour on one side and deposition on the other (fig. 43); however, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 21, 2021, and the survey on April 29, 2013 (fig. 44), indicates about 72 percent of the joint area of interest had detectable change, which means about 28 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition again appears dominant throughout most of the reach between 2013 and 2021 in the DoD, except for localized substantial erosion downstream from the longitudinal spur dikes on the left (northeast) bank and in the troughs in the medium dune features along the upstream middle of the channel (fig. 44). The average difference between the bathymetric surfaces was +2.74 ft (table 8), indicating moderate to substantial channel aggradation between the 2013 and 2021 surveys with a net gain of sediment between 2013 and 2021 of about 111,500 yd³ (table 8). As with 2017, the frequency distribution of bed elevations in 2021 has an almost 5-ft shift towards higher elevations at the centroid of the elevation distribution compared to 2013 (fig. 41). The difference in the distribution likely is the result of the substantially narrower survey area in 2013, and general lack of bank information (fig. 44). The longitudinal spur dikes along the left (northeast) bank showed minor scour on the side that could be surveyed in 2013, but much of the difference is equivocal, and substantial areas behind the spur dikes were not surveyed in 2013 (fig. 44).

The difference between the survey on May 21, 2021, and the earliest survey during flooding on July 26, 2011 (fig. 45), indicates about 82 percent of the joint area of interest had detectable change, which means about 18 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Scour and deposition appear approximately balanced in the reach between 2011 and 2021 in the DoD (fig. 45), with substantial deposition in the thalweg and some areas downstream from the spur dikes on both banks balanced by substantial erosion in the upstream middles of the channel and near pier 15 and bent 16. However, the average difference between the 2011 and 2021 bathymetric surfaces was +2.47 ft (table 8), indicating moderate to substantial channel aggradation with a net gain of sediment between 2011 and 2021 of about 137,200 yd³ (table 8). The frequency distribution of bed elevations in 2021 is most similar to the 2011 distribution above 547 ft, but the 2011 distribution has a higher percentage of cells at a lower channel-bed elevation, particularly below about 545 ft (fig. 41). The longitudinal spur dikes along the left (northeast) bank again showed minor to moderate scour on one side and deposition on the other, as does pier 14 (fig. 45); however, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The vertically averaged velocity vectors indicate mostly uniform flow throughout the channel, ranging from about 3 to 10 ft/s (fig. 46), with locally lower velocities and flow reversal along the banks, particularly downstream from the various spur dikes in the reach. Pier 14 was mostly aligned with flow, causing minimal turbulence downstream (fig. 46); however, flow approached pier 14 from the left side (fig. 46), causing an asymmetric scour hole that is longer and slightly deeper on the left side, similar to observations from previous surveys (fig. 40).

Structures L0550 and A4497 on U.S. Highway 54 at Jefferson City, Missouri

Structures L0550 and A4497 (site 20; table 2) are dual bridges on U.S. Highway 54 crossing the Missouri River at RM 143.9 at Jefferson City, Mo., southeast of Rocheport and west of St. Louis, Mo. (fig. 1). The site was surveyed on May 26, 2021, when the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 534.2 ft (table 6; fig. 47) and streamflow on the Missouri River was about 95,700 ft³/s during the survey (table 6). This site has been the subject of several multibeam surveys (Huizinga, 2014) that provide bathymetry at a variety of flow conditions for comparison. Furthermore, scour countermeasures in the form of a riprap blanket were placed around pier 4 of structures L0550 and A4497 in 2015, and the site near the piers was surveyed before and after the installation of these countermeasures (Huizinga, 2020a).

The survey area was about 1,640 ft long and about 970 ft wide from bank to bank in the main channel (fig. 47). The upstream end of the survey area was about 630 ft upstream from the centerline between structures L0550 and A4497, and piers 3 and 4 of both structures were in the water, although pier 3 was on the extreme edge of the surveyed area on the right (south) bank (fig. 47). The channel-bed elevations ranged from about 504 to 521 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; fig. 48; table 6), except along the toe of the right (south) bank, on the upper banks, and near the spur dikes on the left and right sides of the channel (fig. 47; table 6). A shallow thalweg was present along the right (south) bank throughout the reach and was about 5 to 8 ft deeper than the channel bed in the middle of the channel (fig. 47). Small dunes and ripples were present throughout the survey reach (fig. 47). As in previous surveys (Huizinga, 2012, 2014, 2022a), stone revetment was present on the right (south) bank throughout the reach (fig. 47).

The scour holes that historically had been observed near pier 4 of both structures before 2015 (refer to fig. 31 in Huizinga, 2014) were conspicuously absent in the 2021 survey, having been effectively mitigated by the scour countermeasures installed around the piers in 2015 (figs. 47, 1.7A, 1.7B). The top of the riprap blanket countermeasure was evident around both piers in 2021 but appeared to have settled by 5 to 10 ft at the upstream end (figs. 47, 49, 1.7A, 1.7B), resulting in a shallow scour hole with a minimum channel-bed elevation of about 503 ft around the upstream perimeter of the blanket (fig. 47; table 7). Information from bridge plans indicates that pier 4 of upstream structure L0550 is founded on a caisson on bedrock, having about 44 ft of bed material between the channel bed near the pier and bedrock (fig. 49; table 7). Pier 4 of structure A4497 is founded on shafts drilled 11 ft into bedrock, having about 50 ft of bed material between the channel bed near the pier and bedrock (fig. 50; table 7). The surveyed bed generally was similar to the previous multibeam survey in 2017 (figs. 49 and 50).

The difference between the survey on May 26, 2021, and the previous survey on May 31, 2017 (fig. 51), indicates about 77 percent of the joint area of interest had detectable change, which means about 23 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Erosion appears dominant throughout most of the reach between 2017 and 2021 in the DoD, except in localized troughs of dune features on the left (northeast) side of the channel, and in the thalweg along the right (southwest) bank (fig. 51). The average difference between the bathymetric surfaces was –0.35 ft (table 8), indicating minor channel degradation between the 2017 and 2021 surveys. The net volume of cut in the reach from 2017 to 2021 was about 50,400 yd³, and the net volume of fill was about 34,300 yd³, resulting in a net loss of about 16,100 yd³ of sediment between 2017 and 2021 (table 8). The frequency distribution of bed elevations in 2021 is similar in shape to 2017, but with a higher percentage of cells in the mid-range of channel-bed elevations between 505 and 515 ft (fig. 48). The upstream and right edges of the riprap blanket around the piers showed erosion of 5 to 10 ft, and other localized erosion was evident on the top of the blanket (fig. 51); furthermore, the stone revetment along the right (southwest) bank also showed localized signs of minor scour and deposition (fig. 51); however, the apparent scour and deposition of the revetement and top of the riprap blanket may be the result of minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 26, 2021, and the survey on April 30, 2013 (fig. 52), indicates about 62 percent of the joint area of interest had detectable change, which means about 38 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition appears dominant throughout most of the reach between 2013 and 2021 in the DoD, except for moderate to substantial erosion along the left (northeast) bank (fig. 52). The average difference between the bathymetric surfaces was +0.49 ft (table 8), indicating minor channel aggradation between the 2013 and 2021 surveys, and there was a net gain of about 17,400 yd³ of sediment between 2013 and 2021 (table 8). The frequency distribution of bed elevations in 2021 is again similar in shape to 2013, but with a slightly higher percentage of cells in the mid-range of channel-bed elevations between 505 and 515 ft (fig. 48); furthermore, the centroid of the 2013 distribution is shifted about 2 ft lower than the 2021 distribution below 514 ft. The stone revetment along the right (southwest) bank also showed very localized signs of minor scour and deposition (fig. 52); however, the differences of most of the channel near the revetment were equivocal. As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 21, 2021, and the survey during flooding on July 27, 2011 (fig. 53), indicates about 83 percent of the joint area of interest had detectable change, which means about 17 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition appears dominant in the reach between 2011 and 2021 in the DoD (fig. 53), with substantial deposition of as much as 45 ft in the historical scour hole near pier 4 of both structures. Alternatively, substantial erosion of as much as 20 ft was apparent along the left (northeast) bank (fig. 53). Nevertheless, the average difference between the 2011 and 2021 bathymetric surfaces was +4.52 ft (table 8) with a net gain of sediment between 2011 and 2021 of about 221,100 yd³ (table 8), both of which are by far the largest positive changes observed in the 2021 surveys compared to previous surveys. The frequency distribution of bed elevations in 2011 is unique compared to the other distributions at this site, with a substantially higher percentage of cells at lower channel-bed elevations (fig. 48). The stone revetment along the right (southwest) bank again showed very localized signs of minor scour and deposition (fig. 53); however, the differences of most of the channel near the revetment were equivocal. As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 21, 2021, and an earlier survey on March 1, 2011 (fig. 54), indicates about 72 percent of the joint area of interest had detectable change, which means about 28 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition appears dominant in the reach between March 2011 and 2021 in the DoD (fig. 54), with substantial deposition of as much as 45 ft in the historical scour hole near pier 4 of both structures, but moderate erosion of as much as 15 ft along the left (northeast) bank. However, the average difference between the March 2011 and 2021 bathymetric surfaces was +1.27 ft (table 8), indicating moderate channel aggradation with a net gain of sediment between March 2011 and 2021 of about 45,400 yd³ (table 8). As in the other comparisons at this site, the stone revetment along the right (southwest) bank again showed very localized signs of minor scour and deposition (fig. 54); however, the differences of most of the channel near the revetment were equivocal. As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 21, 2021, and the earliest survey on January 16, 2010 (fig. 55), indicates about 73 percent of the joint area of interest had detectable change, which means about 27 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Despite substantial deposition of as much as 60 ft in the historical scour hole near pier 4 of both bridges, erosion appears dominant in the reach between 2010 and 2021 in the DoD (fig. 55). The average difference between the 2010 and 2021 bathymetric surfaces was –1.21 ft (table 8), indicating moderate channel degradation with a net loss of sediment between 2010 and 2021 of about 45,200 yd³ (table 8). Once again, the stone revetment along the right (southwest) bank again showed very localized signs of minor scour and deposition (fig. 54); however, the differences of most of the channel near the revetment were equivocal. As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The vertically averaged velocity vectors indicate mostly uniform flow throughout the channel, ranging from about 3 to 9 ft/s (fig. 56), with locally lower velocities and flow reversal along the banks downstream from the various spur dikes in the reach. Pier 4 of both structures are mostly aligned with flow but cause moderate turbulence downstream (fig. 56). However, flow velocities seemed unusually variable in all the transects (fig. 56), which is unusual given the general lack of spur dikes and substantial dune features that might cause flow divergence or upwelling in the reach (Best, 2005).

Structure A6288 on State Highway 19 at Hermann, Missouri

Structure A6288 (site 21; table 2) on State Highway 19 crosses the Missouri River at RM 97.9 at Hermann, Mo., east of Jefferson City and west of St. Louis, Mo. (fig. 1). The site was surveyed on May 26, 2021, when the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 497.7 ft (table 6; fig. 57) and streamflow on the Missouri River was about 132,000 ft³/s during the survey (table 6).

The survey area was about 1,840 ft long and about 1,450 ft wide, generally extending from bank to bank in the main channel (fig. 57). The upstream end of the survey area was about 690 ft upstream from the centerline of structure A6288 at pier 5 (fig. 57), and piers 4 through 6 were in the water. The channel-bed elevations ranged from about 463 to 484 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; fig. 58; table 6) except inside the upstream left (north) longitudinal spur dike where a local minimum channel-bed elevation of 442 ft was observed (table 6; fig. 57). A poorly defined thalweg was present along the left (north) bank throughout the length of the surveyed area. Numerous small to medium dunes and ripples were detected throughout the channel (fig. 57). As in previous surveys (Huizinga, 2012, 2014, 2022a), rock outcrops and stone revetment were present on the right (south) bank throughout the reach, and spur dikes were present on both banks (fig. 57).

Minor scour holes were observed near both piers in the main channel, and near pier 6 behind the longitudinal spur dike on the left (north) side (fig. 57). The minor scour hole near pier 6 had a minimum channel-bed elevation of about 469 ft (table 7), which is about 27 ft above the elevation of the bottom of the pier seal course of 442.00 ft (fig. 59; table 7). The minor scour hole near pier 5 had a minimum channel-bed elevation of about 464 ft (table 7), which is about 20 ft above the elevation of the bottom of the pier seal course of 444.50 ft (fig. 59; table 7). The moderate scour hole near pier 4 had a minimum channel-bed elevation of about 460 ft (table 7), which is about 9 ft above the elevation of the bottom of the pier seal course of 451.50 ft (fig. 59; table 7). The scour hole near pier 5 was difficult to discern from nearby dunes and ripples, but the downstream left edge of the footing was visible during the survey (fig. 1.8C). The upstream edge of the footing for pier 4 also was visible during the survey (figs. 57, 59, 1.8E, 1.8F), which likely contributes to the larger scour hole near that pier. The top of the footing may blunt the horseshoe vortex at pier 5, but the exposed front of the footing may exacerbate the vortex at pier 4. Information from bridge plans indicates that piers 4, 5, and 6 are founded on shafts drilled as much as 27 ft into bedrock, having about 68 ft of bed material between the bottom of the scour hole and bedrock at pier 6, about 51 ft of material at pier 5, and about 17 ft of material at pier 4, owing to the sloping nature of the bedrock (fig. 59; table 7). The surveyed bed in 2021 generally was similar to the previous multibeam surveys but tended to fall towards the bottom of the fluctuating range of elevations seen in previous surveys between the longitudinal spur dike on the left (north) side of the channel and pier 4 (fig. 59).

The difference between the survey on May 26, 2021, and the previous survey on May 31, 2017 (fig. 60), indicates about 76 percent of the joint area of interest had detectable change, which means about 24 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition and erosion appear roughly balanced throughout most of the reach between 2017 and 2021 in the DoD, including areas of substantial erosion and deposition behind the longitudinal spur dike on the left (north) bank near the bridge (fig. 60). The scour hole near pier 4 was wider and deeper in 2021 than in 2017 (figs. 59, 60), despite the lower streamflow in 2021 (table 8). The average difference between the bathymetric surfaces was –0.39 ft (table 8), indicating minor channel degradation between the 2017 and 2021 surveys. The net volume of cut in the reach from 2017 to 2021 was about 85,500 yd³, and the net volume of fill was about 58,100 yd³, resulting in a net loss of about 27,400 yd³ of sediment between 2017 and 2021 (table 8). The frequency distribution of bed elevations in 2021 is similar to 2017, but with a 2–3-ft shift of the centroid towards lower elevations (fig. 58). The rock outcrop and stone revetment along the right (south) bank and the longitudinal spur dikes on the left (north) side showed localized signs of minor scour and deposition (fig. 60); however, the apparent scour or deposition may be the result of minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section).

The difference between the survey on May 26, 2021, and the survey on May 2, 2013 (fig. 61), indicates about 71 percent of the joint area of interest had detectable change, which means about 29 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Erosion appears dominant throughout most of the reach between 2013 and 2021 in the DoD, except in localized areas near and downstream from spur dikes on both banks (fig. 61). The average difference between the bathymetric surfaces was –3.05 ft (table 8), which is the largest negative change observed in the 2021 surveys compared to previous surveys. The average difference indicates substantial channel degradation between the 2013 and 2021 surveys, and the net loss of sediment between 2013 and 2021 was about 169,200 yd³ (table 8), which is the most substantial loss between the surveys detailed in this report. The cross section from the 2021 survey along the upstream face of the bridge is about 5–8 ft below the 2013 survey section, except near pier 4 where the difference is near 20 ft (fig. 59). The frequency distribution of bed elevations in 2013 has a higher percentage of cells in a narrower range of channel-bed elevations, causing a shift of the centroid of 4–5 ft higher than 2021 (fig. 58). The rock outcrop and stone revetment on the right (south) bank is within the bounds of uncertainty (fig. 61). A substantial part of the channel on the left (north) side was not surveyed in 2013 because of generally lower flows (table 8) and shallow depths in that area. As with the previous DoDs, deposition or scour apparent on opposing faces of a feature (such as a pier or spur dike) likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section). Nevertheless, the substantial deposition on the longitudinal spur dike just upstream from the bridge (fig. 61) likely is the result of repairs made to the dike between 2013 and 2017, because a similar indication of deposition was not apparent in the comparison between 2017 and 2021 (fig. 60).

The difference between the survey on May 26, 2021, and the survey during flooding on July 28, 2011 (fig. 62), indicates about 73 percent of the joint area of interest had detectable change, which means about 27 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Erosion appears dominant throughout most of the reach between 2011 and 2021 in the DoD, except for substantial deposition in localized areas near and downstream from spur dikes on both banks (fig. 62). The average difference between the bathymetric surfaces was –1.38 ft (table 8), which is the third-largest negative change observed in the 2021 surveys compared to previous surveys. The average difference indicates moderate channel degradation between the 2011 and 2021 surveys, and the net loss of sediment between 2011 and 2021 was about 90,300 yd³ (table 8), which is the second-most substantial loss between the surveys detailed in this report. The rock outcrop and stone revetment along the right (south) bank and the longitudinal spur dikes on the left (north) side showed localized signs of minor scour and deposition (fig. 62); however, as with the previous DoDs, deposition or scour apparent on opposing faces of a feature (such as a pier or spur dike) likely results from minor horizontal positional variances between the surveys (refer to “Uncertainty Estimation” section). The substantial deposition on the longitudinal spur dike just upstream from the bridge observed in the 2013 to 2021 comparison (fig. 61) appears again in the 2011 to 2021 comparison (fig. 62), and likely is the result of repairs made to the dike between 2013 and 2017.

The vertically averaged velocity vectors indicate mostly uniform flow throughout the channel, ranging from about 2 to 9 ft/s (fig. 63), with locally lower velocities and flow reversal along the banks downstream from the various spur dikes in the reach. Flow velocities seemed unusually variable in all the transects (fig. 63), which might be the result of flow divergence and upwelling caused by the spur dikes and small to medium dune features in the reach (Best, 2005).