Purpose and Scope

The purpose of this report is to describe the equipment and methods used to do the surveys and to document the results of the bathymetric and velocimetric surveys of the Missouri River channel near 8 highway bridges at 7 crossings near Kansas City, Mo., completed on August 8–9, 2023 (fig. 1; table 2). The results obtained from the bathymetric and velocimetric surveys of the channel document the channel-bed geometry and velocity distribution at the time of the surveys and provide the characteristics of scour holes that may be useful in developing or modifying predictive guidelines or equations for computing potential scour depth. These data may also be used by MoDOT as a low to moderate floodflow conditions comparison to help assess the bridges for stability and integrity issues with respect to bridge scour. Results are also compared with results from previous surveys at the sites (Huizinga, 2010, 2012, 2016, 2022a).

Description of Study Area

The study area for this report encompasses the Missouri River in and near Kansas City, Mo. (fig. 1). The Missouri River flows through the Kansas City area from west to east, meandering across the floodplain. The river is highly channelized in the Kansas City area; rock revetment and spur dikes placed along the banks maintain the channel alignment, and levees and floodwalls are on the upper banks to limit flooding in the industrial, commercial, and agricultural areas on the floodplains. The site numbering sequence used in Huizinga (2012) is also used in this report for consistency and comparability.

Description of Streamflow Conditions

Data from the streamflow-gaging station (hereinafter referred to as “streamgage”) on the Missouri River at Kansas City, Mo. (station 06893000; U.S. Geological Survey, 2024a; fig. 1), indicate that the Missouri River was in a period of low flow for much of the summer of 2023, with streamflow at or below the 10-percent mean daily value for much of the period from April 15 to July 1, 2023, and near the 25-percent mean daily value for much of the rest of the year (fig. 2A). However, there was a minor rise just after these sites were surveyed on August 8–9, 2023 (fig. 2B). Streamflow as measured at the Kansas City streamgage on the Missouri River ranged from about 44,800 to 47,200 cubic feet per second (ft³/s) during the surveys (fig. 2B). This streamflow has a daily exceedance probability of about 51 to 56 percent (the probability that the indicated streamflow value would be equaled or exceeded on any given day at that streamgage; U.S. Geological Survey, 2024b) and is about half the 90-percent annual exceedance probability (the probability that the indicated streamflow value would be equaled or exceeded within a period of 1 year; also known as the 1-year recurrence interval) flood streamflow of 82,000 ft³/s (U.S. Army Corps of Engineers, 2023, table ES–2). Daily exceedance probabilities were computed using daily streamflow from 1957 to 2024 (U.S. Geological Survey, 2024b).

Streamflow conditions in these daily and annual exceedance ranges are in the low floodflow to nonflood regime. In an analysis of real time scour monitoring data from Jefferson City, Mo., Huizinga (2014) noted that substantial pier scour generally began soon after the onset of hydrograph rise (substantial rise of 8 feet [ft] or more), although the scour often did not reach maximum depth until the peak stage was reached or sometime thereafter (see fig. 35 in Huizinga, 2014). Peak streamflow conditions for the year did not happen during this study, and streamflow was probably near base flow based on the daily exceedance values when the survey was completed. Although scour conditions captured at sites during this study likely do not represent the maximum scour potential at the sites, the cumulative information gathered from multiple surveys from 2010 to 2023 at these sites remains useful for determining scour for a variety of flow conditions, particularly when combined with, or compared to, a scour scenario captured at higher, floodflow conditions.

Description of Equipment and Basic Processing of Data

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 of the Missouri and Mississippi Rivers in Missouri (see report references listed in table 1) and of the Missouri and Yellowstone Rivers in North Dakota (Densmore and others, 2013). MBMS survey methods and data-quality checks were similar to those described in these studies. A brief description of the specific equipment used is given in the following paragraphs; a more complete description of the various system components and methods used in this study are available in 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 was the Norbit iWBMSh (fig. 3), operated at a frequency of 400 kilohertz (kHz) with a curved piezoceramic receiver array which enables bathymetric data to be collected throughout a swath range of 210 degrees. Optimum data are usually collected in a swath of less than 160 degrees (80 degrees on each side of the nadir or straight down below the MBES); nevertheless, the swath can be electronically rotated to either side of the nadir, enabling data to be collected along sloping banks up to a depth just below the water surface. The MBES is the same system used in the previous surveys in 2019 (Huizinga, 2022a). 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, so that the sound velocity at the MBES is sufficiently representative of the 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 what is referred to as a “tightly coupled” configuration (fig. 3A) and two Global Navigation Satellite System (GNSS) receivers (fig. 3B). 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. Realtime 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 2023 surveys.

Like all surveys after 2010 (see report references listed in table 1), the navigation information from the 2023 surveys was postprocessed using the POSPac Mobile Mapping Suite (MMS) software (version 8.7; Applanix Corporation, 2021) to mitigate the effects of degraded positional accuracy caused by GNNS outages when the vessel was near or under a bridge. 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.

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 2022; HYPACK, Inc., 2022) 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 water at various points throughout each study reach were collected using an acoustic Doppler current profiler (ADCP), like previous studies since 2011 (for example, see Huizinga, 2012, 2022a). A Teledyne RD Instruments RiverPro ADCP operating at 1,200 kHz was used to obtain velocities at depth-dependent variable-height increments or “bins” throughout the water column. The RiverPro ADCP operates at depths from about 0.5 to 100 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. Water velocities were measured along sections spanning the width of the channel. 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 that quality data were collected were the same as those used in previous studies using the MBES (methods are detailed in Huizinga, 2010, 2012; Huizinga and others, 2010). A brief summary of—and any differences from—these methods are highlighted below.

Uncertainty Estimation

Like recent bathymetry studies in Missouri (see report references listed in table 1; Rivers and others, 2024), 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 resulting gridded uncertainty is a measure of the variability of the individual points in the cell and is used to determine the CUBE-derived elevation for the cell. Statistics for gridded uncertainty of each of the survey areas are shown in table 4. An example of the spatial distribution of gridded uncertainty typically observed in the survey data at structure A8340 on U.S. Highway 69 is shown in figure 4. 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, 2025).

Table 4.    

Total gridded uncertainty results for bathymetric data at a 1.64-foot grid spacing from surveys on the Missouri River near Kansas City, Missouri, August 8–9, 2023.

[Data are summarized from Huizinga and Rivers (2025).]

Table 4.    Total gridded uncertainty results for bathymetric data at a 1.64-foot grid spacing from surveys on the Missouri River near Kansas City, Missouri, August 8–9, 2023.

Bridge 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
6	A1800	6.56	0.06	0.03	0.10	99.8	99.4	98.2	94.7
7	A8340	6.36	0.06	0.03	0.10	99.8	99.2	97.4	92.1
9	A4060	6.40	0.05	0.03	0.06	100.0	99.8	99.0	96.0
10	A7650	6.14	0.06	0.03	0.07	99.9	99.6	98.3	94.6
11	A5817	6.33	0.06	0.03	0.07	99.9	99.6	98.0	92.3
12	A0767	6.50	0.05	0.03	0.07	99.9	99.7	98.6	94.4
13	A4757/L0568	6.33	0.05	0.03	0.06	99.9	99.7	99.0	96.9

More than 99 percent of the uncertainty values at all the sites were less than 0.50 ft (table 4), 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, 2022). Furthermore, more than 92 percent of the uncertainty values were less than 0.10 ft (table 4). The tops of bridge substructural elements (pier footings and seal courses) typically had uncertainty values of less than 0.10 ft. Like noted in previous surveys using this type of equipment (see, 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) but largest near high-relief features, such as the front or side of a pier footing (fig. 4). The largest uncertainty value in this group of surveys was 6.56 ft (table 4). Occasionally, the uncertainty values were also larger (1.00 ft or greater) in the outermost beams of the multibeam swath where there was overlap 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) can also display larger uncertainty values because substantial bed movement can happen between swaths (fig. 4).

The uncertainty of the gridded data computed using the CUBE method (hereinafter referred to as “gridded uncertainty”) has been decreasing with time compared with data from previous surveys (see table 4 in previous studies of the Kansas City area; Huizinga, 2012, 2016, 2022a). The decrease in gridded uncertainty is primarily because of improvements in data-collection equipment and methods. The tightly coupled configuration of the Norbit iWBMSh used in these surveys decreases some gridded 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 gridded uncertainty of the data in the swath overlap zone that might otherwise experience more substantial bed movement.

The survey at structure A8340 on U.S. Highway 69 had the lowest percentage of bathymetry points with a gridded uncertainty of less than the various thresholds, and one of the higher maximum values (6.36 ft) of gridded uncertainty (table 4). The survey at this site was done with smooth longitudinal swaths, which was also done at nearly all the sites surveyed in this study. The primary anomalies at this site were observed near the outermost beams of adjacent swaths and along the banks where the MBES was used in an electronically tilted configuration to extend the potential coverage in these areas, resulting in higher uncertainty values (fig. 4). Generally, the magnitude and distribution of uncertainty values observed at this site are representative of those observed at all the sites.

Structure A1800 on Interstate 635

Structure A1800 (site 6; table 2) on Interstate 635 crosses the Missouri River at river mile (RM) 374.1, on the northwestern side of Kansas City (fig. 1). The site was surveyed on August 8, 2023, and the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution (from the MBMS during the survey), was 724.9 ft (table 5; fig. 6). Streamflow of the Missouri River was about 44,600 ft³/s during the survey (table 5).

The survey area was about 1,640 ft long and about 755 ft wide, extending across the active channel from bank to bank (fig. 6). The survey area extended about 770 ft upstream from the centerline of structure A1800. The channel-bed elevations in 2023 ranged from about 706 to 716 ft for most of the surveyed area (5 to 95 percentile range of the bathymetric data; fig. 7; table 5). A minor thalweg along the outside of the channel bend on the right (south) bank was about 7 ft deeper than the channel bed on the inside of the bend on the left (north) bank (fig. 6). The channel bed was covered with many small to medium dunes and ripples (fig. 6). As in previous surveys (Huizinga, 2010, 2012, 2016, 2022a), a rock outcrop and stone revetment were on the right (south) bank throughout the reach (fig. 6).

The scour hole immediately upstream from main channel pier 3 had a minimum channel-bed elevation of about 695 ft (table 6; fig. 1.1), about 13 ft below the average channel-bed elevation (the value of the “depth of scour hole from upstream channel bed” in table 6; fig. 6) and about 7 ft below the elevation of the bottom of the pier seal course of 702.28 ft (fig. 8; table 6). Pier 3 is skewed about 15 degrees to approach flow (table 6), and impinging flow is on the right (south) side, resulting in a scour hole that is deeper and longer on that side. Information from bridge plans indicates that pier 3 is founded on shafts drilled 20 ft into bedrock, and about 43 ft of bed material are between the bottom of the scour hole and bedrock at the upstream face of pier 3 (fig. 8; table 6). The cross-section from the 2023 survey on the left side of the upstream bridge face was similar to but below most of the previous cross-sections; however, on the right side of the channel, the current (2023) cross-section was at higher elevation than most of the cross-sections from previous surveys (fig. 8).

The computed difference between the DEM from the survey on August 8, 2023, and the DEM from the previous survey on August 13, 2019, both with a probabilistic threshold mask of 95-percent confidence based on uncertainty (hereinafter, referred to simply as “the difference between the surveys”) (fig. 9), indicates that about 95 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 only about 5 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 7). Deposition is dominant in the thalweg along the right (south) side of the channel and on the left (north) bank from 2019 to 2023 in the DoD (fig. 9); however, the left (north) side of the channel has some erosion between the left bank and pier 3 throughout the reach (fig. 9). The average difference between the bathymetric surfaces from the 2019 and 2023 surveys (the statistical mean value of the gridded raster difference surface [fig. 9]) was +2.36 ft (table 7), indicating substantial channel aggradation. The net volume of cut in the reach from 2019 to 2023 was about 31,700 cubic yards (yd³), and the net volume of fill was about 128,600 yd³, resulting in a net gain of about 96,900 yd³ of sediment between 2019 and 2023 (table 7). As indicated in the previous paragraph, the cross section from the 2023 survey done along the upstream bridge face is below the cross section from the 2019 survey on the left side of the channel, but is substantially higher than the cross section from the 2019 survey on the right side of the channel (fig. 8). The scour hole near pier 3 in 2023 was narrower and shallower than the scour hole from the 2019 survey, as indicated by the deposition near the pier. The frequency distribution of bed elevations in 2023 was narrower than other surveys, with a higher percentage of cells at a narrower range of channel-bed elevations than other surveys (fig. 7). The stone revetement on the right (south) bank showed consistent minor deposition, and the footing and seal course of pier 3 showed minor erosion on the left bank and deposition on the right bank (fig. 9); however, deposition or scour apparent on opposing faces of pier 3 and on the right bank are likely caused by minor horizontal positional variances in data points between the surveys (see “Uncertainty Estimation” section; fig. 5). The previous study (Huizinga, 2022a) had noted that the rock outcrop on the upstream right bank showed signs of nearly uniform minor scour between the 2019 survey and the previous survey in 2015, which may be a factor in the apparent deposition noted here. The erosion apparent on the upstream face of the pier is likely also the result of a minor positional variation in the 2023 survey.

The difference between the surveys from August 8, 2023, and June 2, 2015, (fig. 10), indicated about 81 percent of the joint area of interest had detectable change, which means about 19 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Deposition appears more dominant throughout most of the reach between 2015 and 2023 in the DoD, albeit with some erosion along the left (north) side (fig. 10). The average difference between the bathymetric surfaces was +1.44 ft (table 7), indicating moderate overall channel aggradation between the 2015 and 2023 surveys. The net volume of cut between 2015 and 2023 was about 34,100 yd³, and the net volume of fill was about 84,100 yd³, resulting in a net gain of about 50,000 yd³ of sediment between 2015 and 2023 (table 7). The stone revetment and rock outcrop on the right (south) bank has minor localized areas of erosion and deposition in the 2023 survey, but most of the differences are within the bounds of uncertainty (fig. 10). The scour hole near pier 3 was narrower and shallower in the 2023 survey than in the 2015 survey, as indicated by the deposition near the pier. As with the previous DoD, deposition or scour apparent on opposing sides of pier 3 and its upstream face was likely caused by minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 8, 2023, and during flooding on July 16, 2011 (fig. 11), indicates about 89 percent of the joint area of interest had detectable change, which means only about 11 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Scour and deposition appear roughly balanced throughout most of the reach between 2011 and 2023 in the DoD, with erosion along the left (north) bank and deposition along the right bank (south) side (fig. 11). The average difference between the bathymetric surfaces from the 2011 and 2023 surveys was +0.94 ft (table 7), indicating minor channel aggradation. The net gain of sediment in the reach between 2011 and 2023 was about 36,100 yd³, which substantiates the rough balance of erosion and deposition in the reach (table 7). The cross section from the 2023 survey along the upstream face of the bridge varies from 5 to 15 ft below the 2011 cross section on the left side and 5 to 20 ft above the 2011 cross section on the right side (fig. 8). The frequency distribution of bed elevations in the 2011 survey is somewhat unique compared with distributions from other years at this site; the bed elevations from the 2011 survey had more grid cells at a much wider range of channel-bed elevations than the other surveys (fig. 7). Again, the stone revetment and rock outcrop on the right (south) bank has minor localized areas of erosion and deposition in the 2023 survey, but many of the differences are within the bounds of uncertainty (fig. 11). Similarly, the scour hole near pier 3 was narrower and shallower in the 2023 survey than it was in the 2011 survey, as indicated by the deposition near the pier. As with the previous DoDs, deposition or scour apparent on opposing sides of pier 3 and its upstream face are likely caused by minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 8, 2023, and the earliest on March 15, 2010 (fig. 12), indicates that about 91 percent of the joint area of interest had detectable change, which means that only about 9 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Like 2011, scour and deposition appear roughly balanced throughout most of the reach between 2010 and 2023 in the DoD, with erosion along the left (north) bank and deposition along the right (south) bank (fig. 12). The average difference between the bathymetric surfaces for survey years was –0.35 ft (table 7), indicating minor channel degradation between the 2010 and 2023 surveys. The net loss of sediment from the reach between 2010 and 2023 was about 12,800 yd³, which again substantiates the rough balance of erosion and deposition in the reach (table 7). The cross section from the 2010 survey along the upstream face of the bridge is generally like the 2011 survey section, resulting in similar differences with the 2023 survey section (fig. 8). Furthermore, the frequency distribution of bed elevations in 2010 is like the 2011 distributions at this site (fig. 7). The stone revetment and rock outcrop on the right (south) bank has substantial areas of moderate erosion (fig. 12), which indicates the revetment may have slumped somewhat between the 2010 and 2011 surveys. As in all the previous comparisons, the scour hole near pier 3 was narrower and shallower than it was in the 2010 survey, as indicated by the deposition near the pier. Furthermore, the location of pier 3 in the 2010 survey appears to have a moderate overall positional difference, resulting in substantial apparent erosion on the left (north) side of the pier (see “Uncertainty Estimation” section; fig. 5).

The vertically averaged velocity vectors indicate mostly uniform flow throughout the reach, with velocities ranging from about 3 to 7 feet per second (ft/s; fig. 13). The wake vortices downstream from pier 3 were not pronounced and seemed to be no greater than the general nonuniformity of flow observed in the channel (fig. 13). All sections had minor localized turbulence (fig. 13).

Structure A8340 on U.S. Highway 69

Structure A8340 (site 7; table 2) on U.S. Highway 69 crosses the Missouri River at RM 372.6, on the northwestern side of Kansas City (fig. 1). The site was surveyed on August 8, 2023, and the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 723.3 ft (table 5; fig. 14), and streamflow was about 44,600 ft³/s during the survey (table 5).

The survey area was about 1,550 ft long and about 875 ft wide, extending from bank to bank in the main channel except for a shallow area downstream from the spur dike on the upstream left (north) side (fig. 14). The survey area extended about 680 ft upstream from the centerline of structure A8340 (fig. 14). The channel-bed elevations in 2023 ranged from about 701to 717 ft throughout the survey area (5 to 95 percentile range of the bathymetric data; fig. 15), except near the spur dikes on the left (north) bank (fig. 14; table 5). There was a poorly defined thalweg along the toe of the right (south) bank, which was about 5 to 8 ft deeper than the channel bed in the middle of the river, and the channel was filled with small dunes and ripples (fig. 14). As in previous surveys (Huizinga, 2010, 2012, 2020a, 2022a), a stone revetment was on the right (south) bank throughout the reach, and two spur dikes were on the left (north) bank (fig. 14). Substantial scour holes were downstream from and near the tip of the upstream spur dike on the left bank. The scour hole downstream from the spur dike had the minimum channel-bed elevation of about 685 ft (table 5; fig. 14).

A local scour hole near bent 4 had an approximate minimum channel-bed elevation of about 696 ft, about 10 ft below the average channel bed immediately upstream from the bent (fig. 14; table 6). A local scour hole near bent 5 had a minimum elevation of 708 ft, also about 10 ft below the average channel bed elevation immediately upstream from the pier (fig. 14; table 6). Bent 3 is surrounded by the rock revetment on the right (south) bank, with no scour hole evident nearby (fig. 14; table 6). Information from bridge plans indicates that all the main channel bents of structure A8340 are shafts drilled about 23 to 28 ft into bedrock and have about 58 ft of bed material between the bottom of the scour hole and bedrock at bent 4 (fig. 16; table 6); bents 5 and 3 have about 70 and 83 ft of bed material between the bottom of the scour hole and bedrock, respectively (fig. 16; table 6).

The difference between the surveys on August 8, 2023, and August 13, 2019 (fig. 17), indicates about 96 percent of the joint area of interest had detectable change, which means only about 4 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Deposition appears dominant throughout the reach between 2019 and 2023 in the DoD, except in localized areas of substantial erosion near the ends and downstream from the spur dikes on the left (north) bank (fig. 17). The average difference between the bathymetric surfaces was +1.45 ft (table 7), indicating moderate channel aggradation between the 2019 and 2023 surveys. The net volume of cut in the reach from 2019 to 2023 was about 48,000 yd³, and the net volume of fill was about 112,500 yd³, resulting in a net gain of about 64,500 yd³ of sediment between 2019 and 2023 (table 7). The cross section from the upstream face of the bridge from the 2023 survey is like but above the cross section from the 2019 survey section (fig. 16). The frequency distribution of bed elevations in 2023 is similar in shape to the 2019 distribution but with a higher percentage of cells in a narrower channel-bed elevation range (fig. 15). As with the upstream site at structure A1800, the stone revetment on the right (south) bank showed consistent minor deposition between 2019 and 2023, as do the spur dikes on the left (north) side of the channel (fig. 17); however, this deposition may be the result of a systematic vertical error in the 2019 survey at this site because it is unlikely that the spur dikes have been raised since 2019, and the typical deposition and erosion on opposing faces of these features (indicative of horizontal positional variance) do not exist. Furthermore, the previous study (Huizinga, 2022a) had noted that the right bank revetment showed signs of nearly uniform minor scour between the 2019 survey and the previous one in 2017.

The difference between the surveys on August 8, 2023, and May 22, 2017 (fig. 18), 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 7). Minor to moderate erosion and deposition appear nearly balanced throughout the reach between 2017 and 2023 in the DoD, with erosion being slightly more dominant and with localized substantial erosion and deposition near the ends and downstream from the spur dikes (fig. 18). The average difference between the bathymetric surfaces was –0.35 ft (table 7), indicating minor channel degradation between the 2017 and 2023 surveys, and the net loss of sediment between 2017 and 2023 was about 14,100 yd³ (table 7). The cross section from the upstream face of the bridge from the 2023 survey is the most like the cross section from the 2017 survey section, varying only about 5 ft above and below the 2017 section except near bent 4 (fig. 16). However, the frequency distribution of bed elevations in 2017 is substantially different from the distributions of 2023; the bed elevations of 2017 shifted nearly 10 ft higher than the channel-bed elevations in 2023 (fig. 15). The stone revetment on the right (south) bank shows nearly uniform minor erosion, whereas the spur dikes indicate differences within the bounds of uncertainty (fig. 18). The scour apparent on the right (south) bank may be the result of minor horizontal positional variances that took place between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 8, 2023, and during flooding on July 16, 2011 (fig. 19), again 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 7). Erosion appears dominant in the surveys between 2011 and 2023 in the DoD, but there was also substantial deposition in the channel thalweg and near the location of the old bridge piers near bent 4 (fig. 19). The average difference between the bathymetric surfaces was –0.39 ft (table 7), indicating minor overall degradation between the 2011 and 2023 surveys, with a net loss of sediment of about 15,000 yd³ (table 7). The frequency distribution of bed elevations in 2011 is somewhat unique compared with the other distributions from other survey years at this site, with more grid cells at a much wider range of channel-bed elevations than the other surveys (fig. 15). The stone revetment on the right (south) bank again showed nearly uniform minor scour (fig. 19), like the 2017 to 2023 DoD (fig. 18). Similarly, the spur dikes showed differences within the bounds of uncertainty or minor scour (fig. 19). As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely caused by minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 8, 2023, and the earliest on March 15, 2010 (fig. 20), 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 7). Deposition appears dominant throughout most of the reach between 2010 and 2023 in the DoD, with substantial localized erosion near the ends and between the spur dikes in the study area downstream from the bridge. Substantial deposition downstream from the upstream spur dike and near the location of the old bridge piers near bent 4 also took place (fig. 20). The average difference between the bathymetric surfaces among the surveys was +3.17 ft (table 7), indicating substantial channel aggradation between the 2010 and 2023 surveys, with a net gain of sediment in the reach between 2010 and 2023 of about 110,800 yd³ (table 7). The cross section from along the upstream face of the bridge from the 2010 survey is very similar to the same cross section from the 2011 survey, resulting in similar differences with the 2023 survey section (fig. 16). The frequency distribution of bed elevations in 2010 is roughly like the 2023 distribution at this site, but with a higher percentage of cells at elevations of about 5 to 7 ft lower than in 2023 (fig. 15). The stone revetment on the right (south) bank again showed nearly uniform minor scour (fig. 20), like the previous DoDs for 2017 and 2011 (fig. 18, fig. 19). The spur dikes showed minor scour or differences within the bounds of uncertainty (fig. 20). As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely caused by either minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5), a vertical difference that is greater than the uncertainty of the surveys, or a combination of these.

The vertically averaged velocity vectors indicate mostly uniform flow on the right (south) side of the channel, with a maximum velocity of about 8 ft/s (fig. 21). Local lower velocities and turbulence were observed downstream from the various spur dikes on the left (north) bank (fig. 21). Minimal effect on the velocity vectors was observed downstream from bents 4 and 5 (fig. 21), indicating the piers are appropriately hydrodynamic. All sections had minor localized turbulence (fig. 21).

Structure A4060 on State Highway 9

Structure A4060 (site 9; table 2) on State Highway 9 crosses the Missouri River at RM 365.5, immediately north of downtown Kansas City, Mo. (fig. 1). The site was surveyed on August 8, 2023, when the average water-surface elevation near the bridge, determined by the RTK GNSS tide solution, was 716.2 ft (table 5; fig. 22), and streamflow on the Missouri River was measured at about 46,400 ft³/s during the survey (table 5).

The survey area was about 1,640 ft long and averaged about 900 ft wide, extending from bank to bank in the main channel (fig. 22). The survey area extended about 665 ft upstream from the centerline of structure A4060 at pier 6 (fig. 22). The channel-bed elevations in 2023 ranged from about 695 to 708 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; table 5; fig. 23), except for deeper areas near the end of the upstream spur dike on the left (north) bank and the railroad bridge pier upstream from pier 6 (fig. 22). A poorly defined thalweg along the toe of the right (south) bank was about 5 ft deeper than the middle of the channel, and the channel was filled with small dunes and ripples (fig. 22). As in previous surveys (Huizinga, 2010, 2012, 2016, 2022a), a stone revetment was on the right (south) bank throughout the reach (fig. 22).

The lower flow conditions in 2023 limited the ability to survey on the left (north) bank; nevertheless, a scour hole was found near main channel pier 4 on that bank (fig. 22, fig. 24) that had a minimum channel-bed elevation of 697 ft (table 6). A scour hole near main channel pier 5 (fig. 22) had a minimum channel-bed elevation of about 693 ft (table 6), about 3 ft below the elevation of the bottom of the pier seal course of 696.26 ft (fig. 24; table 6). Near the right (south) main channel pier 6 (fig. 22), a scour hole had a minimum channel-bed elevation of about 690 ft (table 6), about 6 ft below the elevation of the bottom of the pier seal course of 696.26 ft (fig. 24; table 6; fig. 1.3). A substantially deeper scour hole was observed near the upstream railroad bridge pier (fig. 22) that had a minimum elevation of about 679 ft, which is the lowest channel-bed elevation observed in this reach (table 5). This deeper hole was likely caused by flow around the upstream railroad pier that was combined with flow over large sheet piling panels that were lying on the channel bottom on the right (south) side of the pier first observed in the 2010 survey (Huizinga, 2010) and observed in all subsequent surveys (Huizinga 2012, 2016, 2022a). Information from bridge plans indicates that piers 4 through 6 are founded on shafts drilled 20 ft into the bedrock, and about 32 ft of bed material were between the bottom of the scour hole and bedrock at pier 4, about 26 ft of material were between the bottom of the scour hole and bedrock at pier 5, and about 20 ft of material were between the bottom of the scour hole and bedrock at pier 6 (fig. 24; table 6).

The difference between the surveys on August 8, 2023, and August 13, 2019 (fig. 25), indicates about 95 percent of the joint area of interest had detectable change, which means only about 5 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Deposition is clearly dominant throughout most of the reach between 2019 and 2023 in the DoD, except for localized areas of substantial erosion near the end of and downstream from the upstream spur dike on the left (north) bank (fig. 25). Furthermore, erosion and deposition are roughly balanced on the left side of the channel downstream from the bridge (fig. 25). The average difference between the bathymetric surfaces was +2.45 ft (table 7), indicating there was substantial channel aggradation between the 2019 and 2023 surveys. The net volume of cut in the reach from 2019 to 2023 was about 34,700 yd³, whereas the net volume of fill was about 145,100 yd³, resulting in a net gain of about 110,400 yd³ of sediment between 2019 and 2023 (table 7). The frequency distribution of bed elevations in 2023 appears similar in shape to several of the previous surveys; however, the 2019 survey had more grid cells in a substantially wider band of channel-bed elevations (fig. 23), which indicates the 2023 survey had more uniform channel-bed elevations than the 2019 survey. The cross sections from the 2019 and 2023 surveys along the upstream face of the bridge also indicate these same differences (fig. 24); whereas the cross sections are like each other in the left (north) side of the channel, the 2023 cross section is 10 to 20 ft above the 2019 cross section on the right (south) side. The stone revetement on the right (south) bank showed minor to moderate deposition between 2019 and 2023, with more deposition near the toe of the bank throughout the reach, and the footing and seal course of piers 5 and 6 showed minor erosion on one side and deposition on the other (figs. 24, 25); however, as in previous DoDs, deposition or scour apparent on opposing faces of a particular feature is likely caused by minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 8, 2023, and on June 3, 2015 (fig. 26), indicates about 73 percent of the joint area of interest had detectable change in bed geomorphology (table 7), which means about 27 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty. Scour and deposition appear roughly balanced throughout the reach between 2015 and 2023 in the DoD, with deposition generally more prominent on the left (north) side and erosion more prominent on the right (south) side of the channel (fig. 26). Localized areas of substantial erosion are evident near the end of and downstream from the upstream spur dike on the left (north) bank (fig. 26). The average difference between the bathymetric surfaces was +0.24 ft between the 2015 and 2023 surveys (table 7), indicating minor channel aggradation during that time, the net gain of sediment between 2015 and 2023 being only about 8,100 yd³ (table 7). The frequency distribution of bed elevations in 2023 is very similar to 2015, but with a lower percentage of cells at channel-bed elevations between 698 and 703 ft (fig. 23). The 2023 cross section is also like the 2015 cross section, except near pier 4 on the left (north) bank where the 2023 bed elevation is lower than 2015 (fig. 24). The stone revetment on the right (south) bank again showed minor deposition between 2015 and 2023, with more deposition near the toe of the bank throughout the reach (fig. 26). As with previous DoDs, deposition or scour apparent on opposing faces of a feature (such as a pier or spur dike) is likely caused by minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 8, 2023, and during flooding on July 17, 2011 (fig. 27), indicates about 80 percent of the joint area of interest had detectable change, which means about 20 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Scour appears to be dominant throughout most of the reach between 2011 and 2023 in the DoD, with substantial erosion near the end of and downstream from the upstream spur dike, erosion along the left (north) side of the river, and deposition along the right (south) side of the river (fig. 27). The average difference between the bathymetric surfaces of the 2011 and 2023 surveys was –1.25 ft (table 7), indicating moderate channel degradation, and the net loss of sediment was about 47,000 yd³ (table 7). The frequency distribution of bed elevations in 2011 is somewhat unique compared with the frequency distributions from other surveys at this site; the 2011 survey had more grid cells at a much wider range of channel-bed elevations than the other surveys, particularly above 705 ft (fig. 23). Like the other DoDs at this site, the stone revetment on the right (south) bank has localized areas of deposition, with more deposition along the toe of the bank and less farther up the bank, but there are also areas of localized erosion (fig. 27). The scour hole near pier 5 in the 2023 survey was narrower and shallower than the same scour hole in the 2011 survey, as indicated by the deposition near the pier; however, the scour hole near pier 4 was deeper in the 2023 survey (fig. 24 and fig. 27). As with the previous DoDs, deposition or scour apparent on opposing faces of piers 5 and 6 were likely caused by minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 8, 2023, and the earliest on March 16, 2010 (fig. 28), 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 7). Deposition appears dominant throughout most of the reach between 2010 and 2023 in the DoD, with localized substantial erosion near the end of and downstream from the upstream spur dike as well as along the left (north) side and moderate deposition throughout the remainder of the reach (fig. 28). The average difference between the bathymetric surfaces was +1.13 ft (table 7), indicating moderate channel aggradation between the 2010 and 2023 surveys, and the net gain of sediment was about 44,500 yd³, which is like the net loss between 2011 and 2023 (table 7). The frequency distribution of bed elevations in 2010 is like the 2023 frequency distribution at this site (fig. 23). The stone revetment on the right (south) bank has an area of minor to moderate erosion near the upstream end of the study reach and an area of moderate deposition along the toe of the bank (fig. 28); however, much of the bank is within the bounds of uncertainty. Like the 2011 comparison, the scour hole near pier 5 was narrower and shallower in 2023 than it was in the 2010 survey, as indicated by the deposition near the pier. Like the previous DoDs, deposition or scour apparent on opposing faces of piers 5 and 6 is likely caused by minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The vertically averaged velocity vectors indicate mostly uniform flow in the reach, but several sections had localized flow disturbances in their direction and magnitude(fig. 29). Velocities ranged from about 2 to 7 ft/s (fig. 29), with lower velocities and more turbulence observed downstream from the railroad bridge piers and upstream spur dike (fig. 29). All the highway bridge piers were aligned to approach flow, minimizing additional wake vortices downstream (fig. 29).

Structure A7650 on Interstate 35

Structure A7650 (site 10; table 2) on Interstate 35 crosses the Missouri River at RM 364.7, immediately north of downtown Kansas City, Mo. (fig. 1), and is about 4,300 ft downstream from structure A4060 on State Highway 9 (site 9). The site was surveyed on August 8, 2023, when the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 715.4 ft (table 5; fig. 30), and streamflow on the Missouri River was about 46,200 ft³/s during the survey (table 5).

The survey area was about 1,640 ft long and averaged about 875 ft wide, extending from bank to bank across the main channel (fig. 30). The upstream end of the survey area was about 700 ft upstream from the centerline of structure A7650 at the main channel pylon (fig. 30). The channel-bed elevations in 2023 ranged from about 694 to 707 ft in height for most of the surveyed area (5 to 95 percentile range of the bathymetric data; table 5; fig. 31), except near the pylon and downstream from the spur dikes on the downstream left (north) bank (fig. 30; table 5). The channel is generally deeper on the right (south) side of the reach, and there is not a well-defined thalweg (fig. 30). The channel is filled with small dunes and ripples throughout the reach (fig. 30). As in previous surveys (Huizinga, 2012, 2016, 2022a), a stone revetment was on the right (south) bank throughout the reach (fig. 30).

Like previous surveys at this site (Huizinga, 2012, 2016, 2022a), the main channel pylon appears surrounded by piles of rock riprap that serve as a scour countermeasure. The local minimum channel-bed elevation is about 679 ft near the downstream left (north) corner of the study area (table 6; fig. 30; fig. 1.4), which is the minimum channel-bed elevation in the reach at this site (table 5). Although the pylon is skewed about 15 degrees to approaching flow (table 6), with impinging flow on the right (south) side, the scour hole is deeper and longer on the leeward left (north) side, likely because of the contraction and deflection of flow from the protrusion on the left (north) bank (fig. 30). At the upstream face, the scour hole is about 14 ft below the average channel elevation upstream from the pylon (fig. 30, fig. 1.4; table 6). The top of the riprap around the upstream face of the pylon was at an elevation of about 697 ft, which is slightly below the elevation of the bottom of the pylon seal course of 699.50 ft (table 6); the top of the riprap along the sides of the pylon is lower than at the upstream face (fig. 30 and fig. 32; fig. 1.4). The minimum channel-bed elevation near the pylon was about 20 ft below the elevation of the bottom of the pylon seal course (fig. 30; table 6). Information from bridge plans indicates that the pylon is founded on shafts drilled 32 ft into bedrock, with about 32 ft of bed material between the bottom of the scour hole and bedrock (fig. 32; table 6).

The difference between the surveys on August 8, 2023, and August 14, 2019 (fig. 33), indicates about 94 percent of the joint area of interest had detectable change, which means only about 6 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Deposition appears dominant throughout the reach between 2019 and 2023 in the DoD, with localized erosion in the upstream right (south) part of the channel, along the left (north) bank, near the end of and downstream from the spur dike on the downstream left (north) bank, and along the stone revetment along the right (south) bank (fig. 33). The average difference between the bathymetric surfaces was +3.68 ft (table 7), which indicated substantial aggradation of the reach between the 2019 and 2023 surveys. The net volume of cut in the reach from 2019 to 2023 was about 18,600 yd³, and the net volume of fill was about 189,100 yd³, resulting in a net gain of about 170,500 yd³ of sediment between 2019 and 2023, which is the second highest gain of all the comparisons of the bridges in the Kansas City area (table 7). The scour hole near the pylon was slightly deeper on the left (north) side and slightly shallower on the right (south) side in 2023 than in 2019, and the riprap around the pylon appeared to be slightly lower to equivocal with uncertainty in 2023 than in 2019 (fig. 32; fig. 33). The frequency distribution of bed elevations in 2023 is more like 2015 than 2019, but with a higher percentage of cells in the range of 703 to 708 ft than 2015 (fig. 31). The stone revetment along the right (south) bank showed substantial deposition upstream along the toe of the bank and immediately downstream from the bridge in the upstream 60 percent of the reach (fig. 33), which is also evident in the cross section (fig. 32). Farther downstream from the bridge, localized signs of minor scour and deposition are apparent, but most of the indicated changes are within 1–3 ft of the 2019 elevation or within the bounds of uncertainty (fig. 33). Differences on the top of the spur dike were also within the bounds of uncertainty.

The difference between the surveys on August 8, 2023, and June 3, 2015 (fig. 34), indicates about 78 percent of the joint area of interest had detectable change, which means about 22 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Deposition and erosion appear roughly balanced throughout most of the reach between 2015 and 2023 in the DoD, with deposition primarily on the left (north) side and erosion primarily on the right (south) side of the channel (fig. 34). The average difference between the bathymetric surfaces was +1.06 ft (table 7), indicating moderate channel aggradation between the 2015 and 2023 surveys, and the net gain of sediment between 2015 and 2023 was about 38,900 yd³ (table 7). The scour hole near the pylon was generally shallower in 2023 than in 2015, as indicated by the horseshoe-shaped area of deposition around the nose of the pylon; however, the deepest part of the scour hole on the downstream left (north) side was deeper in 2023 (fig. 34). As noted in the previous paragraph, the frequency distribution of bed elevations in 2023 is most like 2015 (fig. 31). Also, as noted in the previous paragraph, there was deposition along the toe of the stone revetment on the right (south) bank in the upstream 60 percent of the reach; other differences to the stone revetment downstream appear to be erosional or within the bounds of uncertainty (fig. 34). Observed differences on the spur dike of the downstream left (north) bank are generally within the bounds of uncertainty, as are most of the differences to the riprap piled around the pylon and on the left (north) bank under the bridge (fig. 34). Upstream and downstream from the bridge, the left (north) bank appears to have experienced some lateral migration between 2015 and 2023, as indicated by the increasing depth of erosion as one moves towards the bank (fig. 34).

The difference between the surveys on August 8, 2023, and the earliest during flooding on July 17, 2011 (fig. 35), 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 7). Deposition appears dominant throughout most of the reach between 2011 and 2023 in the DoD, with localized moderate to substantial erosion along the left (north) bank and downstream from the spur dike (fig. 35). The average difference between the bathymetric surfaces between surveys was +3.18 ft (table 7), indicating substantial aggradation of the reach between the 2011 and 2023 surveys, with a net gain of sediment of about 126,200 yd³ (table 7). The scour hole near the pylon was generally shallower in 2023 than in 2011, as indicated by the horseshoe-shaped area of deposition around the riprap around the nose of the pylon; however, as in other survey comparisons, the deepest part of the scour hole on the downstream left (north) side was deeper in 2023, and the riprap around the pylon appears to have subsided (fig. 32 and fig. 35), as was observed in the previous study (Huizinga, 2022a). As noted in the previous paragraphs, there was deposition along the toe of the stone revetment on the right (south) bank in the upstream 60 percent of the reach, whereas only minor to moderate erosion was observed downstream (fig. 35). Observed differences on the top of the spur dike on the downstream left (north) bank were generally within the bounds of uncertainty, but minor erosion was evident on the upstream face (fig. 35). The rock on the left (north) bank under the bridge had differences also, but they were generally within the bounds of the uncertainty (fig. 35). As in the previous comparison at this site, the left (north) bank upstream and downstream from the bridge appears to have experienced lateral migration between 2011 and 2023, as indicated by the increasing depth of erosion as one moves towards the bank (fig. 35).

The vertically averaged velocity vectors indicate mostly uniform flow in the reach, with velocities ranging from about 2 to 6 ft/s (fig. 36). The pylon is skewed about 15 degrees to approach flow and creates substantial turbulence downstream (fig. 36). The spur dike also causes flow disturbance, and a reduction of flow velocity downstream (fig. 36).

Structure A5817 on State Highway 269

Structure A5817 (site 11; table 2) on State Highway 269 crosses the Missouri River at RM 362.3, northeast of downtown Kansas City, Mo. (fig. 1). The site was surveyed on August 9, 2023, when the average water-surface elevation near the bridge, determined by the RTK GNSS tide solution, was 713.3 ft (table 5; fig. 37) and streamflow on the Missouri River was about 46,700 ft³/s during the survey (table 5).

The survey area was about 1,640 ft long and averaged about 995 ft wide, extending from bank to bank in the main channel (fig. 37). The upstream end of the survey area was about 670 ft upstream from the centerline of structure A5817 at pier 2 (fig. 37). The channel-bed elevations in 2023 ranged from about 690 to 706 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; table 5; fig. 38), except in the well-defined channel thalweg along the left (north) bank and near pier 2 (fig. 37; table 5). Small dunes and ripples were throughout the channel. The area between the spur dikes on the right (south) bank was about 20 ft shallower than the thalweg on the left side of the channel (fig. 37). As in previous surveys (Huizinga, 2010, 2012, 2016, 2022a), a stone revetment was on the left (north) bank throughout the reach (fig. 37). Scour countermeasures in the form of riprap blankets were being installed during the survey, and several construction barges moored near the bridge had to be avoided during the survey.

The historical scour hole near main channel pier 2 was partially mitigated on the upstream side by the riprap blanket being installed during the survey (figs. 37, 39; fig. 1.5); however, a partial scour hole on the downstream right (south) side of pier 2 had a minimum elevation of 686 ft (table 6; fig. 37). This partial scour hole was the result of channel-bed dredging in preparation for the scour countermeasures and was fully mitigated by the completed riprap blanket like the area to the left (north) of the pier (Bryan Hartnagel, Missouri Department of Transportation, written commun., 2023). A minor scour hole was observed near pier 3 (fig. 37), having a minimum channel-bed elevation of about 699 ft, which is about 10 ft above the bottom of the seal course at this pier (table 6). Information from bridge plans indicates that piers 2 and 3 are founded on shafts drilled 20 ft into bedrock, with about 9 ft of bed material between the bottom of the scour hole and bedrock at pier 2 and about 42 ft of material at pier 3 (fig. 39; table 6).

The difference between the surveys on August 9, 2023, and August 14, 2019 (fig. 40), 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 7). Deposition and erosion appear roughly balanced throughout the reach between 2019 and 2023 in the DoD, with alternating areas of erosion and deposition in the area between the left (north) bank and pier 2, primarily deposition between piers 2 and 3, and primarily erosion along the right (south) bank (fig. 40). The scour hole near pier 2 in 2019 has been mostly mitigated by the riprap blanket, except for the substantial “constructed” hole on the downstream right side of the pier (fig. 40). The cross section from along the upstream face of the bridge from the 2023 survey is generally like the lowest cross section from between the left (north) bank and pier 2 and plots among the middle sections between piers 2 and 3 (fig. 39); however, the effect of the riprap blanket around pier 2 is evident. Furthermore, the apparent erosion of the hole dredged for scour countermeasures installation shows that the countermeasures are intended to mitigate scour at a depth beyond the scour observed in the 2019 survey, which had the second deepest hole of all the surveys at pier 2 (fig. 39). The average difference between the bathymetric surfaces was only +0.07 ft (table 7), indicating little-to-no net change between the 2019 and 2023 surveys. The net volume of cut in the reach from 2019 to 2023 was about 44,100 yd³, and the net volume of fill was about 47,600 yd³, resulting in a net gain of only about 3,500 yd³ of sediment between 2019 and 2023, which is the least amount of net change of all the comparisons (table 7). The frequency distribution of bed elevations in 2023 is very similar to the distribution of 2019, but with a slightly higher percentage of cells in the range of 703 to 708 ft (fig. 38). The stone revetment on the left (north) bank showed localized areas of erosion, but nearly all the indicated changes are within 1 ft of the 2019 elevation, or outside the 95-percent confidence interval (fig. 40), and might be the result of a minor positional offset between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 9, 2023, and June 3, 2015 (fig. 41), indicates about 79 percent of the joint area of interest had detectable change, which means about 21 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Erosion appears dominant throughout the center of the reach between 2015 and 2023 in the DoD, with a balance of erosion and deposition along the left (north) and right (south) edges (fig. 41). Because of this, the dredged hole on the downstream right side of pier 2 is again evident (fig. 41). The average difference between the bathymetric surfaces was –1.88 ft (table 7), indicating moderate to substantial channel degradation between the 2015 and 2023 surveys, and the net loss of sediment between 2015 and 2023 was about 77,000 yd³ (table 7). The frequency distribution of bed elevations in 2023 has more grid cells at lower overall elevations than 2015 (fig. 38). The stone revetment on the left (north) bank showed localized areas of deposition (fig. 41); however, nearly all the indicated changes are within 1 ft of the 2015 elevation, or were equivocal with uncertainty, and may result from minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 9, 2023, and during flooding on July 18, 2011 (fig. 42), 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 7). Erosion appears dominant throughout most of the reach between 2011 and 2023 in the DoD, with localized moderate deposition only in the trough of a single large dune in the 2011 survey (fig. 42). The average difference between the bathymetric surfaces was –5.19 ft, the largest average change in surfaces from all the survey comparisons in this study (table 7), indicating substantial channel degradation between the 2011 and 2023 survey. Furthermore, the net loss of sediment was about –239,400 yd³, the largest net loss of sediment from all the survey comparisons in this study (table 7). Accordingly, the frequency distribution of bed elevations in 2011 differs most from the bed elevations from the 2023 survey, with a higher percentage of cells at a higher elevation than 2023 (fig. 38). As with the other DoDs at this site, the stone revetment on the left (north) bank has localized areas of deposition, but most of the bank is within the bounds of uncertainty (fig. 42). The bed near pier 2 was substantially higher in the 2011 survey (fig. 39), and the effects of the riprap blanket near that pier are less evident (fig. 42).

The difference between the surveys on August 9, 2023, and the earliest on March 17, 2010 (fig. 43), 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 7). Erosion again appears dominant throughout most of the reach between 2010 and 2023 in the DoD, but with localized substantial deposition in the thalweg along the left (north) bank (fig. 43). The average difference between the bathymetric surfaces was –1.45 ft (table 7), indicating moderate channel degradation between the 2010 and 2023 surveys, and the net loss of sediment was about 55,000 yd³ (table 7). The stone revetment on the left (north) bank has small areas of erosion, but much of the bank is within the bounds of uncertainty (fig. 43). The dredged hole on the downstream right side of pier 2 is again evident, as is the scour mitigating effects of the riprap blanket (fig. 43).

The vertically averaged velocity vectors indicate mostly uniform flow throughout most of the reach, ranging from about 2 ft/s along the inside of the bend on the right (south) bank to about 7 ft/s in the downstream thalweg along the left (north) bank (fig. 44). Variations in the shape of the stone revetment on the left (north) bank resulted in localized turbulence along that bank (fig. 44).

Structure A0767 on Interstate 435

Structure A0767 (site 12; table 2) on Interstate 435 crosses the Missouri River at RM 360.3, on the northeastern side of Kansas City, Mo. (fig. 1). The site was surveyed on August 9, 2023, when the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 711.4 ft (table 5; fig. 45) and streamflow on the Missouri River was about 46,500 ft³/s during the survey (table 5).

The survey area was about 1,640 ft long and about 930 ft wide, extending across the active channel from the left (north) bank to the right (south) bank in the main channel (fig. 45). The upstream end of the survey area was about 660 ft upstream from the centerline of structure A0767 at pier 7 (fig. 45). Piers 7 and 8 were in the water; however, as in previous surveys (Huizinga, 2010, 2012, 2016, 2020a), pier 8 was immediately downstream from a spur dike and the upstream side of the pier was surrounded by a persistent and substantial debris raft. Furthermore, a persistent sand bar was also downstream from pier 8, as in previous surveys (Huizinga, 2010, 2012, 2016, 2020a), which limited accessibility to that part of the survey area (fig. 45). The channel-bed elevations in 2023 ranged from about 689 to 704 ft for most of the surveyed area (5 to 95 percentile range of the bathymetric data; fig. 46; table 5), except downstream from the end of the spur dike on the right (south) bank. A shallow but wide thalweg (about 3 to 5 ft deep and 150 ft wide) was along the left (north) bank throughout the surveyed area, and small dunes and ripples were detected throughout the channel (fig. 45). As in previous surveys (Huizinga, 2010, 2012, 2016, 2020a), stone revetment and bedrock outcrops were on the left (north) bank throughout the reach (fig. 45).

The scour hole historically observed at pier 7 was not present in the 2023 survey (figs. 45, 47). The most recent previous study (Huizinga, 2022a) details how MoDOT installed scour countermeasures in the form of a riprap blanket around pier 7 in 2020, and that blanket is partly evident in the current (2023) survey (fig. 45; fig. 1.6). A gap in the blanket at the nose of the expanded seal course constructed as part of the countermeasures had a minimum elevation of 682 ft, which is 7 ft below the bottom of the original seal course at that pier (table 6); however, the elevation of the bottom of the modified seal course was not specified in the plans for the countermeasures (Travis Stump, Missouri Department of Transportation, written commun., 2020), but was implied to be below 675 ft. The upstream end of pier 8 was embedded in the rock spur dike and could not be surveyed because of the persistent, substantial debris raft (fig. 45). The top of the footing on the downstream end of the pier was partially visible (fig. 45), but the minimum channel elevation adjacent to the pier footing was about 697 ft, which was about 8 ft higher than the bottom of the seal course elevation of 689.26 ft (table 6). Information from bridge plans indicates that piers 7 and 8 are founded on shafts drilled 20 ft into bedrock, with about 22 ft of bed material between the bottom of the scour hole and bedrock at pier 7 and about 49 ft of material between the bottom of the scour hole and bedrock at pier 8 (fig. 47; table 6). A substantial scour hole was downstream from the end of the spur dike, which contained the minimum channel elevation in the surveyed area of 677 ft (fig. 45; table 5); however, this hole was more than 50 ft away from pier 8, and substantial alluvial material was between the hole and the pier.

The difference between the surveys on August 9, 2023, and August 14, 2019 (fig. 48), indicates about 95 percent of the joint area of interest had detectable change, which means only about 5 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Deposition was dominant throughout most of the reach between 2019 and 2023 in the DoD (fig. 48). Substantial deposition of up to 20 ft was observed along the toe of the right (south) bank upstream from the bridge, as well as downstream from pier 7. Additional deposition of over 30 ft was observed immediately downstream from the spur dike near pier 8 (fig. 48). Localized erosion alternated with deposition in the channel thalweg, and an area of moderate erosion was on the downstream right (south) side of the channel (fig. 48). The scour hole historically observed near pier 7 is effectively mitigated by the riprap blanket, except for a small area of erosion at the nose of pier 7 (fig. 48). The average difference between the bathymetric surfaces was +4.16 ft, which is the largest positive average difference observed in all the surveys in this study (table 7), indicating substantial channel aggradation between the 2019 and 2023 surveys. The net volume of cut in the reach from 2019 to 2023 was about 14,200 yd³, and the net volume of fill was about 207,000 yd³, resulting in a net gain of about 192,800 yd³ of sediment between 2019 and 2023 (table 7), the largest net gain of the surveys in this study (table 7). Accordingly, the frequency distribution of bed elevations in 2023 is the most different from the 2019 survey, with a higher percentage of cells at a higher elevation than 2019 (fig. 46). The cross section along the upstream bridge face for 2023 is like the 2019 cross section, except near the new riprap blanket near pier 7 and between pier 7 and the spur dike (fig. 47). The stone revetment on the left (north) bank showed localized areas of erosion in 2023, but nearly all the indicated changes are within 3 ft of the 2019 elevation (fig. 48) and may be the result of positional variations between the surveys (see “Uncertainty Estimation” section; fig. 5). This positional variation seems apparent in the horizontal shift of the features near the pier in the cross sections from 2023 and the partial survey in October 2020, immediately after the riprap blanket was installed (fig. 47), but in this case it is unknown whether the positional variation applies to the 2023 survey, the October 2020 survey, or both.

The difference between the surveys on August 9, 2023, and June 4, 2015 (fig. 49), indicates about 79 percent of the joint area of interest had detectable change, which means about 21 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Deposition and erosion appear roughly balanced throughout most of the reach between 2015 and 2023 in the DoD, with substantial deposition downstream from the spur dike balanced by moderate erosion upstream, near the end of, and downstream from the end of the spur dike (fig. 49). Downstream from the bridge, deposition was dominant on the left (north) side of the channel, and erosion is dominant on the right (south) side (fig. 49). The effects of the riprap blanket near pier 7 are not evident (fig. 49). The average difference between the bathymetric surfaces was +0.88 ft (table 7), indicating minor channel aggradation between the 2015 and 2023 surveys, with a net gain of sediment of about 33,900 yd³ between 2015 and 2023 (table 7). The frequency distribution of bed elevations in 2023 was most like the distributions of 2015, with a higher percentage of cells at a narrower range of elevations than 2015 (fig. 46). As with the previous DoD, the stone revetment on the left (north) bank showed localized areas of erosion, some areas larger than 5 ft (fig. 49); however, the 2015 survey had erroneous positioning of the data during data collection (Huizinga, 2016) that created an offset of the data along the left (north) bank between surveys.

The difference between the surveys on August 9, 2023, and during flooding on July 18, 2011 (fig. 50), 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 7). Deposition appears dominant throughout most of the reach between 2011 and 2023 in the DoD, with over 20 ft of deposition in the thalweg on the left side of the channel and small areas of erosion along the right (south) bank upstream from the bridge and along the upstream face of the spur dike, as well as over 10 ft of erosion on the right (south) side in the downstream channel (fig. 50). The substantial scour hole near pier 7 in the 2011 survey was clearly mitigated by the riprap blanket (fig. 50). The average difference between the 2011 and 2023 bathymetric surfaces was +2.66 ft (table 7), indicating substantial channel aggradation with a net gain of sediment of about 104,900 yd³ (table 7). The stone revetment on the left (north) bank showed areas of minor erosion and deposition, but much of the bank is within the bounds of uncertainty (fig. 50).

The difference between the surveys on August 9, 2023, and the earliest on March 17, 2010 (fig. 51), 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 7). Deposition and erosion appear roughly balanced throughout most of the reach between 2010 and 2023 in the DoD, with localized areas of moderate deposition of up to 10 ft on the left (north) side of the channel balanced by localized erosion of up to 10 ft on the south side (fig. 51). The average difference between the 2010 and 2023 bathymetric surfaces was +0.73 ft (table 7), indicating minor channel aggradation with a net gain of sediment of about 26,600 yd³ (table 7). The stone revetment on the left (north) bank again showed areas of erosion, with much of the differences being within the bounds of uncertainty (fig. 51). However, the consistent observation of erosion on the left (north) bank in all the preceding comparisons (except 2011) likely implies a positional variation with the 2023 survey, in particular (see “Uncertainty Estimation” section; fig. 5).

The vertically averaged velocity vectors indicate mostly uniform flow throughout most of the reach, ranging from about 2 ft/s along the inside of the bend on the right (south) bank to about 6 ft/s in the downstream thalweg along the left (north) bank (fig. 52). Variations in the stone revetment on the left (north) bank resulted in localized turbulence along that bank, and turbulence and velocity reduction were observed downstream from the spur dike and pier 7 (fig. 52).

Structures A4757 and L0568 on State Highway 291

Structures A4757 and L0568 (site 13; table 2) are dual bridges on State Highway 291, crossing the Missouri River at RM 352.7 on the northeastern side of the Kansas City, Mo., metropolitan area (fig. 1). The site was surveyed on August 9, 2023, when the average water-surface elevation near the bridges, determined by the RTK GNSS tide solution, was 705.6 ft (table 5; fig. 53) and streamflow on the Missouri River was about 46,600 ft³/s during the survey (table 5).

The survey area was about 1,640 ft long and averaged about 890 ft wide, extending from bank to bank in the main channel (fig. 53). The upstream end of the survey area was about 720 ft upstream from the centerline between structures A4757 and L0568 at the piers (fig. 53). The channel-bed elevations in 2023 ranged from about 685 to 697 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; table 5; fig. 54), except in the scour holes near the central main-channel piers (fig. 53; table 5). Many small dunes and ripples were throughout the channel. As in previous surveys (Huizinga, 2010, 2012, 2016, 2022a), a noticeable channel constriction was at and immediately downstream from the bridges, and a stone revetment was on the left (north) bank throughout the reach (fig. 53).

A large scour hole near the main channel piers that nearly encompassed both piers had a minimum channel-bed elevation of about 671 ft (table 6), which was also the minimum channel-bed elevation for the entire reach (table 5). The main channel piers were skewed to the approaching flow by about 25 degrees (table 6), with impinging flow on the right (south) side, which caused a wide scour hole with somewhat unique characteristics (wider, longer, and uniquely shaped) as compared with scour holes at the other surveyed bridges; nevertheless, the scour hole was deeper and longer on the right side. Pier 5 of structure L0568 was in the wake of pier 2C of structure A4757 and, therefore, had a less defined scour hole near it (fig. 53). At the upstream end of pier 2C of structure A4757, the minimum channel-bed elevation was about 671 ft, and near pier 5 of structure L0568, the minimum channel-bed elevation was about 682 ft (fig. 53; table 6). Information from bridge plans indicates that main channel pier 2C of structure A4757 is founded on shafts drilled 20 ft into bedrock (fig. 55), and about 52 ft of bed material are between the bottom of the scour hole and bedrock at the point of minimum elevation of the scour hole near the upstream pier face (fig. 55; table 6). At the point of minimum elevation, the surveyed channel bed was about 5 ft above the bottom of the seal course elevation of 666.26 ft (table 6). Information from bridge plans indicate that main channel pier 5 of structure L0568 is a caisson on bedrock (fig. 56), and about 56 ft of bed material are between the bottom of the scour hole and bedrock at the point of minimum elevation of the scour hole near the downstream right (south) corner (fig. 56; table 6).

The difference between the surveys on August 9, 2023, and August 14, 2019 (fig. 57), 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 7). Deposition appears dominant throughout the reach between 2019 and 2023 in the DoD, with deposition up to 10 ft throughout the reach on the left (north) side, and alternating areas of erosion and deposition on the right side of the channel except immediately downstream from the bridges (fig. 57). The scour holes near the main channel piers are smaller and shallower, as evidenced by the horseshoe-shaped deposition near both piers (fig. 57); the cross section from along the upstream face of each bridge from the 2023 survey also indicates the smaller, shallower scour holes near the piers (fig. 55 and fig. 56). The average difference between the bathymetric surfaces was +2.59 ft (table 7), indicating substantial aggradation between the 2019 and 2023 surveys. The net volume of cut in the reach from 2019 to 2023 was about 11,300 yd³, and the net volume of fill was about 123,000 yd³, resulting in a net gain of about 111,700 yd³ of sediment between 2019 and 2023 (table 7). The frequency distribution of bed elevations in 2023 is unique, with a substantially higher percentage of cells (up to 22 percent for a given 1-ft increment) in the narrow range of elevations between 686 and 691 ft (fig. 54). The stone revetment on the left (north) bank showed localized areas of minor erosion in 2023, but nearly all the indicated changes are within 1 ft of the 2019 elevation, or outside the 95-percent confidence interval (fig. 57) and might be the result of a minor positional offset between the surveys (see “Uncertainty Estimation” section; fig. 5). Furthermore, as in previous DoDs, deposition or scour apparent on opposing faces of the piers is also likely caused by minor horizontal positional variances between the surveys (fig. 5).

The difference between the surveys on August 9, 2023, and June 4, 2015 (fig. 58), indicates about 79 percent of the joint area of interest had detectable change, which means about 21 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Deposition again appears dominant throughout the center of the reach between 2015 and 2023 in the DoD, with deposition clearly dominant in the thalweg on the left (north) side of the channel and a balance of erosion and deposition along the right (south) side (fig. 58). The scour holes near the main channel piers are smaller and shallower, evident in the cross sections (fig. 55 and fig. 56) and by the horseshoe-shaped deposition near both piers in the DoD (fig. 58). The average difference between the bathymetric surfaces was +1.34 ft, indicating moderate channel aggradation between the 2015 and 2023 surveys, with a net gain of sediment of about 50,400 yd³ (table 7). The stone revetment on the left (north) bank showed localized areas of deposition (fig. 58); however, nearly all the indicated changes in 2023 are within 1 ft of the 2015 elevation, or within the bounds of uncertainty, and may result from minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5). Furthermore, as in previous DoDs, deposition or scour apparent on opposing faces of the piers is likely also caused by minor horizontal positional variances between the surveys (fig. 5).

The difference between the surveys on August 9, 2023, and during flooding on July 19, 2011 (fig. 59), indicates about 90 percent of the joint area of interest had detectable change, which means about 10 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 7). Deposition appears dominant throughout most of the reach between 2011 and 2023 in the DoD, with moderate to substantial deposition near the piers and in the thalweg on the left (north) side of the channel and moderate erosion on the right (south) side (fig. 59). The average difference between the bathymetric surfaces was +2.86 ft, indicating substantial channel aggradation between the 2011 and 2023 surveys, with a net gain of sediment of about 122,100 yd³ (table 7). Accordingly, the frequency distribution of bed elevations in 2011 is the most different from the 2023 survey, with a substantially lower percentage of cells (generally less than 5 percent) at a substantially wider range of elevations than 2023 (fig. 54). As with the other DoDs at this site, the stone revetment on the left (north) bank has localized areas of deposition, as well as deposition and scour apparent on opposing faces of the piers (fig. 59), which likely results from minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The difference between the surveys on August 9, 2023, and the earliest on March 18, 2010 (fig. 60), 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 7). Deposition and erosion appear roughly balanced throughout the reach between 2010 and 2023 in the DoD, with localized substantial deposition near the piers and moderate deposition in the downstream left (north) part of the channel and moderate erosion along the right (south) side throughout the reach (fig. 60). The average difference between the bathymetric surfaces between the 2010 and 2023 surveys was +0.24 ft, indicating minor overall channel aggradation during that time, the net gain of sediment being only about 9,700 yd³ (table 7). The stone revetment on the left (north) bank has areas of erosion, but much of the bank is within the bounds of uncertainty (fig. 60). As in previous DoDs, deposition or scour apparent on opposing faces of the piers is also likely results from minor horizontal positional variances between the surveys (see “Uncertainty Estimation” section; fig. 5).

The vertically averaged velocity vectors indicate mostly uniform flow throughout the reach, ranging from about 1 ft/s along the inside of the bend on the right (south) bank to about 4 to 6 ft/s in much of the rest of the channel (fig. 61). Moderate turbulence was observed in several sections, particularly near the bank protrusions near the bridges and downstream from the main channel piers (fig. 61).

General Findings and Implications

Several findings in the site-specific discussions were common to all surveyed bridges, and some more general findings were only evident when they were examined as a set. These general findings are of benefit when assessing scour at the surveyed bridges, as well as other bridges close by or in similar settings.

Comparing Flow Conditions of this Survey with Previous Surveys

Arneson and others (2012) separated long-term aggradation and degradation of a channel from the contraction scour and local scour that happens at a bridge site during floods. Long-term aggradation or degradation refers to changes in channel-bed elevation observed at a particular site as a result of long-term, systemic changes in the overall channel system. Contraction scour refers to the general change in the channel-bed elevation caused by increased material in suspension and transport caused by flood water passing through a constricted bridge opening. Local scour refers to the localized erosion of material caused by flow-vortex action that forms near bridge piers and abutments. Although all the scour processes (long-term, contraction, and local scour) are continually at work, contraction and local scour are generally cyclic for the live-bed scour typically observed in alluvial channels (Arneson and others, 2012) and generally result in a decrease and then subsequent increase of the channel-bed elevation during and after a flood.

Previous studies using an MBES in the Kansas City area and elsewhere (see report references listed in table 1) have shown that the configuration and size of bed forms observed during a particular survey depend on more than the instantaneous streamflow at a given site. Several factors and interactions of factors affect sediment transport conditions and the resultant bed configuration. Although the current (2023) study does not examine all the antecedent conditions that created the observed channel-bed configuration in the most recent (2023) surveys, the following discussion attempts to draw conclusions based on the conditions observed at each site during the most recent and previous surveys.

As discussed in previous reports about Kansas City area bridges (Huizinga, 2010, 2012, 2016, 2020a, 2022a), the durations of most large floods on the Missouri River can be measured in days to weeks because of the contributing large upstream drainage area. Dams on the upper Missouri River upstream from Yankton, South Dakota, can lessen the magnitude and lengthen the duration of a given flood because of streamflow regulation, as was seen during the 2011 flood (Huizinga, 2012), but the substantial drainage area contributing unregulated streamflows downstream from Yankton can create substantial variability in the streamflow conditions at Kansas City. As described in the “Description of Streamflow Conditions” section earlier in this report, the Missouri River was in a period of low flow for much of the summer of 2023, being at or below the 10-percent mean daily value for several periods between April 15 and July 1, 2023, and generally between the 25- and 75-percent mean daily values for much of the rest of the year (fig. 2). Streamflow from the surveys of the current (2023) study was in a low floodflow to nonflood regime and was the lowest streamflow from among all the surveys at these sites.

The average differences between the 2023 and 2019 bathymetric surfaces ranged from 0.07 to 4.16 ft, which indicates overall deposition between the survey dates (table 7), as might be expected based on streamflow at the time of the 2023 survey alone. However, the average differences between the 2023 and 2015 bathymetric surfaces ranged from 1.44 to –1.88 ft, which indicates an overall balance of scour and deposition between those surveys, despite the lower flow conditions in 2023 (table 7). Similarly, the average difference between the 2023 and 2011 bathymetric surfaces ranged from 3.18 to –5.19 ft, which indicates a relative balance of scour and deposition overall, albeit tending towards scour as might be expected with the substantial flood event in 2011 (table 7).

When examining the average channel-bed and water-surface elevations over time (fig. 62), the water-surface elevations from the 2023 surveys were clearly the lowest of the five surveys taken at all the sites, and the channel-bed elevations from the 2023 surveys follow a more consistent and gradual longitudinal decline (fig. 62) in contrast to the channel-bed elevations from previous surveys that have exhibited substantial longitudinal variability. Various cause-and-effect mechanisms for the observed longitudinal variability of channel-bed and water-surface elevations have been hypothesized in the previous studies (Huizinga, 2010, 2012, 2016, 2020a, 2022a), such as effects from tributary inflow from the Kansas River upstream from structure A4649 near downtown Kansas City (fig. 1), straight versus curved reaches, a slug of sediment in parts of the reach during the 2011 flood, and other effects of flooding before the previous surveys. However, the general low-flow conditions experienced in 2023 appear to have had the effect of smoothing away the longitudinal variability observed in the previous surveys, resulting in a channel-bed and water-surface elevation profile that are parallel with each other in 2023 (fig. 62).

The spatially localized minimum channel-bed elevation observed at structure A7650 (site 10) in the 2015 and 2019 surveys (Huizinga, 2022a) was not evident in the 2023 surveys (fig. 62); however, even though the minimum channel-bed elevation was not observed in the 2023 surveys, that does not negate the hypothesis that this site is at or near a local feature that controls sediment deposition and scour. As posited in Huizinga (2016), the feature may be a combination of the constriction at this site (fig. 30) and being near the downstream end of the relatively straight reach of the river between structures A4649 and A7650 (fig. 1). Huizinga (2016) further posited that the sediment unable to deposit near structure A7650 during higher flow conditions might be flushed downstream and deposited at the next downstream site, structure A5817 (site 11), which seems consistent with the average channel-bed elevation profile observed in previous surveys (fig. 62). The constant gradual decline of the channel-bed elevation profile in the 2023 surveys may indicate that the lower streamflow in 2023 did not create the same kind of sediment transport conditions as in the previous surveys.

The relatively consistent shape of the frequency distribution and cumulative percentage curves (hereinafter referred to as “accumulation curves”) between the surveys at site 10 (fig. 31) further substantiates that site 10 may be a channel-sediment control point, allowing minor deposition during lower streamflows as in 2015 and 2023, but flushing the deposition during higher streamflows to make for a relatively consistent bed-elevation distribution during higher streamflow. Alternatively, the variability of the frequency distribution and accumulation curves between the surveys at structure A5817 (site 11) (fig. 38) displays a wider range of elevations at a lower percentage of grid cells, resulting in a wider range of slopes for the accumulation curves not clearly tied to streamflow. Accumulation curves that are steep indicate a channel bed with a narrower range of elevations (more level throughout the reach, such as structure A7560 [fig. 30]), whereas those that are less steep indicate a wider variation of elevations. Accumulation curves with “steps” indicate a channel with distinct groups of elevations, such as a thalweg on one side of the channel and a shallow area on the opposite side (such as those for structure A1800 [fig. 6 and fig. 7] or structures A4757/L0568 [fig. 53 and fig. 54]).

In the previous study (Huizinga, 2022a), an examination of the frequency distributions of the various sites through time (fig. 63) supports the scenario of a sediment control feature being near structure A7650. In the previous surveys (2010–19), the frequency distribution curves for structures A4060 and A7650 tended to have local maxima with a higher percentage of survey-grid cells than other sites in all the surveys (fig. 63A), and the accumulation curves for these two sites were generally steeper than for the other surveys in the area (fig. 63B), which indicates a more level bed with less variation in elevation at these sites. However, in 2023, several other sites also had local maxima (“2023 survey” in fig. 63A), and the accumulation curves all were steep (“2023 survey” in fig. 63B) compared with curves from previous surveys, which is likely the result of lower streamflow conditions during the 2023 surveys. The frequency distributions were generally narrower in the 2023 survey, with a higher percentage of survey-grid cells in a particular elevation range than previous surveys (“2023 survey” in fig. 63A), which resulted in generally steeper accumulation curves (“2023 survey” in fig. 63B). Furthermore, in 2023, the frequency distributions and accumulation curves more consistently decrease in elevation with distance downstream than in previous surveys.

An examination of the dune features at the various sites through time can also explain the different flow regimes during the various surveys. Nearly all the features in the 2023 surveys were small dunes and ripples. At structure A1800 (site 6, fig. 1), the dune features were medium to very large for all the surveys at that site, almost regardless of the streamflow conditions (see fig. 6 in Huizinga, 2010, 2012, 2016, 2022a). Whereas many of the surveys in 2011 had a planar bed area and large to very large dune features that were indicative of high energy flow (Simons and Richardson, 1966), the 2023 surveys were more like the 2015 surveys, being filled throughout with mostly small dune features and superimposed ripples (fig. 6, fig. 14, fig. 22, fig. 30, fig. 37, fig. 45, and fig. 53). The largest dune features from the 2023 surveys were observed at structure A1800 (site 6, fig. 6). The smaller size and amplitude of the dune features in the 2023 survey compared with all the previous surveys indicates less bed material and bedload transport overall (Simons and Richardson, 1966; Simon and Associates, 1985).