Acoustic Backscattered Signals Research Articles

Multi-Layer Perceptron Neural Network Utilizing Adaptive Best-Mass Gravitational Search Algorithm to Classify Sonar Dataset

In this paper, a new Multi-Layer Perceptron Neural Network (MLP NN) classifier is proposed for classifying sonar targets and non-targets from the acoustic backscattered signals. Besides the capabilities of MLP NNs, it uses Back Propagation (BP) and Gradient Descent (GD) for training; therefore, MLP NNs face with not only impertinent classification accuracy but also getting stuck in local minima as well as low-convergence speed. To lift defections, this study uses Adaptive Best Mass Gravitational Search Algorithm (ABGSA) to train MLP NN. This algorithm develops marginal disadvantage of the GSA using the best-collected masses within iterations and expediting exploitation phase. To test the proposed classifier, this algorithm along with the GSA, GD, GA, PSO and compound method (PSOGSA) via three datasets in various dimensions will be assessed. Assessed metrics include convergence speed, fail probability in local minimum and classification accuracy. Finally, as a practical application assumed network classifies sonar dataset. This dataset consists of the backscattered echoes from six different objects: four targets and two non-targets. Results indicate that the new classifier proposes better output in terms of aforementioned criteria than whole proposed benchmarks.

Monitoring of Anthropogenic Sediment Plumes in the Clarion-Clipperton Zone, NE Equatorial Pacific Ocean

The abyssal seafloor in the Clarion-Clipperton Zone (CCZ) in the NE Pacific hosts the largest abundance of polymetallic nodules in the deep sea and is being targeted as an area for potential deep-sea mining. During nodule mining, seafloor sediment will be brought into suspension by mining equipment, resulting in the formation of sediment plumes, which will affect benthic and pelagic life not naturally adapted to any major sediment transport and deposition events. To improve our understanding of sediment plume dispersion and to support the development of plume dispersion models in this specific deep-sea area, we conducted a small-scale, 12-hour disturbance experiment in the German exploration contract area in the CCZ using a chain dredge. Sediment plume dispersion and deposition was monitored using an array of optical and acoustic turbidity sensors and current meters placed on platforms on the seafloor, and by visual inspection of the seafloor before and after dredge deployment. We found that seafloor imagery could be used to qualitatively visualise the redeposited sediment up to a distance of 100 m from the source, and that sensors recording optical and acoustic backscatter are sensitive and adequate tools to monitor the horizontal and vertical dispersion of the generated sediment plume. Optical backscatter signals could be converted into absolute mass concentration of suspended sediment to provide quantitative data on sediment dispersion. Vertical profiles of acoustic backscatter recorded by current profilers provided qualitative insight into the vertical extent of the sediment plume. Our monitoring setup proved to be very useful for the monitoring of this small-scale experiment and can be seen as an exemplary strategy for monitoring studies of future, upscaled mining trials. We recommend that such larger trials include the use of AUVs for repeated seafloor imaging and water column plume mapping (optical and acoustical), as well as the use of in-situ particle size sensors and/or particle cameras to better constrain the effect of suspended particle aggregation on optical and acoustic backscatter signals.

Three-dimensional modeling of sediment resuspension in a large shallow lake

We simulated bottom resuspension events in Lake Erie, using a coupled three-dimensional hydrodynamic and water quality model. Key parameters in the model, including critical bottom shear stress (τcr) and resuspension rate (α) were calibrated and validated by comparing the model output to observations. These included total suspended solid (TSS) concentrations in the bottom boundary layer (RMSE = 0.74 mgL-1) and water column (RMSE = 0.81 mgL-1), and to time series of acoustic backscatter signal (R2 > 0.8) and turbidity (R2 ≈ 0.4) from long-term moorings near the lakebed in 2008–09 and 2013. Signals from phytoplankton, in spring and summer, caused discrepancies between modeled TSS and the observed turbidity data. Although common practice, we show that literature-based or field-observed critical shear stress should not be directly applied in large-scale Reynolds-averaged sediment model as this will likely underestimate resuspension. In agreement with the literature, the model reproduced more frequent and intensive surface-wave driven resuspension in the shallow regions (< ~20 m), particularly in the western basin, compared to the deeper central and eastern basins, where bottom stresses induced by mean currents (τc) were comparable with those due to surface waves (τw). However, on the north-shore of the eastern basin, τc often predominated over τw. We simulated thermocline motion, including up- and down-welling events and swashing of the internal Poincaré wave, to contribute to τc in the central basin and form nepheloid layers.

Complex data labeling with deep learning methods: Lessons from fisheries acoustics

Quantitative and qualitative analysis of acoustic backscattered signals from the seabed bottom to the sea surface is used worldwide for fish stocks assessment and marine ecosystem monitoring. Huge amounts of raw data are collected yet require tedious expert labeling. This paper focuses on a case study where the ground truth labels are non-obvious: echograms labeling, which is time-consuming and critical for the quality of fisheries and ecological analysis. We investigate how these tasks can benefit from supervised learning algorithms and demonstrate that convolutional neural networks trained with non-stationary datasets can be used to stress parts of a new dataset needing human expert correction. Further development of this approach paves the way toward a standardization of the labeling process in fisheries acoustics and is a good case study for non-obvious data labeling processes.

Micronekton distribution in the southwest Pacific (New Caledonia) inferred from shipboard-ADCP backscatter data

Acoustic data are invaluable information sources for characterizing the distribution and abundance of mid-trophic-level organisms (micronekton). These organisms play a pivotal role in the ecosystem as prey of top predators and as predators of low-trophic-level organisms. Although shipboard-ADCP (acoustic Doppler current profiler) acoustic backscatter signal intensity cannot provide an absolute biomass estimate, it may be a useful proxy to investigate variability in the distribution and relative density of micronekton. This study used acoustic recordings data spread across 19 years (1999–2017) from 54 ADCP cruises in New Caledonia's subtropical EEZ (exclusive economic zone) to assess seasonal and interannual variabilities and spatial distribution of micronekton. The dataset was composed of two different ADCPs: 150 kHz for the first period, followed by 75 kHz for more recent years. We examined the 20–120 m averaged scattering layer. Using the few cruises with concurrent EK60 measurements, we proposed that the backscatter from the ADCPs and 70 kHz EK60 were sufficiently closely linked to allow the use of the backscatter signal from the ADCPs in a combined dataset over the full time series. We then designed a GAMM (generalized additive mixed model) model that takes into account the two ADCP devices as well as temporal variability. After accounting for the effect of the devices, we showed that the acoustic signal was mainly driven by diel vertical migration, season, year, and ENSO (El Niño-Southern Oscillation). In a second step, a consensus model between two statistical approaches (GAMM and SVM) (support vector machine) was constructed, linking the nighttime 20–120 m backscatter to the oceanographic and geographic environment. This model showed that sea surface temperature was the main factor driving backscatter variability in the EEZ, with intensified backscatter during the austral summer (December to May) in the northern part of the EEZ. We showed that acoustic density differed significantly, spatially and temporally from micronekton biomass predicted for the same period by the SEAPODYM-MTL (mid-trophic level) ecosystem model. The seasonal cycle given by ADCP data lagged behind the SEAPODYM-MTL seasonal cycle by around three months. Reasons to explain these differences and further needs in observation and modeling were explored in the discussion. In addition to providing new insights for micronekton dynamics in this EEZ (i.e., the science needed for ecosystem-based fisheries management), the data should help improve our ability to model this key trophic component.

Experimental investigation on inversion of ADVP measurement for suspended sediment concentration, a case study

To obtain vertical velocity profile and related suspended sediment concentration (SSC) simultaneously with little interference as far as possible is always of hydraulic researchers’ interest. For this purpose, the experiments with suspended sediment of silica sand were carried out in water vessel in order to investigate the inversion of acoustic backscatter intensity for SSC distribution in acoustic Doppler velocimeter measurement. A Nortek Vectrino Profiler (ADVP) was used in the experiments. The influences of the sediment diameter, the average concentration and the instrument parameter setting on acoustic backscatter signal (AMP) were tested. The results show that AMP values of middle layer are larger than those of the other layers and they have similar logarithmic relationship with SSC as in ADV measurement inversion. The relationship was found to depend on the particle sizes and the sampling cell configuration. The more layered in a same sampling section, the weaker the backscattered intensity and the larger the intensity difference between the middle layer and the other layers. When certain amendment is given to the AMP values on each layer, the ADVP measurements can still be transferred into SSC by using the simple logarithmic relationship. Through the experiments, the acoustic inversion approaches are suggested and the involved formula for sediment concentration profile for the case is established.

Acoustic Methods for Increasing the Cavitation Initiation Pressure Threshold.

Histotripsy is a tissue ablation method that utilizes focused, high-amplitude ultrasound to generate a cavitation bubble cloud that mechanically fractionates tissue. Effective histotripsy depends on the initiation, control, and maintenance of cavitation bubble clouds in the targeted area. In this study, we hypothesized that a low-pressure acoustic pulse sequence applied before and/or during histotripsy therapy would increase the cavitation initiation pressure threshold and the growth of cavitation bubble clouds. This technique could shrink or "sharpen" the focal zone during histotripsy to produce more precise and well-defined lesions with minimal collateral damage. It may also be a way to actively protect the soft tissue from cavitation damage during lithotripsy by increasing the pressure threshold for bubble cloud initiation. We applied these low-amplitude acoustic pulse sequences before and during histotripsy treatments with the pulse repetition frequency of 1 and 100 Hz, in three different mediums: water, tissue phantom agarose gel, and bovine liver in vitro. Acoustic backscatter signals and optical imaging were used to detect and monitor the initiation, maintenance, and growth of the resulting cavitation bubble cloud. The results demonstrated that the use of low-amplitude acoustic pulse sequences could increase the cavitation pressure amplitude threshold by 20% in the targeted area.

Sensitivity analysis of sediment flux derived by laser diffraction and acoustic backscatter within a reservoir

A combination of two indirect methods to measure sediment flux is presented in this study to evaluate suspended sediment transport in a hydropower reservoir. The acoustic backscatter signal (ABS) from an Acoustic Doppler Current Profiler (ADCP) is therefore applied in pre-defined transects within the reservoir in combination with a Laser In-Situ Scattering Transmissometry – stream lined device (LISST-SL). The stationary LISST-SL derived suspended sediment concentration (SSC) measurements are used to calibrate the ABS. From the LISST-SL measurements a time series of SSC is obtained. This enables, in addition, a comprehensive data analysis to evaluate the influence of natural fluctuations of the SSC on the calculated sediment flux, which should be taken into account when assessing sediment transport. Furthermore SSC measurements are done with the LISST-SL close to the reservoir bed. In areas close to the bed no information regarding the ABS is available from the ADCP measurements due to the side-lobe interference. In various studies the information from the last three valid cells is used for extrapolation. However, as result of a comparison of the LISST-SL measurements with extrapolated SSC values from the ADCP measurements it can be seen that, especially in deep reservoirs, this method has to be adapted to the in-situ conditions.

Investigation of the source of histotripsy acoustic backscatter signals

Recent work has demonstrated that acoustic backscatter signals from histotripsy-generated bubble clouds may be used to localize generated bubble clouds and perform non-invasive aberration correction transcranially. However, the primary source of the measured signals, whether from emissions generated during bubble expansion, or scattering of the incoming pulses off of the incipient bubble clouds, remains to be determined and may have important implications for how the acquired signals may be used. Here, we present results from experiments comparing the acoustic emissions and growth-collapse curves of single bubbles generated optically to those generated via histotripsy. Histotripsy bubbles were generated using a 32-element, 1.5 MHz spherical transducer with pulse durations &amp;lt;2-cycles; optical bubbles were nucleated using a pulsed Nd:YAG laser focused at the center of the histotripsy transducer. Optical imaging was used to capture the time evolution of the generated bubbles from inception to collapse. Acoustic emissions from the generated bubbles were captured using the receive-capable histotripsy transducer elements as well as with a commercial hydrophone mounted within. Imaging results indicated that optically nucleated bubbles experienced more rapid growth than histotripsy generated bubbles. Acoustic emissions from both sets of bubbles were comparable, however, suggesting the primary component of the measured histotripsy “backscatter” signal is an emission generated during bubble expansion.

Amplitude calibration of an acoustic backscattered signal from a bottom-moored ADCP based on long-term measurement series

A possible approach to, and preliminary results of, amplitude calibration of acoustic signals backscattered from an ADCP moored at the bottom of the near-shelf zone of the Black Sea is considered. The aim of this work is to obtain vertical profiles of acoustic scattering signal levels, showing the real characteristics of the volume content of suspended sediments in sea water in units of conventional acoustic turbidity for a given signal frequency. In this case, the assumption about the intervals of maximum acoustic transparency and vertical homogeneity of the marine environment in long-term series of ADCP measurements is used. According to this hypothesis, the intervals of the least values of acoustic backscattered signals are detected, an empirical transfer function of the ADCP reception path is constructed, and it is calibrated. Normalized sets of acoustic backscattered signals relative to a signal from a level of conventionally clear water are obtained. New features in the behavior of vertical profiles of an acoustic echo-signal are revealed due to the calibration. The results of this work will be used in subsequent analysis of the vertical and time variations in suspended sediment content in the near-shelf zone of the Black Sea.

On nonhydrostatic coastal model simulations of shear instabilities in a stratified shear flow at high Reynolds number

AbstractThe nonhydrostatic surface and terrain‐following coastal model NHWAVE is utilized to simulate a continually forced stratified shear flow in a straight channel, which is a generic problem to test the existing nonhydrostatic coastal models' capability in resolving shear instabilities in the field scale. The resolved shear instabilities in the shear layer has a Reynolds number of about 1.4 × 106, which is comparable to field observed value. Using the standard Smagorinsky closure with a grid size close to the Ozmidov length scale, simulation results show that the resolved energy cascade exceeds 1 order of magnitude and the evolution and turbulent mixing characteristics are predicted well. Two different approaches are used to estimate the turbulent dissipation rate, namely using the resolved turbulent energy spectrum and the parameterized subgrid turbulent dissipation rate, and the predicted results provide the upper and lower bounds that encompass the measured values. Model results show significantly higher turbulence in braids of shear instabilities, which is similar to field observations while both the subgrid turbulent dissipation rate and resolved vorticity field can be used as surrogates for measured high acoustic backscatter signals. Simulation results also reveal that the surface velocity divergence/convergence is an effective identifier for the front of the density current and the shear instabilities. To guide future numerical studies in more realistic domains, an evaluation on the effects of different grid resolutions and subgrid viscosity on the resolved flow field and subgrid dissipation rate are discussed.

Subharmonic Response of Polymer Contrast Agents Based on the Empirical Mode Decomposition.

The subharmonic threshold for ultrasound contrast agents has been defined as a 20-25 dB difference between the fundamental and subharmonic (2/1) spectral components of the backscatter signal. However, this Fourier-based criterion assumes a linear time-invariant signal. A more appropriate criterion for short cycle and frequency-modulated waveforms is proposed with an adaptive signal-processing approach based on the empirical mode decomposition (EMD) method. The signal is decomposed into an orthogonal basis known as intrinsic mode functions (IMFs) and a subharmonic threshold is defined with respect to the energy ratio of the subharmonic IMF component to that of the incident signal. The method is applied to backscatter data acquired from two polymer-shelled contrast agents, Philips (#38, mean diameter 2.0 [Formula: see text]) and Point Biomedical (#12027, mean diameter 3.9 [Formula: see text]). The acoustic backscatter signals are investigated for a single contrast agent subjected to monofrequency (20 MHz, 20 cycles) and chirp (15-25 MHz, 20 cycles) forcing for incident pressures ranging from 0.5 to 2.4 MPa. In comparison to the spectral peak difference (20 dB) criterion, the EMD method is more sensitive in determining subharmonic signals.

Wintertime water dynamics and moonlight disruption of the acoustic backscatter diurnal signal in an ice‐covered Northeast Greenland fjord

AbstractSix and a half month records from three ice‐tethered Acoustic Doppler Current Profilers deployed in October 2013 in Young Sound fjord in Northeast Greenland are used to analyze the acoustic backscatter signal. The acoustic data suggest a systematic diel vertical migration (DVM) of scatters below the land‐fast ice during polar night. The scatters were likely composed of zooplankton. The acoustic signal pattern typical to DVM persisted in Young Sound throughout the entire winter including the period of civil polar night. However, polynya‐enhanced estuarine‐like cell circulation that occurred during winter disrupted the DVM signal favoring zooplankton to occupy the near‐surface water layer. This suggests that zooplankton avoided spending additional energy crossing the interface with a relatively strong velocity gradient comprised by fjord inflow in the intermediate layer and outflow in the subsurface layer. Instead, the zooplankton tended to remain in the upper 40 m layer where relatively warmer water temperatures associated with upward heat flux during enhanced estuarine‐like circulation could be energetically favorable. Furthermore, our data show moonlight disruption of DVM in the subsurface layer and weaker intensity of vertical migration beneath snow covered land‐fast ice during polar night. Finally, by using existing models for lunar illuminance and light transmission through sea ice and snow cover, we estimated under ice illuminance and compared it with known light sensitivity of Arctic zooplankton species.

A study on the estimating of sediment concentration with turbidity and acoustic backscatter signal for different sediment sizes

Turbidity and acoustic measurement are alternative indirect methods to determine sediment concentration. Acoustic Doppler velocimetry can be used to estimate suspended sediment concentration (SSC) with the signal-to-noise ratio (SNR) in the water. In this study, particles size affecting turbidity and SNR values was investigated using four different sediment size groups (0–50, 50–100, 100–200, and 200–250 micron). The highest turbidity values were determined for small-sized sediment, and they decreased for bigger sediment size. Clay content decreased the relationship between sediment and turbidity and caused a reading error for high concentrations (&amp;gt;4 g.l−1). However, R2 values greater than 0.900 were obtained for all treatments (R2: 0.952, 0.992, 0.987, and 0.977, respectively, from small size to large size group). SNR values had good relationships with SSC values for less than 40 dB and 1 g.l−1 sediment concentrations (R2: 0.990, 0.998, 0.994, and 0.973, respectively, from small size to large size group). SNR values were strongly affected by small changes in sediment concentration but this property can be accepted as advantageous for sensitive measurement. As a result of this study, it could be concluded that turbidity and SNR values can be used for continuous sediment monitoring.

Acoustic methods for modulating the cavitation initiation pressure threshold

The objective of this study is to develop tissue protection methods by modulating bubble cloud initiation pressure thresholds. Specifically, we investigated pressure thresholds to initiate cavitation bubble clouds by the shock scattering mechanism. Cavitation initiation displays a stochastic nature affected by existing nuclei populations in the medium. We hypothesized that by applying proper low pressure pulse sequences before and/or during histotripsy therapy, initiation pressure threshold and growth of cavitation bubble cloud could be modified. We applied histotripsy and cavitation suppressing pulses in both water and agarose gel for pulse repetition rates of 1, 10, and 100 Hz. Acoustic backscatter signals and optical imaging were used to detect and monitor initiation, maintenance, and growth of resulting cavitation bubble cloud. Results demonstrated that the use of cavitation suppressing pulses can increase the cavitation threshold by 20% in the targeted space. Furthermore, we showed these acoustic sequences could modify the shape and density of the bubble cloud. By applying the cavitation suppressing pulses, we were able to generate a dense cavitation bubble cloud in the focus while decreasing scattered cavitation in the peripheral zone.

Bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern North Sea

The presence of a seasonal thermocline likely plays a key role in restraining methane released from a seabed source in the deeper water column, thereby inhibiting exchange to the atmosphere. The bubble plume itself, however, generates an upward motion of fluid, e.g. upwelling and may thereby be partially responsible for an early breakdown of the seasonal thermocline. Measurements at site 22/4b, located at (57°55′N, 1°38′E) in the UK Central North Sea, 200 km east of the Scottish mainland, where gas is still being released since a blow out in 1990, have been used to identify the generation of the seasonal thermocline, and thus, the depth of the upper mixed layer and its breakdown in autumn. Data derived from two landers, containing an Acoustic Doppler Current Profiler and a Conductivity Temperature Depth recorder, were used to determine the mixed layer depth and the breakdown of the thermocline. Mixing of upper layer fluid into the lower layer has been inferred from large amplitude variations in the near-bottom temperature.The ADCPs estimate velocity profiles in four beam directions using Doppler shifted frequency from acoustic pings sent out and received by four different transducers in a specific configuration. Besides that, the intensity of the backscattered sound per transducer is also recorded. Bubbles from the nearby plume contaminate the signal during part of the tidal cycle, but in bubble free periods, the mixed layer depth can be estimated using the acoustic backscatter signal as local maxima. Results show that the thermocline broke down between mid-October and early November, several weeks earlier than the breakdown of the thermocline in nearby/comparable areas, likely caused by bubble-induced downwelling at the site. The early breakdown of the thermocline was accompanied by multiple occurrence of a strong jet-like structure, associated with the seasonal tidal mixing front.

A flocculi model for the acoustic scattering from flocs

The objective of this paper is to suggest a methodology for the practical inversion of acoustic backscatter to obtain suspended mass concentration of marine flocs. Optical data collected using a LISST-100 at the same time as acoustic backscatter (ABS) measurements were made during a series of controlled flocculation laboratory experiments using kaolin (MacDonald et al., 2013) showed a consistent peak in the particle size distribution at around 10–20µm in 50 of the 75 experimental runs. This peak corresponds to the size of the strongest and most tightly bound of component floc structures referred to as ‘flocculi’ (Van Leussen, 1994; Lee et al., 2012). Log-normal distributions fitted to these peaks accounted for between 30.1% and 90.7% of the total volume of particles (median 50.4%). The equivalent mean floc sizes identified from the FLOCView camera system were between 38 and 92µm. It was hypothesised that the acoustic backscatter from the flocculated kaolin was occurring from the flocculi (and not the flocs per se) and that the flocculi could be treated as elastic spheres with density and acoustic wave propagation speeds equal to bulk kaolin. The ABS signals were inverted, using the methodology suggested by Lee and Hanes (1995), using the form function and total scattering cross section defined by Moate and Thorne (2009) for particles with a broad size distribution, to determine the particle size. Excluding the unflocculated runs the particle diameters were 15.5±4.9µm; the particle size tended to increase with concentration and with the ‘level’ of flocculation. The viscous attenuation, which cannot be neglected for particles of this size, was also estimated from the ABS data and was found to be consistent with Urick's (1948) model. It is concluded that the flocculi model of acoustic backscatter is supported by laboratory measurements on a simple suspension of kaolin, and may offer a practical way forward for the interpretation of marine field data.

Do changes in the size of mud flocs affect the acoustic backscatter values recorded by a Vector ADV?

A series of experiments were conducted to examine the effect of mud floc growth on the acoustic back-scatter signal recorded by a Nortek Vector acoustic Doppler velocimeter (ADV). Several studies have shown that calibration equations can be developed to link the backscatter strength with average suspended sediment concentration (SSC) when the sediment particle size distribution remains constant. However, when mud is present, the process of flocculation can alter the suspended particle size distribution. Past studies have shown that it is still unclear as to the degree of dependence of the calibration equation on changes in floc size. Part of the ambiguity lies in the fact that flocs can be porous and rather loosely packed and therefore will not scatter sound waves as a solid particle would. In addition, direct, detailed measurements of floc size have not accompanied experiments examining the dependence of ADV backscatter and suspended sediment concentration. In this set of experiments, direct measurement of the floc size distribution is made with time in a mixing chamber using a floc camera system. A Vector ADV and an OBS are also placed within the tank to measure acoustic backscatter and SSC as the flocs change size with time; concentration in the experiments ranges from 15 to 90mg/l. Results showed that the growth of mud flocs did influence the SNR recorded by the Vector ADV, and that the sensitivity of the SNR signal to changes in floc size was higher for flocs with diameters less than ≈80μm (it kr=1 at a diameter of 80μm). The response of SNR to changes in floc size and SSC was modeled with a modified sonar equation. If properly calibrated, the model was able to capture the functional behavior of SNR with changes in floc size and concentration. Values of the calibration coefficients showed that while changes in floc diameter up to about 80μm did alter the SNR, the change was less than what would be expected from a similar change in the size of solid scatterers.

Asymmetric fluxes of water and sediments in a mesotidal mudflat channel

The hydrodynamics of a small tributary channel and its adjacent mudflat is studied in Willapa Bay, Washington State, USA. Velocity profiles and water levels are simultaneously measured at different locations in the channel and on the mudflat for two weeks. The above tidal flat and channel hydrodynamics differ remarkably during the tidal cycle. When the water surface level is above the tidal flat elevation, the channel is inactive. At this stage, the above tidal flat flow is predominantly aligned along the Bay axis, oscillating with the tide as a standing wave with peak velocities up to 0.3 m/s. When the mudflat becomes emergent, the flow concentrates in the channel. During this stage, current velocities up to 1 m/s are measured during ebb; and up to 0.6 m/s during flood. Standard equations for open-channel flow are utilized to study the channel hydrodynamics. From the continuity equation, a lateral inflow is predicted during ebb, which likely originates from the drainage of the mudflat through the lateral runnels. Both advective acceleration and lateral discharge terms, estimated directly from the velocity profiles, play a significant role in the momentum equation. The computed drag coefficient for bottom friction is small, due to an absence of vegetation and bottom bedforms in the channel. Sediment fluxes are calculated by combining flow and suspended sediment concentration estimated using the acoustic backscatter signal of the instruments. A net export of the sediment from the channel is found during ebb, which is not balanced by the sediment import during flood. When the mudflat is submerged, ebb-flood asymmetries in suspended sediment concentration are present, leading to a net sediment flux toward the inner part of the Willapa Bay. Finally, a residual flow is detected inside the channel at high slack water, probably associated with the thermohaline circulation.

A new methodology for the quantitative visualization of coherent flow structures in alluvial channels using multibeam echo‐sounding (MBES)

In order to investigate the interactions between turbulence and suspended sediment transport in natural aqueous environments, we ideally require a technique that allows simultaneous measurement of fluid velocity and sediment concentration for the whole flow field. Here, we report on development of a methodology using the water column acoustic backscatter signal from a multibeam echo sounder to simultaneously quantify flow velocities and sediment concentrations. The application of this new technique is illustrated with reference to flow over the leeside of an alluvial sand dune, which allows, for the first time in a field study, quantitative visualization of large‐scale, whole flow field, turbulent coherent flow structures associated with the dune leeside that are responsible for suspending bed sediment. This methodology holds great potential for use in a wide range of aqueous geophysical flows.