Wave boundary layer (WBL) dynamics are measured with an Acoustic Concentration and Velocity Profiler (ACVP) across the sheet flow‐dominated wave‐breaking region of regular large‐scale waves breaking as a plunger over a developing breaker bar. Acoustic sheet flow measurements are first evaluated quantitatively in comparison to Conductivity Concentration Meter (CCM+) data used as a reference. The near‐bed orbital velocity field exhibits expected behaviors in terms of wave shape, intrawave WBL thickness, and velocity phase leads. The observed fully turbulent flow regime all across the studied wave‐breaking region supports the model‐predicted transformation of free‐stream velocity asymmetry into near‐bed velocity skewness inside the WBL. Intrawave concentration dynamics reveal the existence of a lower pickup layer and an upper sheet flow layer similar to skewed oscillatory sheet flows, and with similar characteristics in terms of erosion depth and sheet flow layer thickness. Compared to the shoaling region, differences in terms of sheet flow and hydrodynamic properties of the flow are observed at the plunge point, attributed to the locally enhanced wave breaker turbulence. The ACVP‐measured total sheet flow transport rate is decomposed into its current‐, wave‐, and turbulence‐driven components. In the shoaling region, the sand transport is found to be fully dominated by the onshore skewed wave‐driven component with negligible phase lag effects. In the outer surf zone, the total net flux exhibits a three‐layer vertical structure typical of skewed oscillatory sheet flows. However, in the present experiments this structure originates from offshore‐directed undertow‐driven flux, rather than from phase lag effects.

Intense sediment transport experiments were performed in a gravity‐driven open‐channel flow with two sizes of uniformly distributed nonspherical acrylic particles having median diameters of 1.0 and 3.0 mm and a maximum packing volumetric concentration (ϕ) of 0.55. The flow conditions were adapted to each particle size to ensure similar sediment transport flow regimes as sheet flow corresponding to Shields numbers slightly above unity and a suspension number, ratio of settling velocity to friction velocity, near unity for the two experiments. An acoustic scattering‐based system (Acoustic Concentration and Velocity Profiler) and conductivity probes (Conductivity Concentration Profiler [CCP]) with two different vertical resolutions, 1 mm (CCP_1mm) and 2 mm (CCP_2mm) were used to measure instantaneous concentration profiles across the bedload and suspension layers. Measured concentration profiles, bed interface position, and sheet flow layer thicknesses are compared between the two techniques. The capabilities and limitations of both technologies are outlined. Average volumetric sediment concentration profiles were overestimated by 10% with the Acoustic Concentration and Velocity Profiler in the dense sheet layer when ⟨ϕ(z)⟩≳ 0.35, and by 100% with the CCP in the more diluted region when ⟨ϕ(z)⟩≲ 0.015 and ⟨ϕ(z)⟩≲ 0.20 for CCP_1mm and CCP_2mm, respectively. Good agreement is found between the three systems in terms of average and time‐resolved bed level position and sheet layer thickness, validating the different bed interface detection methods on data from the two sensors.

Sand transport predictions represent high social, economic and environmental stakes. Therefore, reliable sand transport models are needed. On a beach, either erosion or accretion can take place. Mechanisms leading to erosion are quite well known whereas accretion is less well predicted. Onshore transport occurs in the Wave Boundary Layer (WBL). Up until now, WBL transport processes have mainly been observed in laboratory conditions, in a controlled environment. This allowed in particular to highlight the three main WBL parameters (near-bed velocity phase shift, non-linearities evolution and WBL thickness) which control bed shear stress and therefore sand transport (Fuhrman et al., 2009). But in order to further improve sand transport models, in situ data are now necessary, to observe what happens under real, full spectrum surface waves. In situ data sets are quite rare because of the difficulty to deploy instruments on beaches, but also because of the metrological challenge bound to the observation of both hydrodynamics and sand transport processes in the Wave Bottom Boundary Layer with high spatial and temporal resolutions in the natural environment. The first in situ deployments, involving a new generation of ultrasonic Doppler sonar system, have been carried out during Winter 2022 at Porsmilin Beach, Brittany, France. This study presents the methodology used to obtain in situ WBL parameters.

Accurate modelling and prediction of sediment transport in aquatic environments is essential for sustainable coastal and riverine management. Current capabilities rely on physical process-based numerical models and fine-scale sediment flux measurements. High-resolution hydroacoustic instrumentation has emerged as a promising tool for such measurements. However, challenges arise due to the inherent complexity of ultrasound scattering processes. This study introduces a numerical modelling using a point-particle approach to simulate the echoes backscattered by such instrumentation in sediment-laden flow conditions. The model considers geometric, statistical, particle cloud, and flow-induced effects on sediment velocity, concentration, and flux estimates using an acoustic concentration and velocity profiler as a reference. The model performance is assessed here under unidirectional constant flow conditions in terms of velocity, concentration, and time-resolved sediment flux estimates for a large range of the particles' advection speed and sampled volume sizes. Application to the estimation of the measurement accuracy of sediment flux in these flows is also considered, with a final error on the flux seen to be partially controlled by the residence time of particles within the sampled volumes. The proposed model provides insights into scattering processes and offers a tool for investigating robust sediment flux estimation techniques in various flow conditions.

Assessing sediment stocks and transfers has become a key issue in recent decades. While the contribution of acoustic methods has led to a better understanding of sediment dynamics, through the measurement of waves and currents, the idea of using them to quantify the sediment fluxes is in full development. However, the use of these methods requires precise knowledge of the scattering properties of the suspended particulate matter present in the water column (also called the form function), as a function of the particle properties (size, shape, density...) and the instrument frequency. Analytical models (also called backscattering model) do exist for simple shapes, but their complexity increases when it comes to real suspensions, preventing accurate inversion of the acoustic intensity into suspended sediment concentration (Stanton and chu, 2000). Therefore, heuristic or semi-analytical models have been developed to overcome these limitations. But this approach has mainly been steered towards the case of homogeneous mineral (sand – Thorne and Hanes, 2002) or organic (zooplankton – Holliday and Pieper, 1995; Stant and Chu, 2000) suspensions. However, in situ sediments in the marine domains are often made of mixed biological and geological origins. This study proposes for the first time a heuristic formulation describing the backscattering properties of a mixture of coarse sand and shell sediments. Different proportions are considered. The method is validated against unsorted in situ samples.