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Detailed measurements are presented of velocities and turbulence under a large‐scale regular plunging breaking wave in a wave flume. Measurements were obtained at 12 cross‐shore locations around a mobile medium‐sand breaker bar. They focused particularly on the dynamics of the wave bottom boundary layer (WBL) and near‐bed turbulent kinetic energy (TKE), measured with an Acoustic Concentration and Velocity Profiler (ACVP). The breaking process and outer flow hydrodynamics are in agreement with previous laboratory and field observations of plunging waves, including a strong undertow in the bar trough region. The WBL thickness matches with previous studies at locations offshore from the bar crest, but it increases near the breaking‐wave plunge point. This relates possibly to breaking‐induced TKE or to the diverging flow at the shoreward slope of the bar. Outer flow TKE is dominated by wave breaking and exhibits strong spatial variation with largest TKE above the breaker bar crest. Below the plunge point, breaking‐induced turbulence invades the WBL during both crest and trough half cycle. This results in an increase in the time‐averaged TKE in the WBL (with a factor 3) and an increase in peak onshore and offshore near‐bed Reynolds stresses (with a factor 2) from shoaling to breaking region. A fraction of locally produced TKE is advected offshore over a distance of a few meters to shoaling locations during the wave trough phase, and travels back onshore during the crest half cycle. The results imply that breaking‐induced turbulence, for large‐scale conditions, may significantly affect near‐bed sediment transport processes.

This study presents novel insights into hydrodynamics and sediment fluxes in large‐scale laboratory experiments with bichromatic wave groups on a relatively steep initial beach slope (1:15). An Acoustic Concentration and Velocity Profiler provided detailed information of velocity and sand concentration near the bed from shoaling up to the outer breaking zone including suspended sediment and sheet flow transport. The morphological evolution was characterized by offshore migration of the outer breaker bar. Decomposition of the total net transport revealed a balance of onshore‐directed, short wave‐related and offshore‐directed, current‐related net transport. The short wave‐related transport mainly occurred as bedload over small vertical extents. It was linked to characteristic intrawave sheet flow layer expansions during short wave crests. The current‐related transport rate featured lower maximum flux magnitudes but occurred over larger vertical extents. As a result, it was larger than the short wave‐related transport rate in all but one cross‐shore position, driving the bar’s offshore migration. Net flux magnitudes of the infragravity component were comparatively low but played a nonnegligible role for total net transport rate in certain cross‐shore positions. Net infragravity flux profiles sometimes featured opposing directions over the vertical. The fluxes were linked to a standing infragravity wave pattern and to the correlation of the short wave envelope, controlling suspension, with the infragravity wave velocity.

Large scale laboratory measurements of sediment dynamics in the swash zone are presented. Two bichromatic wave group conditions were generated, having the same energy content but different wave group period (T g = 15.0 s and 27.7 s). For the shortest wave group, due to bore focussing, the shoreline fluctuates predominantly at the T g time scale, showing a large runup and the presence of wave-swash interactions with strong momentum exchange. In contrast, for the longer wave groups, the swash excursion is dominated by the individual waves. The uprush generally promotes onshore sediment advection with consequent erosion at the rundown location but accretion close to the runup. On the contrary, the backwash promotes seaward sediment advection and accretion at the rundown location. The presence of repeated wave-swash interactions modifies these patterns slightly. A wave overrunning a previous uprush promotes a reduction in onshore sediment advection while weak wave-backwash interactions reduce seaward advection. Consequently, the measured sediment dynamics shows stronger intra-swash cross-shore sediment advection for the swash events produced by the short wave groups. Measurements of the sheet flow layer near the shoreline show that for the shortest wave groups the vertical structure of the concentration