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Beach recovery is key to the continued existence of sandy beaches and is typically driven by the onshore‐directed transport of sediment by short waves during low‐moderate energy conditions. The physical processes governing beach recovery are not well understood, but the theoretically developed dimensionless fall velocity, Ω = H/wsT, was suggested to be important for separating onshore/offshore sediment motion (Dean, 1973). In this paper, the effect of wave period and sediment grain size on short‐wave suspended sediment flux was investigated based on field measurements obtained beneath shoaling and breaking waves at Durras and Vejers beaches. The efficiency of the breaking waves in transporting suspended sediment onshore was roughly the same for the two beaches, despite the wave periods being larger and the mean sediment grain size coarser at Durras beach. The flux efficiencies were, however, shown to be degraded by wave‐current interactions and long/short‐wave interactions at Durras beach, especially. Excluding time series of strong undertow velocities (< −0.1 m/s) and infragravity wave‐energy (>5 m2/s3) resulted in significantly larger flux efficiencies beneath breaking waves at Durras beach compared to Vejers beach. These results indicate that wave‐current interactions and long/short‐wave interactions should be taken into consideration along with the wave period and mean grain size when estimating short‐wave suspended sediment fluxes. The results also showed that plunging breakers were more efficient in suspending sediment and transporting it onshore compared to spilling breakers/surf bores. This finding suggests that wave breaker type also is an important parameter to incorporate when modeling beach recovery.

The existence of sandy beaches relies on the onshore transport of sand by waves during post-storm conditions. Most operational sediment transport models employ wave-averaged terms, and/or the instantaneous cross-shore velocity signal, but the models often fail in predictions of the onshore-directed transport rates. An important reason is that they rarely consider the phase relationships between wave orbital velocity and the suspended sediment concentration. This relationship depends on the intra-wave structure of the bed shear stress and hence on the timing and magnitude of turbulence production in the water column. This paper provides an up-to-date review of recent experimental advances on intra-wave turbulence characteristics, sediment mobilization, and suspended sediment transport in laboratory and natural surf zones. Experimental results generally show that peaks in the suspended sediment concentration are shifted forward on the wave phase with increasing turbulence levels and instantaneous near-bed sediment concentration scales with instantaneous turbulent kinetic energy. The magnitude and intra-wave phase of turbulence production and sediment concentration are shown to depend on wave (breaker) type, seabed configuration, and relative wave height, which opens up the possibility of more robust predictions of transport rates for different wave and beach conditions.

Measurements of near‐bed shear stress were undertaken in the shallow subtidal zone at Durras Beach, NSW, Australia using a sideways‐looking acoustic velocity meter installed within the wave boundary layer. The wave climate was swell‐dominated and wave conditions comprised shoaling and breaking waves as well as surf bores. The sediment at the field site was medium‐grained sand, and observations of bedform geometry were conducted using a pencilbeam‐sonar system. Using frequency‐filtering techniques, the measured stresses were partitioned into terms representing turbulent (Reynolds) stress, stresses due to gravity and infragravity‐scale oscillatory motions, and wave‐turbulence‐mean current cross‐terms. Gravity wave‐orbital scale motions contributed the largest fraction of the stresses, comprising 24% on average, followed by long‐wave advection of vertical orbital motion (16%). The presence of wave orbital‐scale motions near or at the water/sediment interface was likely due to the porous nature of the seabed, facilitating interfacial flow. Shear stresses did not scale with bed roughness but exhibited a linear relationship with the relative wave height. This indicates that for the experimental conditions, surf zone processes overwhelmed bed roughness effects on shear stress and friction. Calculations of the wave friction factor, fw, showed that in a natural surf zone, this was a factor 3–4 larger than conventional predictions. © 2020 John Wiley & Sons, Ltd.

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