Field measurements of shear stress and friction in the surf zone

Abstract

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.

Surf Zone Turbulence and Suspended Sediment Dynamics—A Review

“…It is clear, however, that such imposed cut-off frequencies do not uniquely define, for example, the gravity wave and turbulence frequency bands since gravity wave motion may leak into frequencies above the cut-off, and largescale turbulent motions may exist at frequencies below the cut-off such that an overlap region must be expected. However, the cut-off should identify the spectral bandwidths where the respective motions (wave orbital vs. turbulent) are dominant [36]. Alternatively, Bricker and Monismith [37] devised a phase-method operating in the spectral domain that produced satisfactory results in cases when the wave-induced strain field was weaker than the turbulence strain field.…”

“…Even for strongly breaking waves (γ s > 0.67), <k> was maximum at the seabed, thus overwhelming surface-generated turbulence. When wave-driven longshore currents are strong, wave/current interaction in the bottom boundary layer increases bed shear stress relative to situations without longshore currents [36] and may conceivably also lead to <k>-maxima near the seabed. Considering effects of wave breaker type, observations have shown that for spilling breakers and surf bores, the vertical mixing is slower compared to plunging breakers and <k> tends to decrease more rapidly towards the bed [42,78,79], see also Figure 7.…”