Field measurements of shear stress and friction in the surf zone. Issue 2 (17th November 2020)

• Record Type: Journal Article
• Authors: Aagaard, Troels; Christensen, Drude F.; Hughes, Michael G.
• Is Part Of: Earth surface processes and landforms. Volume 46:Issue 2(2021)
• Page Start: 385
• Page End: 398
• Publication Date: 2020-11-17
• Subjects: coastal processes -- surf zone -- bottom boundary layer -- sediment transport -- field measurements; Geomorphology -- Periodicals; 551.4
• DOI: 10.1002/esp.5031
• Languages: English
• ISSNs: 0197-9337

Abstract

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, f w, showed that in a natural surf zone, this was a factor 3–4 larger than conventional predictions. © 2020 John Wiley & Sons, Ltd. Abstract : Measurements of shear stress and bed friction were undertaken in the surf zone of Durras Beach, NSW, Australia. The bed shear stresses and estimated friction factors were significantly larger than for idealized laboratory environments. These differences involve wave–current interaction, wave breaking, a sloping seabed, and probably, ventilated boundary layers.