Wave boundary layer hydrodynamics and sheet flow properties under large-scale plunging-type breaking waves

Abstract

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.