Wave Boundary Layer (WBL) dynamics are measured with an Acoustic Concentration & 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 behaviours in terms of wave shape, intra-wave 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. Intra-wave concentration dynamics reveal the existence of a lower pick-up 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 3-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.
Bibliographical noteThe authors wish to thank the staff of CIEMLAB, in particular Joaquim Sospedra, Oscar Galego and Ricardo Torres, for their hospitality and hard work during the experimental campaign. This research was funded by the European Community’s Horizon 2020 Programme through the Integrated Infrastructure Initiative HYDRALAB+ COMPLEX (no. 654110), the French DGA funded ANR Astrid Maturation project MESURE (no. ANR-16- ASMA-0005-01) and the SINBAD project funded by STW (12058) in the Netherlands and by EPSRC (EP/J00507X/1, EPJ005541/1) in the UK. Data of this study are available at U. Twente repository doi (10.4121/uuid:753f1d84-36e5-47fa-b74b-55c288545b9b). Comments and remarks raised by the two anonymous reviewers helped to improve the quality of the paper.
- wave breaking
- sheet flow
- coastal sediment transport
- wave hydrodynamics