Surface-Induced Turbulence and Resulting Sand Suspension Beneath Breaking Waves

Abstract:

Breaking waves and bores inject large amounts of turbulence into the water column as vortices, which can travel downward and entrain sand from the bed. Coastal evolution models rarely include the effect of this surface-induced turbulence on sand suspension and subsequent transport to predict surf-zone morphodynamics. Here, we analyze turbulence and suspension measurements beneath non-breaking waves and plunging breakers, collected during the field-scale BARDEXII laboratory experiment using a vertical array of 3 ADVs and 7 OBSs. The array was positioned at a single cross-shore location, but, because of changes in wave conditions and water levels, experienced different degrees of wave breaking. Results show a phase-coupling for both turbulence kinetic energy and sand concentration with the short-wave orbital motion during all conditions, with the highest values when the cross-shore velocity is onshore directed. The vertical turbulence flux under plunging breakers also depends on wave phase, with a downward and upward flux during offshore and onshore directed wave orbital motion, respectively. The plunging jet hits the water surface in the wave trough, resulting in a downward turbulence flux during the offshore directed wave orbital motion. The upward flux during the onshore directed wave orbital motion might represent the injected air bubbles rising to the water surface. This upward flux coincides with the peak in suspension, which, accordingly, reaches higher in the water column than beneath non-breaking waves. Besides a phase-coupling with the short-wave orbital motion, turbulence kinetic energy and sand concentration were also modulated on an infragravity timescale, with high values during the offshore directed infragravity flow. The effect of surface-generated turbulence on the direction and magnitude of short- and infragravity-induced cross-shore sand fluxes will also be discussed. This research is supported by the Dutch Technology Foundation STW, which is part of the Netherlands Organisation for Scientific Research (NWO).