INTERACTION BETWEEN BREAKING/BROKEN WAVES AND INFRAGRAVITY-SCALE PHENOMENA TO CONTROL SEDIMENT SUSPENSION TRANSPORT IN THE SURF ZONE

Abstract

Time series of near-bed sediment, water-level and velocity were measured in the field and laboratory and analysed to investigate (a) the role of breaking/broken waves, (b) the predominance of infragravity-scale transport processes in the surf zone, and (c) the influence of infragravity processes in creating the right conditions for suspension by breaking/broken waves. Instantaneous suspended sediment concentrations were measured using optical backscatter sensors, which were found to be sensitive to sediment size, cohesive sediment in suspension and bubbles. A simple numerical filter was applied to remove the erroneous bubble signal. Suspension was found to be correlated with wave breaker bores and an associated increase in high-frequency velocities, the latter deemed to be caused by surface-generated turbulence. Infragravity-scale sediment transport flux was found to be more significant than incident-scale flux in the transition and inner surf zones. Sediment suspension was further found to be related to the onset of lower water-levels associated with infragravity wave action, which corresponded with a predominance of breaking/broken waves. These breaking/broken waves (which are induced by the low water-level) are deemed to be the cause of this suspension.