Tidal asymmetry in sediment resuspension on a macrotidal beach in northwestern Australia

Abstract

A field experiment was conducted on a macrotidal beach in northwestern Australia to investigate the relation between sediment resuspension and sea bed morphology. Continuous measurements of waves, currents and suspended sediment concentrations were carried out over a period of several hours around high tide, concurrent with half-hourly visual observations of the sea bed morphology. The incident waves had a significant wave height of 0.35 m and period 11 s. Most of the data were collected outside the surf zone where sediment resuspension was mainly by wave groups, and suspended sediment transport was primarily accomplished by mean longshore and cross-shore currents. The sediment resuspension process was further characterised by a pronounced tidal asymmetry with suspended sediment concentrations and transport rates during the falling tide significantly larger than during the rising tide. The asymmetry is attributed to the combined effects of a larger bed roughness and stronger mean nearshore flows during the falling tide. The increase in bed roughness occurred during the rising tide under the influence of decreasing bed shear stresses when post-vortex ripple morphology developed on a previously plane bed. Around high tide, the ripples were fully developed and had attained a ripple height of approximately 0.005 m and a ripple length of 0.06–0.08 m. Application of the Van Rijn [Van Rijn, L.C., 1989. Handbook Sediment Transport by Currents and Waves. Delft Hydraulics, Report H461] sediment transport model further confirmed that enhanced sediment resuspension during the falling tide was primarily the result of the development of ripple morphology, with increased mean current strengths playing a secondary role. The results have major implications for modelling suspended sediment transport on macrotidal beaches.