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

摘要

A field experiment was conducted on a macrotidalbeach in northwestern Australia to investigate the relation betweensediment resuspension and sea bed morphology. Continuousmeasurements of waves, currents and suspended sedimentconcentrations were carried out over a period of several hoursaround high tide, concurrent with half-hourly visual observationsof the sea bed morphology. The incident waves had a significantwave height of 0.35m and period 11s. Most of the data werecollected outside the surf zone where sediment resuspension wasmainly by wave groups, and suspended sediment transport wasprimarily accomplished by mean longshore and cross-shorecurrents. The sediment resuspension process was furthercharacterised by a pronounced tidal asymmetry with suspendedsediment concentrations and transport rates during the falling tidesignificantly larger than during the rising tide. The asymmetry isatributed to the combined effects of a larger bed roughness andstronger mean nearshore flows during the falling tide. The increasein bed roughness occurred during the rising tide under theinfluence of decreasing bed shear stresses when post-vortex ripplemorphology developed on a previously plane bed. Around hightide, the ripples were fully developed and had attained a ripplemorphology developed on a previously plane bed. Around hightide, the ripples were fully developed and had attained a rippleheight of approximately 0.005m and a ripple length of 0.06-0.08m.application of the Van Rijn [Van Rijn, L.C., 1989. HandbookSediment Transport by Currents and Waves. Delft Hydraulics,Report H461] sediment transport model further confirmed thatenhanced sediment resuspension during the falling tide wasprimarily the results of the development of ripple morphology,with increased mean current strengths playing a secondary role.The results have major implications for modelling suspendedsediment transport on macrotidal beaches.