IMPACT OF SEA-BREEZE ACTIVITY ON NEARSHORE AND FORESHORE PROCESSES INSOUTHWESTERN AUSTRALIA

In coastal regions sheltered from the direct impact of swell-and storm -wave activity, locally generated wind waves, particularly those assoc iated with strong sea-breeze activity, play a dominant role in control ling nearshore and foreshore processes. Field data collected from the Perth Metropolitan Coast (western Australia) during a typical summer s ea-breeze cycle, are presented. It is demonstrated that the nearshore environment responds rapidly to an increase in wind speed (up to 12 m s(-1)) during the sea breeze, resulting in considerable changes to the nearshore hydrodynamics and morphology. Incident wave energy increase d during the sea breeze and was associated with development of a wind- wave field with significant wave heights up to 0.9 m. Nearshore curren ts responded to this change in wave climate with the development of ne t offshore near-bed currents and a rapid increase in the mean longshor e current from <0.05 m s(-1) to 1.0 m s(-1). A 10-fold increase in sus pended sediment concentration and a 100-fold increase in the longshore sand transport resulted from the effects of the sea-breeze system. Er osion of the beachface was coincident with the development of the wind -wave held. Sea breeze wave-driven water circulation also completely e roded beach cusps (wavelength 20-30 m), overwhelmed the rip current sy stem associated with the beach cusps and suppressed the infra-gravity wave frequencies in the incident wave and swash record. The beach cusp s reformed after the cessation of the sea breeze. It is demonstrated t hat the beachface is in a constant stage of adjustment to the incident wave energy through the diurnal sea-breeze cycle alternating between dissipative and reflective morphodynamic regimes. The results may be u sed to determine the impact of a medium-sized storm on the beachface. It is clear that the sea-breeze system plays a major role in controlli ng the nearshore and foreshore processes not only in this region, but also on other geographic locations where strong sea breezes are presen t. (C) 1997 Elsevier Science Ltd. All rights reserved.