Edge Waves on the Sydney Coast

Pressure and current oscillations at periods of 40 s to 17 min observed during storm conditions at two locations separated by 560 m in the alongshore direction in the coastal ocean near Sydney, Australia, indicate the existence of infragravity waves having amplitudes of ∼20 cm and velocities of ∼10 cm s−1. The observed infragravity waves appear to be locally forced by the wind wave envelope through radiation stress, yet the observed alongshore phase differences of the infragravity waves are consistent with those predicted from free edge wave theory for low-mode edge waves travelling northward and the relationship of pressure to velocity at each location is also consistent with free edge wave theory. As a function of time, the infragravity wave spectral energy grows and decays in step with the longer-period wind waves, suggesting a continuous transfer of energy. The infragravity waves appear to contain energy in both directly forced and freely propagating (edge wave) oscillations. The edge waves may be generated either by radiation stress as outlined above, by a resonant triad mechanism, or by a combination of the two. An edited version of this paper was published by AGU. Copyright 1987 American Geophysical Union.Science, Faculty ofEarth and Ocean Sciences, Department ofReviewedFacult