Observations of infragravity period oscillations in a small marina

Abstract

We measured water levels in Two Rocks Marina, Western Australia, to investigate infragravity-period (25–300s) oscillations and their forcing mechanisms. Spectral analyses identified four dominant oscillations in the infragravity band, which were generated through excitation of the marina’s natural periods. The oscillations were present at all times, independent of the offshore conditions, indicating that they were forced by a continuous external energy source. The spectral energy of the oscillations increased by ~50 times during storm events (higher incident wave heights), in comparison to calm events (lower incident wave heights). Wave heights of oscillations within the marina were strongly correlated with offshore incident swell wave heights and reached maximum of 0.5m. The groupiness factor of swell waves around the marina was 0.6–0.85. Bound infragravity waves associated with swell wave groups were identified as potential forcing mechanism of infragravity-period oscillations within the marina. The bound infragravity waves have broad frequency spectrum without dominant periods matched the marina’s natural periods however, bound infragravity waves of periods in the proximity of the marina NOPs were adequate to generate oscillations at the NOPs of the marina. Frequencies of the oscillations were independent of the forcing frequency, and determined by the marina׳s geometry.