Observations of infra-gravity period oscillations and their forcing in Western Australia

Strong periodic horizontal water motions, known as oscillations or seiching are identified within Two Rocks Marina, Western Australia, resulting in problematic berthing conditions for moored vessels. It has been suggested that these oscillations are created by infragravity (IG) waves, generated through the mechanism of wave grouping. Furthermore it has been also observed that offshore swell waves can directly contribute for generating IG frequencies inside the marina. To investigate this situation, a numerical study has been carried out using a time-domain Boussinesq type model to simulate the IG waves inside the marina. The model was forced by internally generated swell wave spectra along the offshore boundaries. Having excited the marina by wave forcing, the model was allowed to run with free waves which created a resonance condition in the marina. Results indicated an increase in IG frequency wave energy in the marina. This suggests that IG period oscillations can be generated by offshore swell waves. Natural oscillations of the marina along longitudinal and latitudinal directions can also be identified according to the peak wave energy frequencies and direction of currents. It was found that natural oscillations along longitudinal and latitudinal directions for the marina basin are 208 seconds and 137 seconds respectively.