Influence of offshore topography on infragravity period oscillations in Two Rocks Marina, Western Australia

Abstract

Infragravity (IG) period oscillations in harbours and marinas can often lead to interruption in harbour operations due to excessive vessel movements. Field measurements in Two Rocks Marina in south‐west Australia have shown that IG period oscillations were always present and the amplitude of the oscillations was related to incident swell climate and was enhanced during storm events. The marina is fronted by two shallow, shore-parallel, reef systems located ~3.2 and ~4.7km from the shoreline. The area experiences continuous swell and frequent storm systems, particularly during winter months. This paper describes the application of a Boussinesq wave model, validated using field data, to examine: (1) source of the IG waves incident on the marina; and (2) modal characteristics of the IG period oscillations inside the marina. The cross-shore evolution of the IG wave energy was examined using simulations with different contrasting incident wave conditions, which included measured and idealised wave spectra. The model results indicated that free IG waves were generated as the wind/swell waves propagated over the offshore reef systems independent of the external forcing. During stormy sea condition, the IG energy over the primary and secondary reefs increased by a factor ~10 and ~8 respectively, compared to the IG energy at offshore. The IG wave spectrum near the marina entrance did not contain any major energy peaks, and has an almost constant energy distribution across the IG wave frequencies. However, the frequencies similar to the marina’s natural oscillation periods were excited within the marina. The predicted energy distribution maps and water level snapshots inside the marina identified different oscillation modes, which included mode 1 and mode 2 oscillations corresponding to a partially enclosed water body and, zeroth mode corresponding to an open-ended water body. This study showed that in coastal regions characterised by complex offshore topography, IG waves are generated independent of offshore wave conditions, and harbours located in such environment are at risk of IG period oscillations, depending on their geometry.