Prediction of Coastal Far Infragravity Waves from Sea-Swell Spectra

Abstract

Far infragravity waves (FIGWs) with periods of 2–64 min can reduce the underkeel clearance for vessels in transit to ports and cause resonance in and around harbors and bays that amplifies low-frequency current flows. Sea level records from six tide gauges along the east coast of New Zealand and the Chatham Islands are compared with offshore weather systems and spectral statistics from the gravity wave (GW) band (including both sea and swell waves) from wave buoys and hindcast sources. An empirical formula to predict coastal FIGWs is developed, based on the offshore GW significant wave height, associated peak period, and spectral width parameter. This formula is able to explain the occurrence of most FIGW events, confirming that most of the FIGW energy arriving at the coast is directly associated with GW energy. However, several discrete events were not predicted by the technique, and reverse ray tracing was applied to explore the origin of these events. In the light of these findings, previously reported long wave events on the coast of New Zealand are reexamined using the newly developed long wave predictor.