Identification of breaking events from the responses of a data buoy

Abstract

Conventional data buoys record wave spectral parameters such as the significant height, peak period, and mean direction. Information about the existence of wave groups and breaking waves is also important for the design of ocean structures. To derive such information from data buoy measurements, understanding of the motion characteristics of data buoys under a variety of wave conditions is essential. In the present study, the motion characteristics of a scale-modelled discus data buoy were tested under the action of deep-water breaking waves of different intensities, covering spilling to steep plunging types. The laboratory generation of breaking waves was based on the constructive wave—wave interaction method. The buoy heave and pitch motions were measured using potentiometers and non-contact motion-capturing cameras. The measured time series of the breaking-wave surface elevation and the resulting buoy motions were analysed by continuous wavelet transformation and phase—time methods. A closer insight into the wavelet phases reveals that the breaking events leave a constant phase variation over a wider range of higher frequencies. Through appropriate filtering of the localized frequencies, the phase—time method could also be adopted to detect breaking events in the time histories of the measured wave elevation and motion response. The details of the model, instrumentation, testing conditions, and analysis are presented and discussed in this paper.