Evaluation of the 3D response and performance of an operational wave buoy for coastal wave monitoring

This study analyzes the in situ 3D response and performance of a large, moored, accelerometer-based wave buoy designed for the new Italian buoy network. Deployed in coastal waters near an oceanographic tower in the North Adriatic Sea, the buoy's capability to follow wave motion was evaluated against a stereo-imaging system and a wave gauge. By employing a novel optical system based on stereo view, the 3D buoy displacement (vertical and horizontal motions) was measured, providing insights into buoy dynamics and enabling comparison with the onboard wave sensor. Measurements indicate good agreement between buoy heave and wave elevation in a narrow frequency band of 0.15–0.60 Hz, except around 0.52 Hz, which represents the buoy's natural frequency. Performance degradation at low and high frequencies was attributed to sensor processing as well as buoy and mooring responses, which were addressed by calibrating an empirical response operator for the buoy heave. Furthermore, a phase delay in horizontal motion was observed, likely caused by buoy inertia. For wave statistics, while the buoy effectively captures linear waveforms, it shows reduced sensitivity to second-order nonlinear effects due to its dynamic response, impacting the ability to observe maximum wave elevations. Despite these limitations, buoy data reliably estimate significant wave height and mean wave direction. This study enhances the understanding of moored buoy dynamics and aids in optimising buoy response for coastal wave monitoring.