The velocity field underneath linear and nonlinear breaking rogue waves

During the past decades, a large number of waves of extreme height and abnormal shape, also known as freak or rogue waves, have been recorded in the ocean. Velocities and related forces can be enormous and jeopardise the safety of marine structures. Here, we present an experimental study devoted to investigate the velocity field underneath a breaking rogue wave. The latter is replicated in the laboratory by means of dispersive focussing methods such as the New Wave Theory and nonlinear focussing techniques based on the Nonlinear Schrödinger equation. While the former is basically a liner method, the nonlinear focussing fully accounts for the dynamical evolution of the wave field. Experiments were carried out in the Extreme Air-Sea Interaction flume of the University of Melbourne using a Particle Image Velocimetry (PIV) system to measure the velocity field below the water surface. Measurements show that the mechanism of generation affects the shape of the breaking waves as well as the kinematic field and associated hydrodynamic forces. Particularly, the New Wave Theory leads to higher velocities and a more energetic breaker than the nonlinear focussing.