An experimental study on the effect of wind on the generation of freak waves

Freak wave generation can be explained on the basis of spatial and spatio-temporal concept (Kharif and Pelinovsky, 2003). However the authors pointed out that the role of atmospheric forcing on the generation of these waves have been poorly considered in the past. The paper reports on a series of experiments conducted in the large windwave tank at Marseille-Luminy to investigate the effect of wind on the generation of freak waves. To this end, a PC well controlled submerged wave-maker located under the upstream beach allowed to generate a wave group with frequencies decreasing linearly from 1.3Hz at the front of the group to 0.8Hz at the back of the group. A wave gauge was located at the entrance of the test section and a second one was moved all along the tank at different fetches to determine the spatial evolution of the group. As predicted by the theory, without wind, the waves focused at a precise fetch leading to the occurence of a high amplitude freak wave. Downstream of the point of focusing, the amplitude of the group decreases rapidly and we observed a defocusing of the group. Then the experiments were again conducted with the same conditions for the mechanically generated group (same frequencies and same amplitude) but with a light wind of 4m/s. Surprisingly, we observed a freak wave of large amplitude, occuring at a longer fetch than observed without wind. Moreover, contrarely to the case without wind, the freak wave maintains its coherency and its amplitude as it propagates downstream the previous focusing point. This process cannot presently be explained within the framework of the Miles' theory and then it is needed to improve the theory to take into account of the presence of wind, even for light winds