An Experimental Study of Wave Breaking in Crossing Seas

Breaking poses an upper limit to how large an individual wave can become and is the main mechanism of dissipation of wave energy in the ocean. Understanding how and when waves break is essential for forecasting extreme waves and predicting the resulting loads they exert on fixed structures and floating bodies. When modelling and forecasting extreme wave heights, to predict when a wave may break, parametric wave breaking criteria are currently used. These criteria use properties such as the steepness or the ratio of the fluid speed to crest speed to determine whether a wave will break. Current state-of-the-art wave breaking criteria are capable of predicting when waves will break, when the waves travel in a single mean direction (`following-seas'). In the oceans, it is common to have remotely generated `swell waves', in combination with locally- generated wind-waves that travel in the direction of the wind, resulting in crossing when the two are not aligned. In such `crossing-seas', state-of-art breaking criteria become challenging to apply and may lose their predictive power. We present a series of experiments in which we create crossing and highly directionally spread breaking waves. We examine the effects of crossing and high degrees of spreading on the onset and intensity of wave breaking. Our results show that the onset of breaking, is strongly influenced by directional spreading (both spreading width and crossing angle). As the degree of directional spreading, or crossing angle increases there is a sharp rise in the amplitude at which the onset of wave breaking occurs.