For sailors and swimmers, sea waves are something to be alert to and even fear. They are challenges to celebrate for surfers. But when do waves break? Predicting this phenomenon can lead to advances in applications, including the safety and efficiency of maritime navigation and structures, renewable energy harvesting, climate research, and more.
Researchers from the Faculty of Civil and Environmental Engineering at the Technion-Israel Institute of Technology in Haifa and the University of Melbourne in Australia dealt with this question in an article just published in Physics Fluids under the title “Wave breaking probabilities under wind forcing in open sea and laboratory.”
Technion Prof. Dan Liberzon, marine engineering doctoral Sagi Knobler, and Melbourne doctoral student Ewelina Winiarska, in collaboration with Prof. Alexander Babanin, predict that the study’s findings will help improve our understanding of the dynamics of wave breaking.
Liberzon is a member of the Technion’s Interfaculty Program for Marine Engineering and head of the Technion Sea-Air Interactions Research Laboratory (T-SAIL), which focuses on various aspects of waves evolution under the wind, sea-currents, and atmosphere-sea interactions.
Marine engineer Babanin is a member of his university’s department of infrastructure engineering and a researcher of sea waves and currents.
One of the currently accepted paradigms of the study of waves is that they break when it reaches a threshold steepness at which the wave can no longer maintain its form and collapses. But the team’s findings show that this approach is wrong and that there is no absolute threshold steepness beyond which any wave is doomed to break. They reached this conclusion by developing a new method for the accurate detection of breaking waves based on data collected in a series of observations and experiments in the Black Sea and in the 17.4-meter-long wind-wave flume at the Technion Sea-Air Interactions Research Laboratory (T-SAIL) headed by Liberzon. Flumes are an accurate and effective way to measure flow rate in open channel flow applications. They work by measuring how much the water rises in level before an obstruction of known dimensions and shape.
The breaking of sea waves is one of the most intricate scientific problems in fluid mechanics,” explained Liberzon. “No one doubts there is a connection between the steepness of the wave and the inception of breaking, but we show that the picture is more complex, making it impossible to predict the breaking of the wave according to its steepness alone.”
The breaking, he continued, depends on many complex parameters – the intensity of the wind blowing over the waves, the speed of the wave peak propagation, and other factors. During such complex evolution of the wave, it becomes highly asymmetric both horizontally and vertically. The collapse of the wave begins with the formation of a ‘bump’ at the front side of the wave, from which, depending on the combination of many of the factors previously mentioned, the wave breaks either intensely or gently.
In the current study, the scientists produced detailed statistics of many features for breaking and non-breaking waves using combined experimental data both from the laboratory wind-wave flume at the Technion and waves data from the Black Sea. These detailed statistics will serve as the basis for forecasting which waves will break and when.