Crossing waves are proving to be more extreme than extreme. These extraordinary deep sea waves, about which little is known, can become four times steeper than was thought possible. This emerges from research by TU Delft and other universities published today is Nature.
Long ago, stories circulated about mysterious monster waves that seemed to appear out of nowhere and even overturned large ships. The mythical nature became history when such a monster wave was first recorded at the Draupner platform in the North Sea.
In 2018, Ton van den Bremer and his colleagues at the Universities of Edinburgh and Oxford managed to mimic this Draupner wave in the lab for the first time ever. This gave them the opportunity to study the wave closely. And that gave unexpected insights.
How do cross waves occur
Under certain sea conditions, waves occur from multiple directions. This can happen where two seas meet, or where wind plots change direction, such as in a hurricane. When waves from two directions converge, a cross wave is created, as long as the directions are far enough apart. The study also shows that the farther apart the directions are, the higher the cross wave can become.
Rolling waves break at a certain limit and then reach their maximum steepness. The study shows that waves with a high directional spread can become as much as eighty percent steeper than this limit before they begin to break. These waves can thus become almost twice as high as -gang waves before they begin to break.
Rolling wave (l) and wave with high directional dispersion (r).
Swelling while breaking
The researchers stumbled upon another unusual phenomenon that breaks with existing theories. And that is unprecedented, according to Van den Bremer: -Once a wave starts breaking, you see a white head forming, and there is normally no turning back. But if a wave with a high directional spread starts breaking, the wave can still continue to grow.-The research shows that these huge waves, during the breaking process, can become twice as steep, which was already twice the original limit. Thus, added together, the waves can become four times steeper than was thought possible.
Damage to offshore structures
The knowledge that waves coming from multiple directions can become as much as four times larger than thought can provide guidance for making offshore structures safer. -The three-dimensionality of waves is often overlooked in the design of offshore wind turbines and other structures. Our findings suggest that this leads to designs that are less reliable-, said Mark McAllister of the University of Oxford, who led the experiments and now works as a senior researcher at Wood Thilsted.
Thanks to the innovation vertical sensors it is possible to make accurate 3D measurements of the waves.
Innovation in 3D measurement method
The insights are due to the development of a 3D measurement method in the FloWave lab. -The usual 2D methods to examine waves were not adequate-, says Van den Bremer. The research group designed a new way to get a 3D image of the waves. Ross Calvert of the University of Edinburgh: -For the first time, we managed to measure wave heights with such high spatial resolution over such a large area. This allowed us to understand many more details of the complex breaking of waves.
The research consortium is a collaboration between the University of Oxford, University of Edinburgh, University of Manchester, University College Dublin, Ecole Normale Supérieure Paris-Saclay and TU Delft. The results were published in Nature on Sept. 18, 2024.
M. L. McAllister, S. Draycott, R. Calvert, T. Davey, F. Dias and T. S. van den Bremer. 2024. Three-dimensional wave breaking. Nature.
