Research unravelling the "mystery" of extreme waves
PhD candidate Yuchen He looking out at a rough swell hitting Bondi Beach. Image: Louise Cooper, University of Sydney
Research published in 聽by University of Sydney and Kyoto University researchers has found that extreme or 鈥榬ogue鈥� ocean waves 鈥� colossal walls of water that seemingly appear at random and have the potential to destroy marine infrastructure, carve out coastlines and even threaten life 鈥� could be more frequent in multi-directional wave formations than previously expected.
These enormous 鈥渞ogue鈥� swells have long captured the imagination. Although mathematical modelling has helped scientists and engineers hypothesise different causes, since the 1980s it was thought they were caused by either wave overlap (the superposition principle) or a mechanism which triggers a rogue wave (modulation instability) in one-directional wave systems. In multi-directional systems, known as crossing wave formations or crossing seas, modulation instability聽was believed to be inactive, rendering extreme waves rare or impossible in these conditions.
Catching a rogue wave
To recreate the sea鈥檚 complex conditions, researchers conducted experimental demonstrations in a 30-metre-long wave tank located in the University鈥檚聽School of Civil Engineering.
A mechanical wave generator was employed to create multi-directional waves on the water鈥檚 surface, ultimately resulting in unstable rogue 鈥渂reathers鈥� 鈥� waves that suddenly grow and disappear without any disturbance to the surrounding water.
The research is performed by conducting modelling and experimental demonstrations using the University of Sydney's wave tank. Image: Louise Cooper, University of Sydney
鈥淲hat our physical demonstration has shown us is that multi-directional systems can also rise to extreme waves from modulation instability in the form of breathers even when another wave system is at play,鈥� said Adjunct Professor of Civil Engineering Amin Chabchoub.
鈥淯ntil now, this had not been demonstrated,"聽said lead researcher and PhD student Yuchen He, who is due to complete his PhD at the end of the year.
"The scientific wave community believed that these rogue waves could only be generated in one-directional wave systems 鈥� instead thinking that multi-directional wave formations might cancel out these crucial mechanisms."
鈥淚f you鈥檙e in a boat and get hit by a 25 to 30-metre wave, it鈥檒l most likely be 鈥榞ame over鈥�."
The experimental demonstration showed that crossing water waves, which frontally collide with a crossing angle of 180掳 and cause 鈥榮tanding waves鈥�, also allow breather waves to rise.
Caused most commonly during storms, the researchers caution that extreme waves may pose an increasing risk in the wake of climate change.
Extreme waves can be dangerous to boats and ships. Image: Louise Cooper, University of Sydney
鈥淚f you鈥檙e in a boat and get hit by a 25 to 30-metre wave, it鈥檒l most likely聽 be 鈥榞ame over鈥�. Unfortunately, because of climate change, in the future we will see an increase in storm intensities, with more frequent cases of extreme waves,鈥� said Professor Chabchoub.聽
Mr He said: 鈥淯ltimately, the more we unlock the mystery that surrounds how rogue waves appear from flat, calm, or complex seas, we鈥檙e creating a better understanding of when and where these giants of the sea are likely to appear. Such findings are also crucial in developing a reliable forecast tool in offshore, but also coastal areas. Ultimately, this will benefit maritime navigation, and on and offshore coastal operations.鈥�
DISCLOSURE
The researchers have no conflicts of interest to declare.聽The study was funded by the RFBR and the Laboratory of Dynamical Systems and Applications NRU HSE.