Unique water tank allows scientists to replicate rogue ocean waves

A wave on the Southern Ocean. Photo: Assoc Prof Alessandro Toffoli

By Gilian Aeria

An international team of scientists have demonstrated how ocean winds can generate spontaneous rogue waves. This is the first step to predicting the potentially dangerous phenomena.

Rogue or freak waves are extremely large, steep waves appearing in deep ocean, surging without warning and seemingly at random. They can cause severe damage to ships and structures like oil and gas rigs, but little is understood about how they form.

Lead researcher Associate Professor Alessandro Toffoli said, “Rogue waves were believed to be a sailor’s myth for centuries. However, we now have evidence that rogue waves are causing marine accidents more and more often so the ability to forecast them would be hugely beneficial.”

Assoc Prof Toffoli was recently on an Antarctic Circumnavigating Expedition (ACE) studying extreme wave conditions. In the Southern Ocean, strong winds like the notorious Roaring Forties and Furious Fifties blow around the Antarctic continent, originating the fiercest and most extreme waves on the planet.

“We were able to replicate wave conditions similar to those in the Southern Ocean in the ring-shaped tank because its shape allowed waves to circulate around infinitely – in an ‘unlimited fetch’ condition,” says Assoc Prof Toffoli.

Researchers from the University of Melbourne, Swinburne University of Technology, University of Leuven (Belgium) and University of East Anglia (UK) used a special ring-shaped wave tank called an annular flume at the University of Turin (Italy) to study the statistical properties of wind-generated waves, and therefore the likelihood of rogue wave development.

Previous experiments on wind-generated waves were conducted in conventional longitudinal tanks that were not long enough for rogue wave forms to develop. However, the peculiar shape of this facility allows wind to blow and waves to propagate over an endless distance.

Researchers started with still water in the tank before turning on fans to replicate the steady wind found blowing across the ocean. Wind was blown over the surface for two hours, and the surface elevation of the water measured throughout.

As wind started blowing, an erratic wave field was generated. Rogue waves were allowed to grow and fully mature, and were detected just before the wave height reached a stationary condition.

These measurements allowed the researchers to estimate the probability of finding high, steep waves, which were found to occur more frequently than expected. This provided crucial information about the mathematical likelihood of such eaves occurring.

“Previous estimates put rogue waves occurring once every thirteen hours, but at a certain time during the wave evolution, we found the frequency to be as high as once every hour. With storms lasting anywhere between three to six hours, we could expect to see at least three rogue waves form,” says Assoc Prof Toffoli.

“The next step is to develop a model that demonstrates our experimental observations which would allow us to predict these extreme events.”

The experiment took place at the TurLab facility of the Physics Department of the University of Turin (Italy). The experimental campaign was supported by the European Union through the European High Performance Infrastructures in Turbulence (EuHIT) consortium.