The rogue waves are mysterious and big

Stand on the shore and watch the waves come in, and you’ll notice that most come in almost the same shape. There is a slight difference, but the overwhelming majority is no different from the crowd. With the exception of the most stormy days of the day, they have an almost soothing regularity about them.

Although often, out of the high seas, a rogue comes along the waves. These unusually large waves can strike with surprise and are dangerous even for the largest ships. Research is underway into what these waves create and how they can be identified and tracked ahead of time.

Myth becomes reality

The MS Riverdance was wrecked in 2008 after being hit by a roaring wave off the coast of Blackpool. Attempts to re-float the ship failed, and the ship was eventually wrecked on site. Credit: Susan Noble, CC-BY-SA 2.0

The stories of rogue waves have long been part of maritime folklore and legends. For centuries, sailors have told tales of “walls of water” that would appear on the open sea, shattering ships as huge waves hit them.

They will not be confused with the tsunami. These are usually caused by the displacement of water from volcanoes or earthquakes or similar catastrophes. Tsunamis show more than just a wave of the ocean, only to intensify when they come close to breaking ashore. The roaring waves, in stark contrast, pose a great danger to any ship or platform that stands at high altitudes in the open sea and gets stuck in the path.

Despite many anecdotes of such events, however, mainstream science has been slow to acknowledge the existence of rogue waves. Established models for wave generation and propagation cannot simply account for such behavior. Thus, rogue waves were largely ignored by the scientific literature. A handful of papers and texts saw the idea of ​​strangely large waves, but the idea was seen as little more than a curious myth.

Accurate recording of rogue waves hitting the Dropner platform in 1995 gave rise to a shift in the wider scientific community, shaking up research to better understand rogue waves. Credit: Paolosan, CC-BY-SA-4.0

That all changed in 1995. The North Sea Dropner gas pipeline platform was hit by a rocket wave with a maximum height of 25.6 meters. It was a huge wave, which under normal circumstances was only above the normal significant wave height of 12 meters. The big break was that the Draupner platform was fitted with an accurate laser rangefinder wave recorder, which accurately records the wave height when it passes. Fortunately, the platform has suffered only minor damage, but the captured data has shaken oceanography over the next few years.

Draupner changes wave measurements entirely to scientific consensus; Now there was irrational evidence that the rogue wave really did exist. The study began in earnest, with scientists quickly realizing that rogue waves fell outside the typical Gaussian model used to predict wave height and activity.

In the early 2000’s, scientists determined that rogue waves were not obscure, nor did they occur 1-10,000 years ago. By 2004, the European Space Agency’s satellites were being used to detect rogue waves at sea. After surveying parts of the South Atlantic for just three weeks, researchers found ten rogue waves at a height of 25 meters.

In a relatively short period of time, science has gone from disbelieving in rogue waves to finding them regularly all over the world. This has raised serious concerns, especially in the world of ship design. Modern merchant ships have traditionally been designed to withstand wavelengths of up to 11 meters. Now, it is better understood that waves of more than 20 meters are not unexpected and waves up to 30 meters are possible. This leads to an increase in the minimum wave height of ships that are designed to withstand ships, giving merchant ships a greater chance of surviving in a face-to-face fight with rogue waves.

Hunting for an answer

The modern definition of a rogue wave is a wave that is more than twice the significant wave height. Significant wave heights are determined by taking the average of the top third of the waves in a given ocean state. Thus, a rogue wave is twice as high as other long waves in a given area.

Scientists hope to better understand what causes rogue waves. Instead of a single factor, current research suggests that different factors may play different roles in generating a rogue wave.

An imitation of the Dropner wave performed in 2019. In the first row, the waves meet at an angle of 0,, while in rows 2 and 3 they meet at 60 এবং and 120, respectively. Notice a significant increase in altitude in the third row, where the researchers found that a vertical jet of water rapidly increased the wave height. Credit: McAllister et al 2019, CC-BY-4.0

Constructive intervention is the easiest way to form a rogue wave. It is here that the individual waves traveling through the water meet and merge, where their peaks and valleys line up and the waves interfere structurally to become larger than before. Simulations suggest that the angles at which these wave trains meet play an important role. A simulation shows that the combination of waves at an angle of 120 degrees produces steep rogue waves.

The interaction between wave swells and ocean currents is also suspected to be a way to form rogue waves. The process is thought to have taken place off the coast of South Africa, where large tidal waves are formed in areas of strong Agulhas currents. Where a swell travels in the opposite direction of an ocean current, the current is thought to have a focusing effect on the waves. The current slows down the front of the wave, pushes the subsequent waves closer, and increases the overall wave height.

Other explanations include non-linear phenomena, where a wave in a series of waves can absorb energy from its surroundings and grow on its own. This phenomenon has been successfully demonstrated in laboratory water tank tests.

Research is underway to better predict and understand these wave events. A recent research paper on trying to observe rogue waves in the field aims to explore these effective effects. A wave boy with inertia measurement system and differential GPS was used for high precision to measure a rogue wave off the coast of Canada from 2020 to 2021.

The survey was able to capture data from a rogue wave of 17.6 m trough-to-crest height, in a sea state where the comparatively significant wave height was only 6.05 m. With a height ratio of 2.9 and a crest height of 1.98 times higher than the significant wave height, the authors of the study believe that this is the largest common wave height ever recorded. Significantly, the team found no evidence of non-linear effects on the generation of rogue waves.

It will probably take many years of further research and modeling before the rogue wave phenomenon is truly understood. Efforts can only be expected with a better understanding, with more seafarers and researchers being aware of rogue waves and thus better able to capture data about when and where they will occur.

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