Rogue Waves [47]

Ooh Aah Big Motion In The Ocean

By Richard Birdsey

Intrigued by an article on rogue waves in a recent New Scientist magazine I found out a bit more about the myths, science and reality of this awesome ocean phenomenon. Stories of giant waves taking ships and their crews have been around since ancient times. Many of these legends are being revealed as fact as modern oceanography, marine engineering and complex computer models build up a more realistic picture of these mountains of water.

We often hear about the results (sometimes tragic) of rogue or ‘freak’ natural events but what exactly is a ‘rogue’ wave?

‘Rogue’ is a generic term given to an unusually large wave appearing in a smaller set of waves. Trip reports often talk about the biggest wave (or set of waves) seen that day arriving during a beach launch or exit, damaging boats and threatening life and limb. These waves could be called ‘rogues’. However, the waves I’m talking about are truly monsters. Wave heights (trough to crest) of 17 metres to heights over 30 metres (11 storeys high) are common. When you consider these monster waves may be interspersed among a background of 5 to 7 metre high waves you can start to understand the forces at play. Mariners have accurately measured a few rogue waves, usually by watch officers triangulating wave crests against parts of the vessel. Marine radar, satellite instruments and wave buoys now provide most of the information on rogue waves.

Some of the characteristics of rogue waves are:

  • they are greater than twice the size of the ‘significant wave heights’ of surrounding waves,
  • they are often deep water waves,
  • they may be associated with a very deep trough and other uncommonly large waves moving in a set or ‘train’,
  • they often come unexpectedly from directions other than prevailing wind and waves,
  • they probably last only a short time or distance (minutes or a few hundred metres), and
  • they are unpredictable – though they do occur more frequently in some places in the world.

So How do These Monster Waves Form?

There are a number of factors that generate waves. Underwater seismic movements and other natural phenomena can generate huge waves (called tsunamis), but most waves are generated by wind. Atmospheric variations in air pressure force air down, displacing surface water. As the wind moves laterally across the surface of the water along a pressure gradient it drags or pushes the water with it. These two air movements, vertical and lateral (or ‘shearing’) dump energy into the water. The particles of water don’t actually move much, but the wind-generated energy is transmitted through the water, sometimes at many hundreds of kilometres an hour. As wave height is determined by wind speed, wind duration and fetch (the distance the wind blows uninterrupted over the sea surface) it would be logical to assume that a big wind blowing constantly over a big stretch of water (say the Pacific Ocean) would produce monster waves. Generally in open waters a wave 1.86 times the significant wave height can be expected every 1,000 waves or so. But any resulting big waves would be toppled by winds at about 70 knots and 100 knots of wind would flatten them, so a train of rogue-sized waves couldn’t form. Wave physics is a vastly complex area and I’m not going to weigh into it here.

Some different types of waves are outlined below. “Significant wave height” is the average height of the highest one-third of waves measured by an observer over a period of time.

Deep-water waves (or ‘short waves’)
waves whose length (measured from crest-to-crest or trough-to-trough) is less than the water depth. These waves include wind-generated waves travelling across the open ocean.
Shallow-water waves (or ‘long waves’)
where the length of the wave is greater than the depth of the water. These are wind generated waves that move into shallow coastal waters, tsunamis, and tide waves generated by the interaction of the sun and moons’ gravitational fields.
Capillary waves
rounded and V-shaped wind-generated waves smoothed out and destroyed by the ocean’s surface tension. They have wavelengths less than 11.7 cm.
Sea waves
ocean waves driven to their maximum height by the wind. As the waves move away from the area they are generated in they smooth out into longer (‘swell’) waves.
Tsunamis
are waves generated by underwater seismic movements and shallow-water waves with wave lengths of 160 km or more. Comes from a Japanese word tsu meaning ‘harbour’ and nami meaning ‘wave’. They travel at 500 km/h and are vastly destructive when they come ashore.

The answer probably lies in a complex interaction between wind, current and topography of the seabed. Mechanisms that could generate rogue waves include:

  • Constructive interference. Waves move from their point of generation in sets or ‘trains’. Constructive interference suggests that several different wave trains travelling roughly in the same direction meet at some point and build on top of each other. The energy in the wave trains builds and adds on to the other resulting in a set of large waves, and one huge wave, embedded in the train. This wave will only last a short time as the different trains disentangle themselves and move in their own direction.
  • Focusing of wave energy. As kayakers know, when strong wind-generated waves run into a current going in the opposite direction then dangerous standing waves can form. A rising seabed will further concentrate energy in the currents. This hypothesis suggests that the energy contained in the waves smashing into the counter current can build and accumulate over time, forming huge waves. These waves are thought to be longer-lived than those developed by the constructive interference mechanism.
  • Normal wave height distribution. Wave heights are distributed (like most things) along a bell-shaped curve. Some waves are tiny (occurring at one end of the curve), most occur in the middle of the ‘bell’ and some extremely large waves are generated at the other end of the bell. At sea, most of the waves you encounter are in the middle of the bell (i.e. these are the most common waves) and there is a very low probability of meeting an extremely large wave. But if you do – tough luck. This is the ‘wrong place at the wrong time’ principle.

Whichever of these mechanisms is true (they probably all are in different situations), it is obvious that big seas, big winds and strong currents all are factors in generating monster waves. These factors determine why rogue waves are more commonly associated with some parts of the world. It is of little surprise that these ‘hot-spots’ are some of the most dangerous waterways known. The Agulhas current off the tip of South Africa, the Kuro Shio current off Japan and the Gulf Stream are places where deep ocean paddles are definitely not recommended.

However, the New Scientist article points out two problems with this picture. Firstly, rogue waves are commonly found in places such as the North Sea where there are relatively few fast flowing currents and constructive interference can’t entirely explain their frequency. The second and more pressing problem is that rogue waves appear to be much more common than the bell shaped curve suggests. Complex computer models that attempt to simulate wave patterns also predict that monster waves should be extremely rare. The problem is that studies observing real-life wave patterns (such as those using radar) show that monster waves can occur in some places as frequently as one per week.

Scientists are realising that rogue waves are not as predictable as first thought. Many of the computer models assume linear and predictable outcomes from variables (wind velocity, sea state, wind direction, etc) fed into the model’s algorithms. Scientists now think the sea is more ‘chaotic’ and that chaos theory needs to be introduced into these models. You will probably have heard the chaos adage about how the flap of a butterfly’s wing in Brazil causes a hurricane in Canada. Similar principles are being applied to wave models (e.g. a puff of wind off Cape Horn causes a rogue wave in Japan). How these theories work is far beyond the scope of this article and my ability to explain them. More interesting is how this research is being applied to make the sea a safer place to live, work and play.

The impact these waves have on maritime commerce and industry is huge. Inquiries into maritime disasters are increasingly looking at the possible involvement of rogue waves. Analysis of a number of ship sinkings suggests rogue waves may rip off the ship’s hatches, causing fatal down flooding into the main hold, which then rolls or pitch poles the ship or breaks its back. In any case the wave would come from nowhere and the end would be violent and fast.

Research is being applied to avoid these disasters on three main fronts. First, oceanographic studies and computer models are being combined to try to develop a system to predict when and where rogue waves may form. This will eventually enable maritime authorities to provide an early-warning system for ships and platforms.

Research is also being used in trials to program marine radar systems to identify rogue waves. Land-based radar or satellites might eventually be able to track rogue waves. Similarly, radar on ships can be programmed with calculations used in the models to identify an approaching wave and warn the ship, similar to laser systems used in aircraft to detect wind shear.

Finally, marine architects and engineers are looking at the design of ships, platforms, ports and other structures to gauge their susceptibility to damage from very large waves. Inquiries into the sinking of a number of container and cargo ships have recommended stronger hatches be installed to prevent flooding of the main hold. Complex designs and structures susceptible to wave damage are also being looked at. Drilling rigs may need to be made higher and stronger.

At any rate, the chance of us running into a ‘real’ rogue wave is pretty small. And I’m glad of it!

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