Rogue waves: Fact or fiction?
When our lifeboat crews are called out to a vessel in distress or a capsized kayaker, one of the first things they hear from those they rescue is how they ended up in their situation. The stories are almost always different. Some humorous, others frightening. Some even report getting caught by a rogue wave – a sudden large wall of water that can capsize a boat or sweep someone overboard. They will often say the wave came out of nowhere and took them by surprise.
If you haven’t spent much time at sea, you may have seen rogue waves on the silver screen. They often feature in disaster films, a giant wall of water suddenly blocking out the horizon. Or you may have seen a ship dwarfed by the mountainous wave it’s desperately trying to climb.
But are these rogue waves a real event? Or something that can only be created in a Hollywood special effects house? We spoke to two experts from Plymouth University who have dedicated their academic lives to trying to understand the awesome power of the sea.
Frequency: The number of waves produced in a period of time.
Man overboard: A situation in which a person has fallen from a boat or ship into the water and needs to be rescued.
Rip current: Strong currents running out to sea, which can quickly drag people and debris out to deeper water.
Surf zone: An area of breaking waves.
Swell: Waves that have been generated by a distant weather system rather than local wind. They have more of a defined shape and direction than locally generated wind waves.
What is a rogue wave?
Will Mortimer is a PhD Researcher at University of Plymouth and part of the COAST Engineering Research Group. Will has studied rogue waves as part of his research.
‘The technical definition of a rogue wave is a wave that’s twice the “significant wave height” (the average height of the largest 30% of waves over a certain time period). When two or more ocean waves travelling in different directions come together at a particular point, the waves superimpose upon each other, to cause a transient event of increased amplitude.’
In simpler terms, two or more waves come together and combine their heights into one larger wave. This sudden increase in size can easily catch people off guard.
Can rogue waves be predicted?
You might have heard seas being described as calm, choppy or rough, but the science of seas and how they work has more complexity. To understand what causes rogue waves, you need to understand what a sea-state is. This is the condition of the sea based on factors such as height, size and frequency of the waves.
‘A sea-state describes the energy within ocean waves for a certain time period in a given area,’ explains Will. ‘Energy is distributed over a range of frequencies and wave directions. Well-developed sea-states will see a concentration of wave energy around a particular wave frequency and direction – known as swell. Storm sea-states, which are less developed, will show a spread of energy over a much wider frequency and directional range.’
So a well-developed sea state is one where the waves are relatively the same height and distance apart from each other, travelling in the same direction. A storm sea-state is more chaotic, with waves varying more widely in height, frequency and the direction in which they travel.
‘There is thought to be many ways for rogue waves to form in the ocean. The most commonly explored cause is believed to be “wave-wave” interactions, where waves travelling in different directions cross each other. This could be the result of a storm sea with scattered wave directions, or from two separate swell events meeting at a particular location.’
If we know the causes, can we use them to predict how and when a rogue wave will form? Unfortunately, not yet. ‘Rogue waves are inherently chaotic in their formation, which makes forecasting them an almost impossible task,’ continues Will. ‘There are many tales of experienced mariners being caught unawares by freak waves.
‘Impressive work has been done in the last 20 years to improve forecasting and prediction of rogue wave events. However, forecasting rogue waves by modelling all individual waves and wave interactions in a particular location is extremely computationally expensive.
‘And so, an alternative approach to forecasting rogue waves uses the idea of waves travelling in groups. If the energy held within separate wave groups is understood, then it may be possible to predict a short time window where wave energy will be focused. However, forecasting rogue waves still remains a far from resolved problem.’
Who needs to be aware of rogue waves?
Dr Tim Scott is Lecturer in Ocean Exploration at University of Plymouth, and is part of the Coastal Processes Research Group. If you want to know about waves, he is the person to ask. He explained that it’s not just those out at sea who need to be wary of sudden changes in wave size and height. Those on the shore also need to keep an eye open too.
‘Waves breaking inshore on beaches drive very different variations in water levels in the surf zone and at the shoreline,’ explains Tim. ‘A wave forecast of a given height represents an average of a range of wave heights and frequencies that make up the sea state at moment in time.’
If you are a surfer, you may have seen weather forecasts that give you an idea of the height of the waves. The bigger the waves, the better the surf conditions can be. Many surfers talk about waiting for that perfect ‘set’ of waves, a group with the right size for surfing. These sets of waves can come in from far out to sea, and there can be a gap of up to 15 minutes between them.
‘On a sunny calm day, when a new swell is arriving at the coast from a distant storm, these sets of waves can occur out of relative calm. These can trigger changes of water level and rip current activity in the surf zone, catching out unsuspecting bathers.
‘At the shoreline, the swash zone (where the sea meets the beach) “breathes” at a much slower rate than the waves breaking in the surf zone. Shoreline motions back-and-forth often take longer than 30 seconds and are amplified under high-wave and storm conditions. On a wide beach, when the waves are big, these swash motions can extend hundreds of metres up and down a beach presenting a serious hazard to coastal walkers and beach users.’
How to stay safe
The unpredictable nature of rogue waves means you can’t really prepare for them. But what you can do is to make sure you are prepared for the consequences.
Always wear a lifejacket, so if a rogue wave sweeps you or another crew member overboard, you will be kept afloat. Making sure your training is up to date, especially anything involving man overboard recovery. It will help you to stay calm in these situations and work more effectively to rescue those who have fallen into the sea. And carrying a means of calling for help suitable to your environment and passage will mean that, if the worst does happen, you can raise the alarm and get assistance.
If you’re coastal walking, being aware of your surroundings is key. Keep an eye out for local warning signs and make sure to check tide times if you are walking along the beach or an exposed area. Stick to coastal paths if possible and make sure you have a means of calling for help.
Interested in learning more? Get safety tips on our activity pages.