Understanding rip currents
Dr Tim Scott explains how rip currents are created, what factors affect them, and how we can spot and avoid a rip current
Dr Tim Scott
Associate Professor of Ocean Exploration
School of Biological and Marine Sciences (Faculty of Science and Engineering)
“Rips are strong currents running out to sea, which can quickly drag people and debris away from the shallows of the shoreline and out to deeper water.”
Royal National Lifeboat Institution1
A rip current is a wave generated current that occurs in the surf zone on any beach which has breaking waves.
It is characterised by a flow of water going offshore – from the shoreline out to sea and tends to flow at 1–2mph but can reach 4–5mph, which is faster than an Olympic swimmer.1
As the waves come in and break over shallow sandbars there is a force of water that builds up at the shoreline. This leads to variable heights in the water level as you go along the beach. From areas where waves are breaking, to areas where there are fewer breaking waves, this circulation drives the offshore flowing element of a rip current.
“In the UK, the majority of RNLI Lifeguard incidents involve rip currents. They are a major cause of accidental drowning on beaches all across the world.”1
Where can rip currents be found?
Rip currents are ever-present on wave-exposed coasts. Each year they cause hundreds of drowning deaths and tens of thousands of rescues on beaches worldwide and are therefore the leading deadly hazard to recreational beach users.2
Rips are especially powerful in larger surf, but research has shown that rip currents do not just float offshore like a river flowing out to sea and can also be found around river mouths, estuaries and man-made structures like piers and groynes. It is sensible to never underestimate the power of any water.2
Rip currents quite often flow in circles, like eddies, rotating within the surf zone. These circulating rip currents can often fluctuate in strength, where pulses (which can be associated with large groups of waves) can cause the current to jet out beyond the surf zone, offshore, into deeper water.
What factors influence a rip current?
Rip current flow or rip current hazard is driven and controlled by a number of different environmental factors.
- Wave height: How much energy and how big the approaching waves are affects the strength and the type of currency getting the surf.
- Tidal level: Quite often the sandbars which generate rip currents in this country, certainly in the west coast of Cornwall and Devon, are all located around low tide, so when the tide levels are low you get much more rip current activity compared to the equivalent high tide.
- Beach type: There is an integrated relationship between the current flow and how the beach looks and how the sandbars develop. Sandbars control how the waves break when they come to the beach and that in turn controls the type of rip current circulation.
These different factors can create variable conditions which affects rip current behaviour over time.
- Sandbars can vary from season to season.
- The tides vary daily and there are leap and spring tides every fortnight.
- Waves can vary from seconds to years, so we may have one particularly stormy summer with bigger waves compared to the previous summer. We may also get swell events that come from far away in the Atlantic which come in and can surprise you.
- Within a certain day you also have varying sets or wave groups which arrive at the beach with gaps of maybe thirty seconds to up to ten-to-fifteen minutes.
Rip currents can be classified into three broad categories based on the dominant controlling forcing mechanism.2
- Hydrodynamically-controlled rips are fleeting in both time and space, occur on alongshore-uniform beaches and are essentially driven by hydrodynamic forcing mechanisms.
- Bathymetrically-controlled rips occur at relatively fixed locations and are driven by hydrodynamic processes strongly influenced by natural variability in the shape and structure of coastal systems in both the surf zone and inner shelf zone.
- Boundary-controlled rips are dominated by the influence of rigid lateral boundaries, such as natural headlands or anthropogenic structures (groynes, piers) and are therefore fixed in space and time as they flow against them.
GPS surf zone drifters tracking the direction and magnitude of rip currents on Perranporth beach, Cornwall
How do I spot and avoid a rip current?
Rip currents can be difficult to spot, but are sometimes identified by a channel of churning, choppy water on the sea's surface.
Royal National Lifeboat Institution1
The best recommendation is to always try and swim at a lifeguarded beach.
The lifeguards are experts at understanding and spotting rip currents and they will know the individual beach very intimately. Lifeguards are able to guide you and give you advice and will indicate the safe area to swim between red and yellow flags.
If you're at a beach which is not life guarded, then always be really careful when you're going into the sea, even when the conditions look relatively calm.
As a general rule of thumb, where there are waves and white water it is generally the safer part and where it is calmer and more benign it is generally more dangerous.
What do I do if I’m caught in a rip current?
The RNLI’s advice is:
- Do not try to swim against it or you’ll get exhausted.
- If you can stand, wade don’t swim.
- If you can, swim parallel to the shore until free of the rip and then head for shore.
- Always raise your hand and shout for help.
If you see anyone else in trouble, alert the lifeguards or call 999 or 112 and ask for the coastguard.
Rip current resources
A resource for surf lifesavers and water-users on the basics of rip currents and rip hazards in the UK
“The sea is a constant and it is endlessly fascinating”
A surfer since the age of three, Tim knows first-hand the dangers that rip currents can pose to the inexperienced, after he saved three rookies off Rossnowlagh in Donegal Bay in 2010 as they were being sucked out to sea.
“As surfers, we sit surrounded by waves for hours on end, waiting, observing, feeling the changing conditions. We’re like human drifters. And that’s why when I study data, I look for patterns, those rhythms of nature, and you know instinctively whether they’re ‘real’ or not.”
Coastal Processes Research Group (CPRG)
An internationally recognised group of researchers, specialising in field studies and numerical modelling of coastal processes
CPRG aim to be a leading contributor to the international research community seeking to understand and predict the behaviour of coastal and estuarine systems in support of appropriate management of coastal resources and activities.
The group has links with researchers from a number of different University groups and centres, and also work extensively with researchers in Europe and elsewhere.
Explore, understand and protect our ocean systems at the University of Plymouth
With increasing awareness of the importance of the world's oceans as a natural resource and of their role in the stability of the global climate and environment, our marine science specialists are in high demand.
References
1. Rip currents, RNLI: https://rnli.org/safety/know-the-risks/rip-currents
2. B. Castelle, T. Scott, R.W. Brander, R.J. McCarroll, Rip current types, circulation and hazard: https://www.sciencedirect.com/science/article/pii/S0012825216303117