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When and where are beach rip currents most hazardous?
In recent years scientists and lifeguard agencies from all over the world have been trying to identify when and where rip currents are going to be most hazardous to help reduce global drowning rates. Here in the UK, Plymouth University and the RNLI have teamed up to improve our understanding of beach rip currents on some of the UK’s most hazardous beaches through the measurement of rip currents and analysis of lifeguard incident records.
Video: research team measuring rip currents and basic findings
Further research has shown that the physical characteristics of rip currents can vary in a number of ways that affect the physical hazard to a water-user:
- Rip strength
- Rip circulation
- Rip pulsing
The flow strength, circulation and pulsing characteristics of rip currents vary due to a number of environmental factors that change rip current behaviour on a range of timescales (from seconds to seasons):
- Beach Type and Sandbars – Control wave breaking, shape and location of rip currents.
- Tide – The spring/neap and daily tidal cycles control where the waves are breaking on the beaches.
- Waves – The characteristics of the waves (height, period and direction) drive the rip currents and control rip pulsing and circulation.
Analysis of RNLI high-risk rip current mass-rescue incidents (affecting over 1200 individuals) that occurred between 2006 and 2011 at 20 beaches in Devon and Cornwall with bar/rip morphology showed that:
- 40% of all rip-related mass-rescue incidents occurred on days when low water was around (+/- 10 cm) mean low water MLW (in between neap and spring tides).
- 66% of all rip related mass-rescue incidents occurred when significant wave height wave below 1 m.
- Over half of all rip-related mass-rescue incidents occurred when peak wave period was above 10 secs.
Beach Types
On the vast majority of sandy surf beaches rip currents are generated by waves breaking over complex sandbar patterns known as bar/rip morphology. This type of bar/rip morphology occurs throughout a certain group of intermediate beaches that are identified by a medium beach slope and the presence of sandbars and rip channels. The average wave height on bar/rip intermediate beaches is typically more than 0.8 m.
The shape of a beach is controlled by the local wave and tidal conditions, as well as sediment size and local geology.
There are a huge variety of beaches throughout the UK and, although every beach is unique, they can be classed into a number of beach types.
These range from Reflective (narrow and steep) to Dissipative (wide and flat). Intermediate beaches fall in the middle and are the most dynamic and changeable with the seasons.
Reflective and Dissipative beaches:
- Relatively stable and will often maintain their general shape even under changing wave conditions.
- Many occur along lower energy coasts with smaller waves sheltered from ocean swells.
- The main rip current hazard comes from intermittent topographic rips, often occurring against coastal structures, such as groynes.
- Strong/offshore winds and tidal cutoff are often more persistent hazards than rip currents.
Intermediate beaches:
- More sensitive to changes in wave conditions and can change shape with the seasons.
- Lifeguard incident records show that rip currents are the greatest hazard at intermediate beaches with sandbars, accounting for over 70% of all incidents.
- The most visited beaches due to the recreational appeal of their sand and surf.
- Represented 50% of the beaches patrolled by the RNLI in 2012.
Sandbars
Bar/rip morphology occurs on intermediate beaches, and controls where on the beach rip currents form.
Sand on a beach is constantly moving with every passing wave. Consequently, sandbars change shape and position daily. This change often happens relatively slowly with sandbars appearing to remain in the same place for weeks, but sandbars can adapt very quickly (days) if there is a dramatic change in conditions (e.g., storm event).
There is a seasonal cycle of sand movement on many intermediate beaches. In the winter, when the waves are bigger, sand is transported offshore from the beach to the outer sandbar (below MLWS) and then returned onshore by smaller waves during typical summer conditions.
The sandbar cartoon shows a simplified example of the different types of sandbar shape that exist on intermediate beaches.
An example of the different types of sandbar formations typically found on open-coast beaches with bar/rip morphology. Arrows indicate typical rip current patterns. It is clear that rip current activity, and thus rip current hazard, depends on the type of bar morphology.
Autumn/Winter: after periods of large waves (days-weeks) the outer sandbar will often fill with sand from the inner beach, become straighter and move offshore (away from the beach). This can result in waves ‘closing out’ over the bar with little alongshore variation in wave breaking (waves less ‘peaky’) and the development of a large deep-water trough (‘gutter’) between the outer bar and the beach. This results in less rip current circulation, fewer rip current exits and more longshore currents or ‘side sweep’.
Spring/Summer: after periods of small/medium swell waves (weeks/months) sand will slowly make its way back onshore. The outer sandbar is full of sand and will very slowly move onshore and become more crescentic (three-dimensional). This creates peaks of wave breaking along the beach. Strong beach rip current circulations form under these conditions and they further sculpt the sandbars. Eventually, if these conditions persist, the outer sandbar will merge with the beach creating a very wide beach at low tide with no deep-water trough.
Low-tide sandbars
Throughout intermediate beaches with a large tide range, the most well developed sandbar systems are generally found around low tide.
As water levels fluctuate daily with the tide, the surf zone sweeps up and down the beach (often up to hundreds of meters). During this cycle the water level remains for the longest time at low and high water.
As the tide sits at low water it allows the waves and currents to shape the sandbars and channels, creating the bar/rip morphology.
This effect is less at high water because the tide sweeps back down the beach each day, effectively smoothing the beach and hampering development of bars at high water. It should be noted, however, that there are some beaches where sandbars and channels mainly form at high water.
Finally, as the low tide level changes from day-to-day with the spring/neap lunar cycle, the mean low water level (MLW) is the most common low water elevation. This means that the surf zone at low water will sit at MLW for the longest time over the spring/neap cycle, sculpting the sandbars and rip channels. Therefore, beach rip currents are often at their most active and hazardous at MLW.
Tides
Tides control water levels, dictating where on the beach waves break and for how long. Rip currents are not directly caused by tides, but fluctuating water levels control the amount of wave breaking over sandbars, affecting rip current activity.
The UK has some of the largest tides in the world. Mean spring tidal ranges vary from 1 to 11 m from location-to-location having a huge impact on beach shape, sandbar and rip current formation. The unusually wide variety of tidal environments in the UK presents a real challenge to lifeguards managing rip current risk on public beaches.
Daily tides
Scientific measurements of rip currents show how rip current flow speeds are affected by the tides.
In an example from Perranporth beach, Cornwall, where the sandbar systems are located at low water, the graph shows rip speeds increasing to a maximum at low water and then ‘switching off’ at high water.
In regions with a smaller tide range, the rip currents will still intensify at low water, but may not ‘switch off ‘at high water.
Bottom graph: speed of the rip current in a channel next to the sandbar.
Notice how the rip flow speed peaks at low water.
Spring/neap cycle
During a large low tide (spring tide), the inner sandbars can become very shallow or completely exposed at low water, and wave breaking may increase across the head of the rip channel further offshore, reducing the strength of the inner rip currents.
On the other hand, during a small low tide (neap tide) the sandbars may be too deep for waves to break, leaving the rip currents inactive.
Therefore, the low water level will control the period of exposure to rip hazards. Hazard exposure is maximized if the low water level on a given day coincides with peak rip current activity.
Bar/rip morphology is most commonly found at low water on UK intermediate beaches. However, there are beaches that have bars and rips at high tide, which create the fastest rip currents at high water. There are also many other local factors that can generate rip currents at any stage of the tide. For example, the influence of local geology, rock outcrops, headlands, streams and river inputs, as well as coastal structures, can all modify the beach and wave breaking to form rip currents at any stage of tide.
Waves
We have established that rip currents are controlled by breaking waves and that waves play a key role in shaping the sandbars and rip channels on a beach over a period of days to years.
Anyone who regularly visits the beach will know that wave conditions vary from day-to-day. Recent research has shown that even small changes in the characteristics of the breaking waves can have a large effect on rip current flow speeds, rip pulsing and circulation patterns and hence rip hazards.
Rip flow speed & pulsations
Rip current flow speeds increase with wave height. However, under high-energy wave conditions wave breaking in the head of the rip channel, and on the outer bar further offshore, can limit the strength of rip currents further inshore. Under these conditions, longshore currents often begin to dominate the surf zone, and topographic rips near headlands and mega-rips can become particularly hazardous.
Even under relatively small waves (significant wave heights of less than 1 m) beach rip current speeds can be relatively strong.
Unlike locally generated wind waves, swell waves that have travelled long distances have longer wave periods, are more ordered and form clearly defined groups or sets with long lulls. These long-period swell-waves carry more energy than short-period wind-waves of the same height and drive stronger rip currents.
The formation of swell waves into groups or sets also drives greater fluctuations in rip flow speeds. Under very long-period conditions, rip flows can almost reduce to nothing during ‘lulls’ between wave groups (minutes). This effect can catch bathers off-guard when the wave groups arrive and rip currents pulse.
Video: examples of rip pulsing under low- and high-energy wave conditions, Cornwall
Rip Circulation
We have mentioned that rip currents often flow in a circular pattern, recirculating back towards the beach (rotation) rather than flowing offshore beyond the surf (exit).
On average, beach rip currents around the world have been shown to exit the surf around 10% to 20% of the time. These exits are often associated with the pulsing of the rip current.
Measurements by Plymouth University in the UK have shown that with large variations in waves and tides, the percentage of time that rip currents exit the surf zone from the shoreline can vary from 0 to 50% from day-to-day during periods when rip currents are active.
Observations of active rip currents have shown that rip current exits decrease as wave energy (wave height and wave period) increases. During larger than average wave conditions (typically wave heights of 1.5 – 2 m) rip current exits tend to be less than 10%, with rotational (recirculating) behavior dominating.
During high-energy wave conditions, waves begin to ‘close out’ across the outer bar, breaking across the rip channels. This reduces the proportion of beach rip current exits and encourages recirculation and rotation of floating material back toward the beach. Rip current exits under high-energy conditions are typically associated with pulses in rip flow.
Rip hazard
High-risk, high-exposure rip current scenarios are a balance between rip flow speed, circulation and bather exposure.
Beach rips are especially hazardous to bathers under small swell-wave conditions with a long wave period. Under these conditions rip flow speed, rip exits and rip pulsation are maximized and bather exposure to rip currents remains high.
Bather exposure to rip hazards often becomes limited when waves are large as the broken waves keep them in the shallows, away from stronger currents further offshore.