When waves break on a beach, they push water towards the shoreline. Once that water reaches the shore, it has to find a way to get back out to sea, and it does this by flowing downwards into deeper channels in the surf zone. Once the water is in these deeper areas, it can flow back out to sea away from the shoreline. These deeper channels are called rip currents.
The Different Types of Rip Currents
The most common types of rip currents are FIXED RIPS that are confined to deeper channels between sandbars and are often persistent in a location for periods of days, weeks and even months. They are sometimes referred to as “accretionary”, “low-energy” or “stationary” rip currents and are associated with a period of beach recovery during smaller wave conditions when sand moves back to the beach. The size and shape of the rip channel may change during this period of beach recovery but the location of the rip current does not. It is important to note that the orientation of the rip current channel is not necessarily perpendicular to the beach. These rip currents can vary in width from several meters to 10’s of meters and the distance they flow offshore is usually dictated by the width of the surf zone.
FLASH RIPS, which are also known “high-energy”, “erosional” or “transient” rip currents occur when wave conditions increase suddenly, or during storms, when the water level rises suddenly causing a temporary rip current. Flash rips can appear and re-appear at different locations along the beach, usually during high energy wave conditions. They can be 10’s of metres wide and may extend 10’s of metres seaward of the surf zone. Mega-rips are the largest rip currents and occur during extreme wave events (3m+) and are particularly common on embayed beaches. These rip currents may extend for 100’s of metres offshore.
TOPOGRAPHIC RIPS are also common and are usually semi-permanent in location as their occurrence is related to fixed features in the surf zone such as headlands and man-made structures such as groynes. These rip currents are often given names such as “Backpackers Express” on Sydney’s Bondi Beach due to their persistent location.
MEGA-RIPS are large rip current systems than develop during period of severe wave activity. They are the key driver of coastal erosion and can move vast quantities of sand offshore in a short amount of time. Fortunately, because they only occur when the surf is large and dangerous, very few people go into the water and get caught in mega-rips.
Rip Current Formation and Behaviour
The existence of rip currents is primarily related to patterns of wave breaking. Water level always rises in the vicinity of waves breaking. This phenomenon is called wave set-up and is almost impossible to observe with the naked eye.
The fact that waves break closer to the shoreline means that the water level close to the beach is always a little bit higher than the water level further offshore. Therefore as long as waves are breaking, there is always a tendency for water to flow from high set-up to low set-up, i.e. offshore. Water levels are also higher over shallow sandbars and reefs, as these features concentrate wave breaking, and are lower over deeper areas like troughs, gutters and channels. The presence of these features tends to create localised alongshore variations in shallow and deeper areas and this promotes the movement of water along the beach from areas of high set-up to low set-up.
The occurrence of rip currents is therefore most common on beaches characterised by moderate wave energy and the presence of sand bars and troughs with rip currents being confined to deeper channels between sand bars.
A classic rip current consists of 3 components:
i) a rip feeder which carries water along the beach close to the shoreline;
ii) a rip neck which is a narrow and fast offshore flow formed by the meeting of two feeder currents; and
iii) a rip head which forms seaward of the breaking waves and often forms a plume of disturbed water or suspended sand as the rip current slows down.
While this is a common textbook definition, in reality rip currents take on different forms and types and longshore feeder channels, for example, are not always present.
Contemporary research has questioned the frequency of these traditional idealised flow patterns being reflected in reality. Although research is continuing, it is proposed that many rip currents flow in circulating patterns which return water back onto the adjacent sandbank. This provides further evidence that rip currents are uunpredictable and should be avoided.
How fast do rip currents flow?
Rip flow is quasi-steady meaning that although the flow is continually in the offshore direction, the speed of the rip flow can change. The speed of rip flow is related to the size of breaking waves and the confinement by the channel. Even small stationary rip currents can flow at 0.5-1 ms-1 under normal wave conditions. These speeds make it almost impossible to stand and stay in one place within a rip and are beyond average swimmers capabilities for an extended period of time. Flash rip currents and mega-rips can flow up to 2-3 ms-1, if not stronger.
Generally rip currents increase in speed from the shoreline and reach a maximum in the middle of the rip neck. Aside from the size of breaking waves, there are two factors which affect the speed of a rip current.
The first is the stage of the tide. While it is incorrect to call rip currents “rip tides”, rip flow is affected by the tide. As wave breaking is more pronounced around low tide due to lower water depths, more water enters the rip current, and if the rip current is channelised, it will increase in speed. This is why rip currents always flow fastest around low tide and slowest around high tide. In fact, many rip currents can actually stop flowing at high tide.
The second factor is a phenomenon known as rip pulsing, a sudden acceleration of rip flow which may last for seconds or minutes. Rip pulses are particularly important and play a large role in drowning deaths and rescues as they can cause the rip current to double in speed almost instantly and can extend considerable distances beyond the surf zone. Almost all rip currents display pulsing behaviour. It is generally accepted that rip pulses are related to the occurrence of wave groups, or wave sets - meaning naturally occurring groups of 4-6 larger waves that travel together and break, creating larger levels of wave set-up at the shoreline and over shallow bars. Rip currents generally pulse shortly after a wave set has broken in the surf zone.
Rip Current Survival
If you get caught in a rip current, you need to know your options:
1. For assistance, stay calm, float and raise an arm to attract attention.
2. While floating, rip currents may flow in a circular pattern and return you to an adjacent sandbar.
3. You may escape the rip current by swimming parallel to the beach, towards the breaking waves.
4. You should regularly assess your situation. If your response is ineffective, you may need to adopt an alternative such as staying calm, floating and raising an arm to attract attention.
These may sound like simple options, but rip currents are complex, dynamic processes and all responses also have their pitfalls. You could float on an unpatrolled beach and not be returned to a sandbank with no-one there to help. You can also swim parallel and end up swimming against a longshore current which can flow along a beach; this will see you get tired quickly.Therefore, the best thing you can do is avoid them!
The best thing you can do is stay out of rip current in the first place! The best ways to do this are:
- Always swim between the red and yellow flags
- Learn how to spot rip currents and avoid them
If you are swimming near the red and yellow flags, then the lifesavers can see you and help if you get into trouble. This is particularly important because people can move around in the surfzone or get caught in flash rip currents which can occur anywhere along the beach and sometimes affect people between the flags. This can result in mass rescues, like Black Sunday at Bondi Beach, or the rescue below at Scarborough Beach, WA.
Sunday February 6th, 1938 became known at Black Sunday following 5 drownings and the rescue of approximately 250 bathers in the same incident at Bondi Beach. Initial reports regularly stated the cause as a collapsing sandbar, which is incorrect.
An eyewitness account by Chief Beach Inspector Tom Meagher describe “five or six really big waves in quick succession... knocking many swimmers off their feet. As each wave surged bigger up the beach, the next followed closely on it. There was not the usual break for the water to recede”.
As the set of waves continued breaking and pushing water onto the sandbank, bathers’ feet lost contact with the sandbank and began floating. When the final set wave finally broke, the water began receding rapidly. This phenomenon is known as a flash rip current, and is the true cause of the Black Sunday tragedy. Sandbanks are solid deposits of sand, which can change gradually over time, but not suddenly as suspected here.
Visit the local council’s account of the events from their archives.
Common misconceptions about rip currents
1. Rips are not an undertow.
Studies have confirmed that rip currents do not pull people under water causing them to drown. They simply take a person away from the beach, often eventually back into the wave zone. Rip current related drowning deaths are therefore due to variable factors including:
a. Ability to swim,
b. The instinctive nature to attempt to swim directly back into shore against the rip current which leads to fatigue,
c. The panic which results and the impairment of judgment,
d. Not attending a patrolled beach where services are able to assist/rescue.
2. Rip currents can be hazardous all the time, not just when there are large surf conditions
When waves are larger the speed of the rip current is also higher resulting in increased risk. However, when these conditions exist less people typically enter the water. It's on days when the surf is moderate that more drowning deaths occur as more people go swimming.
3. Calm water is not always ‘safe’ water
Many people end up in a rip current as a result of choosing to swim in an area of calm water on the beach. Unfortunately, where rip currents start on the shore, the water often has a calmer appearance with ripples on the surface indicting a subtle flow of water out to sea. The best area to swim is between the red and yellow flags to ensure you are under supervision of lifeguards.
4. Rip currents can be used wisely depending on your ability in the surf environment
Lifesavers and lifeguards use rip currents to get to people in trouble more quickly and surfers often use a rip current to get out through the wave zone. However, in order to do this, it takes years of training and experience and caution must always be taken.
5. There is no such thing as a “rip tide”
Rip currents are sometimes referred to as ‘rip tides’, which is misleading as tides refer to water level changes over a period of 6-12 hours, whereas rip currents are currents moving over a period of seconds, minutes and hours.