Rip Current Flow Behaviour
How Fast Do Rips Flow?
Rip currents can flow at speeds stronger than most people can swim. For example, a typical channelized rip current will flow on average between 0.3-0.5 m/s under normal wave conditions on a beautiful sunny day. It’s very difficult to stand up in flows that strong and even harder to swim against them - particularly when they pulse.
Rip Pulsing
Rip current flow is unsteady - meaning that the flow speed can fluctuate over time. All rip currents have a tendency to pulse, which is a sudden acceleration in speed where flows may reach over 2 m/s – which are Olympic swim speeds! These pulses are short-lived (less than a minute) and occur after large groups of waves, or wave sets, break and the water level gradients become large, driving the rip flow.
A release of purple dye at Sydney’s Palm Beach shows the offshore extent of a rip pulse (photos Rob Brander)
Rips and Tides
While rip currents are definitely NOT rip tides (please don’t call them that!), rip flow can be influenced by the stage of the tide. On beaches that experience tide ranges (the vertical difference between high and low tide levels) of less than 2 metres, rip current flow tends to be fastest about 1.5 hours either side of low tide because there is more wave breaking when water depths are shallower. However, on beaches with higher tide ranges, the maximum rip flow speeds tend to occur around mid-tide as the rip current channels are exposed at low tide as shown below on beach in south-west France.
Fantastic rip channels stranded at low tide on a beach with large tide range (photo Bruno Castelle)
Rip Current Circulation
The traditional view of rip current flow circulation patterns was that waves break across sandbars and water is transported seaward where it piles up along the shoreline. It then flows along the beach as feeder currents, which meet and turn seawards as a narrow and strong rip neck. The rip then extends, or exits, beyond the surf zone where it slows down and dissipates in an expanding rip head. This is what is shown in (a) below:
However, experiments using GPS attached to drifters have shown that rip current flow does not always exit the surf zone and can in fact recirculate back to the beach in a circular fashion – this is known as circulatory flow behaviour (as shown in b above). It’s very difficult to know which type of circulation a rip will exhibit, as it can change over the order of minutes, but generally most exits are associated with rip pulses.
GPS drifters ready to deploy at Bondi Beach, Sydney, Australia. Note the dark channel rip in the background (photo Rob Brander).
A plot of GPS drifter tracks at Whale Beach, Sydney, Australia. It shows almost 30 drifters over 4 hours. The rips are the hot colours aifh red being the fastest flow speeds (image Jak McCarroll)
The animation below shows the trajectory of GPS drifters released in a rip current at North Cronulla Beach in Sydney, Australia. The perspective is looking down at the beach and the beach is on the left, offshore is on the right and the black dots are the drifters. You can clearly see both circulating and exiting flow and a particularly spectacular ‘firework’ exit at the 30 s mark! Dr Jak McCarroll created this animation during his PhD fieldwork with UNSW Sydney in the School of Biological, Earth and Environmental Sciences back in the early 2010s.
Rip Dye Releases
For years I’ve been releasing harmless purple dye into rip currents to demonstrate rip current flow to my students and the general public. It’s a great way to bring rip currents to ‘life’ and people always remember seeing the dye - even if they see it on a screen! The releases shown below are good examples of both types of circulation patterns It’s also extremely important to realise that the different behaviour of rip current circulation has big implication towards how people should react when they are caught in a rip.
The purple dye shows exiting rip current flow (photo Rob Brander)
Another dye release shows circulating flow behaviour (photo Rob Brander)
A time lapse of a rip current dye release (courtesy Patrick Rynne)
