In late May 2023, two teenagers died on the beach at Bournemouth, UK. Initially, it was thought that a local sightseeing boat, the Dorset Belle, may have been involved. However, in July 2023, it was announced that the cause of the deaths was rip tides (currents). You can read more about the incident here:
Bournemouth beach deaths: No police action to be taken - BBC News
What are rip currents?
On sandy beaches with large waves (beaches that are suitable for surfing), the underwater part of the beach is often characterised by alternating shallow and deep sections referred to as ridges and runnels. Sometimes these channels can either run parallel to the beach or at right angles to it. Strong currents can be present in the channels and when the flow is away from the beach – that is towards the sea – they are known as rip currents. Flow velocities in rip currents often exceed the speed at which people can swim and, in combination with the large water depth, rip currents can easily drag unsuspecting swimmers out to sea.
Rip hazard and risk
In the southwest of England, two-thirds of all the rescues conducted by beach lifeguards of the Royal National Lifeboat Institution (RNLI) are due to rip currents. Furthermore, it is estimated that 35 accidental coastal drownings per year in the UK can be ascribed to rip currents. In Australia and Florida at least 80 and 50 people respectively drown each year in rip currents. As most rip-related drownings occur on beaches without lifeguards we can only speculate on the number of people that drown each year due to rip currents, but they could be in the hundreds.
Figure 1. Components leading to rip currents
What causes rip currents?
All beaches have a complicated series of sand bars and channels (ridges and runnels), with a variety of currents being generated by the breaking waves in the surf zone. As indicated in Figure 1, there are several components that make up the water circulation system in the surf zone. Waves break on the shallow sand bars (1). The breaking waves push large amounts of water towards the shore (2) – this is the flow allowing surf boarders to enjoy long rides. Rip feeder currents (3) then run from the regions of wave breaking, along the shore towards the rip channels. Where there is a gap between sand bars, the rip feeder current turns away from the shore and water starts to flow out to sea. The fastest seaward current occurs where the rip channel is at its narrowest and this is referred to as the rip neck (4). When the rip current creates a break in the sand bars and goes under the line of breaking waves, the flow may spread out and weaken in the region referred to as the rip head (5). Alternatively, the rip flow re-circulates and turns back towards where the waves are breaking (6) and joins the onshore flow caused by the breaking waves.