We defined 3 main types of rip currents, each of which had sub-types based on the mechanisms that create spatial and temporal variability in wave breaking that leads to rip current flow. In total we defined 6 different types of rips and some of the Figures below come from this publication.

Bathymetrically Controlled Rip Currents

This is a fancy term for describing rips that are formed by variations in the underlying bathymetry, such as sand bars, that cause waves to break in different places.

Channelised rip currents form due to the presence of irregular patterns of nearshore morphology such as sand bars and channels. The rips occupy deeper channels between the sand bars and they can stay in place for days, weeks and even months. These are one of the most common types of rips and are the easiest to spot because of their tell tale ‘dark gap’ visual signature such as the rip at Marina Beach, California shown in (b) above. Channel rips can also erode mini-embayments along the shoreline as shown below.

Focused rips are not as common and are formed when the presence of an offshore topographic feature, such as a submarine ridge or canyon, causes incoming waves to bend (refract) and focus their energy and breaking in certain locations. This causes rips to form in fairly consistent locations over time that may be channelised or not. The example shown in (d) above is from Black’s Beach in San Diego, California (photo courtesy of Tom Cozad).

Boundary Rip Currents

Many natural and artificial structures such as headlands, reefs, groynes, jetties and piers often have an almost permanent rip current flowing next to them and in some cases, on both sides. There are two main reasons for this. First, when waves approach the beach and break at an angle, they create an alongshore current that flows along the beach until it reaches an obstacle and is then deflected offshore. This is the deflection boundary rip shown on the right in the diagram below.

At the same time, waves approaching at an angle to a structure will focus wave energy and breaking further along the beach on the other side of the structure with a protected wave shadow zone immediately against it. This results in a water level gradient that drives flow back towards the structure and creates another rip heading offshore. This is the shadow boundary rip shown on the left in the diagram above. Both kinds of rips are often referred to as headland, structural or permanent rips. Most of these rips appear as dark gaps as they tend to occupy a deeper channel.

Hydrodynamically Controlled Rip Currents

These rip currents are not related to nearshore morphology or physical structures and are driven purely by hydrodynamics – waves and other currents. They tend to be variable in occurrence, both spatially and over time, which makes them hard to predict and they also don’t appear as dark gaps between white water. Instead they look like turbulent clouds and streaks of sand and water. There are several types of hydrodynamically controlled rips as shown below.

Flash rips are common on beaches and as the name implies, they tend to form suddenly and don’t last long (a few minutes or less) before forming again somewhere else along the beach. The simple explanation is that some randomly large waves, or a set of large waves, will break causing the water level to rise quickly and create an offshore flow - a flash rip.  As shown in (b) above of Zuma Beach, California, they appear as narrow bands of turbulent and sandy water heading offshore. They can initiate from the shoreline, but can also flow off the seaward slope of sandbars.

Shear instability rips are not common and only occur under specific conditions. When a very large swell wave event occurs and approaches the shoreline at a strong angle (as shown in (d) above at Huntington Beach, California) a very strong alongshore current is generated. As this current flows, different sections of the current move at different speeds – that’s where the ‘shear’ comes in. At some point this causes the current to become unstable resulting in eddy-like rips that spin out offshore like this one in Western Australia. Although they are not common, we’re seeing more images like this thanks to more drone coverage. They can flow fast enough to carry suspended sand from the breaking waves offshore.

Other Types of Rip Currents

Mega Rips

These rips only occur on embayed (or pocket) beaches when waves are very large, such as during major storms.  In these cases, massive mega rips can form along one or both headlands, or out from the middle of the beach, depending on the length and geometry of the embayment. These rips aren’t dangerous for people, because no-one would be swimming, but they can cause significant beach erosion

Swash Rips

The term ‘swash’ refers to the combined uprush and backwash when waves break at the shoreline. Swash rips are not technically rips, but they do involve the offshore flow of water, albeit not very far offshore. Most beaches that are very steep are characterised by features known as beach cusps. When waves break on these beaches the water rushes up the beach and the cusps act to concentrate the water in a very strong backwash, which can extend into deeper water. I often think the terminology ‘undertow’ may have originated with these types of rips.