Rip Science

A rip current is a strong, narrow seaward flow of water that begins close to shore and extends through the surf zone and sometimes seaward of the breaking waves.

Rip Current Formation and Occurrence

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 (Figure 1). 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.

Figure 1: Longshore variations in wave height and wave set up resulting in rip current formation. (source: Komar, 1998)

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Rip Current Types

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 (Figure 2). 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 (Figure 3). Furthermore, under certain conditions rip flow does not always extend past the surf zone, but returns shorewards in a wide circle (Figure 4).

Figure 2: Idealised schematic of a rip current system showing longshore rip feeder currents, rip neck and rip head. (source: Komar, 1998)

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Figure 3: Rip current with no feeder channels © Rob Brander

Figure 4:
Circulating rip currents occuring under conditions with outer longshore sandbank.
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The most common types of rip currents are FIXED RIPS that are confined to deeper channels between sandbars and are often persistent in 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.

Figure 5:
A large flash rip which is 'shearing' beyond the surf zone
Image: Tom Cozad
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Topographic rip currents 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” and the “Bronte Express” in Sydney’s eastern suburbs due to their persistent location.

Figure 4: Bondi Express

“Swash” rip currents are smaller, usually occur on steep beaches and are associated with strong backwash from the beach face due to larger waves. They do not extend very far from the shoreline.

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Rip Current Identification

Fixed rips are the most common type of rip in Australia and are best identified by looking for narrow and persistent dark gaps in the surf zone. The reason for this is that fixed rips occupy deeper channels and deep water is always darker. Furthermore, waves break less in deeper water and more on the shallow sandbars adjacent to the rip current. Fixed rips will therefore appear as a gap between areas of concentrated whitewater. This unfortunately “lures” people into swimming in these areas as the water appears clear and calm.

Flash rips look different and appear as streaks of white, turbulent water often with clouds of sand in suspension. They often extend past the lines of breaking waves. When rip flow is particularly strong, the interaction of water moving seaward with the rip current and incoming waves often creates a bumpy or disturbed surface. With stronger flows it is also possible to observe turbulent streaks of water, seaweed, suspended sand and floating objects moving offshore in the rip currents. This is more common with flash rip currents.

Rip feeders and rip necks should generally not be identified by looking for murky or turbid water, but the rip head is often characterised by plumes of sediment as the rip current enters cleaner water offshore and slows down, and concentrates its load of suspended sediments. Finally, rip currents that have persisted in the same location for days or weeks can cause localised beach erosion at the shoreline creating noticeable cusp-shaped rip current embayments.

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Rip Currents Behaviour

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 place in 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, which is 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, which are 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.

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