Rip current

Definition of Rip current: Rip currents are wave-generated, concentrated seaward-directed currents that originate in the surf zone. This is the common definition for Rip current, other definitions can be discussed in the article

The above general definition includes a wide family of currents. It includes, for example, a longshore current that separates from the shore at a strong curvature of the shoreline. An important class consists of currents driven by alongshore variations in wave breaking and the resulting differences in wave set-up. These variations may arise from self-organized nearshore morphology, offshore bathymetric features or coastal structures. Other rip currents result from the offshore deflection of longshore currents or from transient hydrodynamic processes.

Rip currents are a major hazard to swimmers and can contribute to beach erosion by transporting sediment offshore. They also play an important role in the exchange of water and suspended material between the surf zone and offshore waters. Rip currents can occur on any coast where the wave breaker location varies alongshore. Rip-current activity (morphodynamic rips) on open coasts typically increases with a shore-normal wave incidence, higher wave height, longer wave period, and lower tide level.

Morphodynamic rips

Fig. 1. Schematic view of a rip cell. Beach: yellow, sea: blue; shallow water: light blue; nearshore bars: hatched; flow pattern: red arrows.

Numerous different morphological patterns can arise in the surf zone as a result of morphodynamic feedback processes, as described in the article Rhythmic shoreline features. Rip cells are one member of this large family of self-organized surf zone patterns. When waves break over a nearshore sandbar, the decrease in the cross-shore component of radiation stress produces an onshore-directed force that raises the mean water level between the bar and the shoreline. This wave-induced water-level increase is known as wave set-up. On an approximately longshore-uniform coast, the cross-shore radiation-stress gradient associated with wave breaking is largely balanced by the pressure gradient due to wave set-up. When wave breaking varies alongshore, differences in set-up develop. The resulting alongshore pressure gradients drive currents towards regions with weaker breaking and lower set-up, where the water can return seaward as a concentrated rip current.

These currents form circulation cells that strengthen and consolidate the hydrodynamic imbalances by imprinting the rip flow circulation in the nearshore bathymetry: (1) a rip channel is scoured at the location of dominant seaward flow, in which the seaward rip flow is concentrated; (2) beach embayment may develop opposite the rip channel as a result of spatial variations in wave action and sediment transport; (3) onshore horns are accreted at the inner bar where the waves are attenuated most strongly. The resulting morphology reinforces the alongshore differences in wave breaking and the associated rip circulation (positive morphodynamic feedback). The corresponding flow circulations and rhythmic shoreline features are schematically displayed in Fig. 1. The presence of rip currents in the field is illustrated for two different beaches in Fig. 2 and 3. The rips are situated between the zones where waves are breaking on the nearshore sandbar.

Fig. 2. Rip currents at Burrill Beach, South Coast NSW, Australia. The strength of the current is illustrated by the offshore plume of sediment that is stirred up from the seabed. The beach facing the rip is most exposed to wave breaking. Photo credit Daryl Gama. https://www.scienceofthesurf.com/

Fig. 3. Rip cell at the barred coast of North Holland, The Netherlands. Photo credit Rens Jacobs https://beeldbank.rws.nl/

Channel rips arise on a barred sandy shoreface under moderate wave conditions when the direction of incident waves is close to shore-normal. The rip-channel bathymetry persists under such conditions, but disappears when the nearshore sandbars are straightened by highly energetic, obliquely incident waves. In case of oblique wave incidence the rip cell pattern travels slowly in the direction of the littoral drift. Morphodynamic rips usually have a regular spacing along the coast with wavelengths in the range 50 to 500 meters (Short and Brander, 1999).

Waves propagating against a rip current become steeper and can be refracted towards the rip channel. The resulting increase in wave breaking within the channel can raise the local wave set-up, reducing the alongshore pressure gradients that drive the feeder currents. This provides a negative feedback on rip-current strength (Yu and Slinn, 2003). When the opposing rip current causes breaking of the highest waves, this negative feedback is weakened. The alternating strengthening and weakening of this feedback can produce a slow pulsation of the rip flow. Other causes of rip-current fluctuations include the response to wave groups, standing infragravity wave patterns, vortices generated by rip-current shear instability, and tidal modulation. Rip currents may also be modulated by the occurrence of low-frequency surf zone eddies (periods longer than about 4 minutes).

Rip currents at low tide are usually stronger than at high tide. A possible explanation is that during low water, the reduced water depth over the bar crests enhances wave breaking relative to the deeper rip channels. This strengthens the alongshore differences in wave set-up that drive rip circulation. This occurs especially in the presence of a bar-trough morphology that often develops after storms, when a storm-built bar migrates back onshore. A more frequent occurrence of morphodynamic rips can thus be expected in such situations (Dalrymple et al., 2011).

Boundary rips

Fig. 4. Rip currents induced by a groyne under oblique wave incidence. Updrift side (right): Deflection rip current. Downdrift side (left): Shadow rip current. Beach: yellow; sea: blue; shallow water: light blue; wave breaking: white; rip currents: red arrows; dotted dark blue lines: wave rays. Deflection rips can transport large quantities of sediment offshore well beyond the depth of closure.

Boundary rips are related to the presence of hard topographic structures (headlands, groynes, jetties). Such structures may locally shield the beach from obliquely incident waves. In the wave shadow zones, the radiation stress is not in balance with the wave set-up at the beach. The excess pressure of the wave set-up generates a rip current that runs along the structure - a so-called shadow rip. Shadow rips often form circulation cells in which part of the seaward flow returns shoreward through the breaker zone (Castelle et al. 2016).

The oblique wave incidence also generates a longshore current that causes accretion at the updrift side of the structure and erosion at the downdrift side. The longshore current at the updrift side is deflected offshore by the structure. This deflected current is called a deflection rip. It can extend far offshore, up to a few times the surf zone width (McCarroll et al., 2014). The shadow rip and deflection rip are schematically depicted in Fig. 4.

The offshore extent of a deflection rip depends strongly on the relative groyne length [math]\displaystyle{ L_g/X_s }[/math], where [math]\displaystyle{ L_g }[/math] is groyne length and [math]\displaystyle{ X_s }[/math] is surf-zone width. For short groynes ([math]\displaystyle{ L_g/X_s < 0.5 }[/math]), offshore deflection is generally limited. Strong deflection currents can develop for intermediate relative lengths ([math]\displaystyle{ 0.5 < L_g/X_s < 1.25 }[/math]), although much of the flow may continue alongshore. For longer groynes ([math]\displaystyle{ L_g/X_s > 1.25 }[/math]), the deflected flow is more likely to extend beyond the surf zone, while its velocity generally decreases. These offshore rip flows along groynes can be a significant conduit for transporting sediment offshore, especially in cases of high waves with strong oblique incidence (Scott et al., 2016). This offshore sediment transport by groyne-induced boundary rips can cause substantial sand losses from groyne compartments, reducing the effectiveness of groyne fields in preventing continuing beach erosion.

Other rip types

According to the more general definition several other mechanisms can produce rip currents.

One type of rip current is related with the presence of bathymetric features outside the surf zone, such as rocky outcrops, submarine canyons, offshore shore-parallel breakwaters, or an island near the coast. Such features perturb the alongshore uniformity of the incident wave field and the uniformity of mean water levels at the beach, by wave shielding effects, by affecting local wave dissipation, or by creating specific wave diffraction and refraction patterns. Perturbation of longshore uniformity induces rip currents that can become channelized. However, unlike morphodynamic rips, the channels are not self-organizing with alongshore periodicity. They are often referred to as forced rips; those associated with wave focusing are sometimes called focused rips. The presence and strength of these forced rips depend strongly on offshore wave conditions.

Rip currents can also develop without morphodynamic interaction. These rip currents are known in the literature under various names: transient rips, hydrodynamic rips, or flash rips. They are episodic or quasi-periodic, typically lasting from less than a minute to several minutes, and their location can shift along the beach. Transient rip currents can occur on beaches with both oblique and normal wave incidence. The driving mechanism is not yet fully elucidated. There are indications that flash rips are associated with flow vorticity generated by shear instability of the alongshore current or by temporal and spatial alongshore non-uniformity of the wave breaking process. The simplest type of transient rip current could be the one formed at the intersection nodes of wave trains generated by regular waves with slightly different wave periods and directions. Transient rip currents are generally short-lived and spatially variable, but they can nevertheless generate strong offshore flows and pose a hazard to swimmers.

Relevance of rip currents

• Rip currents are among the leading causes of swimming accidents and drowning at wave-exposed beaches
• Rip currents can accelerate coastal erosion by transporting beach sand offshore, especially in the presence of structures such as groynes
• Rip currents contribute to the mixing and flushing of nearshore waters and to the regulation of nearshore water quality
• Rip currents contribute to the exchange of organisms between nearshore and offshore waters

Recommended reviews on rip currents

Castelle, B., Scott, T., Brander, R.W. and McCarroll, R.J. 2016. Rip current types, circulation and hazard. Earth Science Reviews 163: 1–21

MacMahan, J.H., Reniers, A.J.H.M., Thornton, E.B. and Stanton, T.P. 2004. Surf zone eddies coupled with rip current morphology. Journal of Geophysical Research 109, C07004

Largier, J.L. 2022. Rip Currents and the Influence of Morphology on Wave-Driven Cross-Shore Circulation. Ch. 8.05. Treatise on Geomorphology, 2nd edition, Volume 8. Elsevier.

Recommended website: https://www.scienceofthesurf.com/

Related articles

Rhythmic shoreline features

Currents

Nearshore sandbars

Shoreface profile

References

The main author of this article is Job Dronkers Please note that others may also have edited the contents of this article. Citation: Job Dronkers (2026): Rip current. Available from http://www.coastalwiki.org/wiki/Rip_current [accessed on 14-11-2026] For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Dronkers J