Rip Current Science

Rip currents are driven by the action of breaking waves and are found on a range of beach types along ocean, sea, and lake coastlines--anywhere that breaking waves are present! Rip currents may occur on long, straight beaches, embayed beaches, beaches with sandbars and deeper channels, and flat and featureless beaches. They are also frequently found next to both natural and man-made hard structures such as headlands, groins, jetties, and piers.

Breaking waves alone are not enough to form a rip current: rip currents are formed when there are alongshore variations in wave breaking. In particular, rip currents tend to form in regions with less wave breaking sandwiched between regions of greater wave breaking. To understand why this is the case, we need to examine a phenomenon known as wave setup.

When waves break along a shoreline or over a shallow sandbar, they “push” water toward shore, resulting in an increase in water level near the beach known as wave setup. Wave setup is often about 10% of the breaking wave height and is difficult to see with the naked eye, but can be measured with oceanographic instruments. When wave breaking varies along the beach, for example as a result of a gap in a sandbar, resulting variations in wave setup result in rip currents. In places where there is more wave breaking, there is a corresponding higher setup, or water level, while places where there is less wave breaking have lower setup. The water essentially flows downhill, along the shore from the regions of high setup (more wave breaking) to the regions of lower setup (less wave breaking), where the currents converge and flow offshore as a rip current.

Video shows an example of rip current formation with water flowing from regions of greater wave breaking (H) to regions of lower wave breaking (L) and then seaward. Source: NOAA

Rip currents are part of nearshore circulation cells that transport water between the surf zone and areas offshore. While rip current circulation patterns have some common features, the details of the structure of these currents can vary widely.

Breaking waves in the surf zone result in onshore movement of water and patterns of wave setup that drive longshore “feeder currents” flowing along the shoreline, converging and turning seaward to form a narrow and fast-flowing rip-neck. The rip-neck extends through the surf zone and outside of the breaking waves where it eventually decelerates and dissipates as an expanding rip-head. This water can then be transported shoreward again by breaking waves, completing the cell. In some cases the rip-neck is narrow and flows perpendicular to the beach and extends well beyond the surf zone (see figure below).  In other cases, the rip may flow at an angle away from the beach, or may even meander.

Other rip currents are characterized by recirculating flow within the surf zone with only occasional exits of flow offshore (see figure below). Both types of circulation patterns exist and have significant implications for swimmer escape strategies and the transport of sand, larvae and water borne material.

Rip current structure and different types of rip current circulation patterns showing both surf zone and offshore exchange via exiting flow (left) and re-circulation within the surf zone (right). Source: Castelle et al., 2016.

 

Many different types of rip currents can occur on beaches in the United States and around the world. Scientists and beachgoers use a range of terms to describe these various rip currents types, and usage often is inconsistent. The following list of rip current types follows a scientific classification from Castelle et al. (2016) that differentiates rip currents based on their forcing mechanisms: the different causes of the alongshore variations in breaking waves that are crucial for the formation of rip currents.

Bathymetrically-controlled rip currents

The location and persistence of rip currents in many locations is controlled by the bathymetry, or shape of the bottom.  This can mean the bathymetry near the beach, including sandbars that may shift in position and exhibit different configurations, or the offshore bathymetric features that are relatively fixed, including submarine canyons, submarine ridges, and reefs. Bathymetrically-controlled rip currents may be referred to as either channelized or focused:

Channelized rip currents are the most documented and well understood rip current type. They occupy deeper channels that interrupt shallow, mostly shore-parallel sandbars found on many beaches. They can persist in a relatively fixed location for days, weeks, or months and are relatively easy to identify (see figure below). Channelized rip currents typically range in width from 5 to 100 yards, have depths on the order of 3 to 10 feet or more, and may be spaced anywhere from 50 to 500 yards apart, with greater spacing often occurring for large wave heights.

 

Examples of channelized rip currents in the Outer Banks, NC (top),  Marina Beach, CA (middle) and Pensacola Beach, FL (bottom). Source: M. Moulton (top); R. Brander (middle and bottom)

The dominant visual characteristic of channelized rips is a narrow path of darker water between areas of whitewater associated with breaking waves ( see figure below). While the darker water appears seemingly calm, the surface is often characterized by a choppy, rippled texture. When channel rip currents remain in the same place for a long period of time, they can also erode the shoreline creating pronounced scallops, sometimes referred to as rip embayments or megacusps. Channelized rips have also been referred to as bar-gap, fixed, and accretionary rip currents.

Focused rip currents, like channel rips, also occur at relatively fixed locations, but are controlled by alongshore variations in breaking waves created by offshore bathymetric features such as submarine canyons, submerged ridges, or offshore sand bars. These features result in wave patterns that tend to focus wave breaking and rip current formation at a particular location. However, as the direction of wave approach changes, so too can the location of focused rip currents. Focused rip currents can occur along flat, featureless beaches where they typically appear as offshore directed plumes of turbulent water and sediment (see figure below).

An example of a focused rip current caused by offshore bathymetry at Blacks Beach, California. Source: Tom Cozad

 

Rip currents often occur adjacent to man-made structures such as groins, jetties, and piers as well as natural geologic features such as headlands and rock/reef outcrops ( see figure below). These structurally-controlled rip currents are persistent in location and can occur even during small breaking wave conditions. Structural rips sometimes are associated with deeper water adjacent to structures, and in these cases appear as areas of darker water with reduced wave breaking activity.

Structural rips also have been referred to as boundary, topographic, permanent, and headland rip currents. They can be grouped into two types based on how and where they form. In some cases, these types can exist simultaneously on each side of the structure.

Shadow rip currents flow on the down-wave side of a rigid structure or boundary (i.e., the sheltered side). On straight beaches, the presence of a rigid obstacle creates alongshore variations in breaking waves due to the wave shadowing effect of the obstacle. Wave setup tends to be higher away from the structure due to larger waves, and lower closer to the structure due to smaller waves, as this region is more protected from wave energy. This pressure gradient forces water “downhill” in the alongshore direction back toward the obstacle and then offshore as a rip current alongside the obstacle (see figure below).

Deflection rip currents occur on the up-wave or exposed side of a rigid structure or boundary. On straight beaches, waves approaching and breaking against the shoreline at an angle can create strong alongshore currents that are physically deflected seaward when they reach the structure and flow offshore adjacent to it (see figure below).

Examples of structural rip currents occurring adjacent to a groin (left) or rock reefs (right). Source: Maria Estella Carelli (left); R. Brander (right)

 

As waves approach the shoreline, they usually break at an angle, generating a longshore current that flows along the shoreline (parallel) to the beach. When the longshore current (moving along the shore) encounters coastal structure (such as a groin, jetty, or pier) it is deflected in an offshore direction. This offshore-directed flow of water is called a rip current.

Hydrodynamically-Controlled Rip Currents

Unlike bathymetrically and boundary-controlled rip currents, some rip currents do not rely on the presence of nearshore or offshore bottom features or the presence of structures. These rip currents are controlled solely by hydrodynamics -- wave and current interactions -- and can occur on featureless straight beaches. It is not possible to predict the exact location and timing of hydrodynamic rip currents, but it may be possible to estimate the likelihood of their formation on a given beach.

There are several mechanisms by which hydrodynamic rips form. For example, waves originating from two different sources may approach the beach from different directions, and interference between the different wave trains causes higher breaking waves in some locations than in others. This variability in wave breaking leads to wave setup patterns that drive rip currents. In addition, waves that break in short sections, or “short-crested waves”, can drive small swirling flows or eddies that can merge to form “flash” or “transient” rip currents, lasting for several minutes. At other times, “shear instability” rip currents may form when eddies are shed off of longshore currents flowing along the beach as a result of waves breaking at an angle to the coast.

Hydrodynamically-controlled rip currents often present different visual characteristics than channelized rip currents and typically appear as narrow bands of turbulent whitewater and sand extending offshore of the surf zone ( see figure below). Less is known about their flow speed and duration due to a lack of field measurements; however, they still represent a risk to swimmers and waders due to their unpredictability and ability to carry people offshore into deeper water.

Examples of flash rips. The top photo shows turbulent water and sand exiting seaward under stormy conditions. The bottom photo shows a similar plume heading offshore during good weather. Both pictures are from Rehoboth Beach, DE.  Source: R. Brander (top), Wendy Carey (bottom).

 

 

While the best way to avoid getting caught in a rip current is always to swim near a lifeguard, however lifeguards are not everywhere at all times. Therefore a key aspect of being safe at a beach with breaking waves is to understand what rip currents are and how to avoid them by learning how to identify them.

The video below shows a channelized rip current.  Note the area in the center of the video where there is a gap in the line of breaking waves. Also note the slightly discolored water, sand and foam traveling seaward past the breaking waves. Source: NOAA

 

The following list provides some tips on how to best spot rip currents:

• It is much easier to spot rip currents from an elevated position overlooking the beach. This might be from a parking lot, beach access, sand dune, or headland (see figure below).

 

A picture of a channelized rip current taken from an elevated position on the dune beach access in the Outer Banks, NC. Source: G. Dusek

 

• Always watch the water for several minutes as rip current conditions can change.
• Channelized rip currents are the easiest to identify as they typically appear as darker, narrow gaps of water heading offshore between areas of breaking waves and whitewater. They can appear as darker paths heading out through the surf so look for gaps in the lines of breaking waves (see figure below).

Two examples of channelized rip currents showing dark gaps in the lines of breaking waves. Source: R. Brander

 

• Look for narrow regions of choppy, rippled water heading offshore. Waves moving toward shore may steepen as a result of opposing offshore rip current flows, leading to a different surface water texture (see figure below).

A rip current indicated by choppy, rippled water in the location where there are gaps in the lines of breaking waves. Source: R. Brander

 

• Look for plumes of sand or foam offshore of the breakers, often moved away from shore by rip currents (see figure below).
• Some types of rips, such as flash rips, can appear as narrow sections of turbulent whitewater heading offshore.

Rip currents (foreground and middle of photo)  extending away from shore indicated by foam and sand traveling seaward, offshore of the breaking waves. Source: R. Brander

 

• Look for pronounced scalloped embayments on the beach, which can be caused by channelized rip currents eroding the shoreline (see figure below).

Channelized rip currents can stay in the same place for days or weeks and can erode out scalloped embayments in the beach like the one in the middle of the photograph (Source: Rob Brander).

• Wear polarized sunglasses to help identify contrasting colors in the water; deep rip current channels stand out as darker water.
• Ask a lifeguard if there are any rip currents and to point them out for you.

Rip currents can occur along any coastline with breaking waves.

Flow speed and behavior: Rip currents flow at speeds that can quickly transport unsuspecting swimmers considerable distances offshore. Channel rip currents typically flow about 1 to 2 feet per second and may reach speeds up to 8 feet per second! Even in weaker rip currents, it can be difficult to stand in place at waist depth against persistent flows and the combination of breaking waves and currents can be disorienting. The speeds of the strongest rips are on par with the top speeds of some of the fastest Olympic swimmers in history. Even for a strong swimmer, attempting to swim back to shore against the seaward flowing current can quickly result in exhaustion.

Not all parts of a rip current flow at the same speed. For example, in a classic rip current system, flow speeds gradually increase from the alongshore feeders and and peak about half way offshore in the rip-neck before eventually slowing down and stopping in the rip head. Similarly, rip flow speeds tend to be fastest toward the middle of the rip current and are at their maximum just below the surface of the water.

Rip pulsing: Rip current flow is also unsteady, which means it changes over time. A rip pulse is a sudden acceleration in flow speed that lasts for a short time period (30 seconds to a minute) and is associated with the breaking of incoming wave groups (sets of larger waves). When groups of large waves break, water depths can rapidly increase due to increased wave setup and unwary swimmers and waders can suddenly lose their footing, be swept into a rip current, and carried offshore by the subsequent rip current pulse. During rip pulses, the speed of the flow can more than double. It is extremely important to understand that changes in rip current speed can change rapidly with random increases in incoming wave heights and water levels. Rip pulses are often what lead to people getting in trouble in rip currents.

Tidal modulation: The speed and presence of rip currents on ocean beaches is related to the stage of the tide because the tide affects the amount of wave breaking, which is what drives rip currents. On beaches with a tide range of less than 6 feet, rip currents often flow faster about an hour or two on either side of low tide, when water levels are shallower and breaking wave activity is at its peak. Rips may stop flowing completely around high tide because the greater water depths can dramatically reduce the breaking wave activity and decrease any alongshore variations in wave setup or water level (see figure below)

On beaches with large tidal ranges, the tide may drop low enough that bars become exposed (dry) at low tide. In this case, rips shut off once the bars are exposed and the maximum rip speeds occur at mid tide, when the water level is low enough that there is strong wave breaking on the bars but not so low that the bar is dry. Regardless of the tidal range, rip currents have not been found to be stronger during a falling outgoing tide, versus a rising incoming tide; only the water level of the tide is important to controlling rips, not the direction of the tide.

Wave breaking shown at the same location in Duck, NC at low and high tide on the same day.  Note the wave breaking over the bar at low tide (left) driving rip currents compared to wave breaking only on shore at high tide (right) and no observable rips. Source: US Army Corps of Engineers Field Research Facility

 

Rip currents are often incorrectly referred to as rip tides. Rip currents are not tides, so this term can cause confusion. Tides are a large-scale ocean process that typically leads to the slow rise and fall of water level over a period of 6-12 hours. Rip currents are not a rise and fall of water level, but rather concentrated currents that move water in a particular direction. As described in Section 6, the speed of rip current flow can be modulated by the level of the tide, which affects wave breaking.

Strong and concentrated currents often occur in tidal inlets, mouths of estuaries and harbor openings associated with both the incoming flood tide and outgoing ebb tide.  While these currents can be very strong and also represent a hazard, they are best referred to as tidal currents rather than rip tides.

Rip Currents or Undertow? Rip currents are not the only way that water brought toward the shoreline by breaking waves is returned seaward. Offshore movement of water located close to the bottom also moves water out. This phenomenon is known as undertow or a near bed return flow. The term undertow has sometimes been used mistakenly to refer to rip currents and has contributed to the myth that rip currents can pull a person under the water. Undertow and rip currents are separate phenomena and neither will pull a person under the water! Undertow is usually slower than rip currents and typically does not present a hazard to swimmers.

For some swimmers in distress, the false sensation of being pulled under by a rip current results from a combination of panic, exhaustion, and being hit by breaking waves.

Undertow also is often confused with the backwash of broken waves at the shoreline. After a wave breaks and runs up the beach face, the returning water is called backwash and can be quite strong, particularly on steep beaches and in the embayments of beach cusps. While this backwash will not carry water very far offshore, it can be strong enough to knock people over, particularly small children and the elderly, and carry them into deeper water.

Collapsing Sandbars? Some mass rescue events and fatal drownings have been incorrectly attributed to the collapse of a sandbar, which suddenly left waders in deeper water. Sandbars do not collapse. Sandbars are large masses of sand and while they do shift in location, this is a slow and gradual process taking days to months under normal conditions, or as short as several hours during a large storm. In many cases, the real cause of swimmer distress in these situations is a group of larger waves, which causes a small rise in water level (wave setup), causing waders to lose footing. This is often associated with a rip current pulse or increase in flow speed as described in Section 6, which can present a further hazard.

• Backwash: The seaward return of water following the uprush of waves on the shoreline. Also called backrush or run down.
• Beach cusp: One of a series of crescentic undulations along the shoreline spaced at more or less regular intervals. Cusps are characterized by ridge-like horns separated by shallower embayments.
• Bed Return Flow: A term used to describe the seaward movement of water near the bottom. Often referred to as undertow.
• Breaker: A wave that has become so steep that the crest of the wave topples forward, moving faster than the main body of the wave. A breaking wave.
• Breaker zone: The zone within which waves approaching the coastline commence breaking, typically in water depths around 10 to 20 ft.  Denotes the seaward extent of the surf zone. Can often be confined to crests of sandbars.
• Embaymenrt: A large indentation in a shoreline forming an open bay bound by headlands.
• Estuary: A semi-enclosed coastal body of water that has a free connection with the open sea and where freshwater and saltwater mix. The seawater is usually brackish or measurably diluted with freshwater.
• Feeder current: The currents that flow along the shoreline, converge and form the offshore-directed neck of a rip current.
• Groin: A shore-protection structure (often built to trap littoral drift or slow erosion of the shore). It is narrow in width (measured parallel to the shore) and its length may vary from tens to hundreds of yards (extending from a point landward of the shoreline out into the water). Groins may be classified as permeable (with openings through them) or impermeable (a solid or nearly solid structure).
• Jetty: On open seacoasts, a structure extending into a body of water to direct and confine the stream or tidal flow to a selected channel, or to prevent the navigation channel from infilling with sediment. Jetties are built at the mouth of a river or entrance to a bay to help deepen and stabilize a channel and facilitate navigation.
• Littoral currents: A current running along the shoreline (parallel) to the beach and generally caused by waves striking the shore at an angle.
• Littoral drift: The sedimentary material moved along the shoreline (parallel) in the nearshore zone by waves and currents.
• Longshore current: A current located in the surf zone, moving generally along the shoreline (parallel), generated by waves breaking at an angle with the shoreline, also called an alongshore or littoral current.
• Mega-rip currents: A type of extremely large rip current resulting from the circulation cell of an entire embayment or pocket beach.
• Nearshore: The region extending seaward from the shoreline, beyond the surf zone to where waves first start to interact with the bottom.
• Rip Head: That part of a rip current circulation typically located beyond the breakers, marked by a spreading out or fanning of the circulation. It is here where the velocity and strength of the rip current circulation begins to weaken considerably.
• Rip Neck: That part of a rip current circulation located in the surf zone, marked by a narrow band of swiftly moving, seaward flowing water. It is here where velocity of the circulation is at a maximum.
• Rip Channel: A deeper channel cut by the seaward flow of a rip current, usually between sandbars.
• Rip Current:  A strong, narrow and concentrated seaward flow of water that extends from close to the shoreline, through the surf zone and variable distances beyond.
• Rip Tide: This term has inconsistent usage, and is not used widely in the scientific community. Rip currents are not rip tides. A tidal current is a distinct and separate type of current, and especially strong and variable tidal currents are sometimes called a "rip tide" or "tidal rip.: These strong tidal currents occur in areas where tidal flow is constricted like inlets, mouths of estuaries, embayments and harbors. These currents may cause drowning deaths, but these tidal currents are a separate and distinct phenomenon from rip currents.
• Runup: The rush of water up a beach due to the breaking of a wave. The amount of runup is the vertical height above stillwater level that the rush of water reaches. Also referred to as uprush.
• Sandbar: An offshore ridge or mound of sand which is submerged (at least at high tide) and either separated from the beach by a short distance, or connected to the beach. They are often shore-parallel, but exhibit various shapes and forms and are also common at the mouths of rivers or estuaries.
• Shoreline: The intersection of the ocean water surface with the shore or beach. On tidal coasts, the shoreline position changes with the rise and fall of the tide.
• Significant Wave Height: The average wave height of the one-third highest waves over a period of time..
• Surf Zone:  Area of water between the high tide level on the beach and the seaward extent of breaking waves.
• The combined uprush and backwash of waves after they break on a shoreline. The swash may be deflected by beach cusps, concentrating the downrush in cusp embayments.
• Swash Zone:The region of the beach that is alternately wet and dry as waves run up and down.
• Swell: Wind-generated waves that are no longer directly forced by local winds.  Often they have traveled out of their source region, usually over a considerable distance. Swell waves exhibit a more regular and longer period (typically between 8 and 20 seconds) with flatter crests than choppy, locally generated wind waves.
• Tide: The periodic rising and falling of the water resulting from the gravitational attraction of the Moon and Sun acting upon the world’s oceans as the Earth rotates.
• Tide Range: The vertical elevation difference between water level between high and low tide.
• Undertow: A scientific term used to describe the seaward return of water close to the bottom surface that is driven by breaking waves. Often referred to as bed return flow. Undertow is often mistakenly used to describe rip currents. It is also often associated with the strong backwash after breaking waves.
• Uprush: The landward rush of water up the beach after a wave as broken.
• Wave Height: The vertical distance between the crest and the preceding trough of a wave.
• Wave Period: The time taken for two successive wave crests to pass a fixed point.
• Wind Waves: Waves generated by, and directly attributable to local winds, as opposed to swell waves.  They typically have wave periods of less than 8 seconds.

A number of excellent reviews of rip currents exist in the scientific literature that provide a more detailed overview of the material provided on this site:

• Brander, R.W. (2015). Rip Currents. In: Sea and Ocean Hazards, Risks and Disasters (Eds. J. Ellis, D. Sherman). Treatise in Hazards and Disasters. Elsevier, p. 335-380.
• Castelle, B., Scott, T., Brander, R.W., McCarroll, R.J.  (2016). Rip current type, circulation and hazard. Earth Science Reviews, 163: 1-21.
• Dalrymple, R.A., MacMahan, J.H, Reniers, A.J.H.M., Nelko, V. (2011). Rip currents. Annual Review of Fluid Mechanics, 43:551-581
• MacMahan, J.H., Thornton, E.B., Reniers, A.J.H.M. (2006). Rip current review. Coastal Engineering, 53:191:208.

This review was created by Dr. Gregory Dusek (NOAA National Ocean Service), Dr. Melissa Moulton (University of Washington, APL) and Dr. Rob Brander (UNSW Sydney) in July 2018.