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A rip current, often simply called a rip (or misleadingly a ''
rip tide A rip tide, or riptide, is a strong offshore current that is caused by the tide pulling water through an inlet along a barrier beach, at a lagoon or inland marina where tide water flows steadily out to sea during ebb tide. It is a strong tidal flo ...
''), is a specific kind of water current that can occur near beaches with breaking waves. A rip is a strong, localized, and narrow current of water which moves directly away from the
shore A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past a ...
, cutting through the lines of breaking waves like a river running out to sea. The current in a rip is strongest and fastest next to the surface of the water. Rip currents can be hazardous to people in the water. Swimmers who are caught in a rip current and who do not understand what is happening, or who may not have the necessary water skills, may panic, or they may exhaust themselves by trying to swim directly against the flow of water. Because of these factors, rip currents are the leading cause of rescues by lifeguards at beaches, and in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
, they are the cause of an average of 46 deaths by drowning per year. A rip current is not the same thing as undertow, although some people use the term incorrectly when they are talking about a rip current. Contrary to popular belief, neither rip nor undertow can pull a person down and hold them under the water. A rip simply carries floating objects, including people, out to just beyond the zone of the breaking waves, at which point the current dissipates and releases everything it is carrying.


Causes and occurrence

A rip current forms because
wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...
and breaking waves push surface water towards the land, and this causes a slight rise in the water level along the shore. This excess water will tend to flow back to the open water via the route of least resistance. When there is a local area which is slightly deeper, or a break in an offshore sand bar or reef, this can allow water to flow offshore more easily, and this will initiate a rip current through that gap. Water that has been pushed up near the beach flows along the shore towards the outgoing rip as "feeder currents", and then the excess water flows out at a right angle to the beach, in a tight current called the "neck" of the rip. The "neck" is where the flow is most rapid. When the water in the rip current reaches outside of the lines of breaking waves, the flow disperses sideways, loses power, and dissipates in what is known as the "head" of the rip. Rip currents can form by the coasts of oceans, seas, and large lakes, whenever there are waves of sufficient energy. Rip currents often occur on a gradually shelving shore, where breaking waves approach the shore parallel to it, or where underwater topography encourages outflow at one specific area. The location of rip currents can be difficult to predict; some tend to recur always in the same places, but others can appear and disappear suddenly at various locations along the beach. The appearance and disappearance of rip currents is dependent on the bottom topography and the exact direction from which the surf and swells are coming. Rip currents occur wherever there is strong longshore variability in wave breaking. This variability may be caused by such features as sandbars, by
pier image:Brighton Pier, Brighton, East Sussex, England-2Oct2011 (1).jpg, Seaside pleasure pier in Brighton, England. The first seaside piers were built in England in the early 19th century. A pier is a raised structure that rises above a body of ...
s and
jetties A jetty is a structure that projects from land out into water. A jetty may serve as a breakwater, as a walkway, or both; or, in pairs, as a means of constricting a channel. The term derives from the French word ', "thrown", signifying somet ...
, and even by crossing wave trains, and are often located in places where there is a gap in a
reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock out ...
, or low area on a
sandbar In oceanography, geomorphology, and Earth science, geoscience, a shoal is a natural submerged ridge, bank (geography), bank, or bar that consists of, or is covered by, sand or other unconsolidated material and rises from the bed of a body o ...
. Rip currents, once they have formed, may deepen the channel through a sandbar. Rip currents are usually quite narrow, but they tend to be more common, wider, and faster, when and where breaking waves are large and powerful. Local underwater
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...
makes some beaches more likely to have rip currents; a few beaches are notorious in this respect. Although ''rip tide'' is a misnomer, in areas of significant tidal range, rip currents may only occur at certain stages of the tide, when the water is shallow enough to cause the waves to break over a sand bar, but deep enough for the broken wave to flow over the bar. (In parts of the world with a big difference between high tide and low tide, and where the shoreline shelves gently, the distance between a
bar Bar or BAR may refer to: Food and drink * Bar (establishment), selling alcoholic beverages * Candy bar * Chocolate bar Science and technology * Bar (river morphology), a deposit of sediment * Bar (tropical cyclone), a layer of cloud * Bar (u ...
and the shoreline may vary from a few meters to a kilometer or more, depending whether it is high tide or low tide.) A fairly common misconception is that rip currents can pull a swimmer down, under the surface of the water. This is not true, and in reality a rip current is strongest close to the surface, as the flow near the bottom is slowed by friction. The surface of a rip current can often appear to be a relatively smooth area of water, without any breaking waves, and this deceptive appearance may cause some beach-goers to believe that it is a suitable place to enter the water.


Technical description

A more detailed and technical description of rip currents requires understanding the concept of
radiation stress In fluid dynamics, the radiation stress is the depth-integrated – and thereafter phase-averaged – excess momentum flux caused by the presence of the surface gravity waves, which is exerted on the mean flow. The radiation stresses behave as ...
. Radiation stress is the force (or momentum flux) that is exerted on the water column by the presence of the wave. When a wave reaches shallow water and
shoals In oceanography, geomorphology, and geoscience, a shoal is a natural submerged ridge, bank, or bar that consists of, or is covered by, sand or other unconsolidated material and rises from the bed of a body of water to near the surface. I ...
, it increases in height prior to breaking. During this increase in height, radiation stress increases, because of the force exerted by the weight of the water that has been pushed upwards. To balance this, the local mean surface level drops; this is known as the ''setdown''. When the wave breaks and starts reducing in height, the radiation stress decreases as the amount of water that is elevated decreases. When this happens, the mean surface level increases — this is known as the ''setup''. In the formation of a rip current, a wave propagates over a sandbar with a gap in it. When this happens, most of the wave breaks on the sandbar, leading to "setup". However, the part of the wave that propagates over the gap does not break, and thus the "setdown" continues in that part. Because of this phenomenon, the mean water surface over the rest of the sandbar is higher than that which is over the gap, and the result is that a strong flow issues outward through the gap. This strong flow is the rip current. The vorticity and inertia of rip currents have been studied. From a model of the vorticity of a rip current done at Scripps Institute of Oceanography, it was found that as a fast rip current extends away from shallow water, the vorticity of the current increases, and the width of the current decreases. This model also acknowledges that friction plays a role and waves are irregular in nature. From data from Sector-Scanning Doppler Sonar at Scripps Institute of Oceanography, it was found that rip currents in La Jolla, California, lasted several minutes, that they reoccurred one to four times per hour, and that they created a wedge with a 45° arch and a radius of 200–400 meters.


Visible characteristics

Rip currents have a characteristic appearance, and, with some experience, they can be visually identified from the shore before entering the water. This is helpful to lifeguards, swimmers, surfers, boaters, divers and other water users, who may need to avoid a rip, or in some cases make use of the flow. Rip currents often look somewhat like a road or river running straight out to sea, and they are easiest to notice and identify when the zone of breaking waves is viewed from a high vantage point. The following are some visual characteristics that can be used to identify a rip: * A noticeable break in the pattern of the waves — the water often looks flat at the rip, in contrast to the lines of breaking waves on either side of the rip. * A "river" of foam — the surface of the rip sometimes looks foamy, because the current is carrying foam from the surf out to open water. * Different color — the rip may differ in color from the surrounding water; it is often more opaque, cloudier, or muddier, and so, depending on the angle of the sun, the rip may show as darker or lighter than the surrounding water. * It is sometimes possible to see that foam or floating debris on the surface of the rip is moving out, away from the shore. In contrast, in the surrounding areas of breaking waves, floating objects and foam are being pushed towards the shore. These characteristics are helpful in learning to recognize and understand the nature of rip currents. Learning these signs can enable a person to recognize the presence and position of rips before entering the water. In the United States, some beaches have signs created by the
National Oceanic and Atmospheric Administration The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...
(NOAA) and
United States Lifesaving Association The United States Lifesaving Association is a nonprofit professional association of beach lifeguards and open water rescuers in the United States. History The United States Lifesaving Association's predecessor organization was founded in 1956 ...
, explaining what a rip current is and how to escape one. These signs are titled, "Rip Currents; Break the Grip of the Rip". Two of these signs are shown in the image at the top of this article. Beachgoers can also get information from lifeguards, who are always watching for rip currents, and who will move their safety flags so that swimmers can avoid rips.


Danger to swimmers

Rip currents are a potential source of danger for people in shallow water with breaking waves, whether this is in seas, oceans or large lakes. Rip currents are the proximate cause of 80% of rescues carried out by beach lifeguards. Rip currents typically flow at about , but they can be as fast as , which is faster than any human can swim. However, most rip currents are fairly narrow, and even the widest rip currents are not very wide; swimmers usually can exit the rip easily by swimming at a right angle to the flow, parallel to the beach. Swimmers who are unaware of this fact may exhaust themselves trying unsuccessfully to swim directly against the flow. The flow of the current also fades out completely at the head of the rip, outside the zone of the breaking waves, so there is a definite limit to how far the swimmer will be taken out to sea by the flow of a rip current. In a rip current, death by
drowning Drowning is a type of suffocation induced by the submersion of the mouth and nose in a liquid. Most instances of fatal drowning occur alone or in situations where others present are either unaware of the victim's situation or unable to offer a ...
occurs when a person has limited water skills and panics, or when a swimmer persists in trying to swim to shore against a strong rip current, and thus eventually becomes exhausted and drowns. According to NOAA, over a 10-year average, rip currents cause 46 deaths annually in the United States, and 64 people died in rip currents in 2013. However, the
United States Lifesaving Association The United States Lifesaving Association is a nonprofit professional association of beach lifeguards and open water rescuers in the United States. History The United States Lifesaving Association's predecessor organization was founded in 1956 ...
"estimates that the annual number of deaths due to rip currents on our nation's beaches exceeds 100." A study published in 2013 in Australia revealed that rips killed more people on Australian territory than bushfires, floods, cyclones and shark attacks combined.


Management

People caught in a rip current may notice that they are moving away from the shore quite rapidly. Often, it is not possible to swim directly back to shore against a rip current, so this is not recommended. Contrary to popular misunderstanding, a rip does not pull a swimmer under the water; it simply carries the swimmer away from the shore in a narrow band of moving water. A rip current is like a moving
treadmill A treadmill is a device generally used for walking, running, or climbing while staying in the same place. Treadmills were introduced before the development of powered machines to harness the power of animals or humans to do work, often a type o ...
, which the swimmer can get out of quite easily by swimming at a right angle, across the current, i.e. parallel to the shore in either direction. Rip currents are usually not very wide, so getting out of one only takes a few strokes. Once out of the rip current, getting back to shore is also not difficult, since waves are breaking, and floating objects (including swimmers) will be pushed by the waves towards the shore. As an alternative, people who are caught in a strong rip can simply relax (either floating or
treading water Treading water or water treading is what a Human swimming, swimmer can do while in a vertical position to keep their head above the surface of the water, while not providing sufficient directional thrust to overcome inertia and propel the swimmer ...
) and allow the current to carry them until it dissipates completely once it is beyond the surf line. Then the person can signal for help, or swim back through the surf, doing so diagonally, away from the rip and towards the shore. It is necessary for coastal swimmers not only to understand the danger of rip currents, but to learn how to recognize them, and how to deal with them. And whenever possible, it is also necessary that people enter the water only in areas where lifeguards are on duty.


Uses

Experienced and knowledgeable water users, including surfers, body boarders,
divers Diver or divers may refer to: *Diving (sport), the sport of performing acrobatics while jumping or falling into water *Practitioner of underwater diving, including: **scuba diving, **freediving, **surface-supplied diving, **saturation diving, a ...
, surf lifesavers and kayakers, when they wish to get out beyond the breaking waves, will sometimes use a rip current as a rapid and effortless means of transportation.


See also

*
Cross sea A cross sea (also referred to as a squared sea or square waves) is a sea state of wind-generated ocean waves that form nonparallel wave systems. Cross seas have a large amount of directional spreading. This may occur when water waves from one w ...
* Longshore drift * Rip current statementwarnings issued by the U.S.
National Weather Service The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...
*
Undertow (water waves) In physical oceanography, undertow is the undercurrent that is moving offshore when waves are approaching the shore. Undertow is a natural and universal feature for almost any large body of water: it is a return flow compensating for the o ...


References


External links


NOAA glossary of terms used in describing rip currents
* * by Surf Life Saving Australia showcasing several signs to spot rips. {{DEFAULTSORT:Rip Current Physical oceanography Bodies of water Surf lifesaving Oceanographical terminology