Rip currents

A rip current, often referred to simply as a rip, or by the misnomer rip tide, is a specific kind of water current which 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, cutting through the lines of breaking waves like a river running out to sea, and is strongest near the surface of the water.

Rip currents can be hazardous to people in the water. Swimmers who are caught in a rip and who do not understand what is going on, and who may not have the necessary water skills, may panic, or exhaust themselves by trying to swim directly against the flow of water. Because of these factors, rips are the leading cause of rescues by lifeguards at beaches, and in the US rips 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 latter term incorrectly when they mean 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 beyond the zone of the breaking waves.

A rip current forms because wind and breaking waves push surface water towards the land, and this causes a slight rise in the water level along the shore, which will tend to flow back to the open water by the route of least resistance. When there is a local area which is slightly deeper or a break in an offshore 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 shore towards the outgoing rip as feeder currents, and then flows out at approximately a right angle to the beach in a tight current called the "neck" of the rip, 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 occur on a gently shelving shore where breaking waves approach near perpendicular to the average shoreline, or where underwater topography encourages outflow at a specific area. They can form at the coasts of oceans, seas, and large lakes when there are waves with sufficient energy. The location of rip currents can be difficult to predict. While some tend to recur always in the same place, others can appear and disappear suddenly at various locations along the beach. This will depend on the bottom topography and the direction of the waves.

Rip currents can potentially occur wherever there is strong longshore variability in wave breaking. This variability may be caused by such features as sandbars (as shown in the animated diagram), by piers and jetties, and even by crossing wave trains, and are often located in places such as where there is a gap in a reef or low area on a sandbar, and may deepen the channel through a sandbar once formed. Rip currents are usually quite narrow, but tend to be more common, wider and faster, when and where breaking waves are large and powerful. Local underwater topography makes some beaches more likely to have rip currents; a few beaches are notorious in this respect.

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Wikipedia