Rip current
A rip is a strong, localized, and narrow current of water that moves directly away from the shore by cutting through the lines of breaking waves, like a river flowing out to sea.
Rip currents can form by the coasts of oceans, seas, and large lakes, whenever there are waves of sufficient energy.
The appearance and disappearance of rip currents is dependent upon the bottom topography and the direction from which the surf and swells are coming.
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.
[3] 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.
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.
[4] A more detailed and technical description of rip currents requires understanding the concept of radiation stress.
Radiation stress is the force (or momentum flux) that is exerted on the water column by the presence of the wave.
[6] 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.
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.
Learning these signs can enable a person to recognize the presence and position of rips before entering the water, which is an important skill as studies show the majority of people are unable to identify a rip current and therefore unable to identify safe places to swim.
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.
According to the NOAA rip currents caused an average of 71 deaths annually in the United States over the ten years ending in 2022 (with 69 in 2022).
[11] A 2013 Australian study found that rips killed more people in Australia than bushfires, floods, cyclones and shark attacks combined.
[13] As an alternative, people who are caught in a strong rip can simply relax, either floating or treading water, and allow the current to carry them until it dissipates completely once it is beyond the surf line.
[7] In a planned trial in a large rip current at Muriwai Beach in New Zealand, an Australian researcher from the School of Biological, Earth and Environmental Sciences, UNSW Sydney found that "just swim to the side" would not work as the rip current was too wide to see its sides, and said that, despite a rescue boat being near, he was unable to relax and not panic.
[14] Experienced and knowledgeable water users, including surfers, body boarders, divers, 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.
