Rip current

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"Riptide" redirects here. For other uses, see Riptide (disambiguation).
Rip current mechanism: breakers cross sand bars off the shore, the water travels back out to sea through the gap in the sand bars, creating a fast "rip" current
A small rip current, seen from above

A rip current, commonly referred to simply as a rip, or by the misnomer "rip tide", is a current or flow within a larger body of water; specifically a strong localized (narrow) current of water near the surface that moves directly away from the shore, cutting through the lines of breaking waves.[1] Rip currents can occur at any beach where there are breaking waves, on oceans, seas, and large lakes. The location of rip currents is often unpredictable; they can appear and disappear at various locations in the water next to a beach.[2]

Rip currents can be hazardous to people who are in the water. Swimmers who are caught in a rip and who do not understand what is going on, may panic, or exhaust themselves by trying to swim against the flow of water. Rips are the leading cause of rescues by lifeguards from beaches, and in the US, rips are responsible for an average of 46 deaths from drowning each year.

Rips tend to be more common, wider and faster, wherever (and whenever) breaking waves are large and powerful. Varying underwater topography makes some beaches more likely to have rip currents; a few beaches are notorious in this respect.

Causes and occurrence[edit]

When wind and waves push water toward the shore, that water is often forced sideways by the oncoming waves. This water streams along the shoreline until it finds a suitable exit route back out to the open water, straight out, at a right angle to the beach. This return flow of water is called a rip current. Rip currents are usually quite narrow, and are located in places such as where there is a gap in a sandbar, under piers, and along jetties.

A fairly common misconception is that rip currents are somehow able to pull a swimmer vertically down, under the surface of the water. This is not true, and in reality a rip current is strongest close to the surface. This strong surface flow tends to damp the effect of incoming waves, leading to the illusion of a calm part of the sea, without waves, which may possibly attract some swimmers to that area. The offshore path taken by a rip current can be demonstrated by placing colored dye at the start of a current at the shoreline.[3]

A more detailed description involves a quantity known as radiation stress. This is the force (or momentum flux) exerted on the water column by the presence of the wave. As a wave shoals and increases in wave height prior to breaking, radiation stress increases. To balance this, the mean surface (the water level with the wave averaged out) decreases—this is known as setdown. As the wave breaks and continues to reduce in height, the radiation stress decreases. To balance this force, the mean surface increases—this is known as setup. As a wave propagates over a sandbar with a gap (as shown in the lead image), the wave breaks on the bar, leading to setup. However, the part of the wave that propagates over the gap does not break, and thus setdown will continue. Thus, the mean surface over the bars is higher than that over the gap, and a strong flow issues outward through the gap.

Rip currents can potentially occur wherever strong longshore variability in wave breaking exists. This variability may be caused by such features as sandbars (as shown in the animated diagram above), by piers and jetties, and even by crossing wave trains.

Danger to swimmers[edit]

Photo of sign reading "HANAKAPIAI BEACH WARNING! DO NOT GO NEAR THE WATER UNSEEN CURRENTS HAVE KILLED 83 (displayed as 16 groups of 4 vertical lines with 1 diagonal line marking a group of 5 and three additional lines) VISITORS
Warning sign on the trail to Hanakapiai Beach.

Rip currents are a potential source of danger for people who are in shallow water with breaking waves in seas, oceans and lakes.[4] Rip currents are the cause of 80% of the rescues carried out by beach lifeguards.[5]

Rip currents typically flow at 0.5 metres per second (1–2 feet per second), but they can be as fast as 2.5 metres per second (8 feet per second), which is faster than any human can swim. However, most rip current are fairly narrow, and even the widest rip currents are not very wide; swimmers can easily exit the rip by swimming just a few strokes at a right angle to the flow, parallel to the beach. If the swimmer is unaware of this fact, he or she may exhaust themselves trying unsuccessfully to swim against the flow.[2] The pull of the current also fades out completely at the head of the rip, outside the zone of the breaking waves, so there is 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 occurs when a person has limited water skills, when they panic, or when a swimmer persists in trying to swim to shore against a strong rip current, thus eventually becoming exhausted.

Over a 10-year average, rip currents cause 46 deaths annually in the United States; 64 people died in rip currents in 2013.[6]

Recognizing and identifying rips[edit]

Rip currents have a characteristic appearance, and this means that with practice, and using careful observation, lifeguards, beach goers, and water users can learn to notice and identify rips, thus generally being able to successfully avoid them. Rip currents often look almost like a river running out to sea, away from the shore. The following are some characteristics that can be used to visually identify a rip when standing on the beach:[4]

  • There is a noticeable break in the pattern of the waves: it often looks quite flat where the rip is, in contrast to the lines of breaking waves on either side of the rip.
  • A "river" of foam: the surface of the rip often looks foamy, because the water is churned up.
  • Different color: the rip usually differs in color from the surrounding water; it is often more opaque or cloudier and so the rip may be 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.

These characteristics are helpful in learning to recognize rip currents before entering the water.

Escaping a rip[edit]

A swimmer or floater caught in a rip current may notice that he or she is moving away from the shore quite rapidly. It is often not possible to swim directly back to shore against a rip current, so this is not recommended. However, contrary to some popular misunderstandings, a rip is not capable of pulling a swimmer under the water; instead it carries the swimmer away from the shore in a narrow channel of water.[1] The rip is like a treadmill, which the swimmer needs to step off of. One effective response is to remain calm and swim parallel to the shore until the swimmer exits the rip current, which is usually not very wide. Once out of the rip, swimming back to shore is fairly easy in areas where waves are breaking and where floating objects (including swimmers) are being pushed towards the shore.[7]

As an alternative, swimmers or floaters who are caught in a strong rip can instead relax and calmly float on their back or tread water. Eventually the rip current will lose its momentum, and the swimmer is then able to swim at a leisurely pace, in a diagonal direction, away from the rip but back to shore.[8]

It is necessary for coastal swimmers to understand the danger of rip currents, to learn how to recognize them and how to escape from them, and if possible to swim only in areas where lifeguards are on duty.[4]

Uses[edit]

Experienced and knowledgeable water people such as surfers, body boarders and kayakers, who wish to leave the shoreline and get out to the zone just beyond the breaking waves, sometimes use rip currents as a rapid means of transportation.[9]

See also[edit]

References[edit]

  1. ^ a b Rip Current Characteristics College of Earth, Ocean, and Environment, University of Delaware. Retrieved 16 January 2009.
  2. ^ a b "United States Lifesaving Association's - Rip Currents". www.usla.org. Retrieved 8 July 2009. 
  3. ^ Don't get sucked in by the rip... Youtube.com
  4. ^ a b c Rip Currents Safety US National Weather Service. Retrieved 16 January 2009.
  5. ^ "NWS Rip Current Awareness Home Page". www.ripcurrents.noaa.gov. Retrieved 6 June 2010. 
  6. ^ http://www.nws.noaa.gov/os/hazstats.shtml
  7. ^ Science of the Surf Educational video
  8. ^ Rip Current Safety Tips US Lifeguard Association.
  9. ^ http://www.paddling.net/guidelines/showArticle.html?163