Rip current
Encyclopedia
A rip current, commonly referred to by the misnomer rip tide, is a strong channel of water flowing sea
ward from near the shore
, typically through the surf line. Typical flow is at 0.5 metres per second
(1–2 feet per second), and can be as fast as 2.5 metres per second (8 feet per second). They can move to different locations on a beach break, up to tens of metres (a few hundred feet) a day. They can occur at any beach with breaking waves, including the world's ocean
s, sea
s, and large lake
s.
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 an exit back to the sea or open lake water. The resulting rip current is usually narrow and located in a trench between sandbars, under pier
s or along jetties
. A common misconception is that ordinary undertow
or even rip currents are strong enough to pull someone under the surface of the water; in reality the current is strongest at the surface. This strong surface flow tends to dampen incoming waves, leading to the illusion of a particularly calm part of the sea, which may possibly lure some swimmers into the area. The off-shore path taken by a rip current can be demonstrated by placing colored dye at the start of a current at the shoreline.
Rip currents are stronger when the surf is rough (such as during high onshore winds, or when a strong hurricane is far offshore) or when the tide
is low.
A more theoretical 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 above), 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 will issue outward through the gap.
Rip currents can potentially occur wherever strong longshore variability in wave breaking exists. This variability may be caused by sandbars (as above) or even by crossing wave trains.
Rip currents are a source of danger for people in ocean and lake surf. They can be extremely dangerous, dragging swimmers away from the beach
. Death by drowning
comes following exhaustion while fighting the river
or ocean current
.
Although a rare event, rip currents can be deadly for non-swimmers as well: a person standing waist deep in water can be dragged into deeper waters, where they can drown if they are unable to swim and are not wearing a flotation device
. Varying topography
makes some beaches more likely to have rip currents; a few are notorious.
Rip currents cause more than 100 deaths annually in the United States
. Rip currents cause 80% of rescues needed by beach lifeguard
s.
which the swimmer needs to step off. The swimmer should remain calm and swim parallel to the shore until he or she is outside of the current. Locations to aim for are places where waves are breaking. In these areas, floating objects are generally transported towards the shore.
A swimmer in a strong rip, who is unable to swim away from it, should relax and calmly float or tread water to conserve energy. Eventually the rip will lose strength, and the swimmer can swim at a leisurely pace, in a diagonal direction, away from the rip but back to shore. Coastal swimmers should understand the danger of rip currents, learn how to recognize them and how to escape from them, and swim in areas where lifeguards are operating, whenever possible.
Sea
A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean...
ward from near the shore
Shore
A shore or 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 and present, while the beach is at the edge of the shore,...
, typically through the surf line. Typical flow is at 0.5 metres per second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
(1–2 feet per second), and can be as fast as 2.5 metres per second (8 feet per second). They can move to different locations on a beach break, up to tens of metres (a few hundred feet) a day. They can occur at any beach with breaking waves, including the world's ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
s, sea
Sea
A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean...
s, and large lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
s.
Causes and occurrence
When windWind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
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 an exit back to the sea or open lake water. The resulting rip current is usually narrow and located in a trench between sandbars, under pier
Pier
A pier is a raised structure, including bridge and building supports and walkways, over water, typically supported by widely spread piles or pillars...
s or along jetties
Jetty
A jetty is any of a variety of structures used in river, dock, and maritime works that are generally carried out in pairs from river banks, or in continuation of river channels at their outlets into deep water; or out into docks, and outside their entrances; or for forming basins along the...
. A common misconception is that ordinary undertow
Undertow (wave action)
Undertow is a subsurface flow of water returning seaward from shore as result of wave action. This type of shore current can play a role in material deposition such as creating sand bars....
or even rip currents are strong enough to pull someone under the surface of the water; in reality the current is strongest at the surface. This strong surface flow tends to dampen incoming waves, leading to the illusion of a particularly calm part of the sea, which may possibly lure some swimmers into the area. The off-shore path taken by a rip current can be demonstrated by placing colored dye at the start of a current at the shoreline.
Rip currents are stronger when the surf is rough (such as during high onshore winds, or when a strong hurricane is far offshore) or when the tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
is low.
A more theoretical description involves a quantity known as radiation stress
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...
. This is the force (or momentum flux) exerted on the water column by the presence of the wave. As a wave shoals
Wave shoaling
In fluid dynamics, wave shoaling is the effect by which surface waves entering shallower water increase in wave height . It is caused by the fact that the group velocity, which is also the wave-energy transport velocity, decreases with the reduction of water depth...
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 above), 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 will issue outward through the gap.
Rip currents can potentially occur wherever strong longshore variability in wave breaking exists. This variability may be caused by sandbars (as above) or even by crossing wave trains.
Dangers
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...
. Death by drowning
Drowning
Drowning is death from asphyxia due to suffocation caused by water entering the lungs and preventing the absorption of oxygen leading to cerebral hypoxia....
comes following exhaustion while fighting the river
Current (stream)
A current, in a river or stream, is the flow of water influenced by gravity as the water moves downhill to reduce its potential energy. The current varies spatially as well as temporally within the stream, dependent upon the flow volume of water, stream gradient, and channel geometrics...
or ocean current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
.
Although a rare event, rip currents can be deadly for non-swimmers as well: a person standing waist deep in water can be dragged into deeper waters, where they can drown if they are unable to swim and are not wearing a flotation device
Personal flotation device
A personal flotation device is a device designed to assist a wearer, either conscious or unconscious, to keep afloat.Devices designed and approved by authorities for use by...
. Varying topography
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
makes some beaches more likely to have rip currents; a few are notorious.
Rip currents cause more than 100 deaths annually in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. Rip currents cause 80% of rescues needed by beach lifeguard
Lifeguard
A lifeguard supervises the safety and rescue of swimmers, surfers, and other water sports participants such as in a swimming pool, water park, or beach. Lifeguards are strong swimmers and trained in first aid, certified in water rescue using a variety of aids and equipment depending on...
s.
Escaping a rip current
A swimmer caught in a rip current should not attempt to swim back to shore directly against the rip. This risks exhaustion and drowning. A rip does not pull a swimmer under water; it carries the swimmer away from the shore in a narrow channel of water. The rip is like a treadmillTreadmill
A treadmill is an exercise machine for running or walking while staying in one place. The word treadmill traditionally refers to a type of mill which was operated by a person or animal treading steps of a wheel to grind grain...
which the swimmer needs to step off. The swimmer should remain calm and swim parallel to the shore until he or she is outside of the current. Locations to aim for are places where waves are breaking. In these areas, floating objects are generally transported towards the shore.
A swimmer in a strong rip, who is unable to swim away from it, should relax and calmly float or tread water to conserve energy. Eventually the rip will lose strength, and the swimmer can swim at a leisurely pace, in a diagonal direction, away from the rip but back to shore. Coastal swimmers should understand the danger of rip currents, learn how to recognize them and how to escape from them, and swim in areas where lifeguards are operating, whenever possible.