A rip current
, rip tide
is a strong surface flow of water
from near the shore
(not to be confused with an undertow
). Although rip currents would exist even without the tides
, tides can make an existing rip much more dangerous—especially low tide. Typical flow is at 0.5 meters per second
(1-2 feet per second), and can be as fast as 2.5 meters per second (8 feet per second). Rip currents 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 oceans
, and large lakes
such as the Great Lakes
and the United States
Recognizing a rip current
Rip currents can be recognized by unusually calm waters, caused by the channel of water flowing out. The color of the water may be different from the surrounding area. Additionally, the waterline is lower on the shore near a rip current. It is advisable to look for the existence of a rip current before heading into the water.
Rip currents can also be extremely useful for surfers as they save the effort of having to paddle out to catch a set. They can also be used by lifeguards (swimming or on paddleboards), who can use them to get out from the shore to perform a rescue much quicker than they could by swimming/paddling through the waves.
Rip currents can be extremely dangerous, dragging swimmers
away from the beach
and leading to death by drowning
when they attempt to fight the current
and become exhausted
. Although a rare event, rip currents can be deadly for non-swimmers as well: a person standing waist deep in water can be dragged out into deeper waters, where they can drown if they are unable to swim and are not wearing a flotation device
. Some beaches are more likely to have strong rip currents than others, and a few are particularly well known for them, the overall topography
of the area being the main factor.
Rip currents cause approximately 100 deaths annually in the United States, more than all other natural hazards except heat and floods. Over 80% of rescues by surf beach lifeguards are due to rip currents totaling 18,000 lifeguard rescues a year.
Causes and occurrence
While the precise conditions leading to a rip current are not known, the general picture is as follows. When wind and waves push water towards the shore, the previous backwash is often pushed sideways by the oncoming waves. This water streams along the shoreline until it finds an exit back to the sea. The resulting rip current is usually narrow and located in a trench between sandbars
, under piers
or along jetties
. While a common misconception is that a rip occurring under the water, instead of on top — an undertow — is strong enough to drag people under the surface of the water; the current is actually strongest at the surface, and can dampen incoming waves, leading to the illusion of a particularly calm area, luring some swimmers in.
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 sea 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 sea 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 sea 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.
Surviving an encounter with a rip current
When caught in a rip current, one should not fight it. Rather, swim parallel
to the shoreline in order to leave it, as rip currents are generally only 30 to 100 ft (9–30 m) wide, thus swimming out of one via swimming parallel would only take a few minutes. While floating
until the current disperses into deeper waters is another method of surviving such a dangerous incident, it may leave the swimmer farther away from shore.
Posted warnings, where available, should always be heeded. It is advisable to stay at least 30 m (100 ft) away from piers and jetties, which impede waves, encouraging rip currents to form. Also, check the local newspaper and internet for tide timetables. (Beware that tides can be substantially different at beaches relatively close to each other.) Never go into the water without lifeguard supervision from two hours before to four hours after the daily maximum low tide, especially at night. Always swim with a friend and follow all lifeguard warnings or signage.