rip current

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A rip current is a strong channel of water flowing away from the shoreline, typically through the surf line, and can occur on any shore that has breaking waves.^[1][2] The water flows seaward from near the shore. 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 meters (a few hundred feet) a day. They can occur at any beach with breaking waves, including the world's oceans, seas, and large lakes such as the Great Lakes in the United States of America and Canada.

A sign warns hikers on the trail to Hanakapiai Beach.

Dangers

Rip currents are a source of danger for people in ocean and lake surf.^[3] They can be extremely dangerous, dragging swimmers away from the beach and leading to death by drowning when they attempt to fight the River or ocean current and become exhausted. Although a rare event, rip currents can be deadly for non-swimmers as well^{[citation needed]}: 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.^[4] Over 80% of rescues by surf beach lifeguards are due to rip currents.

A rip current mechanism

Causes and occurrence

When wind and waves push water towards 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 piers or along jetties. A common misconception is that 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. The off-shore path taken by a rip current can be demonstrated by placing coloured dye at the start of a current at the shoreline, as seen here [1]

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.

References