Rip current

A rip current is a powerful, narrow stream of water that flows straight out from the shore, pushing through the lines of breaking waves like a river heading to sea. It starts when wind and waves drive surface water toward the beach, raising the water level slightly along the coast. That extra water then seeks the easiest path back out, often through a deeper spot like a break in a sandbar or reef.

These currents are strongest and fastest near the water’s surface. They can be dangerous for swimmers. People caught in a rip who panic or try to swim directly against it may exhaust themselves, which is why rip currents are the main reason lifeguards perform rescues. In the United States, they caused an average of 71 drownings each year from 2013 to 2022. A rip current is not the same as undertow—a subsurface current that pulls water back out after waves break. Despite common belief, neither can pull a person down and hold them underwater. A rip simply carries floating objects, including people, beyond the breaking waves, where the current weakens and releases them.

Rip currents form when waves break unevenly along the shore. The water pushed toward land flows sideways along the beach as “feeder currents” until it reaches the rip’s outgoing “neck,” where the flow is fastest. Once past the breaking waves, the current spreads out and fades in the “head.” They can appear on ocean, sea, or large lake coasts wherever waves have enough energy. They often occur on gently sloping shores where waves approach parallel to the beach, or where underwater features like sandbars, reefs, piers, or jetties create a path of least resistance. Some rips recur in the same spots, while others appear and vanish suddenly, depending on bottom shape and wave direction.

Rip currents are usually narrow but become wider and faster when waves are large and powerful. Local underwater topography makes some beaches more prone to them—Hanakāpīʻai Beach is a well-known example. Though “rip tide” is a misnomer, in areas with big tidal ranges, rips may only happen at certain tide stages when water is shallow enough for waves to break over a bar but deep enough for the broken wave to flow over it. On gently shelving shores, the distance between a sandbar and the shoreline can vary from a few meters to over a kilometer between high and low tide.

A common misconception is that rip currents pull swimmers down. In reality, the current is strongest near the surface; friction slows the flow near the bottom. The surface of a rip often looks like a smooth, calm patch with no breaking waves, which can mislead beachgoers into thinking it’s a safe place to enter the water.

Technically, rip currents involve radiation stress—the force waves exert on the water column. As a wave shoals and rises before breaking, radiation stress increases, causing a local drop in mean water level called “setdown.” When the wave breaks and shrinks, radiation stress decreases, raising the mean level—this is “setup.” When a wave passes over a sandbar with a gap, most of it breaks on the bar, creating setup, while the part over the gap doesn’t break, so setdown continues there. The resulting difference in water height drives a strong outward flow through the gap—the rip current. The vorticity and inertia of these currents have been studied using models.