How Rip Currents Work

Why Rip Currents Form

Rip currents are anomalous occurrences, but they are born out of ordinary, everyday ocean waves. On the most basic level, you can think of ocean waves as travelling fluctuations in water level. Some external force (usually the wind) pushes on the ocean, creating a swell of water, which is passed along the ocean's surface. The energy of the wave, which may be built up by additional wind pressure, is passed from water molecule to water molecule. The water itself doesn't actually travel; only the energy keeps going.

Eventually, some waves meet up with land. In areas with a rocky shore, the water surge "crashes" as it is deflected. On a sandy beach with a gently sloping shore, the swell simply pushes uphill. The climb up the beach drains all the energy of the surge, and the water eventually flows downhill, back to the ocean -- in other words, the water finds its own level again.

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Ordinarily, this receding flow of water moves with minimal force. The slight slope of the beach effectively spreads out the force over a great distance, so it's not particularly strong at any one point. And since it's weaker than the opposing force of incoming waves, the receding flow usually won't carry you out to sea.

A rip current occurs when the receding flow becomes concentrated in a particular area at a particular time. There are a number of things that can cause this, but the most common is a break in a sandbar. Sandbars are long, narrow hills of accumulated sand along the outer part of the shore. They are formed by the motion of waves and tides.

When a large sandbar forms, it can produce a sort of basin along the ocean shore. Waves move up against the sandbar with enough force to push water into the basin, but the receding water has a hard time making it back over the sandbar to return to sea. This is something like a bathtub with the drain plugged up: Just as the water in a bathtub is being pulled downward by gravity but is blocked by the drain plug, the receding wave is being pulled outward by the ocean (and by gravity), but is kept in by the sandbar.

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A simplified version of a shore with a sandbar. Sandbars sometimes peak out above the water, but more often they will be submerged just below the surface.

In some cases, the backward pressure of the receding water may be strong enough to break through part of the sandbar. Other times, the water flows along parallel to the beach until it reaches a low point on the sandbar. In either case, the water that has piled up in the basin rushes out to sea once it finds an opening, just as the water in your bathtub rushes out when you unplug the drain.

The resulting rip current sucks in water from the basin and spits it out on the other side of the sandbar.

In the next section, we'll examine this water flow in greater detail and find out what you should do if a rip current pulls you out into the ocean.