How Rip Currents Work

Rip currents are primarily caused by the moon's gravitational pull. See more beach pictures.

Rip currents are responsible for about 150 deaths every year in the United States. In Florida, they kill more people annually than thunderstorms, hurricanes and tornadoes combined. They are the number-one concern for beach lifeguards: About 80 percent of all beach rescues are related to rip currents.

Despite these startling statistics, many swimmers don't know anything about rip currents, and they have no idea how to survive when caught in one. In this article, we'll find out what causes rip currents, how you can recognize them and what you should do if one takes you out to sea.

A rip current is a narrow, powerful current of water running perpendicular to the beach, out into the ocean. These currents may extend 200 to 2,500 feet (61 to 762 m) lengthwise, but they are typically less than 30 feet (9 m) wide. Rip currents can move at a pretty good speed, often 5 miles per hour (8 kph) or faster.

These currents are often called "riptides," but this is a misnomer. Tides are the rising and falling of water levels in the ocean. They are primarily caused by the moon's gravitational pull, and they change gradually and predictably every day. Rip currents are caused by the shape of the shoreline itself, and they may be sudden and unexpected.

Rip currents may also be referred to as "undertow," which is just as inaccurate. Undertow describes a current of water that pulls you down to the ocean bottom. Rip currents move along the surface of the water, pulling you straight out into the ocean, but not underneath the water's surface. A rip current may knock you off your feet in shallow water, however, and if you thrash around and get disoriented, you may end up being pulled along the ocean bottom. But if you relax your body, the current should keep you near the surface.

Rip currents are terrifying because they catch you off guard: One minute you're bobbing along peacefully in the surf, the next you're being dragged out to sea at top speed. They occur in all sorts of weather and on a wide range of beaches. Unlike violent, crashing waves, you probably won't notice a rip current until you're right in the middle of it.

In the next section, we'll find out what causes this frightening phenomenon and see why it claims so many lives every year.




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.

Escaping a Rip Current

In the last section, we saw that rip currents occur when water rushes through a low point in a sandbar. Since waves keep pushing more water into the basin between the sandbar and the beach, the rip current may continue for several minutes, or even several hours. Some rip currents are brief occurrences, but others are long-term fixtures of an area.

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Typically, the strongest part of a rip current is the direct line between the water's edge and the sandbar opening, but the current will also pull in water from either side of the basin. In this way, a rip current might pull you sideways, parallel to the beach, before it pulls you outward, away from the beach.

Once the receding wave makes its way through the sandbar opening and meets up with water at its own level, its pressure immediately drops. Overall, the water flow pattern has a mushroom shape.

Depending on its severity, you may be able to see a rip current from the beach. Strong rip currents disrupt incoming waves and stir up sand from the ocean floor. When you're at the beach, keep an eye out for narrow, muddy streaks in the ocean where there aren't any waves breaking.

If you get caught up in a rip current, it's crucial that you keep your wits about you. Your first instinct may be to swim against the current, back to shallow waters. In most cases, even if you're a strong swimmer, this will only wear you out. The current is too strong to fight head-on.

Instead, swim sideways, parallel to the beach (see illustration below). This will get you out of the narrow outward current, so you can swim back in with the waves helping you along. If it's too hard to swim sideways while you're being dragged through the water, just wait until the current carries you past the sandbar. The water will be much calmer there, and you can get clear of the rip current before heading back in.

People drown when they thrash about in the water or expend all of their energy swimming. To survive a rip current, or any crisis in the water, you have to keep calm, and you have to conserve your energy. If you don't think you can swim all the way back to the beach, get past the rip current and tread water. Call for help, signal to people on the beach and, if all else fails, wait for the waves to carry you in.

If you're on the beach and see somebody else caught in a rip current, call for help from a lifeguard or the police. Don't immediately dive in and swim out to the person. It's too risky to swim out there yourself unless you have a raft, boogie board or life preserver with you.

The most effective way to fight rip currents is to follow basic swimming safety rules: Never go in the ocean alone, and if you aren't a strong swimmer, stick to shallow waters (although even shallow waters can be dangerous). Ideally, you should only swim in areas where there is a lifeguard or strong swimmer on the beach who can keep an eye on you.

If you plan to swim in the ocean anytime soon, it's a good idea to learn everything you can about rip currents. After all, they are the beach's number-one killer.

To find out more about rip currents and ocean safety, check out the links on the next page.

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