Ask a Scientist

Ask a scientist: What causes rip currents?

CorrespondentAugust 25, 2013 

Spencer Rogers, a coastal erosion and construction specialist with the North Carolina Sea Grant, has been studying rip currents since 1978. ( photo)


Spencer Rogers, a coastal erosion and construction specialist with the North Carolina Sea Grant, has been studying rip currents since 1978. Here he explains why they can be so dangerous. Questions and answers have been edited.

Q: What causes rip currents?

Rip currents have several different causes depending on the local beach conditions. In North Carolina, most rip currents are caused by waves breaking on the offshore sandbar, which forces water to pile up between the bar and the beach, higher than the ocean. The water flows along the beach until it finds a low elevation in the bar, where the current turns offshore and can become dangerous. The faster the water is collected the faster it scours a deeper hole in the sandbar.

Rip currents are present on most N.C. beaches every day but are too slow to be a threat to swimmers. However, certain combinations of wave height, wave period (the time between wave crests) and tide elevation can tune the system to generate very fast and dangerous currents directed offshore into deep water.

Q: How big and fast are rip currents?

When rip currents become dangerous they can be faster than the best Olympic swimmers.

Q: How can you tell when there is a rip current in the water?

You can ask a lifeguard on the beach. Alternatively, you can learn to identify rip currents by looking for differences in the surf zone. Look for darker, deeper water; fewer breaking waves; or different-colored water or floating debris that is visibly moving offshore. Some dangerous rip currents appear suddenly and disappear quickly so if you see a potential rip current location, it is best to avoid swimming near the area, even if it is not immediately dangerous.

Q: What is being done to study the science of rip currents?

I love to talk about the science, but first I think it is important to know what to do if you get caught in a rip current: Do not panic. Do not swim against the current. Do not overexert yourself swimming. Do swim parallel to the beach until you are out of the relatively narrow current before heading back to the beach. If getting tired, float. Do call for help if needed. Never attempt to rescue someone else unless you have some kind of float like a lifejacket, bogie board, surfboard or even a cooler.

Recent research on West Coast and other high-energy beaches have found that many rip currents set up circulation cells that redirect the currents back to the beach after five to 15 minutes. There may be a 90 percent chance that you can get back to the beach by just floating, but there remains a 10 percent chance that you will be pushed much farther offshore.

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