by Emma Shie Nuss, Audrey Casper, Christine M. Baker, Melissa Moulton and Walter Torres, The Conversation
If you’ve ever gone into the ocean for a swim and suddenly realized that the shore is getting farther away, not closer, you may have encountered a rip current. Common on coastlines around the world, these strong currents flow from shore to sea at speeds of up to several feet per second.
It is important to know what rip currents are and how to find them, because they are the main cause of drowning in the surf zone near the coast. According to a recent estimate, rip currents accounted for 435 drownings in the US since 2017.
National Weather Service offices serving coastal communities issue forecasts that predict where and when rip currents are likely to occur. Those predictions draw on decades of research into the physics of rip currents. Many scholars, including our research group, are looking for new ways to discover more about rip currents—including their important role in coastal marine ecosystems.
Not all rip currents are the same
All rip currents have similar effects, but they can form in many ways.
One type of rip, known as a bathymetric or channel rip current, forms when there are gaps between breaking waves. As the waves break, they push the water towards the shore and slightly raise the water level.
When waves break over a sandbar, but not in a deeper channel that passes through the sandbar, the excess water pushed by the waves toward the shore exits back into the ocean through the channel. The escaping water flow acts like a conveyor belt, moving water, unsuspecting swimmers and small marine organisms on the beach.
Another type, known as a passing or flash rip current, forms when the surf is choppy. The edges of the breaking waves push the water and make it spin, like a speeding ice skater hitting someone.
This creates eddies known as eddies, which can combine to form larger eddies, with currents acting like temporary conveyor belts. Flash rip currents are an active area of research.
Swim, float, call for help
Choosing beaches with lifeguards and paying attention to beach flag warnings is the best way to avoid getting swept away by the current. However, if you do get caught by one, here are some methods to get back to shore safely.
Think of a rip current as a raging river that sweeps the surf off the coast. Swimming against the current will tire you out and put you at risk of drowning. Instead, swim parallel to shore—think of going to the “bank of the river”—until you get out of the rip current. Once you’re done fighting it, you can swim back to shore.
Another strategy is to float until the rip current takes you offshore beyond the waves. The rip current slows down here, so you can swim away from the rip current and back to shore.
If you believe you are in danger, try to stay calm. Wave your arms and ask for help. If you see someone caught in a rip current, throw them a flotation device and alert the lifeguard.
Predicting rip currents
The National Oceanic and Atmospheric Administration’s rip current hazard model provides advance forecasts of the likelihood of encountering dangerous rip currents given wave conditions at specific beaches. NOAA continues to work to make these hazard forecasts more accurate, including in continued collaboration with the US Lifesaving Association. This partnership will work to compare modeled predictions with lifeguard reports of rip current hazards and to recalibrate the model for different regions and waves.
At the University of Washington, we evaluate NOAA hazard forecasts against the latest rip current science. This will help us evaluate predictions for different types of rip currents, such as unexpected flash rips.
To measure rip currents, we sometimes put on scuba gear and fight the waves to place instruments in the surf. But this work can be expensive, and it relies on knowing where the rips will occur in advance. That is not possible for flash rips, so we need different methods to analyze them.
We use supercomputers and huge wave tanks the size of an Olympic swimming pool, with paddles at one end that generate waves, to simulate flash rips. Tank laboratory experiments and computer simulations allow us to control the types of waves we create and facilitate the collection of large amounts of data. This work improves our understanding of the relationship between wave conditions and flash rips, which can help improve hazard predictions.
Expressways for marine life
Rip currents are not just a safety issue. Scientists are beginning to better understand the important ecological role they play in the redistribution of small marine organisms, as well as plastic, pollutants, sediments and coastal debris.
Many marine organisms, including oysters, barnacles, fish and coral, rely on ocean currents during their larval stage to find suitable habitat. These organisms swim up or down or attach to floating or sinking material and are transported by many processes in the ocean.
Rip currents are an important mechanism for dispersing larvae into deeper water or recirculating them in shallow waters. Rip current type and behavior can affect the movement of marine organisms.
Water temperature and salinity can change the behavior of rip currents—and send organisms on a different route—by changing the water’s density. Our team analyzed images taken from low-flying aircraft and found that warmer rip currents carry water away from the coast above, while cooler rip currents spread below the surface in different patterns.
Our research team and other scientists use computer simulations and numerical “larvae” to investigate how temperature, salinity and other factors affect the transport of marine organisms. With a better understanding of these surf-zone conveyor belts, we aim to help swimmers stay safe and assess how rip currents affect near-shore aquatic ecosystems.
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Citation: Rip currents are dangerous for swimmers but also important ecologically—how scientists are working to understand them (2023, July 24) retrieved on July 24, 2023 from https://phys.org/news/2023-07-rip-currents-dangerous-swimmers-ecologically.html
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