Riptide: the deadliest trick of the ocean

The rapids near Melbourne, Florida after Hurricane Jenny. According to the data of the American Life Saving Association (USLA), more than 65,438+000 people on the shore drown every year because of these powerful river-like waterways. Right here in America. According to the data of the American Oceanographic Association (USLA), nearly half of all rescues carried out by lifeguards on the beach are related to rapids. Globally, sharks usually kill 6 people every year. [Find out your chances of dying from various phenomena]

A common view about how they work is that the tearing current pulls swimmers underwater; In fact, they are strong and narrow currents flowing from the beach.

"Essentially, they are rivers of the ocean," said Wendy Carey, an expert on coastal disasters at the Delaware Ocean Aid Advisory Service Center of the University of Delaware.

"People began to sink because they were afraid. They feel that the current is dragging them down. " No current can drag you into the sea.

There are many different types of tearing flow, and there are many ways to form it. When a large number of swells roll in, the wave height will change rapidly, thus triggering the tearing flow. Tearing flow may also occur where there is a gap in the sandbar; There, water is injected into the sea. These waterways in the sandbar are close to the beach. When water returns to the ocean, it follows the path of least resistance, usually through these channels. Carey said that strong tearing flow often appears next to buildings such as docks, breakwaters and groins.

Breaking wave is the key factor of all tearing flows. If there were no waves, there would be no rapids, "Carey said. [Summer Science: How are the waves formed? ]

The risk of tearing flow is determined by many factors, including weather, tides, local changes in beach shape and how waves break offshore. Some beaches have rapids almost all the time, while others can hardly see dangerous currents.

These powerful and very limited currents will take unsuspecting swimmers to the sea. Ocean currents usually move at a speed of 1 to 2 feet per second (0.3 to 0.6 meters per second), but stronger ocean currents can be pulled at a speed of 8 feet per second (1.6 meters per second). Carey said that this is the same speed as the record freestyle athlete in the Olympic Games. Even Olympic swimmers will find themselves going backwards in the rapids. Olympic gold medalist michael phelps can swim the butterfly at a speed of 6.5 feet per second. )

Torrential water can accelerate rapidly in a short time. The unsteady flow of rapids is similar to standing in a river on land. Kelly said, "A strong current can sweep you away from your feet." Adults standing in waist-deep rapids will find it difficult to stay in the same place, she said.

Rapids is an inappropriate term. A strong torrent can trigger a sudden torrent, but it is most dangerous near the low tide, when the sea is far from the beach. Carey said that in the past, torrent was sometimes called torrent, which was a mistake. "Tide is actually a slow change of water level, which does not cause tearing flow by itself," she said.

Scientists have been studying rapids for over 100 years. In the past ten years, the progress of measurement technology has provided many new insights for studying how these complex currents work. Researchers now throw drifting objects equipped with GPS into the waves to accurately track the movement and speed of the tearing flow. Acoustic Doppler current profile (similar to sonar) reveals the internal work of tearing current. This acoustic Doppler current profiler emits high-frequency pulse sound waves, which impact and bounce particles in water. The instrument measures the frequency of the returned signal-if the particles (and surrounding water) are moving away from the instrument, the signal will have a lower frequency, if Carey says the victim himself. Additional reporting by Becky Hosking, senior writer of Field Science

Supplementary resources

National Weather Service: Rip's current security, photos and frequently asked questions; University of Delaware Ocean Fund: Rip current information; American lifesaving society: Rip's current survival guide.