The "disappeared him" scene at the beach, the killer behind it is actually...

Audit expert: Yi Jingkun

Zhihu famous geography and meteorology blogger

In the hot summer, many people will choose to go to the seaside for vacation, enjoy the beach and sunbathe. However, the seemingly calm sea water may also hide dangers.

On July 5, a girl posted a message asking for help online. Her boyfriend was playing in the water at a beach in Riyue Bay when he walked to the point where the water reached his lower abdomen. Then, a wave hit him, and the highest wave was over his head. "I couldn't see him for 15 minutes."

Source | Weibo

Finally, after nearly two days of searching, her boyfriend's body was found at the beach of Shimei Bay, 9 kilometers away from the accident site. Although after the incident, relevant staff of the Coast Guard Corps of the Public Security Department of Hainan Province said that due to the complicated situation at sea, it was impossible to verify the cause of the accident. However, the distance where the body was found sparked a discussion among netizens about "offshore currents".

Rip current source: Rip Current – ​​Отбойное течение. Как не утонуть в океане

1

What are rip currents?

Rip Current is a narrow and strong stream like a beam, which consists of three parts: rip root, stream neck and stream head. It flows vertically or nearly vertically to the coast, toward the depths of the ocean, and its width is usually no more than 10 meters. Since the rip current has a very fast flow rate (0.5~2m/s, sometimes up to 3m/s), and the surface is calm and not easy to detect, it only takes a few seconds for it to bring swimmers on the shore to the open sea area . The stream neck is the most dangerous place, which is not only narrow and fast, but also the negative pressure generated by the high speed will "suck" nearby swimmers in, which is extremely dangerous.

Source: China Ocean Development Research Center

The main reason for its appearance is that when a large amount of seawater rushes to the coast due to waves or tides, underwater obstacles can prevent the seawater from rushing directly back into the sea, but subsequent waves continue to push, and the seawater has to move in a direction parallel to the beach and stop only when it finds an opening around the obstacle. Eventually, it converges to form one or several powerful currents that rush into the sea. Therefore, rip currents are mostly random, and no one knows where they will appear.

Source: China Maritime

A rip current is a current that rushes toward an opening like water flowing out of a sewer. Because the rip current is moving faster than the water on either side of it, it may pick up sediment on the beach. At this point, a rip current tends to become a dark or muddy line extending from the beach into the ocean. A rip current often appears calmer than the surrounding water . Once it passes an obstacle, such as a sandbar or between bridge piers, it loses momentum and stops flowing.

Like most beaches on lakes or rivers, beaches without breaking waves will not produce rip currents, but beaches subject to extreme weather such as typhoons can develop rip currents. As typhoons form and develop far out at sea, they can generate powerful waves that hit the coast with great energy. These waves can pile up sand bars and form offshore caves—deep depressions in the seafloor. Strong waves, deep offshore caves, and sand bars can combine to produce strong rip currents.

For example, rip currents are easily formed in the middle area of ​​a crescent-shaped sandbar, beaches with rigid obstacles are also prone to rip currents, and rip currents often occur in bag-shaped bays.

Schematic diagram of rip current formation Source: People's Daily Online

2

What are the more obvious distinguishing features?

Rip currents are different from waves and tides. They are a specific type of water flow associated with the rapid movement of tidal water through estuaries and harbors. **Rip currents are usually easier to identify high up on the shore. They are mostly distributed in relatively calm waters where turbid mud or white foam drift away from the shore and flow into deep water. **Typically, there are 4 types of rip currents:

1) Sudden rip currents: They can form suddenly and disappear just as quickly, usually occurring when there is a sudden increase in wave conditions or during storms.

2) Stationary rip currents: These form between sand bars and deeper channels and can stay in the same place for days, weeks or months. They are the most common type of rip currents.

3) Mega rip currents: These are the largest type of rip currents and form during periods of extreme wave activity, such as hurricanes or heavy rains.

4) Permanent rip currents: occur where there are permanent obstructions, such as piers, docks, or coral reefs.

Enhanced photo (green) shows a clear source of rip currents: NOAA

3

What should you do if you are accidentally caught in a rip current?

Rip currents can move at up to 2.4 meters per second, which is faster than an Olympic swimmer! Rip currents are dangerous to even the strongest swimmers. Therefore, panicked swimmers who try to fight rip currents by swimming directly back to shore are at risk of drowning due to fatigue. In the United States alone, lifeguards rescue tens of thousands of people from rip currents each year, and conservative estimates are that at least 100 people die from rip currents each year. So if you are caught in a rip current, don't fight it! Swim parallel to the shore and then swim back to land at a certain angle .

Source: South China Sea Bureau, Ministry of Natural Resources

If you are caught in a rip current, here are some life-saving safety rules:

1) Keep a cool head, conserve your energy and think clearly about your situation.

2) Never go against the current.

3) Think of it like a treadmill that you can’t turn off and you need to get on top of it.

4) Swim out of the current in the direction of the shoreline.

5) If you cannot swim out of the rip current, stay afloat or calmly tread water. When you are out of the current, swim toward shore.

6) If you still cannot reach the shore, wave your arms and shout for help to attract attention.

7) If you see someone caught in a rip current, seek help from a lifeguard and throw floating objects to the victim. (Do not rush into the water to rescue)

Therefore, when you go to the beach, you should learn about the local weather and terrain in advance and stay away from dangerous waters. If you are unfortunate enough to encounter a rip current, you should stay calm and use the methods described in this article to correctly escape and rescue.