Watch out for crowd turbulence

A disaster can occur anytime one of the authorities relax tension.

LEE KYOUNG-HEEThe author is the head of the Innovation Lab of the JoongAng Ilbo. Every year, hundreds of people come to Mecca, Saudi Arabia, for the Islamic Hajj pilgrimage. The highlight of the pilgrimage is the “Stoning of the Devil” ritual, which refers to throwing stones at three pillars on the Jamarat Bridge. In 2006, a large crowd gathered on the bridge, killing more than 300 people. Professor Dirk Helbing and other crowd dynamics experts analyzed footage from the disaster and discovered the phenomenon of “crowd turbulence.”

Turbulence can be described as disorderly and abnormal movement of a fluid. When a faucet is turned on a little bit, water falls vertically straight down, but when the flow is increased, some water droplets spray out irregularly. Similarly, when density increases, people irregularly fluctuate, as if they are a mass of a liquid. It is also called a “crowd quake,” as it is uncontrollable and threatening like an earthquake.

If there are more than 7 people per 1 square meter (11 square feet), crowd turbulence is formed, and people caught in the turbulence would be swept up by the wave in unpredictable directions. In this density, people struggle to secure enough space to breathe. As force is transferred to adjacent people, it becomes physically amplified.

When crowd turbulence occurs, it can lead to a serious disaster if only a few people lose balance and fall. Because of the sudden void left from a person who falls, those who don’t have support would fall on top, tumbling like dominos.

Once a crowd turbulence takes effect, it is hard to reverse. Crowd dynamics experts unanimously instruct that it should be prevented by maintaining a safe distance between people and making a crowd flow smoothly by implementing one-way traffic.

There are conspiracy theories of intentionally inducing the disaster in Itaewon, for instance, accusing men pushing people at the scene. It is quite unscientific that a few people in the crowd deliberately induced the crush. In a dense crowd so packed that people struggle to breathe, each person is just a drop of water in the stream.

After the 2006 Jamarat Bridge disaster, Saudi Arabia dispersed the attendants through the expansion and renovation of the bridge. But in 2015, hundreds were killed on the way to Jamarat. It is an example that even after a few years free of accidents, a disaster can occur anytime one of the authorities relax tension.