MRS Paddling Academy | “Accident Case Report” Series – Vol. 4

“When water falls over the dam, it creates a vortex. The vortex is so strong that it’s almost impossible to control your body in the water. That’s why low-head dams are also called ‘drowning machines.’”

Case Studies
What is a low-head dam? Why are they called “drowning machines”?

Accident Review

On November 16, a villager accidentally fell into the river while fishing. On November 23, the villager’s family called the Blue Sky Rescue Team for help. Five team members went to the Zhanghe River’s Daming section for recovery operations.

Data shows that “water rescue” is the main rescue project of the Blue Sky Rescue Team | Source: The Paper

Images from the scene show the team trapped downstream of the dam, with water constantly surging over the dam, creating a dangerous rolling current.

According to a professional rescuer at the scene, the dam’s significant height difference and rapid water flow created a “boil line” in the water. type of water flow is especially dangerous—people caught in it are repeatedly pushed down, even with life jackets, until they are too exhausted to escape.

Accident Review

On April 21, a dragon boat team reached a dam on the Taohua River near Lujia Village. Three boats had stopped due to the elevated water levels after recent rainfall. The first dragon boat tried to cross the dam twice but failed both times, eventually capsizing, and its occupants were thrown into the water. As the second boat approached to rescue the fallen rowers, it too was caught by the vortex and capsized, resulting in more people falling into the river.

At the time, Li Han was on a dragon boat that was about to reach the shore. Upon seeing the first dragon boat capsize and people falling into the water, they quickly rushed to rescue them. Li Han said that they extended their paddles to the people in the water, trying to pull them up, but then realized their own boat was caught in a whirlpool, causing it to turn sideways. the people in the water tried to climb aboard, their boat quickly capsized as well. After falling into the water, Li Han was trapped under the overturned boat. He tried to swim out several times but couldn’t get free.

Video footage taken by bystanders shows multiple people struggling in the river. At this point, the second racing boat had already approached the shore. However, when faced with the people in the water and the cries for help from those onshore, they rowed back into the river. According to Tan Gui’an, Secretary General of the Guilin Dragon Boat Association, as the other racing boat got closer, “The people in the water were so desperate to survive that they grabbed onto the boat, causing it to capsize too.” Eyewitnesses said, “The current was too strong, and there were whirlpools, which caused their dragon boat to capsize as well.”

Both racing boats capsized, with 57 people falling into the water.

One local rescuer told a reporter that dams had been built on the Lijiang River to allow boats to navigate during the dry season. The sand under the accident site had been hollowed out, creating strong undercurrents and uneven depths. “Many people wanted to help, but it was impossible. Some were trapped under the boats, while others clung to the boats but became too exhausted to hold on. We could only watch as they were swept away by the water.”

Risk Analysis

Both accidents were primarily caused by the low-head dam at the accident site. But why is the vortex at a low-head dam so dangerous?

This diagram of a low-head dam (copyright © MRS) illustrates the dam’s side view.

As the water flows over the dam and falls, it gains energy and momentum, creating a violent swirling motion below the dam, akin to the churning of a washing machine or boiling water.

This diagram of a low-head dam (copyright © MRS) illustrates the dam’s side view.

This rolling water, while often calm on the surface, hides powerful undercurrents beneath. Anyone caught in this vortex will be pulled and pushed by the swirling water, making it nearly impossible to control their movement. Even wearing a life jacket does not guarantee that one’s head will remain above water.

Player Tips

When rafting in the wild, we often encounter similar water hazards, commonly referred to as “water holes.” The danger of low-head dams lies in their continuous circular motion. The smooth structure of the dam creates a wide backflow area, and the water falling from the dam drags everything back into the vortex, creating a dangerous cycle.

This diagram of a low-head dam (copyright © MRS) illustrates the dam’s side view.

Since it is artificially constructed, this type of man-made horizon water hole features uniform and regular ledges. These smooth ledges create a broad rollback zone. When the water from a low-head dam churns and disturbs, a large “whirlpool” forms at the intersection between the rollback water and the falling water. The width of the rollback indicates the suction power of the whirlpool. The water that falls into the river pulls the rollback water downward. If there is enough drop or flow, the falling water generates enough pull that objects, which would normally be carried downstream, are instead drawn back into the whirlpool and trapped in a repeating cycle.

People who would typically be able to float on the surface find it difficult to control their direction and are sucked underwater. In the turbulent, swirling waters, it is easy to lose orientation. Even if someone intends to swim outwards, it is often difficult to escape. The person in the water will quickly exhaust their strength while struggling to get free.

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