Dam in China trapped millions of tonnes of water, rising 7 feet daily and threatening millions. How did engineers avert disaster?

China averted a potentially catastrophic flood in June 2008 after engineers and military personnel carved an artificial drainage channel through a massive landslide dam formed by the 7.9-magnitude Wenchuan earthquake in Sichuan province on May 12. The quake-triggered blockage on the Tongkou River created a rapidly filling lake that threatened millions of people downstream. Authorities evacuated residents, airlifted heavy machinery into the mountainous region and deliberately released the water through an engineered channel, bringing the lake under control without flood-related casualties.

The operation at Tangjiashan became an important case study in managing unstable landslide dams. Its lessons on drainage-channel design, erosion prediction and downstream flood modelling were subsequently applied during the 2018 Baige landslide crisis in China.

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How the 2008 Wenchuan earthquake created a dangerous natural dam

On May 12, 2008, the Wenchuan earthquake struck the Longmenshan mountains along the eastern margin of the Tibetan Plateau in Sichuan province. The violent ground movement triggered an estimated 60,000 to 200,000 landslides across the mountainous terrain, including a massive collapse on Tangjiashan Mountain.

The landslide deposited approximately 20.4 million cubic metres of rock, soil and shattered mountain debris across the Tongkou River, a fast-flowing tributary of the Fujiang River. The resulting co-seismic landslide dam, also known as a natural earth-fill dam, stood between 82 and 124 metres high.

As the river became blocked, water accumulated behind the debris to form the Tangjiashan Landslide-Dammed Lake, also called Quake Lake. It became the largest of the 257 landslide dams formed by the earthquake. The lake's projected maximum capacity was 315 million cubic metres.

Why the Tangjiashan landslide dam threatened millions

The immediate problem was the instability of the natural barrier. Unlike an engineered concrete dam, the landslide blockage had no reinforced core, designed spillway or internal stabilisation system. It consisted of loose rock and soil that could be rapidly eroded by flowing water.

Heavy seasonal monsoon rainfall and continuing aftershocks added to the danger, while the lake's water level was rising by as much as two metres a day. Geologists feared that once the water overtopped the dam, it could erode the debris, enlarge the breach and release a destructive surge downstream.

The location of the blockage heightened the risk. The dam lay just 3.5 kilometres upstream of Beichuan town and 85 kilometres upstream of Mianyang, a major urban centre. An uncontrolled breach was estimated to threaten between 1.3 million and 2.5 million people downstream.

How authorities evacuated residents before releasing the water

Faced with uncertainty over when and how the natural dam might fail, Chinese authorities organised a large-scale evacuation before attempting to drain the lake.

Beichuan town was cleared of its remaining population. In Mianyang and surrounding towns, authorities evacuated more than 250,000 residents. Approximately 5,000 villagers upstream were also relocated because of the risk of rising water and backwater inundation.

Evacuation planning included a designated 36-metre flood line, representing the calculated peak flood height under the planned drainage scenario. Thousands of residents were moved to higher ground above this level.

The evacuation was a critical part of the operation because engineers intended to allow water to flow through the artificial channel and erode the landslide dam. Even a planned release could produce dangerous downstream flows, making it necessary to clear exposed communities before drainage began.

How engineers and the military controlled the lake's release

Rather than attempting to reinforce the unstable landslide dam, authorities adopted a strategy of artificially controlled failure. The objective was to create a drainage route through which water could escape before the natural barrier failed unpredictably.

Airlifting heavy machinery to the mountain

Landslides had destroyed surrounding roads, making it difficult to transport equipment to the dam. Military helicopters, including heavy-lift Mi-26 aircraft, were used to carry bulldozers, excavators and fuel to the site.

Carving an artificial drainage channel

Engineering troops worked in shifts around the clock despite the continuing threat of aftershocks. Using heavy machinery, dynamite and anti-tank rockets, they cut an artificial spillway approximately 475 metres long across the crest of the landslide dam.

The channel was designed to give the rising lake a designated outlet rather than allowing water to spill unpredictably over the natural barrier.

Releasing the water in June 2008

On June 5, the lake reached the mouth of the artificial channel and water began flowing through it. As anticipated, the current gradually eroded the loose sediment, widening and deepening the drainage route.

By June 10, the discharge had reached approximately 6,500 to 7,110 cubic metres per second. The resulting torrent flowed through downstream settlements that had been evacuated ahead of the operation.

The lake level subsequently fell below the danger threshold, neutralising the immediate threat to Mianyang. The operation was completed without casualties resulting from the flood.

What lessons did the Tangjiashan operation teach engineers?

The Tangjiashan crisis provided important information about how landslide dams behave during controlled drainage. Studies by the United States Geological Survey and the Chinese Academy of Sciences, along with subsequent research, examined the relationship between channel design, the composition of the landslide debris and the risk of an uncontrolled breach.

Drainage-channel design matters

A simple, straight trench can allow water to accelerate and erode the channel unpredictably. Research following the Tangjiashan operation highlighted the importance of channel geometry, including cross-sections that are wider at the entrance and narrower in the middle, and a reduced gradient to help regulate flow and erosion.

The composition of the landslide affects how quickly a dam erodes

Landslide dams contain varying combinations of soil, fractured rock, large blocks and fine sediment. These materials respond differently to flowing water, affecting the rate at which a drainage channel deepens and the timing of a possible breach.

The Tangjiashan case contributed to the development of numerical models used to estimate dam-breach flood scenarios and improve predictions of drainage behaviour.

Flood planning must account for the entire river network

A breach can generate flows capable of affecting smaller natural dams or engineered barriers farther downstream. Subsequent research has therefore emphasised the importance of modelling connected river systems rather than assessing the main landslide dam in isolation.

How China applied the lessons during the 2018 Baige landslide crisis

The controlled-drainage approach used at Tangjiashan was applied again during the 2018 Baige landslide crisis along the border of Sichuan province and Tibet.

In October and November 2018, successive rockslides near Baige village blocked the Jinsha River, the upper stretch of the Yangtze River. The resulting landslide dam formed a barrier lake containing more than 500 million cubic metres of water, according to the supplied account.

The blockage threatened downstream communities and hydroelectric infrastructure if the dam failed suddenly. Authorities declared an emergency and used data models to assess the dam's internal geology and the potential flood risk.

Emergency teams transported heavy excavators to the site and carved a deep, engineered spillway across the crest of the natural barrier. As the lake rose to the channel, water began flowing through the designated route, eroding the debris and draining the reservoir.

Although the high flows caused manageable impacts on downstream infrastructure, the operation averted the feared catastrophic collapse, according to the supplied account.

The two cases illustrate the importance of combining early evacuation, rapid deployment of heavy machinery, engineered drainage and flood modelling when dealing with unstable landslide dams. Tangjiashan demonstrated how a natural blockage could be deliberately breached to reduce the danger of an uncontrolled failure, while the Baige crisis provided another application of the controlled-drainage strategy.