A dam born from violence
On May 12, 2008, a magnitude 8.0 earthquake struck Sichuan province. The immense seismic energy dislodged a mountainside, sending a catastrophic landslide into the Tongkou River valley. This mass of rock and soil—an estimated 20.4 million cubic meters of debris—crashed across the river, forming a massive natural dam in a matter of minutes. The Tangjiashan Landslide Dam was immense, stretching approximately 600 meters across the river and 800 meters along its length. Its height ranged from 82 to 124 meters, creating a formidable and unstable barrier. The material was a chaotic mix of fragmented siltstone, mudstone, and soil, creating a structure with no engineered stability.
With the Tongkou River blocked, water began to back up immediately, forming what became known as Tangjiashan Lake. The water level rose rapidly, at an initial rate of 1.2 meters per day. Within weeks, the lake held a staggering volume of water, with estimates reaching up to 316 million cubic meters. This accidental reservoir posed a direct and catastrophic threat to more than 1.3 million people living in downstream communities, including the major city of Mianyang. The situation was a race against time as the rising water crept closer to the top of the unconsolidated dam.
Engineering a solution
The threat of a dam breach was imminent. An uncontrolled failure would release a wall of water with devastating consequences. In response, a massive engineering and evacuation effort began. Hundreds of soldiers and engineers were airlifted to the remote and inaccessible dam site. Their mission was to dig a spillway across the top of the dam to allow for a controlled release of water. Working around the clock with heavy machinery, they excavated a diversion channel 12 to 13 meters deep.
As the work proceeded, authorities implemented one of the largest evacuations in recent history, moving more than 250,000 people to safer ground as a primary precaution. On June 7, 2008, water began to flow through the newly created channel. The moment came on June 10, when the flow was intentionally increased. The water discharge surged, reaching a peak flow rate of approximately 6,500 cubic meters per second. The controlled breach was successful. Over the next few days, a massive volume of water was drained from the lake, lowering the water level by over 27 meters and averting a large-scale disaster. The carefully engineered channel eroded rapidly as designed, widening to over 100 meters and successfully draining the immediate threat.