There is a 93% chance that Tangjiashan barrier lake will collapse, and the Chinese government has responded by relocating over 250,000 living downstream and conducting an emergency evacuation drill that could involve another one million or so people.

The above information was released during a press conference held in the night of June 3 and broadcasted live on state television.

The Tangjiashan barrier lake – located in the Beichuan county near Mianyang city, Sichuan – was formed following the May 12 earthquake and subsequent aftershocks that triggered landslides and blocked river flows. It is one of 34 such lakes formed after the quake and classified as the most dangerous.

By 17:00 on June 3, the water level at Tangjiashan, holding 205.5 million cubic of water, had reached 737.33 meters, only some 2.37 meters away from its overflowing point near the diversion channel dug by risk mitigation team, according to the Water Resources Ministry.

Liu Ning, chief engineer of the Ministry, disclosed that if rainfall continued, chances of the lake to burst were 93%, as every two millimeters of rainfall upstream could increase the water level by one meter. Frequent aftershocks also added the risk, he said.

Liu said at the press conference that though mitigation team had identified the weak spot and dug up diversion channel to avert flows in the event of a collapse to minimize damages, there were other uncertainties involved.

Among the uncertainties listed by Liu included further potential landslides that could land earth and debris into the lake and change the situation; more weak spots in the banks for leakage; and weather condition.

He said three strategies had been deployed to avert damages – around the clock monitoring at the lake, risk mitigation work of digging diversion channel, and the evacuation of those living downstream.

"We have two priorities, to ensure the safety of people’s lives and to restore stability in life for those affected as soon as possible.

"However, the former is more urgent. Homes can be rebuilt, but lives lost cannot be revived," he said at the press conference. Liu had been stationed at the Tangjiashan site since May 26.

Risk mitigation team working at the lake had also evacuated by June 1 after they had completed digging diversion channel. Only about a dozen of observation personnel were left on higher grounds of the lake to monitor the situation. 

Mianyang city deputy mayor Lin Xin, who was also present at the press conference, said based on recommendations from the experts, the city had evacuated over 250,000 people to safer grounds within 28 hours between May 30 and 31.

He said the affected were transferred to 250 relocation spots equipped with basic living necessities, such as tents, food, water, and medical services. He said the evacuation was based on a "one-third collapsed" contingency plan.

The Chinese authorities had earlier revealed three contingency plans based on the degree of collapse and the possible route of flows as a result, thus the affected areas and number of people. The plans included one-third, half and total collapse, and the number to be evacuated would be 158,600, 1.2 million and 1.3 million people respectively.

Lin said at the press conference that on June 1, the Mianyang local government had also performed a drill based on the "half-collapse" scenario to ensure that if the worst happened, the city would be prepared.