Chinese engineers are working round the clock to try to stop thousands of people from being flooded out by water in a dam that was created by landslides during the earthquake, The Times has learnt.

The swelling lake that has risen to 70m (230ft) would threaten, if it burst, the five million residents of Mianyang county, home to the second-biggest city in southwestern Sichuan province. It is expected that the water will be released to avert a collapse.

China has pledged to save every life it can, mobilising 130,000 troops and countless medical staff and helpers nationwide. Two people were pulled out alive from the rubble yesterday — a 60-year-old woman, who survived by drinking rainwater, and a man rescued from a flattened power plant.

David Petley, Wilson Professor in Hazard and Risk at Durham University, said that if the water were to seep through and erode the barrier, it could send bricks, trees and other debris downstream. “You can’t be sure that the town below would be destroyed, but it would certainly be a very dangerous place to be,” he said.

One plan is to build up earth barriers halfway across the Beichuan river, at several points downstream from the reservoir. These barriers would divert the water, slowing its flow and reducing the danger to communities in its path. Once these were ready, some or all of the water could be released over the next day or two, possibly by blasting away part of the landslide.

Professor Petley said that the usual method of breaching a dam, familiar to military engineers, would be to build a channel across the top, starting from the dry side and excavating towards the water. “The channel has to be lined with big rocks to stop the water corroding it and causing a huge flood, but the key thing with this method is that only the water at the top of the lake will flow out and you control the flow.”

More than a thousand dams of different sizes lie in the area hit by the 8.0-magnitude earthquake. Eleven teams of about ten specialists each have been sent to carry out checks on all of the dams. Another six teams have been assigned to monitor lakes formed by landslides, which have blocked rivers in at least 21 places.

More than 2,000 people were moved at the weekend from a village in Hongyuang township after the blocked Qingzhu river burst its banks and began flooding their homes.

At one dam in Fuxing Township, a team of engineers from northeastern Liaoning Province was using lorries and diggers to shore up a small reservoir that had sprung a leak during the tremor.

Dai Yuxin, leading the team, told The Times: “It only looks simple. By the morning we will have moved in 8,000 cubic metres of earth. It is as if we are building a second dam here.”

His team had driven for six days across China to reach the earthquake area. “If the dam bursts then it would be like another earthquake for the people here,” he said. Officials have already helped the farmers downstream to bring in their crops — just in case.

Gu Junyuan, chief engineer at the State Electricity Regulatory Commission, has said that several dams weakened by the earthquake are under 24-hour observation for signs of collapse and may not be able to withstand strong aftershocks or flooding. “The earthquake this time has caused damage at various levels to reservoirs and dams. Safety experts have been put in place to monitor the operation of the dams 24 hours a day.”

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