快猫短视频

IS A historic disaster about to be repeated? More than 200 years ago, an earthquake in the Chinese province of Sichuan triggered a landslide that dammed the Dadu river. Ten days later, on 10 June 1786, the blockage collapsed under the accumulating weight of the monsoon rains. Some 50 million cubic metres of water rushed down the valley, and more than 100,000 people are believed to have died in the largest 鈥渜uake lake鈥� disaster on record.

Last month鈥檚 magnitude 7.9 earthquake, which is already thought to have killed at least 70,000 people, occurred just 200 kilometres to the north. It too triggered extensive landslides that blocked river valleys, creating 30-plus lakes across the region.

Now, as efforts to drain these new quake lakes continue, fears of a flood disaster similar to the one in 1786 remain. Monsoon rains are threatening to swell river flow in the region, and many of the lakes could be filling up unnoticed. 鈥淎n unexpected collapse of at least one landslide dam is surely likely,鈥� says geographer and landslide expert David Petley at the University of Durham, UK. Meanwhile, the damage likely to have been sustained in the quake by the region鈥檚 many hydroelectric dams is raising questions about the government鈥檚 plans for hydropower in the region (see 鈥淐hina鈥檚 power house鈥�).

As 快猫短视频 went to press, fears of a quake-lake breach were greatest for the Tangjiashan lake, created by a landslip in a steep river valley near the town of Beichuan, which sits on a tributary of the Fu river. Last weekend, Chinese engineers finished digging a 475-metre spillway through the landslip, designed to allow the 40 million cubic metres of water already in the lake to empty safely.

Petley, who surveyed similar lakes that formed after a major earthquake in Pakistan in 2005, admires the Chinese engineers鈥� efforts to make the lake safe, but warns that the danger is not over. Water pouring down the steep spillway may start to erode the fine-grained landslip material. 鈥淚t is incredibly hard to predict what will happen,鈥� he says. As the bed of the spillway erodes, it releases more water, allowing more erosion in a dangerous feedback loop. 鈥淭he water then collects the debris in the dam and creates a fast, dense and violent debris flow,鈥� Petley says.

In the worst case, the lake would empty within a few hours, sending a 鈥渧ery large flood wave鈥� down the valley. The authorities have announced plans to evacuate more than a million people if water flowing from the lake gets out of hand. 鈥淚 just hope they understand how quickly it could happen,鈥� says Petley.

As attention focuses on the Tangjiashan lake, other potentially lethal quake lakes in the region may have been missed. The Chinese government says that around 30 lakes have been formed by landslips that have blocked rivers, and officials insist they are all being monitored for the risk of a breach. 鈥淏ut given the vast areas involved, I worry that there might be one lurking in the mountains that has not been spotted yet,鈥� Petley says. It would be an extraordinary achievement if the authorities were able to ensure that none of the quake lakes collapsed, he adds.

鈥淚t would be an extraordinary achievement if the authorities were able to ensure that none of the quake lakes collapsed鈥�

The monsoon season is likely to bring more landslides, as mountain slopes destabilised by the quake become saturated, and Petley predicts these could in turn form new quake lakes. The danger for mountain communities could last for years, he says.

For the many man-made dams of the region, however, the timing of the quake has been fortunate, according to Martin Wieland, chairman of the seismic safety committee of the International Commission on Large Dams (ICOLD), the main dam-builders鈥� body. That鈥檚 because in mid-May most of the reservoirs were empty, awaiting the monsoon rains. If the quake had happened during the monsoon season, many of the dams would have been much more vulnerable. Over the past decade, China has built hundreds of hydroelectric dams in the mountainous west of Sichuan. A series of dams cascade down the Min river, a major tributary of the Yangtze, which runs along the Longmen Shan fault that ruptured last month (see Map).

None of the Min river dams burst, but at a meeting of ICOLD in Bulgaria on Monday, Chinese delegates revealed that 1583 dams and reservoirs had been damaged by the quake 鈥� many more than previously thought. The damage ranges from cracks and leakage to power generation failure. Some had been subject to stresses 鈥渇ar beyond鈥� those they had been designed to withstand, according to Xu Zeping, director of the China Institute of Water Resources and Hydropower Research.

Of the dams close to the epicentre, the two-year-old Zipingpu dam has had the most attention so far. It can hold back up to 1.1 billion cubic metres of water, and is one of the world鈥檚 largest examples of a new design of dam combining a rock core with a concrete face. These concrete face rock-fill dams are cheap to build and increasingly popular, especially in China.

Zipingpu is the first large dam of its type to be subjected to severe ground shaking, and so far the signs are good, Wieland says. Large cracks formed in the concrete face, breaking its waterproof seal, but this does not appear to have damaged the integrity of the dam, and it should be able to return to service after repairs.

According to Xu, dam safety experts have inspected all the other dams in the region, but it remains unclear how thoroughly. For example, one of the biggest dams, the 132-metre-high Shapai dam, just 12 kilometres from the epicentre, has so far only been examined from the air because it is still inaccessible otherwise. Xu says it is 鈥渂asically in a safe condition鈥�.

A bigger problem may be the 400 or so small dams in the quake zone which have not been designed for earthquakes, Wieland says. 鈥淚f some of them are severely damaged, they may be eroded during a flood,鈥� he warns, and they could be almost as vulnerable as the quake lakes.

Even for larger dams, engineers still hold their breath when quakes hit. Wieland, who has written industry studies on designing dams to withstand earthquakes, has previously pointed out that it is not possible to reliably predict the behaviour of dams during strong quakes. During several recent earthquakes 鈥� most recently the Bhuj quake in India in 2001 鈥� most reservoirs were almost empty. If they had been full, they might have collapsed and killed large numbers of people.

Though China鈥檚 dams survived this time, the country鈥檚 extensive network of dams may be vulnerable in future. In 1975, a dam burst during a monsoon in Henan province in central China, causing a tidal wave that destroyed a bigger dam downstream at Banqiao. A wall of water 12 kilometres wide and 6 metres high crashed into the town of Huaiban, killing between 80,000 and 200,000 people, though official secrecy at the time meant it was years before the world learned of the disaster.

China today is a much more open society, as the media coverage of the quake has shown. Its people may think differently now about allowing dams to be built in such vulnerable regions 鈥� and learn the lessons of their tragic history.