This month’s 7.9 magnitude tremor spawned 34 so-called quake lakes, according to the International Association of Hydraulic Engineering and Research expert. The vast pools of water were created when the earthquake triggered landslides down plunging valleys, clogging rivers and turning them into fast-rising lakes. Twenty-eight quake lakes are at risk of bursting, according to Chinese state media agency Xinhua. But the one at Tangjiashan – on the Jianjiang river above the town of Beichuan – is the most precarious.
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The delicate, tortuous work involves heavy machinery gingerly shifting debris from the dam, and engineers blasting dynamite to carefully punch holes in the mountain of rubble and soil – although experts warn this risks further destabilising the structure. Nearly 160,000 people in the disaster zone have already been evacuated in case the Tangjiashan quake lake bursts.
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Troops and engineers are racing to carve a 500 metre (1,640 ft) channel out of the landscape and divert the water towards the Fujiang river. They aim to complete the giant sluice and begin draining the 300 million cubic metre capacity lake within 10 days. “Once the water begins to flow over the top of the dam there’s nothing you can do to stop it,” said Dr Alex Densmore, of Durham University’s Institute of Hazard and Risk Research.
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Little wonder then that Premier Wen Jiabao says he regards draining the swelling quake lakes at China’s ground zero as the nation’s most urgent task.