China quakes, but the dams don't break - News review 08
By Fred Pearce, New Scientist, Magazine issue 2687. 28 December 2008
Some 80,000 people died, mostly in landslides and collapsing buildings, in the earthquake which rocked China's Sichuan province on 12 May, yet it could have been so much worse. Probably nowhere on Earth are there so many large dams in an area of such seismic risk. Engineers are astounded that none of the 390 hydroelectric dams damaged in the magnitude 7.9 quake actually collapsed.
The biggest potential disaster was averted at the 158-metre-high Zipingpu dam, 17 kilometres from the quake's epicentre. Holding back more than a cubic kilometre of water, it is just upstream of Dujiangyan, a town of half a million people. The hydroelectric dam was the largest of a new, cheap design with a rock core and concrete face. As the tight valley sides juddered, the structure was squeezed and ended up 18 centimetres downstream, and 70 cm lower. The concrete was ripped apart but the core of the dam survived.
But had the quake occurred two months later during the monsoon - when the reservoir would have been full and the stresses on it greater - Zipingpu and other dams would probably have failed. As it was, engineers swiftly emptied the reservoir and the structure now awaits repair. Undeterred by this narrow escape, officials in Beijing announced in December that yet more dams are to be built in the Sichuan mountains.
Luck and engineering skill also saved people in Beichuan, where a huge "quake lake" formed on a tributary of the river Fu after it was blocked by a landslide. The hastily named Tangjiashan Lake grew for days, until engineers dug a channel through the debris to drain it. Foreign experts predicted that the water flow in the crude channel would erode its banks and cause an uncontrolled rush of water downstream.
On the fourth day of draining, the channel flow increased 30-fold in 4 hours, flooding parts of Beichuan and washing away downstream dams. It was close, but a million people were saved from disaster. It was announced this month that Beichuan's 160,000 people are to be moved to safer terrain.
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