Engineers Face Testing Times as Thousands Flee Dam Threat

Clive Cookson in London
Thursday, May 29, 2008

Originally published in The Financial Times

The modern world has never faced the threat of dangerous dams on
anything like the scale of the crisis now unfolding in Sichuan in the
aftermath of the earthquake two weeks ago, engineers say.

In addition to about 380 existing man-made dams that were significantly
damaged by the 7.9-magnitude quake, the Chinese authorities are faced
with an estimated 35 new "natural" dams formed when hills and mountains
collapsed into rivers, according to the official Xinhua news agency.

The greatest immediate threat comes from the Tangjiashan "quake lake"
on the Jianhe River, where a landslide is holding back 130m cubic
metres of water. As many as 1,000 soldiers and engineers are trying
desperately to dig channels through the mud and rocks, using explosives
and mechanical equipment airlifted to the site, to allow water to drain
away without causing a catastrophic failure that could inundate more
than 1m people living downstream. But western experts are reassured by
the fact that "the Chinese are as good as anyone on the planet at this
sort of engineering", as Ian Cluckie, professor of hydrology at Bristol
University, put it.

"Huge numbers of Chinese attend the regular international conference on
dam engineering," Andy Hughes, director of dams and water resources for
Atkins, the multinational engineering company, said. "They have
certainly brought themselves up to the best international standards."

Even so, the scale of China's worst natural disaster for more than 30
years, and the disruption of transport over a wide area of Sichuan, is
making life hard for the Chinese authorities. "The worst problem for
them is the sheer amount of work - and deciding on the order of
priorities for the emergency operations," Prof Cluckie said. "It will
be a scary few weeks or even months," he added, "before they can be
confident that all the dams are safe."

Prof Hughes said natural dams formed by landslides were potentially
much more dangerous and unpredictable than man-made dams, because they
were random "lumps of hillside" rather than carefully engineered
structures with spillways to carry away excess water.

At the same time as carving drainage channels through the Tangjiashan
dam, the authorities are evacuating people who live further down the
Jianhe river. Xinhua, the Chinese news agency, said the 158,000 people
at greatest risk had been moved but up to 1.3m were threatened if all
the water in the lake were released.

"Human factors come into play in an evacuation" from a newly formed
dam, Prof Hughes said. "People living downstream may not recognise that
there is a real danger because they are not used to having a dam above

The last big natural disaster that produced dangerous landslide dams -
though not on such a large scale as in Sichuan today - was the 1980
Mount St Helens eruption in the US. Three large lakes in the upper
Toutle river, formed by volcanic debris dams, required rapid
modifications to their outlets in order to prevent catastrophic
failure. The US Army Corps of Engineers built spillways and tunnels to
prevent the lakes overtopping their natural dams - and these are still
monitored closely today, to provide early warning of any impending

Engineers say the Tangjiashan lake emergency should not divert all
attention from damaged man-made dams. Most prominent is the
156-metre-high Zipingpu dam, completed in 2006.

The concrete layer that seals the giant dam's rock core from the water
has cracked and is leaking, though Wen Jiabao, the Chinese premier,
said the main structure was safe when he visited last Saturday.

The damaged dam crisis is unlikely to have significant long-term
commercial or environmental consequen-ces. Chinese engineers expect to
restore almost all the man-made dams to full operation, as sources of
hydropower and water for homes, industry and agriculture. And the
landscape of mountainous earthquake zones such as Sichuan has been
reshaped naturally by landslide dams and lakes over millions of years.

In the short term there is a lot of safety work to be done but, Prof
Hughes said, "the fact that we have seen no catastrophic failures is
testimony to the resilience of Chinese dams in a severe earthquake".