Tuesday, August 21, 2012

Floods test Three Gorges Dam

Floods test Three Gorges Dam
By Deng Quanlun
August 15, 2012


In late July, the world's largest dam faced the toughest challenge in
its chequered nine-year history. Deng Quanlan reports on a continuing

It was the fourth Yangtze flood of the year and, on July 24, the water
level at Chaotianmen in Chongqing, western China, hit 187.92 metres -
the highest flood peak since 1981. Up to 71,200 cubic metres of water
flowed past every second, 28 times faster than the Yellow River and with
volumes greater than those recorded during the huge floods of 1998 and
2010. This was the biggest test for the Three Gorges Dam in its
nine-year history.

Official reports from Chongqing city indicate its outlying districts of
Jiangjin and Yongchuan were badly impacted by the floods. In total,
around 163,000 people were affected, including more than 82,000
evacuated from their homes.

Though this was the mammoth dam's most serious test to date, in terms of
what the structure is built to withstand it was a moderate challenge at
worst; the kind of thing its operators expect to see every 20 years.
After all, the barrage was designed to be able to cope with
once-in-a-millennium floods on a daily basis, and to be able to
withstand a once-in-10-millennia flood - plus 10%.

Monitoring showed that, when the flood peak reached the dam,
displacement, seepage and deformation were all within normal ranges, and
all safety indicators remained stable. The dam had already seen three
flood peaks earlier that month.

However, as the Three Gorges Dam released more water, the flood risk
further down the Yangtze increased. The water level in China's largest
freshwater lake, Poyang, rose to its highest level for two years. Some
2,200 people were assigned to monitor dykes in the lakeside areas of
Jiujiang, Shangrao and Nanchang, as well as the stretch of the Yangtze
in Jiujiang.

If the rainy season in the upper Yangtze coincides with that of the
lower and middle parts of the river, severe flooding can hit the plains
downstream. Most years, that doesn't happen - but this year it did:
"That's the situation we're seeing now," said a Yangtze flood-prevention
official, "and so there's a lot of pressure on the lower and middle
reaches of the river."

The Three Gorges project has always been controversial, from early
discussions and initial approval to construction and operation. Renowned
hydro-engineer Huang Wanli once predicted that the dam would cause
Chongqing's ports to become blocked up with silt and gravel within a

Since the Three Gorges reservoir was filled, there have been no
repetitions of the severe flooding of 1998, which killed more than 3,700
people and left 15 million homeless. But the controversy hanging over
the dam has spread to questions about its impacts on drought and the
downstream climate. These complex issues, which interlink with hot
topics like climate change, have sustained the dam as a focus of public
debate in China.

In recent years, the Yangtze basin - not usually short of water - has
been hit by two serious droughts: first the 2006 summer drought in
Sichuan and Chongqing and, more recently, the March 2010 drought, also
in the south-west. Some people blamed these disasters on the Three
Gorges Dam.

Former chief engineer at Hunan's water authority Nie Fangrong argues
that, though the droughts were caused by a decline in rainfall, they
were exacerbated by ecosystem changes wrought in the lower Yangtze River
by the Three Gorges dam. But Cao Guangjing, chief executive of dam
developer the Three Gorges Group, has pointed out that the structure
stores water between August and October every year, and releases
increasing amounts of water from January onwards, when the reservoir
level drops. During that period, the dam is actually helping to relieve
drought, rather than holding water back, he said.

Other critics blame the barrage for changes in the climate around the
reservoir and downstream. They say it has created a low-pressure zone
which affects normal atmospheric flows, triggering extreme weather
events such as the 2006 and 2010 droughts, and the 2008 blizzards in
southern China. The Chinese Meteorological Administration and other
experts have said this is nothing to do with the dam: global climate
change is causing extreme weather. Liu Min, head of the Wuhan Regional
Climate Centre at Hubei Meteorological Bureau, said there is no data to
show that the dam has caused drought on the lower Yangtze.

The dam's most important role - flood prevention - has also been a
source of controversy. Informed sources say that local governments
downstream of the dam have all asked the operators to release less water
in order to reduce flood risks, meaning the dam is under pressure from
both sides.

The dam has long faced such difficulties. Upstream, the city of
Chongqing complains that the dam makes flood prevention more difficult -
that "Chongqing drowns to save Wuhan". Downstream, there are complaints
that it continues to release water even when there are flood risks. Cai
Qihua, head of the Yangtze River Commission, said this is a
misunderstanding: when the dam holds back floodwaters, the reservoir
level does rise, but this has little or no impact on Chongqing upstream.
The rising waters in Chongqing are due to water coming from the Jin, Min
and Jialing rivers, and water backing up at the Tongluo Gorge,
downstream of Chongqing, Cai said.

The dam's role in flood prevention is to control water coming from the
upper reaches of the Yangtze. When it comes to regional flooding
downstream, it can only play an indirect role. Weng Lida, formerly head
of the Yangtze River Commission's Water Resources Protection Bureau,
explained that the dam can retain water from upstream, but if there is
heavy rain downstream, there is nothing it can do. Nor can the dam store
all of the floodwaters - it can only hold back those which cannot be
safely released into the rivers downstream. "The flood prevention
ability of the dam is limited - it can't do everything," said Weng.

And since the dam can't do everything, it is crucial to make sure that
flood-control mechanisms along the length of the river are integrated.
"Different regions want to handle floods differently," added Weng.
"Flood prevention on the Yangtze isn't just about the Three Gorges -
it's made up of groups of reservoirs along the main river and its
tributaries, and flood-diversion areas. Both the river basin and its
regions need to be considered. That means that regional management needs
to follow instructions from basin-level management, with the Yangtze
Flood Prevention Office overseeing things."

The Yangtze Institute of Survey, Planning Design and Research has
calculated that, in the long-term, capacity of the Three Gorges
reservoir and the upstream control reservoirs will reach almost 100
billion cubic metres, and flood prevention reservoirs 50 billion cubic
metres. It would be of huge benefit to flood prevention if these can be
managed in unison. Yangtze River Commission experts say that preventing
floods on the lower Yangtze requires comprehensive flood-prevention work.

In May 2011, China's State Council - the highest state organ - admitted
in a statement that "while the Three Gorges Dam has brought great
benefits, there are still urgent issues with relocations, environmental
protection and the prevention of geological disasters; and it has had a
certain impact on navigation, irrigation and water supplies."

Given the new century has already seen a series of "once-in-a"
disasters, perhaps the incredible Three Gorges Dam is only just starting
to be tested.

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