Sediment

Water official raises Yangtze flood-control concerns

Three Gorges Probe
December 20, 2001

A senior Chinese water official has raised concerns about flood control on the Yangtze River, even after the Three Gorges dam is built.

Cai Qihua, director of the Changjiang Water Resources Commission (Changjiang shui ziyuan weiyuan hui), told a recent meeting of academics and officials in Wuhan that the dam “will not completely solve the Yangtze River flood problem.”

In the event of another catastrophic flood on the scale of China’s 1954 tragedy, up to 40 billion m3 of flood water will have nowhere to go even after the Three Gorges dam is completed, Ms. Cai said. (The 1954 flood is considered a “200-year flood” – of such severity it occurs usually once every 200 years.)

Constructing a more comprehensive “flood-control security system” (including dams, dykes, and overflow flood plains) has to be the first priority in managing and developing the river, says Ms. Cai.

“We have no reason to be optimistic about the current situation, although we have achieved a lot in the construction of flood control projects and facilities in the Yangtze basin,” she is quoted as saying in the Dec. 3, 2001, report on the Changjiang Water Resources Commission’s Web site.

Flood control in the Yangtze River valley is far from secure, warns Ms. Cai, who identifies the Jingjiang area as particularly weak. This section of the river, stretching from Zhicheng city in Hubei province to Chenglingji city in Hunan province, is considered the most dangerous section of the Yangtze because of the river’s narrow and meandering course between the cities of Yichang and Wuhan.

While the Three Gorges dam will increase protection of the Jingjiang stretch from a one in 10-year flood (a relatively frequent, but smaller flood) to a one in 100-year flood (a less frequent, but larger flood), “the Three Gorges dam is far from a final solution to the floods on the Yangtze River,” says Ms. Cai. The reason, she explained, is that the dam itself will alter the complex hydraulic regime of the river. Its mammoth structure is likely to affect the relationship between water and sediment in the river, impacting on downstream erosion and siltation, the pattern of water storage and discharge in the main channel, and the relationship between the river and lakes linked to it that act as overflow storage areas during floods.

According to Ms. Cai, once the Three Gorges reservoir is filled and sediment builds up behind the dam, the river downstream will have a stronger scouring capacity, leading to deepening of the riverbed. Mathematical models of the sedimentation process show the sections most likely affected will be the Jingjiang area and the region between Chenglingji and Wuhan.

Ms. Cai says the Yangtze’s flood control security system must include more than the Three Gorges dam. Other additions include US$850 million earmarked by the state to build or strengthen 30,000 km of dykes along the main channel of the river and its tributaries after a major flood in 1998. These embankment projects are expected to be completed before the 2003 flood season, when the Three Gorges reservoir is to be filled to the 135-metre level.

Second, an additional flood diversion area, as large as 50 billion cubic meters, that has been planned “in an effort to store extra flood water from the Yangtze.”

But even after all that is done, more dams upstream of the Three Gorges will have to be built says Ms. Cai. The proposed Xiangjiaba and Xiluodu dams on the Jinsha River and the Tingzikou dam on the Jialing River will not only store some flood water but will also reduce the amount of sediment reaching the Three Gorges reservoir and building up behind the dam.

Because of the nature of the mighty river, Ms. Cai conceded, even these “engineering structures” will not completely solve the Yangtze flood problem. “Non-engineering measures” will need to be employed, including the resettlement of populations currently living on the Yangtze’s flood overflow areas, to make “more room for the Yangtze.”

In her speech, Ms. Cai also discussed the south-north water transfer project, which will divert Yangtze River water to north China; the west-east power transmission project, which will see more big dams built upstream of the Three Gorges; and the measures undertaken to tackle water pollution in the Yangtze and to prevent soil erosion in its upper reaches.

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