Sediment

Three Gorges dam implicated in serious embankment breaches

Kelly Haggart and Mu Lan

March 31, 2006

Officials in Hunan province, racing to repair ruptures in earthen embankments near Yueyang, point to the big dam 400 kilometres upriver as the main cause of the dangerous situation.

Officials in Hunan province, racing to repair a secondary Yangtze embankment that has been breached in heavy rain, point to the Three Gorges dam as the “fundamental cause” of the dangerous situation. After days of torrential rainfall, the Yangtze has burst its banks in three places in the city of Yueyang, about 400 kilometres downstream of the big dam, official media reports said. The longest collapsed section stretches for 600 metres, provincial water-resources officials were quoted as saying.

Collapsed earthen embankment near Yueyang

In some places, the breaches that have opened up in the earthen embankment beside the river are just 60 metres away from the main Yangtze dyke, a much stronger flood-control structure that runs parallel to the river further inland.

Liu Guhua, head of the Yueyang city flood control headquarters, described the collapses as the most serious of recent years. Xinhua quotes Mr. Liu as saying that if the embankment near the river continues to cave in, the safety of the main Yangtze dyke — designed to protect millions of people living behind it — will be threatened. In a Chinese-language report today (Mar. 31), Xinhua quotes Mr. Liu as saying that the Three Gorges dam is the “fundamental cause” of the problem. (Xinhua’s English news service made no mention of the dam in its coverage of the story.)

When the dam began holding back water in June 2003, it also began holding back sediment. Carrying a lighter silt load, the river below the dam is both clearer and faster flowing. It has a stronger “scouring capacity” as a result, with more power to erode riverbanks even hundreds of kilometres downstream of the dam. The New Beijing News (Xin jing bao, Mar. 30) quotes a Yueyang flood-control official, Wu Wensheng, as making the same point — that since the Three Gorges reservoir was filled to the initial level of 135 metres above sea level in mid-2003, the section of the Yangtze that includes Yueyang has been subject to more severe erosion.

Mr. Wu said city officials were moving swiftly to carry out repairs to the damaged embankment, an urgent task as the flood season commences. Several years ago, Three Gorges Probe reported the concerns of a top water official who warned of the very danger now confronting Yueyang. In early 2003, Cai Qihua, director of the Changjiang (Yangtze) Water Resources Commission, said that once the Three Gorges reservoir began storing water later that year, 60 per cent of the sediment carried by the murky Yangtze would be deposited on the riverbed behind the dam. The result would be a faster, more erosive river downstream that could threaten flood-control embankments with collapse.

“In specific sections of the river, the alterations in river regime and in the riverbed could be tremendous,” CCTV quoted Ms. Cai as saying. She said one of the areas likely to be most affected was the lower Jingjiang (another name for the section of the Yangtze that has just been hit), “where the scouring is likely to be as much as five or six metres deep.”

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