January 24, 2003
The builders of the Three Gorges project are showing new concern about the prospect of a dangerous buildup of silt in the massive reservoir behind the dam, and are still discussing basic aspects of the dam’s operating regime and likely impacts.
To study sedimentation and other factors that will influence how Yangtze floods behave after the dam is completed, a US$22-million (180 million yuan RMB) model of the reservoir is to be built over an area of 40 hectares, China Central Television (Zhongyang dianshi tai) has reported. The huge model, whose location was not specified in the Jan. 10 report, will be used to simulate floods in a 1,000-kilometre section of the river.
After the Three Gorges reservoir is filled to the 135-metre level in June, 60 per cent of the sediment carried by the turbid Yangtze will be deposited on the riverbed behind the dam, CCTV quoted a top water official, Cai Qihua, as saying. With a reduced silt load, the river below the dam will flow more quickly and have a greatly increased “scouring capacity,” CCTV noted. The faster, more erosive river could cause riverbanks to collapse, and threaten the vital flood-control embankments that protect millions of people downstream of the dam.
Cai Qihua, who is director of the Changjiang Water Resources Commission, was quoted as saying that “in specific sections of the river, the alterations in river regime and in the riverbed could be tremendous.”
She said one of the areas likely to be most affected would be the lower Jingjiang River – the section of the Yangtze from the city of Zhicheng in Hubei province to Chenglingji in Hunan – “where the scouring is likely to be as much as five or six metres deep.” The effects of the river’s increased scouring capacity may be felt as far downstream as Datong in Anhui province,” 1,400 km from the dam, she said.
Meanwhile, a large dam is being built on a Yangtze tributary 350 km upstream of the dam specifically to keep sediment from entering the Three Gorges reservoir, the Chongqing Morning Post (Chongqing chenbao) reported on Jan. 19.
Construction of the concrete gravity dam on the Zhuxi River near the city of Wanzhou began in November, the newspaper said. It is scheduled to reach a height of 138 metres above sea level by June, and attain a final height of 170 metres when completed in 2005.
Though the Zhuxi River is small, the high rate of erosion in the river valley causes a big problem, with the river depositing 772,000 tonnes of silt in the Yangtze every year. All of this sediment will be trapped behind the new dam and prevented from flowing into the Yangtze, the newspaper said, but it gave no details of any plans to deal with the silt that will accumulate behind the Zhuxi dam itself.
The Three Gorges Project Daily said that while it may sound like a good idea to prevent sediment from entering the Three Gorges reservoir by building more big dams upstream, a leading member of the Chinese Academy of Sciences cautions that the strategy is likely to trigger other problems.
Lin Bingnan, who was in charge of the sedimentation research carried out in the mid-1980s for the Three Gorges feasibility study, said that reducing the Yangtze’s silt load in this way would strengthen the river’s scouring capacity not just below the dam but in the reservoir itself.
“So there are some questions to be asked,” he said. In the long term, is the river going to become ever wider in the reservoir area, eroding riverbanks and causing collapses and landslides? What changes will occur to the river regime? Will the river become more or less meandering? Will the flood-season danger zones shift as a result?
Prof. Lin also said he was seriously concerned about some of the construction of new settlements in the reservoir area. In the urban districts of Chongqing and Fuling, roads are being built right alongside the river, and even on land reclaimed from the river.
“This is a dangerous practice, if other cities and towns along the reservoir follow suit,” he said. “This is likely to make the flood water levels higher, and we will face serious consequences if the levels are even one metre higher than the norm. This is a serious hidden problem.”
But Prof. Lin did express optimism that with more big dams upstream, the water level in the Three Gorges reservoir could be raised directly to 175 metres on completion of the dam in 2009. According to the original Chinese feasibility study, the reservoir was to be filled to its “normal pool level” of 175 metres only after operating for 10 years at 156 metres so the dam’s impact on sedimentation could be monitored.
Prof. Lin also advised that the reservoir should be called on to fulfill its flood-control function only during major inundations. The reservoir’s storage capacity should be carefully protected, he said, and it will fill quickly with silt if it is used to control every little flood. (Closing the dam to hold floodwater back, rather than opening it to flush the water downstream, would cause more sediment to settle in the reservoir, raising the riverbed and reducing the reservoir’s floodwater storage capacity.)
Prof. Lin warned that while he feels certain the Three Gorges sedimentation problems can be fixed, vigilance will be essential. The solutions will never mean “tianxia taiping” (peace under heaven), he said, but rather “we should be well prepared for danger even in peacetime, and take the new problems with sedimentation fully into account.”