January 31, 2003
Three of China’s top water engineers have called for monitoring stations to be set up below the Three Gorges dam to study the changes in the Yangtze River that will begin occurring after the reservoir is filled in June, an official newspaper reports.
Zheng Shouren, Wen Fubo and Han Qiwei, senior members of the prestigious Chinese Academy of Engineering, have warned that once the dam begins holding back water, the downstream river regime is likely to be substantially altered. They made the appeal for sustained scrutiny of these changes at a conference on Yangtze Valley management held in Wuhan, capital of Hubei, the province where the dam is located.
A year ago, two of the scientists – Mr. Zheng and Mr. Wen – also urged the central government to undertake a geological-safety inspection of all the new settlements being built in the area to house people displaced by the dam. Mr. Wen, former director of the Changjiang Water Resources Commission (CWRC), and Mr. Zheng Shouren, current manager of the CWRC’s engineering group, predicted that impounding a huge body of water in the reservoir is likely to activate at least 760 landslips, putting lives at risk in the new towns.
Mr. Zheng told the recent conference in Wuhan that as much as 60 per cent of the sediment carried by the Yangtze will be trapped in the reservoir behind the dam, the China Water Conservancy Daily (Zhongguo shuili bao) reported on Jan. 26. With a reduced silt load, the river will flow more quickly and have a greatly increased scouring capacity, he said. And as more and more water is impounded in the reservoir, the river’s erosive power below the dam will increase.
Mr. Han, a sedimentation expert with the Beijing-based Institute of Water Resources and Hydropower Research, said the scouring capacity of the Yangtze below the dam is likely to be much greater even than that of the raging river during the severe floods of 1998.
The increasingly violent scouring will cause the river channel to widen, threatening flood-protection dykes and increasing the risk of a disastrous flood in the middle and lower reaches of the Yangtze, Mr. Han said. The scouring would also deepen the riverbed, causing the river to hold more floodwater and release less into Dongting Lake, which acts as a natural flood retention area.
Dongting, China’s second-largest freshwater lake, located 500 kilometres downstream of the dam, can fluctuate in size from 600 square kilometres in the dry season to 2,800 sq km in the flood season. It supports a productive fishery, including significant aquaculture ventures, which experts have warned could be devastated by a drastically reduced inflow of floodwater and nutrient-rich sediment.
Mathematical models can give a general idea of the dam’s likely impact on downstream areas, Mr. Han noted. But he said that on-the-spot monitoring stations are urgently needed, especially in the Jingjiang section of the Yangtze below the dam, to pinpoint exactly where the scouring will take place, and where riverbanks and flood embankments are most at risk.
The models have suggested that the most severe scouring will take place in the lower Jingjiang section (from Ouchikou to Chenglingji), where it is likely to continue occurring for the next 50 years and to deepen the riverbed by up to five metres, Mr. Han said. The second most affected region will be the upper Jingjiang section (from Taipingkou to Ouchikou), where the scouring could deepen the riverbed by at least three metres and occur for the next 30 to 40 years.
Cai Qihua, current director of the CWRC, wrote recently that changes in the shape of the river "inevitably will be dramatic in certain regions." Noting that the Three Gorges project "will have a tremendous impact on the environment," she said the main issues upstream of the dam will be "the environmental capacity for resettlement, sedimentation in the reservoir and the triggering of geological disasters."
In the middle reaches of the Yangtze below the dam, the impacts will centre on "erosion of the river channel and the relationship between the river and the lakes along the river," she wrote in an article posted Jan. 8 on the CWRC Web site.
Categories: Three Gorges Probe