(July 30, 2013) Fan Xiao, a Chinese geologist and chief engineer of the Regional Geology Investigation Team of the Sichuan Geology and Mineral Bureau, says analysis of the recent landslide in Yunnan Province indicates that impoundment of the nearby Xiluodu Dam reservoir most likely caused the event and that more can be expected when the reservoir is filled again. Sharply rising or rapidly falling reservoir water levels pose a threat to geological stability, he says, and can trigger disaster.
By Fan Xiao, Chengdu
As local media outlets such as Sichuan Online and Sichuan News Network have reported, a giant landslide occurred in the newly filled Xiluodu Dam reservoir located on the lower Jinsha River around 17:00, July 27 (Beijing time).
The landslide, measuring 200 metres wide, 250 metres high and 120,000 cubic metres in total volume, ripped through Huangping Village of Huanghua Town in Yongshan County of Yunnan Province before flowing into the Jinsha River, where it caused tsunami waves as high as more than 20 metres, washing over to the opposite shore of the Jinsha.
According to news posted by local governments in both Yunnan and Sichuan provinces, at least 12 people, caught by the waves, remain missing, including laborers working on the construction of a pier, passers-by, and five teenagers between 13 and 16 years of age.
The Xiluodu Dam, on completion, will stand as the second largest hydropower project in China behind the Three Gorges Dam. The height of the Xiluodu reaches 278 metres, with a crest elevation of 610 metres, its reservoir capacity is 12.67 billion cubic metres and its total installed capacity is 13,860 MW. The dam is located on the Jinsha River (the upper Yangtze River) on the boundary between the provinces of Yunnan and Sichuan.
Impoundment of the Xiluodu Dam began on May 4, 2013, when, beginning at 440 metres above sea level, it was filled to a height of 540 metres. The first phase of impoundment lasted only 51 days but the increase in water level reached 100 metres, far higher than that of the Three Gorges reservoir which only increased 65 metres in its first phase of impoundment, while the Xiangjiaba Dam reservoir (on the upper Yangtze) increased 76 metres. Due to rapid filling of the Xiluodu reservoir, shipping traffic below the dam in the Luzhou section of the Yangtze was severely affected. Run-off from upstream was only 35-56% of the average for the same period in 2012; the water level of the Erlangtan in downstream Luzhou City declined to 0.18 metres from 1.4 metres during the period of reservoir filling.
Huangping Village of Huanghua Town in Yongshan County of Yunnan Province, where the landslide occurred, is located in the backwater zone of the Xiluodu Dam reservoir. According to the dam’s impoundment schedule, the reservoir’s water level will continue to rise until it reaches 560 metres above sea level in late August of this year (the water level reached 540 metres in June). Currently, the water level is already at 554.5 metres due to the impact of the flood season. In other words, the cumulative increase of water levels has reached 115 metres in total; meanwhile, more than 5.1 billion cubic metres of the reservoir’s storage capacity had already been used up by early May.
Lessons need to be drawn from landslides that have occurred in the Three Gorges reservoir area and landslides in China’s other large reservoir regions. As these cases demonstrated, periods in which water levels rise sharply or fall rapidly are usually dangerous because they can trigger geological disasters or exacerbate geological instability. These cases include the big landslide in Qianjiangping when the Three Gorges reservoir was filled to 135 metres for the first time in July 2003, and the large landslide at Gongjiafang when the Three Gorges reservoir was filled to 172 metres for the first time in November 2008, and so on and so forth.
Both the spatial and temporal analysis of the landslide that occurred in Yongshan County in Yunnan Province last Saturday indicate that the impoundment of the Xiluodu Dam reservoir most likely caused this incident. More such incidents will occur again and again in the future because, according to the impoundment schedule, the Xiluodu reservoir will be filled to its NPL (normal pool level) of 600 metres (above sea level) by 2014.
This landslide reminds us, once again, that when impounding and operating large reservoirs in a geologically disaster-prone region like western China, full consideration should not only be given to the interests and needs of hydroelectric power production, but also to the environmental impact and the effect on geological hazards. In the first phase of reservoir impoundment, especially, it’s imperative to allow for a sufficient observation and grace period, and to be really careful in regards to the timing and rate of filling, and the dropping of water levels. During reservoir operation, the timing, volume and rate of filling and lowering of water levels should also fully consider the environmental impacts on downstream areas. And, generally, the monitoring and management of reservoir induced seismicity and other geological disasters must be strengthened.
Fan Xiao is the Chief Engineer of the Regional Geology Investigation Team of the Sichuan Geology and Mineral Bureau
Fan Xiao | Probe International
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