A 30 percent loss of output from two massive dams on the upper Yangtze, in combination with a 6.1M earthquake upstream and south-west of the two stricken hydro plants, heightens fears in an intensely seismic-prone region that dam construction and reservoir filling are quake triggers.
Operating for only seven years, the two plants are upstream sisters of the Three Gorges complex. Against its generating capacity of 22.6GW, the two plants are rated at 13.86 GW and 6.45GW respectively, ranking them at No3 and No 9 of the world’s hydro dams now generating. Giving context, the world’s most powerful nuclear plant, Japan’s Kashiwazaki-Kariwa, is rated at 8.2GW.
The operator CYPC reported without comment that water flows through the 200 kilometre long Xiluodu reservoir upstream of the larger of the pair, fell by 44.68% to 13.061 billion cubic metres over the three months.
(Image: Three Gorges Corporation website)
On 21 May this year, 270 miles upstream and south-west of the two stricken hydro plants, the city of Dali close to Tibet suffered a 6.1 magnitude earthquake. Geologists put its epicentre at up to 10 kilometres deep. Deaths from the quake were officially registered by Chinese authorities at three, but with 12,882 properties damaged.
During its filling in 2013, the Xiluodu reservoir suffered at least one catastrophic landslide, as a result of a 6.5 magnitude quake centred 40 kilometres away. The quake reportedly killed 600 and displaced 300,000 more.
Both hydro plants lie 50 miles apart, on the upper Yangtze, Asia’s longest river at 3,915 miles or 6,300 kilometres. Known in its upper reaches as the Jinsha, the river rises in Yunnan province in China’s far south west.
Mountainous Yunnan is riddled with deep geological flaws, in the shadow of the Himalayas and Tibet where the Eurasian and Indian tectonic plates collide.
The province’s decade-long boom in building dams and filling their mega-reservoirs has caused fears among Chinese geologists that reservoir construction may actively cause quakes.
“..when water flows quickly into resulting reservoirs, it can change the stress on faults deep underground, either from the sheer weight of the water, or when water infiltrates the rocks through cracks and pores. These events might accelerate a fault’s natural ‘seismic clock’, hastening an earthquake that is already building, or increase the chance of one occurring at all”.
Qiu cited Fan Xiao, an engineer at the Sichuan Bureau of Geology & Mineral Resources, noting increasing numbers of small quakes close to the Xiluodu reservoir between late 2012 and mid-2014. She reported Fan’s suspicions as being confirmed by Hu Xian-ming, a geophysicist at the Sichuan Earthquake Administration in Chengdu. “There are serious concerns for deadly quakes in future”, the journal quoted Hu as saying.
At least three quakes in the region over eighteen months included one in April 2014 at Yongshan, only seven kilometres from the Xiluodu reservoir. All the quakes had coincided with reservoir filling, according to reports.
Five hundred miles downstream of the Xiluodu reservoir, the Three Gorges dam draws on a different catchment area. It suffered only a 6.67% drop in water flow during 2021’s second quarter, CYPC told London investors last week. Its generation output was virtually unchanged, at 22,291GWh. Less than twenty miles further downstream, Gezhouba, CYPC’s fourth dam, saw production rise by 6.9% to 5,074GWh.
Hemmed in by 2,000 metre mountains, the Xiluodu dam took ten years to build. It is 285 metres high, and its reservoir has a capacity of 12.7 billion, just over half considered as active energy storage. The plant’s eighteen turbines were all commissioned within twelve months after December 2013.
Construction of the Xiangjiaba hydro dam began in 2006. Its first turbine span six years later, and the last of eight was commissioned in July 2014.
(Image: Three Gorges Corporation website)
Adding to the Yangtze’s hydro woes are diminishing melt water flows off the Himalayas, known to climate scientists as the ‘world’s third Pole’. Up to 16% of Himalayan melt waters have been lost since the 1980s, according to a paper in Nature Climate Change last month.
The Energyst asked the China Yangtze Power Company what caused the two dams’ 44 % drop in flows and their resulting c.30% drop in generation. We also sought to know what steps it could take in relation to earthquakes. At time of publication the company had not replied.