The May 12 earthquake in southwest China caused a crisis at the Zipingpu dam. Situated on the Min River, it is the largest hydroelectric station in Sichuan province. Power generation equipment was damaged and ground to a halt. The flood gates could not be opened and the waters behind the dam rose quickly as heavy rains began to fall. The dam was only made safe on May 17, when engineers were able to force the gates open.
On May 23 there were still collapsed buildings a kilometre away from the dam, and fissures in the ground that ran toward the dam and its reservoir. In some places the cracks were 20 centimetres wide. And things didn’t look good at the dam either. Armed police were controlling access, but cracks were visible and many concrete pillars had fallen. Fissures had opened on the hillsides behind the dam. Rope teams were working to patch up cracks in the face of the dam and ensure it would not collapse in the imminent flood season.
Yong Yang, a geologist who was at the scene soon after the quake, said that on one side of the dam a 200-metre stretch of ground had subsided, the machine room that raised the sluice gates had been damaged and the cliffs around the reservoir were in danger of collapsing. The dam and its reservoir had taken a beating.
The reservoir was designed to hold 1.12 billion cubic metres of water, but at the time of the quake it was at less than one-third of capacity. When work started on the dam in 2001 it was supposed to be able to withstand an intensity VII (very strong) earthquake on the Mercalli scale. The Wenchuan quake measured XI (very disastrous), and the epicentre was less than 20 kilometres away. It is a miracle that the 156-metre dam withstood the powerful and averted a flood.
Dams on the upper reaches of the Min River were damaged, but would remain stable as long as there were no strong aftershocks, experts from the China Electricity Regulatory Commission, the China Hydroelectric Consulting Group and the Ministry of Water Resources said on May 20. The Zipingpu dam could hold back the extra water.
But Yang remains cautious. The external damage to the dams on the Min River does not look too serious, but internal damage needs to be further evaluated. The dams, including Zipingpu, lie on an active fault line. Continued aftershocks, and the coming flood season, will present a major challenge. Immediate worries about these expensive dams, some already built and some under construction, may have passed, but longer-term issues remain.
The Yalong River, Dadu River and Jinsha River, which border the provinces of Yunnan and Sichuan, were all affected by the quake. By 2020 there are slated to be 10 huge dams on these river, with a total generating capacity five times that of the Three Gorges dam.
Work started on the Zipingpu dam in 2001. The following year, Li Youcai, a retired senior engineer from the provincial seismological authorities, issued a warning about the potential dangers.
In early 2002, Li read a report about the potential intensity of earthquakes at the dam site. The document, a 1989 paper by the Earthquake Analysis and Prediction Centre at the China Earthquake Administration, said that the strongest earthquake the dam would suffer would be of intensity VII. Li, however, found problems with that conclusion.
“[The report] should have said IX or higher,” said Li. “The earth’s crust in that area is not stable and the project should have been classed as potentially dangerous. It leaves the life and property of the millions of people on the Chengdu plains at constant risk.”
Li’s doubts were raised by his research into major fault lines in the area. The report barely covered these, but after looking into the structure and activity of historical earthquakes and ancient buildings, Li concluded that several large active fault lines met in the area. Accumulated stresses, he realised, made it a likely site for a 7.5 magnitude earthquake.
Li submitted his opinions to the China Earthquake Administration, and three of its experts met with him, but rejected his views. In 2006 Li found a response from a committee at the Earthquake Analysis and Prediction Centre, written in 2003. “Although it is possible there may be a one-off quake of intensity IX or over in the next 5,000 years, the basic intensity for the area can be set at VII,” it said. But Li disagreed. Observations of intensity IV earthquakes over the past decade indicated a risk of quakes with an intensity greater than VII in the near future.
Li submitted a new report to the provincial authorities on September 16, 2006. Again, it was refuted. Li said that the seismological authorities were responsible for legislation and regulations on earthquake safety evaluation, the actual evaluation and handling complaints on the process. This ran against legal principles and could be extremely dangerous, he said.
In late March 2008 Li submitted a petition calling for further analysis of the project, greater awareness of the earthquake risk and increased monitoring at the Zipingpu dam. When, little more than a month later, Wenchuan was hit by a massive 8.0 magnitude earthquake, there was nothing more Li could say.
Do reservoirs cause earthquakes?
After the Wenchuan earthquakes there was a fierce debate about the role of the dam. Some people said the fact that the dam withstood the earthquake was an amazing feat. The other said it was nature’s warning to a reckless mankind, and that the Zipingpu reservoir may have even caused the earthquake. The debate centres on the issue of whether or not reservoirs can cause earthquakes.
One of Li’s articles, written before the earthquake, said that since the dam was situated at the meeting point of three active faults, the weight of the water could trigger an earthquake. Fan Xiaori, the head engineer at the Sichuan Bureau of Geological Exploration’s surveying team, wrote in China National Geographic that the possibility of the reservoir triggering the earthquake could not be ignored.
Fan lists seven factors that indicate a reservoir might trigger an earthquake: a dam higher than 100 metres with a reservoir of more than 1 billion cubic metres; active fault lines in the area; a reservoir lying in or near a Cenozoic or Mesozoic rift-subsidence basin that has risen or fallen in recent times; unusual stress gradients deep underground; cracks in rocks that absorb water; a history of earthquakes in the area; and the close proximity of hot springs. The more of these conditions are met, the more likely it is that a reservoir will trigger an earthquake. According to Fan, the Zipingpu dam meets the first six conditions. He believes further research and analysis of the data from the earthquake is required. “Until a final conclusion is reached, the possibility that the reservoir induced the earthquake cannot be excluded.”
The main opponent of this view is the water authorities. Speaking at a press conference in May, the head of the Sichuan Water Resources Office, Zhu Bing, denied the dams were to blame. The deputy secretary of China Hydroelectric Engineering, Zhang Boting, has said that reservoirs may help trigger earthquakes, but not create the conditions for earthquakes. In his opinion, the dams are not to blame.
According to Zhang, it is not only possible to build reservoirs in earthquake-prone areas, but this also helps to reduce the strength of quakes. Reservoirs might cause the quakes to become more frequent, but they will dampen them like a quilt over the ground, he argues.
The debate continues, but after the earthquake it is clear that monitoring and preparation for earthquakes need to be stepped up at Zipingpu and other dams in southwest China.
Li Xiaoming is a reporter at Scientific Times
This report is adapted, edited and translated with permission of Scientific Times
Homepage photo caption: Cracks in the Zipingpu dam, Li Xiaoming