Dams and Earthquakes

Three Gorges project unaffected; has the design capability to withstand an earthquake of seismic intensity 7

China News Service (Zhongguo xinwenshe)

May 29, 2008

“The Three Gorges reservoir area – from Honghuabao in Jiangjin City of Chongqing to the dam site downstream – has not been affected and is generally stable,” says Li Yongan, general manager of the Three Gorges Corporation.

Reported by Jiang Shixiang and Xu Hua

Translated by Three Gorges Probe

“The Three Gorges reservoir area – from Honghuabao in Jiangjin City of Chongqing to the dam site downstream – has not been affected and is generally stable,”  Li Yongan, general manager of the Three Gorges Corporation, was quoted as saying. According to Mr. Li, the distortion of the dam caused by the powerful Wenchuan earthquake was very small, with all structures remaining as normal. The dam project is operating normally and smoothly, and construction is carrying on according to plan.

Zheng Shouren, a member of the Chinese Academy of Engineering, chief designer of the Three Gorges project, and chief engineer of the Changjiang Water Resources Commission, also argued that the Three Gorges dam area has weak seismic activity, is relatively stable geologically and is subject to almost no probability of big earthquakes.

During the period from the late 1950s to the mid-1980s, the Institute of Geophysics of the Chinese Academy of Sciences and State Bureau of Seismology each conducted studies and assessments of the seismic intensities in the Three Gorges area, and concluded that ‚ “the maximum seismic intensity1 would be under 6 (VI) in the Three Gorges area within a hundred-year period.”  However, considering the size and importance of the dam project, Three Gorges was designed to resist an earthquake with a seismic intensity of 7 (VII) occurring with a frequency of one in ten thousand years – equivalent to the earthquake resistance standard for nuclear power plants.

With the highest magnitude and devastating consequences, the Wenchuan earthquake is the biggest to have occurred since construction began on the Three Gorges project 16 years ago. When the earthquake occurred, people at the Three Gorges dam site felt strong shaking. Fortunately, the dam was not affected, the shiplock is operating normally and navigation is “business as usual,” with no boats backlogged upstream or downstream of the shiplock. The Three Gorges reservoir is also operating normally, and none of the new ports built in the reservoir or the geological disaster prevention projects (editor’s note: such as bank and landslide prevention works) have been affected. All engineers and workers in the construction site evacuated from the scene in a timely manner, and no one was hurt after the earthquake occurred. On May 13, the day after the earthquake, construction work resumed on the dam site.

Based on a calculation by the Bureau of Seismology of Hubei Province, the seismic intensity created by the earthquake in the Three Gorges dam site area was measured at 4 (IV), much lower than the standard 7 (VII) which the Three Gorges project is designed to withstand. Moreover, at a distance of about 700 kilometers away, the Three Gorges reservoir is not in the same geological tectonic plate on which Wenchuan (the epicentre) is located. This is why the powerful earthquake has had no significant impact on either the Three Gorges reservoir or the dam itself.

According to officials at the Three Gorges Reservoir Geological Disaster Prevention Headquarters, there are 3,028 volunteer landslide monitoring groups (editor’s note: run by local villagers), and 251 government-run monitoring stations in the Three Gorges area. Results from both the volunteer monitors and the government monitoring stations indicate that no sign of distortion of existing landslides or newly activated landslides had been reported in the period from May 12, when the Wenchuan earthquake occurred, to May 22, 2008. According to one of the officials in charge of the Headquarters “the powerful Wenchuan earthquake created no damage to the Three Gorges dam project, so the incident will have little effect on the schedule to fill the reservoir to a higher water level.”

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1 Seismic intensity is a way of measuring or rating the effects of an earthquake at different sites – the degree of damage on the earth’s surface and to man-made structures, such as dams and buildings, for example. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end. The Intensity scale differs from the Richter Magnitude scale in that the effects of any one earthquake vary greatly from place to place, so there may be many intensity values measured from a single earthquake.

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