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Effects of the May 12, 2008 Sichuan earthquake on dams – a preliminary assessment
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Effects of the May 12, 2008 Sichuan earthquake

on dams – a preliminary assessment

 

Dr. Martin Wieland

Chairman, ICOLD Committee on Seismic Aspects of Dam Design

May 16, 2008

 

The earthquake of May 12, 2008 had a magnitude of 7.9 and because of its shallow focal depth of about 10 km the ground shaking in the epicentral region must have been very severe.

 

The following assessment is based on information obtained from dam and earthquake engineers in China and other countries, and the review of reports published by the mass media. The author had visited the epicentral region in Dujiangyan in January 2007 where the central parts of an over 2000 year old irrigation scheme are located, which is still in operation. He has also visited the Three Gorges Dam at the end of April 2008 in connection with the International Evaluation Symposium of the 233 m high Shuibuya concrete face rockfill dam (CFRD) in Yichang, the world's highest CFRD.

 

Sichuan province is the province with the largest hydropower potential in China and several large projects such as the 240 m high Ertan arch dam had been completed at the Yalong River and numerous other projects are either under construction or in the planning phase along the Yangtze river and its tributaries.

 

Shortly after the devastating earthquake reports appeared on cracks, which were detected at the concrete face of the 150 m high Zipingpu CFRD located upstream of Dujiangyan City. The dam was completed in 2006 and the design reservoir volume is 1.1 billion m3. At the time of the earthquake the reservoir volume was 0.32 billion m3, i.e. the water level in the reservoir was rather low. Therefore, it was also possible to detect theses cracks, which otherwise would not have been visible if the reservoir were full. As modern CFRDs such as Zipingpu dam are designed to withstand severe leakages in the concrete face, and in view of the low water level in the reservoir, the dam can be considered stable as reported by Chinese dam experts who inspected the dam after the earthquake. It is expected that the amount of leakage has increased substantially after the earthquake. According to Chinese dam design regulations, all CFRDs must be equipped with a bottom outlet, which allows the lowering of the reservoir in the case of a severe earthquake or other critical incidents. Lowering of the remaining 0.32 billion m3 reservoir may take a few weeks. As considerable damage was observed it will be necessary to perform a thorough inspection of the dam before the start of the next approaching rainy season. In view of the numerous aftershocks and the observed damage it will be necessary to closely monitor the behaviour of the dam. As Zipingpu is the first modern and large CFRD, which was subjected to severe ground shaking, the engineers involved in the design and construction of CFRD are very much interested in getting a full picture on the earthquake behaviour of this type of dam.

 

The other project reported in the media is the Three Gorges Dam, which is located at the Yangtze River in Hubei province near the City of Yichang. The dam is located several hundred kilometers away from the epicentre of the Sichuan earthquake. As the dam was designed against earthquake action and shaking table tests with a model of a typical section were performed to check the seismic design, it is very unlikely that any damage has occurred at this dam. It has to be pointed out that due to the fact that new buildings are designed for much lower seismic actions than large dams, severe damage to buildings would have to be observed in the villages and cities surrounding the Three Gorges Project. As older buildings in rural areas have not been designed against earthquakes, they would have been damaged first before any damage would be expected in a massive concrete dam. No such building damages have been reported it can be assumed that the Three Gorges Dam has also not suffered any damage.

 

According to different sources about 400 dams have been affected by the Sichuan earthquake. These dams must all be inspected. Most of these dams are small embankment dams. Many of them may not have been designed for earthquake action or if they were designed for earthquakes then most likely design criteria and methods of dynamic analysis were used for the older dams, which are outdated today. The seismic safety of these dams is a concern and it is expected that quite many of hem will need repair and strengthening. This was also the case after the January 26, 2001 Bhuj earthquake (Mw 7.6) in Gujarat province in India, where some 240 dams - mainly small embankment dams for water supply and irrigation - had to be strengthened. At that time no dam failure with a catastrophic flood occurred as at the time of the earthquake the water levels in the reservoirs were very low.

 

Besides the Zipingpu and the Three Gorges dams, concerns about damage and the safety of the Taipingyi hydropower plant, located upstream of the Zipingpu dam, were mentioned in the media. However, no reliable information on this project is available at this time. As most of the dams are located in mountainous regions, where many mass movements occurred, access to the damaged dams may be difficult. Therefore, further news on damaged dams and their safety will appear in the coming days.

 

Due to the large number of landslides it is expected that some smaller rivers might be blocked. The sudden failure of such 'earthquake' dams may have similar consequences as the failure of one of the smaller damaged dams.

 

The Chinese authorities are very much concerned about the safety of their dams and as there are many more projects in the pipeline, safety is the prerequisite for their acceptance among the affected people. This is also true elsewhere.

 

 

Dr. Martin Wieland

Poyry Energy Ltd.

Hardturmstrasse 161

CH-8037 Zurich/Switzerland

Tel. +41 76 356 28 62

Fax +41 44 355 55 61

E-mail: martin.wieland@poyry.com

Web: www.poyry.com

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