HYDROPOWER  DAM SAFETY & EARTHQUAKES

2014-08-05  （Myanmar Green Energy Summit 2014） （Yangon，Myanmar）

By： ZHANG Bting

CONTANT

1,Dam safety is an issue that human being must face

Scientifically, there is no any project with a 100% safe reliability in the world. So it can be said that no dam is absolutely safe in the world.

Although the collapse of a dam would be a huge disaster, no country dares to say that they do not need dam.

Why?

With the development of human civilization and the contradiction between human needs and water resource, there is no society that can live and develop without dam.

Just as Myanmar Daw Aung San Suu Kyi mentioned in her Irrawaddy Appeal worried that the problems, like the floods in rainy seasons, droughts in the dry seasons and salt-water intrusion, are caused by the spatial and temporal distribution of the natural water resource.

Up to now, the only means to solve the water distribution (flood and shortage) problems is building dams (reservoir) to store water in the rainy seasons, and make it good use in the dry seasons.

According to a research by the UN in 2007, the human development index of a country is almost proportional to their reservoir storage capacity.

In many countries including China and Myanmar, the national energy department is in the charge of hydropower development.

But，why should we build large dams and reservoirs ?

Besides the generation of power, the reservoir  can regulate the time and space  distribution of the natural water resource. In order to regulate water resources, we need high dams and large reservoirs. At the same time, the threat of dam safety is also inevitable.

China have the most numbers of dams in the world, some of  them  have confronted some accidents and more than 99.% of which are micro dams

Only 2 big dam collapse accidents happened. One is Banqiao Dam in Henan in August, 1975. The other is Gouhou Dam in 1992. Both the collapses have caused serious casualties and property loss.

According to the reports after the dam collapses, these two incidents have obvious precursor. If necessary measures had been taken, the two accidents should have been avoided. One reason for the Banqiao dam accidents was due to the exceptionally large storms, and the sluice gates had been out of services for many years and could not be open during the emergency . The other reason for Gouhou dam was that the dam management staff did not pay enough attention to the leakage of the concrete face earth dam.

Teable of  the  Dam Collapse Accidents in The World

According to China’s experience, all the dam collapse accidents have one common feature that is nearly all the accident dams have no power generation function. Generally, the dams with hydropower  station have more effective management.

Besides, compared to the pure public welfare dams, the dams with hydropower station gain economic benefits from the power and can maintain the dams better.

Therefore, one of the most important experiences of the dam safety is that the big dam had better to build the hydropower station. The construction of hydropower not only can improve the comprehensive utilization level but also be helpful to dam safety.

2, Hydropower & the reservoir earthquakes

The reservoir earthquake issue could be make clear in two aspects. One is the earthquake threat to the dam and the other is reservoir-induced earthquake.

The first aspect is most important.

According to the lessons from the history, even a small dam collapse may cause a big disaster or thousands of deaths.

So if the construction of the hydropower dam cannot be effective against the threat of earthquake, it must be admitted that the construction of hydropower  may magnify the disaster  of the natural earthquake.

However, it is luckily that the modern science and technology are capable to protect all the buildings in the earthquake prone area.

Now, this engineering technology of earthquake resistance from theory to practice have nearly to a conclusion.

  2.1  There is safety assurance for dam construction in high seismic risk zone

It is easy to imagine that the dam is stronger than the high-rise buildings of complex structure. Los Angeles in USA, Tokyo in Japan and the Tangshan in China are in the high seismic risk zone. But why dare engineers to build high-rise buildings in these cities?

Because the engineers master and apply the scientific seismic resistance technology.

It must be emphasized in the scientific earthquake resistance principle that construction should avoid all the active earthquake faults. If the buildings are built on the active faults, it may collapse by tear even in a small earthquake.

Keeping away from the active faults, the safety of a building will depend on their seismic resistance capacity.

In all, the building located on the active fault (not a general fault zone) is hard to resist the tear of earthquake. Meanwhile the buildings without earthquake-resistance capacity may also cause huge deaths in the earthquake. In China, the earthquake-resistance standard was not considered in many civil buildings in the past, so the earthquakes have often caused large loss. In developed countries, the situation was much better. However, in our dam construction the earthquake-resistance technology has been fully used, because China is a late comer in the hydropower  construction compared with the developed countries and the technology in China developed rapidly.

A few years ago, many persons worried about some hydropower dams would be destroyed by earthquake, especially the Zipingpu Dam in the Min River, Sichuan province China. However, the Zipingpu dam only 17 km,near from the  Wenchuan  earthquake  center， has stood the earthquake on May 12, 2008.  Moreover  its reservoir  became the important  escape  way  during the earthquake.

Thankfully, the success of Zipingpu dam in resisting large earthquake was not a special case.

All the dams and hydropower stations in the area (more than 2000) stood the earthquake.

From this fact many hydropower engineers  believe that the issue of dam seismic resistance could be solved successfully.

2.2 The nature of reservoir earthquake is triggering the natural earthquake in the making

Reservoir’s water pressure is not the reason for the reservoir earthquake. The energy of reservoir earthquake is mainly from the natural earthquake in the same zone.

In China, some persons thought that the water stored in the Three Gorges Reservoir is over to 30 billion tons and they felt particularly terrible.

However，the truth is that water is distributed in more than 1000 square kilometers.

The crust only bears 30 tones per square meter and only 3 kg per square centimeter, which is less than the pressure from a truck.

So, if a truck cannot cause the earthquake, can the reservoir do?

According to the classification of China Earthquake Administration, the earthquakes induced by reservoir do not belong to the artificial earthquake.

Please note: the difference between artificial and natural earthquake is the seismic energy source.

Artificial earthquake is manly caused by the explosion. The pressure and energy density of this explosion are often hundreds of times higher than the pressure of reservoirs water.

Some persons say that the depth of the water in a reservoir is different so than the intensity of pressure should not be calculated in average. However, because the surface of a reservoir is horizontal, the total volume of water and soil on the same unit crust area should be equal.

Obviously, where the water is more deep the rock-soil is less and where the water is less, the rock-soil is more. The relative density of rock is heavier than that of water. Overall, the reservoir and the surrounding rock-soil form a comprehensive pressure to the crust. That is, where the water is deeper the intensity of pressure is smaller. Therefore, no matter how large the reservoir is and how deep the water is, the reservoir cannot cause the earthquake by pressing.

Although the reservoir cannot cause earthquake by pressing, it is possible that the reservoir can induce the earthquake.

Why?   The nature of the earthquake is that the movement of crust plate is impeded. The reasons of the reservoir induced earthquake are that the seepage of the reservoir impact the friction between the plates and provide the premise of natural tectonic earthquakes.

The reason of tectonic (structural) earthquake broke out, mainly is the accumulation of  block tectonic activity of ground stress, changes make fault produced a sudden fracture or dislocation.

Fault (rupture) earthquake condition is that the accumulated of in-situ stress to overcome the friction between the plate (fault).

So, When the bigger the friction of the fault, the greater the force needed. In this case, the earthquake is not likely to happen. However, once occur the earthquake energy released larger.

If the smaller friction  the fault, the less the load needed, the more prone to earthquake, but at the same time,  the earthquake  will release of energy  lower.

In all, the essence of reservoir earthquake is the seepage of reservoir reduces the friction between the plates, that lead to the nature of tectonic earthquakes occurred in advance

The key point of reservoir influence on the natural tectonic earthquake is not the weight of water but is the effect of seepage.

The classification of China Earthquake Administration also proves this conclusion.

The induced earthquake is natural earthquake caused by the seepage of reservoir storage or oilfield water injection.

Because the  quake energy is not caused by human being, the induced earthquake is not belong to artificial earthquake. In fact, it is natural structural earthquake caused by the movement of plates.

From the nature, we can see that the construction of reservoir will not increase the risk of the original earthquake , while it can, in advance  release the earthquake energy in the making.

The truth is that the energy of the induced earthquake is not from human being but existing  in nature.

In the 1970s, the international community had tried  to reduce the earthquake hazards through high pressure injection from deep well . However, it is difficult to control the time and the magnitude of the induced earthquake. So it was easily lead to public misunderstanding, therefore this disaster reducing method of earthquake was terminated.

That is the reason why we do not use injection water to release the earthquake energy and reduce the disaster today.  However to build reservoir and regulate the time and space distribution of the water resource that is our human society have to do. So in facet, our human society, never stop reducing the hazards by releasing the earthquake energy in advance, not through deep well injection, but by the reservoir seepage (induced earthquake).

Summary

1. The construction of large dam and reservoirs is necessary for human civilization and social development.  So, we must face the dam safety issue.

2. Through improving the construction and management level of the dam, the dam collapse accident should be able to avoided.

3. The construction of large dam or reservoirs had batter to build hydropower station.  It will not only have clean energy but also improve the dam safety.

4、As long as make buildings away from the active earthquake fault, and to ensure the structure of the seismic resistance capacity, the safety of the buildings (including dam) can be guaranteed, even in high risk earthquake zone.

5、 Reservoirs could not press out earthquake, the essence of reservoir  earthquake is the  seepage triggering  the nature  structural earthquake. So the disaster is not from hydropower construction, on the contrary, it make the releasing of seismic energy early and disaster mitigation, in some extent.

  Thanks

                                Prof.  ZHANG Boting

          Chinese Society for Hydropower Engineering