FLOOD DEFENSE: The Three Gorges Dam on July 31. In this year's flooding season, the dam, China's largest flood-control project, stored 15 billion cubic meters of floodwater (ZHENG JIAYU)

After successfully containing the biggest flood this century, the Three Gorges Dam, on the upper reaches of the Yangtze River in central China's Hubei Province, has encountered another flood, that of public opinion: Will this dam be able to hold floods that are "once-in-10,000-years" or "once-in-1,000-years" or "once-in-a-century"? Should numerous hazards this year, including land and mudslides and mountain torrents, be blamed on accumulation of sediment caused by the construction of the Three Gorges Dam? To get answers to these questions, journalists from Xinhua News Agency interviewed Cao Guangjing, Board Chairman of the China Three Gorges Corp., the operator of the Three Gorges Dam.

Netizens have collected several articles about the flood-control capacity of the Three Gorges Dam. In these articles, its capacity was differently described as tackling flooding "once-in-10,000-years," "once-in-1,000-years" or "once-in-a-century." Can you give the specifics of the flood control capacity of the dam?

Cao Guangjing: "Once-in-x-years" is a statistical concept based on the hydrology records of the Yangtze River and other historical information. For instance, the so-called "once-in-10,000-years" refers to flooding with peak flow as much as 113,000 cubic meters a second, while "once-in-1,000-years" is 98,800 cubic meters a second and "once-in-a-century" is 83,700 cubic meters a second. But "once-in-a-century" doesn't mean a flood of a given size would occur exactly every 100 years but means, over a very long period of time, it happens every 100 years on average. The peak flow this year reached 70,000 cubic meters a second, which can be termed "once-in-15-years."

The Three Gorges Dam itself was designed to a standard of containing flood "once-in-1,000-years" and was tested at a standard of 1.1 times of a flood "once-in-10,000-years." That's to say, even facing a flood of such a magnitude that it occurred only once in 10,000 years, the main body of the dam would still be safe. When facing a flood of the size of "once-in-1,000-years," the dam would ensure safety of the downstream Jingjiang River valley by adjusting discharge. Even if facing a "once-in-10,000-years" flood, the dam would function together with the Jingjiang flood-diversion area to ensure the safety of the Jianghan Plain area.

In the past few years and also at our press conference on July 19, we have been elaborating on different anti-flooding effects the dam would have in the circumstance of "once-in-a-century," "once-in-1,000-years" and even "once-in-10,000-years" floods.

What was the anti-flooding effect this year?

Practice this year shows the Three Gorges project has been very successful. In 1998 Yangtze flooding, the flow peaked at 63,000 cubic meters a second and the government had to provide more than 10 billion yuan ($1.47 billion), millions of people and hundreds of thousands of servicepersons to fight the flood. This year, the biggest flow into the Three Gorges Reservoir was 70,000 cubic meters a second on July 20. The dam discharged water at about 40,000 cubic meters a second, and residents in the downstream area handled it with ease. With the dam adjusting and reducing discharge, water levels never rose above alert lines in the middle and lower reaches of the Yangtze River. By rough estimation, it avoided the necessity for hundreds of thousands of people to patrol the river, which saved tens of millions of yuan in loss of work time and food allowances. Meanwhile, only one third of the flood-control capacity of the Three Gorges Reservoir was used and the situation did not become critical.

When the flood peak was over, we could see trash floating everywhere in the reservoir. It's reported this will threaten the safety of the dam. What's the real condition?

SHARP CONTRAST: Thanks to the Three Gorges Dam, the Jingjiang section of the Yangtze River is now under much less pressure when dealing with flooding. The top picture was taken on July 22, 2010. The picture below was taken in 1998 (XINHUA)

It's an international problem dealing with floating trash in reservoirs on waterways. Floating trash has a great influence on quality of water as well as on navigation and power generation. But it won't affect the safety of the Three Gorges Dam. In every flooding season, floating trash would appear in the Three Gorges Reservoir or nearby tributaries of the Yangtze River whenever there was a deluge. This year, after the flood peak, the flotation area in front of the dam exceeded 10,000 square meters. Every year, the China Three Gorges Corp. deploys a large quantity of manpower and capital to clear the trash.

On July 19, Chongqing upstream from the Three Gorges Dam encountered its biggest flood in history, and Chaotianmen Wharf in the city's downtown area was inundated. There were rumors the situation in Chongqing was related to floodwater storage and detention at the Three Gorges Dam during the flooding. What is your comment?

When building the dam, we used the alpine landform of the upstream and relocated residents below the designated water line to form a certain capacity for the reservoir. On July 19, the water level reached 185 meters in Chongqing, while the water level in the dam was about 150 meters. The backwater of the reservoir didn't even reach Chongqing. So the flood detention didn't have any impact on flood discharge in the city.

The middle and lower reaches of the Yangtze River are flat and have a large population. If they were submerged, there would be a huge loss. The upstream is mostly mountainous areas, which has larger areas to handle flooding. The anti-flooding function of the Three Gorges Dam is mainly to protect the middle- and lower-reaches areas.

The Yellow River turned into "an overground river" after the construction of the Sanmenxia Dam in the 1950s due to sediment deposits. As a result, the problem was the center of discussion before building the Three Gorges Dam. How serious is it?

The basic solution for sediment deposits is to adjust the discharge. We followed the problem closely during experimental water storage of the reservoir. Recently, with reservoir construction and soil conservation efforts in upstream areas, the amount of sediment in the reservoir has decreased greatly, and is now only 40 percent of the expectation during the dam's designing. We previously estimated the capacity of reservoir would be 85 percent of the original amount after being used for a century, but judging from the current condition, it may be more than 90 percent.

Some foreigners said, "China used to say the water line is 180 meters and the reservoir capacity reaches about 20 billion cubic meters. But the final water line is just 175 meters and reservoir capacity exceeds 22.1 billion cubic meters." How do you explain the differences?

There were several proposals about the water line during feasibility studies, such as 180 meters, 170 meters and 160 meters. In the final plan, which was approved by the Central Government in 1992, it is 145-175 meters according to needs of flood control. The reservoir's flood-control capacity, which is 22.1 billion cubic meters, is strictly calculated based on that plan.

In this flooding season, there were many hazards in the Yangtze River valley, including rainstorms, mountain torrents, land and mudslides, and urban flooding. Some people have attributed this to the water storage of the Three Gorges Reservoir. Is it true?

The Three Gorges Dam is partly blamable for landslides along sections of the Yangtze River and some tributaries affected by water storage. Rainstorms and mountain torrents have nothing to do with the dam. It's a bias of interpretation to connect these hazards with the dam.

Before the dam was built, hundreds of thousands of dams had been built around the world. Although having the largest power generation installed capacity worldwide, the water storage capacity of the Three Gorges Reservoir ranks only 22nd in the world. Operation of large reservoirs in the past shows, in the early period of water storage, areas around the new reservoir were unsteady. During that time, geological hazards happened frequently. After years of water storage and drainage, these areas stabilized gradually. We can say that no country in the world has done such comprehensive reconnaissance of and planning for the conditions of geological hazards alongside a reservoir and paid such a price for managing them as China. Since the first water storage in 2003, although there have been a few landslides, there have never been casualties or huge property losses. This is a marvelous achievement.