China's Dams

Why is the flood control capacity of the Three Gorges Dam project being questioned again?

Because the project’s flood control capacity doesn’t work.

Recent coverage of heavy rain in the middle reaches of China’s Three Gorges Dam region have prompted some media outlets, and the nation’s Netizens, to question the dam’s ability to hold back floods — the main benefit of the dam touted by the Chinese government to justify its existence. A doubt that this new report by Chinese geologist and environmentalist, Fan Xiao (the former chief engineer of the Regional Geological Survey Team of the Sichuan Geology and Mineral Bureau), substantiates: Fan contends that, in reality, it is self-evident the Three Gorges Dam cannot hold back floods downstream of the dam and that if a full river flood throughout the Yangtze valley were to occur (such as the 1954 flood that killed 30,000 outright and more through starvation and disease), its shortcomings would become abundantly clear. Luck, so far, has been on the government’s side in terms of the type of flood that would truly test the dam’s flood control capabilities and reveal the extent of its limitations. Although the dam can do little to alleviate the floods currently in the middle reaches of the region, dam operators can hold back some water from moving downstream and appear to be doing just that (see the water table below) which the government can point to and say, “See. It is working!” What is working is luck. Were storms to strike both the upper and lower reaches of the Yangtze valley simultaneously, the situation would worsen dramatically. So far, the Three Gorges Dam’s flood control capacity has not been put to the test, says Fan.

By Fan Xiao for Probe International

In early July of this year, heavy rainstorms caused severe floods in the middle and lower Yangtze valley. It is no wonder that the flood control capacity of the Three Gorges Dam project has become a hot topic for the public once again, especially on the Internet. These challenges to the flood control capability of the Three Gorges Dam project started with the original feasibility study of the project, which was influenced by political factors and by powerful interest groups. This so-called scientific feasibility study existed in name only, with leading scientists and experts who had dissident opinions excluded and all different views suppressed. To push the project ahead, the proponents of the project launched rounds of one-sided propaganda campaigns, stressing that the dam project would “ensure the safety of the middle and lower reaches of the Yangtze in terms of flood control” and play “an irreplaceable key role [in protecting people in the valley from floods].” The government as well as the official media avoided mentioning the project’s negative impacts as well as its shortcomings.

As early as 1988, Professor Lu Qinkan, former deputy chief engineer at the Ministry of Water Resources and a flood control and sedimentation expert, who had refused to sign the feasibility report as a flood control expert, made his arguments crystal clear:

“There are three types of floods that have occurred since 1949 along the Yangtze River:

1) Big floods throughout the entire river, such as in 1954;

2) Floods that are serious only at the upper reaches, such as the floods in 1981; and

3) Floods that have occurred due to local storms only at the lower and middle reaches, such as the floods in 1991.

The Three Gorges Dam project can only control floods from the Chuanjiang River in the upper reaches, and cannot control the floods from many of the large tributaries in the lower and middle reaches such as the Xiang, Zishui, Yuanshui, Lishui, Han and Gan rivers. The second type of flood, represented by the 1981 flood, was very serious in the upper reaches. The peak discharge at Chongqing of 85,700 m3 per second diminished to 70,800 m3 per second by the time it reached Yichang further down river as a result of channel storage. The lower and middle reaches were not affected at all. Therefore, the Three Gorges project is hardly necessary for this type of flood. It is unnecessary to state the obvious, that the Three Gorges Dam project is useless for the third type of flood.”

Therefore, it is clear that the Three Gorges dam project cannot lower water levels in the city of Wuhan, the capital of Hubei Province, nor can it reduce the severity of the floods in its surrounding areas. For floods that occur in both Jiangxi and Anhui provinces, downstream of Wuhan, the Three Gorges Dam project would be totally useless. So the dam’s flood-control capacity is limited.

How does a reservoir’s flood control capacity function? It works like this: when the peak flood comes, the reservoir temporarily blocks and stores the floodwaters in order to reduce the volume of the flooding water, and then discharges it later.  In order to make room to store the floodwaters, the reservoir’s water level must be lowered to the “flood control level” prior to the arrival of the flood season. Thus the storage capacity is also called “flood control capacity.”

Since the flood flow in a river often exceeds the flood control capacity of the reservoir, and a peak may last a long time, or perhaps there is a very short interval between two peaks, it is impossible for the reservoir to hold too much floodwater. Otherwise, the second peak that follows would lead to overtopping and even dam collapse because the reservoir would have no way of storing the floodwaters. This is why the reservoir must discharge the floodwaters it stores as soon as possible, so that the water level can drop to the “flood control level” to ensure that there is sufficient capacity to impound the peak which follows.

Regarding a reservoir’s flood-control ability, an important indicator is the ratio of the total flood-control capacity and the total flow in the main flood season. If the total flood-control capacity is far less than the flood volume (i.e., the ratio is far less than 1), it would be like using a small cup to regulate a large pond of water.

The dam authority claims that the dam project has a flood control capacity of 22.15 billion cubic metres, which is calculated based on the space between the “flood-control level” of 145 metres and the NPL (normal pool level) of 175 metres. However, as Dr. Wang Weiluo has pointed out, the ratio of Three Gorges’ flood-control capacity (22.15 billion cubic metres) and the total flow of 244.4 billion cubic metres in a 60-day period at the Yichang Hydrological Station [for example, during the 1981 flood which was considered a one-in-a-hundred year flood] is as low as 0.091: 1. In other words, the Three Gorges Dam project’s flood control capacity is equivalent to only 9% of the total flow in a one-in-a-hundred-year flood. This is why people have concluded that the Three Gorges project’s flood control capacity is limited.

However, even the officially claimed figure of the 22.15 billion cubic metres flood control capacity is an exaggeration because of a calculation error. As early as 2000, Zhang Guangduo, chief expert of the Three Gorges project and double-member of both the Chinese Science Academy and the Chinese Engineering Academy, said to Guo Shuyan, vice-director of the Three Gorges Project Construction Committee:

“Perhaps you know that the flood control capacity of the Three Gorges project is smaller than declared by us. The research was done by Qinghua University. After learning this, Vice-chairman Qian Zhengying questioned the Changjiang Water Resources Commission. The Changjiang Water Resources Commission has also admitted this is true. However, we can sort this problem out by lowering the flood control level to 135 metres, even though this would affect shipping on the river. But keep in mind, never, ever let the public know this…”

Furthermore, according to Dr. Wang Weiluo, Li Peng wrote in his “Three Gorges Diary” (page 42), that with a NPL of 180 metres, (the Three Gorges project) has a flood control capacity of nearly 20 billion cubic metres. But, Dr. Wang points out, “actually the Three Gorges project has a normal water level of 175 metres, five metres lower than the 180 metres that Li Peng claims, so the flood control capacity should definitely be less than 20 billion cubic metres. How then and why did the dam authority declare the current flood control capacity is 22.15 billion cubic metres, 2.15 billion cubic metres more than what Li Peng said?”

In addition, according to Zhang Guangdou’s statement, the flood control capacity would become as large as 22.15 billion cubic metres if the flood control level were lowered to 135 metres. But of course, there is no way to achieve this goal because it would cause a tremendous loss of electricity output as well as shipping business on the Yangtze River. In fact, since the Three Gorges project began operation, the flood control level has been above 145 metres, and has never been as low as 135 metres. There is an interesting question here: with a flood control level of 145 metres, how big would the Three Gorges’ flood control capacity be? Remembering Zhang Guangdou’s words, this might be another state secret, so never, ever let the public know.

In fact, without the Three Gorges Dam and reservoir, the river itself has a great ability to reduce peaks and store floodwaters. Dr. Wang Weiluo provided a good example: in the 1981 flood, the peak discharge at Chongqing of 85,700 m3 per second diminished to 70,800 m3 per second at Yichang further down river as a result of channel storage. Although high water levels appeared in Shashi, Chenglingji and other cities below the dam, they failed to form a flood, so Wuhan, further downstream of the dam, was not threatened at all. It means that without the dam project, the Three Gorges, in a natural river state, absorbed as much as 10.2 billion cubic metres of floodwaters coming from the upstream area.

Only in 2010, the flow rate reached 70,000 cubic metres per second, the largest since the completion of the Three Gorges project, but the peak duration and the amount of floodwater in 2010 was not as severe as the 1998, 1981, and 1954 floods. As of July 20 this year, the largest flood peak has only been about 50,000 cubic metres per second. So, the Three Gorges project has yet to face its true flood control test.

As stated above, the Three Gorges project can do nothing to control Yangtze River floods below the dam in general, and in the middle and lower reaches in particular. The question remains unanswered: if big floods occur throughout the entire Yangtze valley simultaneously and the peaks are sustained over a long period, like the floods of 1954 and 1998, what will the Three Gorges Dam operators do? Store floodwaters, or discharge the water? This is truly a dilemma and a challenge for the dam authority.

Nobody knows.

 

Three Gorges reservoir levels July 2016-07-20

Date Water levels

(upstream) metres

Water levels

(downstream) metres

Rate of flow

(inflow) (m3/s)

Rate of flow

(outflow) (m3/s)

July 1, 2016 148.73 68.59 48500 31700
02 150.51 67.27 38000 30100
03 151.53 67.04 30000 29000
04 151.31 67.59 23000 29500
05 150.24 67.50 20800 28700
06 149.38 68.08 19500 25600
07 149.17 67.34 23000 20100
08 149.82 67.10 26000 21200
09 150.78 67.11 22500 21200
10 150.22 67.15 20200 20900
11 150.35 67.23 21000 21200
12 150.50 67.29 21000 21400
13 150.54 67.41 21300 22200
14 151.93 67.23 23000 20400
15 153.13 67.64 30000 21100
16 154.18 67.56 32000 24000
17 154.67 68.16 27800 26300
18 154.92 68.21 25000 26100
19 155.44 67.91 30000 24000
20 156.85 67.88 39000 23600

Compiled by Probe International

Further Reading

Dai Qing: The expensive Three Gorges flood control project

Press Release: Leaked documents reveal officials fear environmental crisis in dam reservoir

After the Three Gorges Dam: What have we learned?

Half empty: China’s vanishing “kidneys”

The limited benefits of flood control (An interview included in the original Chinese edition of Yangtze! Yangtze!)

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