January 29, 2007
Maoping landslide on the Qing River (Photo: Wang Jianfeng)
Dams and flood control: Safety insurance or Damocles sword?
It goes without saying that flood control is one of the most important functions a dam project can fulfill. However, it is unrealistic to build a dam expecting it to achieve a permanent solution to a flood problem.
The Three Gorges reservoir, for example, has a total storage capacity of 22.15 billion cubic metres. But if one takes into account the fact that 16.5 billion cubic metres of this capacity is given over to power generation and navigation on the Yangtze River, the project really has a flood-control capacity of only 5.65 billion cubic metres. And even if its entire capacity of 22.15 billion cubic metres were to be dedicated to flood control, the dam could not do all that much because the total volume of flood water during the one-month flood peak on the Yangtze can be as much as 100 billion cubic metres. Moreover, the dam is useless in controlling the floods that occur in the downstream region below the dam.
Huang Wanli warned that Chongqing harbour would silt up with coarse pebbles after the Three Gorges reservoir was filled, causing more frequent and severe flooding
On flood control, Professor Huang Wanli strongly questioned the wisdom of building the Three Gorges dam. He warned that Chongqing harbour at the tail end of the Three Gorges reservoir would silt up with coarse pebbles after the reservoir was filled, causing more frequent and severe flooding in the densely populated region of Sichuan province and Chongqing municipality.
Prof. Huang pointed to the flood disaster that occurred in Ankang, Shaanxi province, in 1983. The city is located 200 kilometres upstream of the Danjiangkou dam on the Han River, a major Yangtze tributary. After the Danjiangkou dam was built in 1969, coarse pebbles began to accumulate in the river section below Ankang. When major rainstorms hit the region from July 27-31, 1983, the level of the Han River rose precipitously.
The entire city of Ankang was flooded on July 31, 1983, with disastrous consequences. Thousands died
Ankang was caught in a “pincer attack” from upstream and downstream areas. Due to the heavy rainfall, operators of the Shiquan reservoir upstream of Ankang were forced to release water just as the water level in deep gorges below the city was also rising rapidly. As a result, the entire city of Ankang was flooded on July 31, 1983, with disastrous consequences. Thousands of people died, including those who had tried to save themselves by climbing up to the fourth floor of apartment buildings.
In 2004, a similar flood disaster occurred in Chongqing municipality’s Kai county, in the heart of the Three Gorges reservoir area. A severe rainstorm hit the county and surrounding area on Sept 6, 2004. The Xiao River rose to 171.5 metres, 5.6 metres higher than the “warning” level and 2.3 metres above the historic high on the river. The old county seat, which is to be relocated in 2007, was completely flooded, with water rising in the streets to a depth of 11 metres.
The big floods in Kai county worried Three Gorges project officials. As Lu Chun, vice-director of the office of the Three Gorges Project Construction Committee, observed: “How will we be able to deal with floods once the Three Gorges reservoir is filled to 175 metres [above sea level] given that when the reservoir was only at the 139-metre level, flood water had nowhere to go on the Xiao River? How will Kai county cope if a ‘500-year flood’ occurs in the region?”
Whether or not the Three Gorges project will in fact reduce the severity of floods on the Yangtze is still a matter of debate, but the dam was built to a height of 185 metres anyway. As the water level in the Three Gorges reservoir rises, however, this question will be answered in time. What we do not want to see is Prof. Huang Wanli’s warnings coming true.
Poorly built, dangerous reservoirs
Another major problem with China’s dams and reservoirs is that many have been poorly constructed and are dangerous, but are still operating nonetheless. Some of these facilities constitute a major hidden danger in the country’s water-resources system.
Statistics compiled by water experts reveal that as many as 36 per cent of China’s reservoirs are poorly constructed or dangerous
Statistics compiled by water experts reveal that as many as 36 per cent of China’s reservoirs are poorly constructed or dangerous. Half of those are medium or large in size, and about 700 of them are in urgent need of repair. Roughly 10,000 small reservoirs are in the same sorry state. Such poorly built and dangerous reservoirs are time bombs waiting to explode in the event of a severe flood or other unexpected occurrence.
Unfortunately, Chinese history provides no shortage of lessons on dam failures. One of the worst was the collapse of the Banqiao and Shimantan dams in central China. In August 1975, exceptional rainfall caused devastating floods in the Huai River valley in Henan province. The two medium-sized dams collapsed, along with 60 smaller reservoirs. The tragedy affected 12 million people, and claimed as many as 200,000 lives.
Wang Shucheng, the minister of water resources, has said that 235 dams (233 small, two medium-sized) have collapsed in China since 1991. Of these, 147 burst when flood water overtopped the dams — 71 of which turned out to have been poorly constructed. Mismanagement was cited as the cause of the other collapses.
China leads the world in terms of number and scale of dam projects, but it has lagged behind developed countries in the safety management of dams and reservoirs. In 1991, the government issued a “regulation on safety management of dams and reservoirs” and another on “methods of management on the downgrading and dismantling of reservoirs” in 2003, but problems have arisen with the implementation of these policies. Moreover, financial and technical difficulties persist with the management of the poorly built and dangerous reservoirs.
The high cost of geological disasters
The river valleys in which most of China’s big dams are planned, under construction or already built — including the Min, Dadu, Yalong, Jinsha, Lancang-Mekong and Nu — are located in the transition belt between the Qinghai-Tibetan Plateau and the Yunnan-Guizhou Plateau and Sichuan Basin. The geology in this area is unstable, and geological disasters are frequent.
A number of hydropower projects have been built in this region of southwest China without due regard for the danger
Due to the dramatic variations in topography and landscape, regions such as this are seen, both inside and outside China, as holding tremendous potential for hydropower development. But at the same time, the risk of geological disasters is particularly high, and a number of hydropower projects have been built in this region of southwest China without due regard for the danger.
There are several well-known earthquake zones and seismically active belts in the region, and an average of one quake registering at least 6 on the Richter scale strikes every 10 years. This is also the region of China that is most plagued by landslides, riverbank collapses and mud-rock flows.
For example, in 1989, during construction of the Manwan dam on the Lancang (Mekong) River in Yunnan province, excavation work on the left bank triggered a massive riverbank collapse, which cut off a road on the top of the dam, brought the construction work to a halt and added 140 million yuan (US$17.5 million) to the cost of the project. Since 1993, when the first Manwan generators went into operation, more than 100 riverbank collapses and landslides have been caused by the big changes in water level during the regular operation of the reservoir.
In March 1995, for instance, 51 riverbank collapses and landslides occurred over the course of one week in Jingdong county alone due to the sudden drop of water level from 991 metres to 940 metres. According to official statistics, 2,958 local people had to be resettled for a second time because of geological disasters triggered by the dam — almost as many as had to be relocated for the dam in the first place (3,042).
In 1996, the Geheyan dam was completed on the Qing River, a tributary of the Yangtze below the Three Gorges. When the Geheyan reservoir was filled for the first time in 1993, rising from 132 metres to 200 metres, deformations began to appear in the Maoping landslide located 66 kilometres upstream of the dam. The landslide had been stable for years and had shown no signs of deformation before the filling of the reservoir.
Maoping landslide began to slip after Geheyan reservoir was filled (Photo: Wang Jianfeng)
But now, in the past few years, the huge Maoping landslide, which has a volume of 24 million cubic metres, has started slipping again. If it were to slide into the river, the Qing would be completely blocked and make another big dam upstream unworkable. This is the 233-metre-high Shuibuya dam, currently under construction 90 kilometres upstream of Geheyan and scheduled to be completed in 2008.
In 2001, at the Zipingpu dam site on the Min River, excavation work on the slopes, combined with several days of rain, triggered large-scale landslides and mud-rock flows, blocking a national highway and causing other economic losses.
Dam-induced seismicity is another major problem. As many as 15 earthquakes triggered by dams have been recorded in China.
One of the most serious such tremors occurred near the Xinfengjiang reservoir on the Dong River in Guangdong province. Seismic activity was detected just a month after the reservoir was filled in 1959. And then, on May 7, 1962, a powerful earthquake registering 6.1 on the Richter scale shook the area, with the epicentre only 1.1 kilometres upstream of the dam. The quake killed six people, destroyed 1,800 houses and caused an 82-metre-long crack to open in the structure of the dam, rendering it unworkable. The incident was ranked as one of the world’s six most powerful earthquakes above 6 on the Richter scale that have been triggered by dams.
Two big earthquakes, also above magnitude 6, were reported in Dayao, Yunnan province, in July and October 2003. The tremors damaged 54 large and medium-scale reservoirs built on tributaries of the Jinsha River. Many of the structures developed cracks and began to leak water, forcing the evacuation of people living downstream.
When a dam is being planned, the potential impact of seismic activity is considered. But in a geologically complex area prone to disasters, this does not mean the dam will be safe
When a dam is being planned and built, the potential impact of seismic activity is considered, and measures to protect the dam are proposed. However, in a geologically complex area prone to disasters, such as southwest China, this does not mean that the dam will be safe. And this does not mean that the reservoir area or the region below the big dam will be safe.
In China, both the feasibility study and environmental impact study for a dam will focus mainly on a geological assessment of the proposed site and the foundation on which the powerhouses will be built. Little attention will be paid to the valley as a whole where this development will take place.
Furthermore, even when a more comprehensive geological assessment of the region is made, the costs of the potential geological disasters are not often taken into account, either in the cost-benefit analysis or the decision-making process.
A longer version of this article was published in the November 2004 issue of Chinese National Geographic magazine (Zhongguo guojia dili).