Three Gorges Probe
August 24, 1998
As the Yangtze River Valley is engulfed by China’s worst flooding in more than four decades, debate over whether the Three Gorges dam would have stopped this year’s flood has been revived.
According to official estimates, 2,500 have died since this year’s flooding began. Unofficial estimates are many times higher. Apart from those who have died, one fifth of the nation’s population has been affected, 12 million people being evacuated, and crops on 11 million acres or three percent of the national cropland have been destroyed. According to the government, the damages could reach $4.8 billion.
As in every flood season, Three Gorges Project proponents have come out in full force promoting the dam as the answer. Lu Youmei, Chairman of the Three Gorges Development Corporation claimed on August 2, "If the Three Gorges Project had already been completed, the problems of flood control would have already been solved."
However, flood control experts and environmental specialists in both China and abroad argue that this is wishful thinking that has no basis in fact.
Because the flooding this year happened along the entire length of the Yangtze River valley, from the upper stream of Chongqing to the lower reaches of Hubei, Hunan and Jiangxi provinces, there is no way that a single reservoir at the Three Gorges could have protected the region. Dr. Yegang Wu, senior scientist from South Florida Water Management District, points out that the Three Gorges reservoir is currently designed to have a capacity of 20 billion cubic metres at the flood level. Yet this capacity is equivalent to only 4% of the total runoff water in the region. "It is obvious from common sense that this capacity could not hold back floodwaters enough to reduce significantly the flood risk at the lower reaches," Wu says.
When it comes to managing floods, experts argue that dams are not a silver bullet. Effective flood management instead includes dyking, flood proofing, flood warning systems, diversion areas, and development restrictions in floodplain and designated diversion areas.
Many environmental specialists are concerned that such systems have been neglected along the Yangtze River since the central government concentrated its investment on building the Three Gorges dam.
One of the major threats to the flood management along the Yangtze is the build-up of sediment in the river and its tributaries, largely caused by deforestation and soil erosion along the upper reaches of the Yangtze River, especially upstream of the Three Gorges area.
Forest cover in the upper Yangtze is barely ten percent, and in many counties the remaining forest cover is immature trees. Many areas are regressing from forest to bush, to grassy slopes to bare rock. In all, approximately 40 million tons of sediment are eroding into the Yangtze every year.
Wang Shengfu, a senior engineer of the Yangtze River Water Conservation Committee points out in a Xinhua story that the massive accumulation of sediment in the river and flood diversion lakes has lowered their capacity to contain water. Large lakes along the river have been shrinking and small ones have been disappearing. In Hubei province, the number of lakes that played a vital role in absorbing flood waters have declined from 1,332 in the early 1950’s to 843 in the 1980’s, says Wang.
Large freshwater lakes likes Dongting and Poyang lakes in the lower Yangtze river basin, for example, have been important natural flood water retention areas: Dongting can fluctuate from 600 sq km to 2800 sq km in the flood season. But over the past thirty years, Dr. Joseph Larson, a US wetlands expert at the University of Massachusetts, who has studied the Yangtze for years, says the lake’s surface area has been reduced by half, losing much of its storage capacity.
The reclamation of wetlands and floodplains for agriculture and development is also diminishing the flood control capacity of the river valley. Population settlement since the major flood 40 years ago in the region between the river and main dykes (beaches) and in the overflow diversion areas has been unrelenting. The Yangtze basin is home to 400 million people, making it one of the most densely populated river basins on earth.
For instance, the crucial Jingjiang overflow diversion area near the city of Wuhan is now home of over 300,000 people. This makes it particularly difficult to inundate the area when huge floods occur. (In fact, these 300,000 people were evacuated two weeks ago, in anticipation of the needing to dynamite the Jingjiang dyke.)
Experts argue that this year’s flood damages would not have been so catastrophic had the government bolstered the flood control capacity of the river valley, rather than simply building more dams.
Philip Williams, President of the International River Network (IRN) and a hydrologist, dismissed the argument that the Three Gorges dam will protect the population and property from disastrous floods.
According to Williams, dams do not reduce flood damage, they simply transfer damaging floodwaters from one part of the river basin to another. In the case of Three Gorges dam, the dam operators would have to choose between flooding out large numbers of people living around the reservoir or large numbers of people downstream and in the overflow diversion areas below the dam.
Furthermore, he points out that with multi-purpose dams, such as the Three Gorges, the need for electrical power usually prevails over the need for flood control. Dam operators come under strong political pressure to delay emptying the reservoir prior to the flood season, or to keep the reservoir higher than normal in order to maximize electricity revenues. The reservoir is often full at a time when flood storage is required.
For these various reasons, dam operators often delay making required large flow releases during major floods. Often they wait until the last minute, letting levels rise, hoping the flood will abate. If the floods do not abate, then the operators have to make large emergency releases which can cause disastrous floods downstream.
Actual operating experience with large flood control dams also show that when large floods occur there is a high risk that the spillway gates do not work as designed, due to operator errors or malfunctions.
Added to this, dams threaten the integrity of dykes downstream because they release silt-free or clear water from their dams, which has more erosive power than the natural flow. Dr. Wu at South Florida Water District, meanwhile, argues that Three Gorges dam builders have not addressed these issues adequately. He is worried that the Yangtze dam might even make flood damages worse.
Wu points out that the dam will create a long, yet very narrow reservoir with Chongqing, a city of 13 million people at its tail. This reservoir has a limited capacity to store flood waters and restricted capacity to release them rapidly in the case of emergency. The worst scenario, Wu says, is that if it holds the water, Chongqing may be flooded. But if it releases the water, it may worsen the flooding in lower reaches.
Categories: Three Gorges Probe