The Yangtze river’s periodic flooding was one of the reasons China built the Three Gorges dam, the largest addition to the country’s network of tens of thousands of dams.
The idea was that by storing and then carefully releasing river waters, the country would be able to prevent catastrophes like the floods of 1931, in which millions of people died in cities along the mighty river, or the 1998 floods, that left millions homeless. “The Three Gorges Dam is instrumental in our flood control efforts,” a deputy water resources minister said soon after the completion of the controversial project.
China has about 98,000 dams, most of them small-scale and constructed before the 1970s. In addition to flood control, these dams play an important role in hydroelectric power generation, and ensuring water security. But flooding in recent years due to unusually heavy downpours has drawn more attention to the challenges of managing dams in an era of climate change, when extreme rainfall poses new risks for surrounding communities.
Last month, the Yihetan dam, a major dam in central China’s Henan Province, was breached and “seriously damaged” after the province was hit by torrential rain. Two other dams in north China’s Inner Mongolia region were breached and collapsed after overflowing, affecting more than 16,000 people. Heavy rainfall last year even prompted concerns about the stability of Three Gorges after waters rose above its flood-prevention level, talk that Chinese officials quickly rebuffed.
“Because of climate change there could be higher and higher precipitation, that’s something that may have not been considered during the process of designing the dam,” said Wen Wang, a professor of hydrology at Hohai University in Nanjing, adding that it’s a major issue engineers are paying attention to now, yet one that’s hard to construct for given the difficult of accurately forecasting volumes of extreme rain.
While’s it’s difficult to link any one extreme event to global warming, the new report from the Intergovernmental Panel on Climate Change published Monday (Aug. 9) says that rainfall and floods seen as “once in a decade” events are going to become increasingly more frequent. In east Asia, the IPCC predicts with high confidence that heavy rainfall will increase in frequency and intensity.
Zhengzhou city, in central China’s Henan province, for example, recorded what is typically nearly a year’s worth of rain on a single day. Torrential rain on July 20 caused floods that left more than 300 dead, including many who were trapped in car parks or in the inundated subway system.
Johnny Chan, professor of atmospheric sciences at City University of Hong Kong, said that such levels of precipitation are “unheard of in that part of the country.” But, as the Earth gets warmer, speeding evaporation, this may not remain unusual. “You can expect to see higher and higher frequency of heavy rain,” said Chan.
According to a report commissioned by the National Climate Center, part of China Meteorological Administration, mean temperatures in the country were above normal in every season in 2020. Meanwhile, the annual rainfall for the whole country in 2020 ranked as the fourth-highest since 1951. The heavy rainfall caused extreme floods across southern, central, and eastern China. As a result, water levels at the Three Gorges Dam, the world’s largest hydroelectric plant, located in central China and completed in 2009, rose 6.5 feet above its flood-prevention level.
One of the dams that collapsed this year, the Xinfa dam in Inner Mongolia, was “well constructed and prepared very well (for floods)” said Mohammad Heiderzahad, an associate professor of civil engineering at Brunel University in London. Heiderzahad, who is a dam engineer himself, explained that even so the dam collapsed quickly despite having two spillways and an emergency bottom outlet, which allows for water to be released safely when a dam is in danger of overflowing.
This may have been due to the fact that the unprecedented level of rainfall exceeded the Probable Maximum Flood the dam was designed for, a term that refers to an estimation of the largest flood conceivable where the dam is being constructed.
“If unprecedented flooding is going to be the (new) norm, then large reservoirs would put communities at a big risk” said Heiderzahad. One mitigation step he suggested is adding another emergency spillway under already existing dams to prevent them from overflowing.
In one case last year, officials were forced to blow up a dam that was at risk over overflowing.
Wang, of Hohai University, disagreed that size itself is a risk. Instead, he said collaboration between different reservoirs, and the use of engineering and non-engineering methods of handling water overflow, is what matters. When it appears that engineering methods cannot handle the level of rainfall, dams must empty their reservoirs, he said, a move that is highly reliant on accurate forecasts of precipitation. Learning from last year’s floods, for example, engineers at Three Gorges expelled 90% of the dam’s capacity at the beginning of June this year to brace for the flooding season.
Others note that the age of a dam may be a crucial factor to consider. “The aging dam landscape faces new temperature, snow, discharge, and floods patterns that increase the risk of hydrological failure,” noted a 2021 paper in Nature Communications that described dams as playing a positive role in reducing flood vulnerability. ”To maintain historical levels of flood protection in the face of climate change, new dam release operations will be required.”
If more dams have to release water to accommodate more intense rainfall, however, that could result in flooding in downstream areas, particularly if those flows coincide with rainfall.
“But we have to make a balance,” said Wang. “If the reservoirs are not emptied to prepare for high precipitation events, maybe the dam will collapse, which will cause more and more loss of people and properties.”