Environment, Talking Point

Renewed scrutiny over dam

After Covid-19, China grapples with the challenges of flood relief


The Three Gorges Dam has faced its sternest stress test

More than two months of torrential rain have created a major crisis along the Yangtze, China’s longest river. Higher up the watercourse in Sichuan, rising waters have even touched the toes of the 71-metre Leshan Giant Buddha, where floodwaters lapped the base of the world’s largest stone Buddha for the first time since 1949. Sandbars and islands further down the Yangtze have been swallowed up, with river transport largely suspended. The city of Chongqing has been sounding the highest flood alerts, while the reservoir behind the nearby Three Gorges Dam has hit record highs. The dam was built with the intention of protecting people from “once in a century” floods but it has been forced to release massive torrents of water. Is that a signal that the Yangtze’s flood defences are failing?

Has this year seen China’s worst flooding?

The country has battled several “once in century” floods in the past decade, prompting questions over whether this relates more to climate change or poor flood control measures (see WiC331). The Chinese authorities have pointed out this is the wettest year on record for the Yangtze River Basin, which has endured at least six major waves of rainfall over the summer monsoon. Rain in the middle and lower basin reached its highest levels since 1961 from early June to mid-July, according to the China Meteorological Administration. Water levels had already reached historic peaks in at least 53 associated rivers over the summer and another huge belt of rain swept through the Yangtze’s upper basin this month, taking total rainfall to 60% more than last year.

The good news is that technology including river-monitoring cameras that transmit warnings to flood inspectors has kept down casualties, according to Xinhua. The number of dead or missing from June to early August fell to 219 – less than half the average figure in each of the past five years, the ministry of emergency management said last week. Extreme wet weather in 1998 was responsible for more than 4,000 deaths, for instance.

But this year’s deluge has caused record amounts of economic damage – nearly Rmb180 billion ($26 billion) as of mid-August, based on the most recent estimates from the government.

Isn’t the Three Gorges Dam supposed to prevent this from happening?

China’s meiyu, or ‘plum rains’, have wreaked havoc for generations, with 1998 and 2016 suffering some of the most severe cases of flooding in the recent past. In response the government has built a defensive redoubt of nearly a thousand reservoirs and hundreds of thousands of levees.

The Three Gorges Dam is the centrepiece of the flood control network. Completed in 2006 as the world’s largest hydroelectric power plant, its primary role is to regulate water flow along the Yangtze. Water levels in the dam’s 600km reservoir are kept high during dry season to support electricity generation at the hydropower plant. Then they are lowered before the summer rains to make space for the incoming floodwaters.

This year the rains have been so torrential that the Three Gorges has struggled to control the water flow in the intended way. Officials have defended the dam, saying that the flooding would have been a lot worse downstream without it because the volume of water released has been less than the amount surging in. But the rainstorms have seen the reservoir fill up at its fastest pace ever, flushing in as much as 75,000 cubic metres of water per second and bringing water levels close to the dam’s design capacity threshold of 175 metres. That has put pressure on the dam’s structure, with Xinhua reporting “displacement, seepage and deformation” after the second wave of rains in July. It has also forced the discharge of a tidal wave of floodwater further down the river, flooding lower-lying towns and villages.

Is there a design fault in the dam?

Satellite images from Google Maps last year seemed to show that the dam wall was distorted, sparking speculation that it might burst. China Three Gorges Corp, its operator, rebutted the suggestion fiercely, insisting that the dam was operating “safely and reliably”. Other commentators told state media that minor deformations in the dam’s structure were acceptable within an elastic range (see WiC460).

However, the intensity of this year’s rains has reinforced the view that the Three Gorges Dam isn’t capable of absorbing the entirety of excess water in the upper reaches of the basin during periods of extreme rainfall, even if it does help to alleviate the risks of flooding during more normal weather.

During “once-a-century floods” more than 244 billion cubic metres of water – or about twice the volume of the Dead Sea – flows into the reservoir at the Three Gorges, Fan Xiao, a longstanding critic of the project, told CNN earlier this month. But the reservoir can only store about 9% of this inflow, Fan says, describing the situation as “like using a small cup to deal with a big tub of water”.

Other analysts say that too much is expected of the dam and that a tendency at official level to over-focus on mega projects has meant underinvestment in other, smaller defences further upstream. Heavy rain in the regions downstream of the Three Gorges Dam also raises river levels and causes flooding that the dam has less power to avert.

All the same, the dam was built to hold back water flow when the Yangtze’s middle and lower reaches are running high. This year the downpour in the upper reaches of the basin has flushed record run-offs into the reservoir, forcing officials to raise discharge volumes to new highs. Last Thursday 48,800 cubic metres of water a second were being released from the reservoir in raging plumes and torrents, although some experts were still warning that discharge volumes might need to be higher to reduce the threat of an “overtopping” of the dam wall.

What’s the impact lower down the river?

The discharged water is putting greater pressure on flood defences downstream and forcing the authorities into efforts to divert floodwater into emergency reservoirs or simply disperse it over nearby towns and countryside.

The Mengwa Flood Diversion Area in Anhui – home to four townships and 200,000 people – was one of the districts to be sacrificed last month, after officials opened 13 sluice gates on the Huaihe river, a major tributary of the Yangtze, China Daily reported. The Huaihe’s total drainage area is about the size of New Zealand, the newspaper added.

These diversionary tactics are designed to protect the major cities but they often mean that poorer parts of the country pay much of the price – an awkward trade-off for a government that is supposed to be narrowing the rural-urban divide.

Xi Jinping, China’s leader, has been visiting areas of Anhui where the sluices were opened, deluging dozens of villages and huge areas of farmland (another factor in his call to limit food wastage; see WiC507). He made sure to point out that the Chinese have been “fighting natural disasters for millennia”. Yet the disruption comes at a time when the government can least afford it, interrupting efforts to reboot economic growth in the wake of the Covid-19 pandemic. Floods like these typically push up food prices as well, another constraint on the campaign to boost consumer spending in China.

Is the flooding set to get worse?

Governments of the past have talked grandiosely about bending nature to their will – “make the high mountain bow its head and make the river yield its way”, as Mao Zedong once put it.

Xi has sounded more circumspect, arguing that the fight against flooding “needs to respect nature, and follow the laws of nature, and to co-exist with nature harmoniously”. Yet green campaigners are warning that the situation is going to get much worse because of climate change, with a warmer atmosphere bringing heavier and more frequent rains.

“I think the sustained heavy rainfall in the Yangtze River Basin is happening against the larger backdrop of global warming,” Jia Xiaolong, deputy director of the state-backed National Climate Centre, told state television news at the end of July.

Flood-prone regions along the Yangtze look particularly exposed to changes in weather patterns, especially provinces with large populations, such as Hubei, Anhui, Hunan and Jiangxi. Rising economic output in much of the region also means much higher costs of disruption in extreme situations. A sobering study last year by Sustainability, an academic journal, estimated that disruption from flooding had already reduced manufacturing output by an average of 28% a year in China between 2003 and 2010, bringing annual losses of Rmb15.4 trillion to the wider economy.

Perhaps that’s why this year’s floods have spurred promises of another major round of dam construction, with the National Development and Reform Commission announcing a further Rmb1.29 trillion ($184 billion) in spending on 150 major “water projects” – normally a reference to more dams, reservoirs and dykes, Bloomberg reported this month.

Yet flood control experts are urging a wider range of approaches to minimising the risks, like the reintroduction of wetlands and natural storage basins along the Yangtze, as well as the promotion of ‘sponge cities’ in urban areas.

‘Sponge city’ projects are seen as a way of catching and absorbing more of the rainwater at source, rather than channelling it into poorly built drainage systems that can’t cope with sudden downpours. Some city governments have been stipulating that constructors of larger apartment blocks and factories install underground water tanks as well. These holding tanks should be capable of storing overflows of rainwater, which are then released into rivers once the emergency has passed.

These kinds of initiatives have been talked about for a while (see WiC202 and 334), with frustrations expressed that more hasn’t been achieved in frequently flooded places like Wuhan. But another year of widespread damage highlights how rapid urban development has worsened the situation by stripping out the grassland, woods and lakes that absorb heavy rainfall more naturally. Better understanding of the economic costs should make investment in sponge city projects more of a priority. Expect more determined pressure on municipal governments to prevent rainwater from pooling on concrete surfaces or running off directly into waterways – in pursuit of State Council goals for the capture and reuse of 70% of the rain that falls on four-fifths of China’s urban land by 2030.

In the meantime Three Gorges Dam bosses are watching the weather forecasts for signs that the summer rains are heading north, bringing misery to other provinces but reducing some of the strain on the worst afflicted stretches of the Yangtze.

In better news this week, the latest surge of floodwater in the upper basin seems to have passed its peak too, bringing water levels in the reservoir down from their record highs.

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