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There is serious concern that China's Three Gorges Dam may fail due to heavy flooding.

And that if it fails, it would kill millions of people, perhaps 100 million according to some claims, and displace even more. Up to ~400 million people live downstream of the dam.

It would arguably be the biggest non-pandemic disaster in history and have devastating second-order effects, including:

  • exacerbation of the coronavirus,
  • famine,
  • nuclear radiation worse than Fukushima,
  • further instability in Great Power dynamics, and
  • global economic depression/collapse.

It is no exaggeration to say that a sudden collapse of this dam could be the spark of cascading civilizational collapse. It is worth examining even if the risk is small.

From the National Review:

China has been suffering through record rains the past weeks, leading to the worst flooding in the country in decades. There is little relief in sight, and the Yangtze River is now above flood level, according to China’s Ministry of Water Resources. A few days ago, officials admitted that certain “peripheral” structures of the massive Three Gorges Dam deformed due to the building water pressure. Stunning pictures of water being released to relieve pressure are raising the specter of whether the entire dam could fail (some good photos here). Some online satellite photos purporting to show the buckling of the dam, however, should be viewed with skepticism.
Still, the damage that has already occurred from the record deluge is significant, with numerous cities upriver from the dam already flooded[...]
All that would be dwarfed if the Three Gorges Dam failed. The dam was built from 1994 to 2006, at a cost of $31 billion and displacing 1.4 million people for its construction, precisely to lessen the risk of devastating flooding along the Yangtze, a perennial problem in China since ancient times. The river’s basin accounts for nearly half of China’s agricultural output, and it runs through major cities, such as Wuhan, with 10 million people.
Chinese authorities have already evacuated 38 million people downriver. The dam can hold back waters to a level of 175 meters above sea level; according to the Bureau of Hydrology of the Chanjiang (Yangtze) Water Resources Commission, the latest (Friday) height at the dam was 158.85 meters, down from 164 meters on Tuesday. Yet more rain is predicted, and if smaller, older dams upriver from Three Gorges overflow or fail, then the pressure on the main dam could quickly overwhelm either its capacity or even its structural integrity.
While an outright failure of the dam may not be the primary danger, nonetheless its geopolitical consequences are staggering to contemplate. It would be a black swan of epic proportions, China’s Chernobyl moment. A tsunami-like wave from a breach in the Three Gorges Dam could wipe out millions of acres of farmland right before the autumn harvest, possibly leading to famine-like conditions. As it is also the world’s largest hydroelectric power station, a failure would lead to huge power outages. Low-lying cities of millions along the Yangtze’s banks cities could become uninhabitable and the death toll could be staggering.

Related Questions:

  • What is the base rate of dams collapsing from heavy rains?
  • What is the average level of rainfall in China/the Yangtze river basin each year at a given period of time (particularly in the summer/the month of August)?
  • How much future rainfall is projected in the Yangtze river basin?
  • How much rainfall would need to fall in the Yangtze river basin in order for Three Gorges Dam to fail?
  • How many people would die if Three Gorges Dam collapsed?
    • How many people currently live within the area of projected flooding if Three Gorges Dam collapses?
    • How many deaths from starvation would occur if Three Gorges Dam collapsed?
      • How much food is produced in the path of projected floodwaters?
    • How many nuclear power plants would be destroyed?
  • What are second-order effects of Three Gorges Dam collapsing?

Here is the accompanying Metaculus question for this post (pending mod approval). Please submit estimates to it as well!

A good answer to this question displays original information, modelling, thinking, and also provides a probability estimate. I invite a plethora of amateur Fermi calculation reasoning. I do hope to see at least a handful of answers from people who are dam experts, engineers, hydrogeologists, physicists, weather forecasters, or otherwise have expertise in relevant domains.


$50 to the best answer to this question by the end of July, as judged by upvotes + personal judgement.

$25 'Fastest Updater' prize, awarded at my discretion. This will be given to whoever is judged to have exhibited the most of some combination of: volume of contribution, most amount of justified changes to their probability estimate, collecting useful info, biggest update to their explicit models in either direction, causing the biggest update to others' models, most amount of changes to other's probability estimates. 'Having lots of updates rather than one correct prior'.

Prize pool contributions welcome!

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Good Judgement Open might have you covered here; see: Will China's Three Gorges Dam fail before 1 October 2020?.

Current crowd probability: 3%. (note the timeline).

Some comments I've curated from that question:

https://asiatimes.com/2020/07/three-gorges-dam-deformed-but-safe-say-operators/ Release appears to be controlled. https://www.upi.com/Top_News/World-News/2020/07/23/China-braces-for-impact-after-mass-flooding-at-Three-Gorges-Dam/2221595525864/ Revising today after considering this further. The chance that the country doesn't divert water to prevent the dam from failing, even if upstream dams burst, seems very slim.

There have been heavy rains in the region which continue as of the time of writing. The dam is 181m high and the design maximum water level is 176m. Dams are designed to last hundreds of years, though climate change could mean that the original design assumptions have become outdated. There is a very slight chance of the dam "failing" within the next few months and releasing a sudden rapid uncontrolled flow downstream - just above zero. Some reports say the dam was built to hold 145 meters of water but actually that figure refers to the level at which water is released downstream in order to smooth out flood flows and maintain capacity in the reservoir. Discharge in recent days and weeks has been between 20 and 30 thousand cumecs, but this has gone up to 40k in the past, so there is still some cushion. Probably the greatest risk is of failure of one or more major dams upstream, unleashing a flood surge that could overtop the dam.
(Says a civil engineer)

This is a gravity dam, and it relies upon the construction itself to stand. It was 50 ft Above flood level. I see this as a concern without being a high probability event.

This question talks about the failure of a $ 32 Billion project completed in 2012. We have roughly 9 weeks from today till when the question is resolved. 5% for Yes is a good baseline to start. While the dam is currently holding more water than it is designed for, water can always be released if things get bad. The reason why they would be holding more water is in order to prevent the catchment areas from getting flooded.

This looks unlikely, but how unlikely seems difficult to estimate: on the one hand, quality of construction in China is poor and cutting corners is a way of life. On the other this is a flagship project, which means that there must have been stringent quality controls (in contrast with the standard situation in China). Unfortunately this is inside view. I did not try to make a historic review of dams failing around the world or in China. However the Banqiao dam failure in 1975 readily comes to mind: https://en.wikipedia.org/wiki/Banqiao_Dam https://en.wikipedia.org/wiki/1975_Banqiao_Dam_failure and catastrophic floods have been a common recurrence along Chinese history.

There were 3,523 incidents of dam failure from 1954 to 2013 (He et al. 2008; Zhao 2014) that caused significant loss of life and economic losses in china. This averages out to 67 dam failures, of various sizes per year. There are approx 87,000 dams in total. Given this, the three gorges dam broadly speaking has .08 chance of failing this year. There is significant flooding atm, which could increase probability of failure, but the concrete dam wall is 181 m (594 ft) high above the rock basis and has a max capacity water level of 175 m (wiki) Water level currently seems to be at 145-7 m from the articles I can find, which is well within capacity. The dam has passed quality checks, is a relatively new project (old dams fail more often) and there's a large amount of research done on seismic activity in the area. https://www.sciencedirect.com/science/article/pii/S1674984715300756; https://journal.probeinternational.org/2014/04/07/three-gorges-dam-triggers-frequent-seismic-activities/ https://www.slu.edu/news/2018/september/earthquake-research.php There are rumors of buckling and deformation and a google earth image going around (https://www.foxnews.com/world/integrity-of-chinas-three-gorges-dam-questioned-despite-china-officials-dismissing-it-as-safe) but I looked myself, and google earth currently shows shows no buckling, nor could I find any inconsistencies, so Ima say probs not, image seems fake. Long story short, is the dam gonna prevent flooding downstream? Maybe not, its effectiveness at doing so seems questionable based off the articles. This, however, isn't the issue at hand. We're asking is the worlds largest dam gonna fail in the next three months after passing safety checks, having research available about seismic activity in the area and currently within capacity? Highly unlikely. If upstream dams start to fail and/or if water levels breach capacity, then it gets more likely. But til then, low low chance.

One of the things I've been thinking of....dam failure means any amount of water that they didn't intend to let through passing the dam. So Im wondering, is there a higher percent chance of something small happening (whups, a couple gallons seeped through, or we lost a couple thousand gallons over the edge, our bad) or is it an all or nothing deal where when she goes, she goes, rip wuhan. Good point on the time it takes for flood level to get there. Also, thinking of ways it could potentially fail, I could conceivably imagine a scenario where the dam is at or slightly over capacity due to flooding, seismic activity happens thats unprecedented and the concrete slips off the bottom rock. There's history of other large concrete dams doing so (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4997708/) but seems to be only at filling? There's also this article: https://asiatimes.com/2020/07/three-gorges-dam-deformed-but-safe-say-operators/ which is recent and does mention some deformation to 'non structural parts of the dam' I don't exactly know what that means lol. And then finally, there was one scientist dude who has been talking about failure for a while, but stating cracks in the concrete during early stages of the building process and instances of substandard concrete, not buckling as the internet seems intent on portraying lol. https://www.rfa.org/english/news/china/threegorges-safety-07082019085631.html Ah, one more thing: https://damsafety.org/dam-failures#:~:text=Dam failures are most likely,the top of a dam.&text=National statistics show that overtopping,of all U.S. dam failures. The way dams are most likely to fail (this is US based, but i read a scientific study about chinese dams that was saying the same things) overtopping number one reason of failure, within the very low percentage chance of a dam failing. Slipping second most likely. So if the dam does fail, it's most likely gonna either overfill or slip. Will be good to pay attention to water levels upstream, precipitation, and how much output their letting through in anticipation.

This is a topic I have some subject-knowledge on, and I think the question requires clarification: The Forestry reference to "sudden, rapid, and uncontrolled release of impounded water" is included in their definition of “Dam failure” and the key element of the failure is the release of the impounded water. The dam is designed to manage a 1:100 year flood, derived statistically, by having the impoundment reserve capacity and controlled discharge of this amount of water. Floods greater than the 1:100 year value are managed by the sluice gates, turbine channel flow and, ultimately, by the dam’s spillway. The spillway is the lowest part of the dam crest and is designed to permit much larger flows (beyond the “Probable Maximum Flood”) As defined, I think the question asks whether there will be a failure of the dam which releases the water impounded below the spillway level, e.g., structural/geotechnical failure, undermining or uncontrolled bypass, which is highly unlikely. However, the question may be interpreted to ask whether the dam will be ‘overtopped’, with uncontrolled, rapid release over the spillway – which is quite probable this year. BTW the dam was built to reduce the frequency of flooding downstream, where millions have died from flooding of the Yangtze River. From a flood risk management perspective the dam is small at 1:100 year capacity. Negative press in competing or developed countries focussed on displacing 1.3M people in the interest of power production, not on flood risk management. Also, “The Interpreter” article is accurate in describing older dams in China (and around the world) as being potential ‘black swans’: these dams were often not designed to spill “probable maximum floods” and their failures may well jeopardize life downstream.

The dam is already controversial, so any story on it will be far reaching and potentially exaggerated. The Chinese have admitted to some movement to the dam but say it's within normal parameters. While under scrutiny for COVID and struggling with it's international image I'd like to think that evacuations would be in place if the risk was high. Because this may be a naive thought and catastrophic accidents have occurred in the past due to bureaucratic failings in similar regimes (think Chernobyl), I have input 2%.

The 3GD is a gravity dam, but the blocks are resting on the riverbed, not dug in. This is causing deformation throughout the structure. Construction is likely shoddy and the quality team that inspected the dam were from the same company that built it - not independently done. The CCP came out yesterday(?) to say that there is some deformation in the dam, but nothing to worry about. This alone from the CCP is unusual. In my estimation, the CCP would rather flood Wuhan downstream than see their flagship fall. That said, they may not have a choice. As the heavy rains in the region continue, water is making its way back into the Yangtze and thus the reservoir. Not all of the rain we've seen in the past has yet entered the reservoir. The water level is already at 164m, 175m is the warning of collapse, 185m and it's gone. The final and most likely catastrophic failure is the spillways being damaged - we're seeing this on at least two of the spillways on the livestream. A large chunk appears to be broken off spillway #6 and there is evidence of cavitation in spillway #2. If the damage to the spillways continues, the dam will fail, and badly.

(This last comment is from a new and unexperienced forecaster, with a Brier score worse than the aggregate. He still only gives 15%)

My own impression is that the aggregate seems correct, i.e., a 1:30 bet seems roughly fair.

I also somewhat disagree with "It is worth examining even if the risk is small;" it seems to me that decisions will be taken by the CCP, and that there is probably no leverage to be found here.

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Here's an informative prediction writeup from Metaculus user beala.

I like this writeup.

From what I understand, since Three Gorges is a gravity dam, meaning it uses the weight of the dam to hold back water rather than its tensile strength, a failure or collapse would not necessarily be catastrophic one. So if some portion falls, the rest will stay standing. That means there's a distribution of severity within failures/collapses, it's not just a binary outcome.

If somebody here is a native Chinese speaker and understands Chinese internet well, it might be helpful to jot your thoughts down, especially if you're following the situation closely.

I might also be interested in having a short chat with you and do a collaboration thing where you explain nuances of the Chinese internet's current opinions, and I try to translate the most important information into a way that's legible for forecasters.

I'm a native speaker but I don't go on Chinese internet much and I don't understand cultural context enough to navigate it well. (I know very little about 3Gorges and I don't have a lot of experience with this type of "cultural context translations" but I've talked to and written very short reports about specific covid-19 experiences from China, Ecuador, Brazil and a few other countries).

Here are my existing notes.

FYI, the final paragraph is duplicated.

metaculus link is broken

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I read in Tweeter (so it is not very good source) that one of the problem of the 3GD is cavitation inside discharge tubes. Cavitation is happening when the speed of the waterflow is above 10 meter per second and water creates "vacuum bubbles" which later collapse and create shockwaves which are able to destroy even strongest materials. The discharge channels are inside the body of the dam as we can see on photos and if there will be a problem, they will affect the dam from inside without overtoping. Obviously, such channels could be closed but this will slow water release and increase chances of the use of the emergency spillway. Such spillway itself could be fragile (like in the case of Oroville dam) and could undermine the dam if damaged.

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