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How the Three Gorges Dam Handles Extreme Floods
Author’s Note: If you want to watch the video for this first, you can watch it below
I started writing about the Three Gorges Dam because I thought it was a pretty cool piece of engineering work. Since I had done videos about the Taipei MRT and the High Speed Rail, I thought this would be a nice followup.
I did know that the dam was built with some controversy and I would eventually make a video about those relocation issues. But when I posted the first video, I was surprised to get a lot of questions about the recent floods during the monsoon season. This video was a follow up to help answer some of those questions.
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While researching my earlier video on the Three Gorges Dam, I came across speculation on the possibility of the dam collapsing. The Chinese government issued a bunch of statements about the situation but as is the case nowadays, most people did not believe those statements.
I find this whole thing to be a bit weird as the dam has been around for a while. I know the Chinese government does not have the best reputation for truth-telling, but we don’t really need to take their words at face value to recognize that the dam is doing fine right now. There is a lot of publicly available data on how it works, the way it is built, and prior “extreme” situations that the dam has survived.
In this video, I want to talk about how the dam manages extreme flood risks in a new age of climate change.
The Three Gorges Dam is the world’s biggest dam. I spent 15 minutes talking about this $22 billion piece of infrastructure in the first video I made a while ago.
In this section I want to dive a bit more into the design details engineered such that to keep the dam from being overwhelmed by major floods and stresses.
The dam is designed to handle incoming floods of up to 90,000 cubic meters of water per second. It has never seen a flood approach those maximums. The check amount (designed maximum) is 124,000 cubic meters of water per second. This is nearly four times larger than the amount of water going over Wikipedia’s largest waterfall - the Inga falls in the Congo.
The dam has three sections. On the left and right are the powerhouses. The center area handles the discharge of water. This 483 meter long stretch is broken up in 23 subsections.
When the dam discharges water, it does so using one or more of the three types of water outlets built into the dam structure. These outlets are arranged in a unique three tier structure:
22 “Crest Outlets” at the top of the dam. Placed 158 meters high on the dam’s structure and are 8 meters wide
23 “Deep Outlets” - which despite their name are placed in the middle of the dam’s height, at around 90 meters high on the dam structure. They are 9 meters tall and 7 meters wide
22 bottom outlets, 56 meters high and protruding farthest outwards
The deep outlets are designed to be used for the longest period of water discharge. They were designed to spray water such in a way that it would not interfere with the bottom outlets, if those were to be opened.
For the bottom outlets, the two main concerns were the huge amounts of sediment in the water and the potential of the water eroding away the riverbed.
A slot was set up at the outlet’s entrance to catch sediment and special concrete was poured to prevent sediment from eroding the outlet’s structure and compromising it.
The Three Gorges Dam’s discharge structures have been operating for over 15 years without issues. And crucially, all three of the outlets have been used before.
In contrast, I want to refer to a previous high-profile potential dam collapse scenario: the 2017 Oroville dam incident. One of America's tallest dams and a cornerstone of the California water system.
The overarching issue with this rather old dam was that on paper it had four big water outlets for handling flood scenarios. But the big backup spillway people counted on for when the Big One came had not actually ever been tested.
So when five days of storming in February 2017 brought the dam to the brink of collapse, some 188,000 people downstream were forced to evacuate their homes.
Annual Operating Procedures
Another reason why dams fail are improper or inflexible operating procedures. In this section I want to discuss how the dam is operated.
Flood season in the Yangtze basin normally starts in May and ends in September. During this period of time, the area receives 80% of its total annual precipitation and 70% of water runoff - the amount of excess water not absorbed into the ground. The rest of the year, runoff and precipitation are relatively small.
The dam’s procedures are structured around this schedule.
Every June at the start of flood season, the dam releases water until the reservoir falls to a level of 145 meters. At the normal water level of 175 meters, the reservoir stores 39 billion cubic meters of water. Cutting it to 145 meters reduces the reservoir to 22 billion cubic meters, thus freeing about 17 billion cubic meters of water of flood control capacity.
Throughout those three months, the water level is maintained at 145 meters. Rain and precipitation bring floods. The dam catches those, stores it, and then releases the water at a more manageable time. The dam is also generating the most electricity at this period of time due to the amount of water currently passing through its powerhouses.
At the end of flood season in September, the dam changes operations and slowly gathers water over the next two months. Reservoir levels cannot rise over 162 meters until after the end of September. After September, it can rise to the final maximum amount of 175 meters.
This amount of 175 meters is maintained from November into January. Thereafter, water is slowly released until it hits 155 meters at around April. This 155 meters is maintained for the rest of the month.
At May’s end, the dam releases the last 10 meters to hit the 145 meters necessary for handling flood season again.
It has to strike a balance. On one hand, you need to have spare capacity to handle any floods during flood season. But the opposite situation has its own problems too. Too low a level of water can cause navigation issues for ships and water supply issues for the cities downstream. And the reservoir water itself is needed for agriculture and residential uses as well.
Climate change will likely affect the dam’s reservoir performance and hydropower production in the near future. Few studies have been conducted on this, but the records show the change.
The mean temperature in the Three Gorges basin is projected to increase by some 1.5 to 5.4 degrees Celsius. This means that precipitation is projected to increase by 4-12% over the next 100 years.
Precipitation on the north side of the Three Gorges has been getting more intense and sustained, especially after 1980. Bigger rainstorms are happening in the September-October period, after the flood season is supposed to have ended.
Considering that these off-season rains take place during a period of time when the Dam is supposed to be raising its water levels to its maximum 175 meter height, it could be of some risk. It may require some modification to the operating procedures going forward.
The challenge is that these off-season rains are very unpredictable. Sometimes you get record highs. Sometimes you get the normal behavior. This is something that will require further study down the line
I think the first time this collapse situation started to stick in people’s heads was last year in July 2019.
A number of Google Maps images were circulated in some WeChat groups that claimed to show the dam’s structure warping. China’s online community had some concern.
The dam’s owner is the publicly traded China Three Gorges Corporation. They quickly put out a message and some new satellite pictures that showed that the dam was fine. This seems to have calmed down most people’s fears.
Heavy floods and rains in China in 2020 seemed to have brought the fears back up though. Heavy precipitation across the Yangtze basin during the 2020 flood season caused floods in multiple Chinese cities. The Three Gorges Dam took on the brunt of preventing these floods from causing serious damage.
Five floods were recorded as hitting the Three Gorges Dam. The fifth flood was particularly strong. Per Chinese news sources, on August 20th 2020, the dam saw an inflow of 75,000 cubic meters of water per second. The reservoir level rose to 168 meters high, which Xinhua cites as a record.
Yes it is the most the dam has ever seen. But not by all that much. In 2012, the dam withstood a 5% frequency flood - meaning one in twenty years - without incident. At its worst, this 20-year flood sent 71,200 cubic meters of water per second at the dam.
And the 75,000 is still far below the designed parameters of 90,000 cubic meters flood discharge amount (124,000 cubic meter check amount).
The reservoir added 18.2 billion cubic meters of water during the 2020 flooding season. It sounds like a lot but per the operating procedures that I mentioned earlier, 17 billion cubic meters of water were released in the prior six months as the dam prepared for flood season. It has the capacity.
There seems to have been some criticism from people. The criticism goes something like this: Since several cities downstream did indeed experience some flooding (Yichang seems to be the biggest victim here), the dam failed to do its job.
I find this criticism to be setting up a straw man argument. As I pointed out in my previous video, it had been known from the very beginning that the dam was not placed to cover 100% of the Yangtze’s watershed. The 1998 and 1954 floods occurred as a result of extensive rains downstream from where the dam had been.
Instead, the Three Gorges needs to be seen as just one player within a bigger system.
The Yangtze System
The Three Gorges Dam should not be seen as the final solution for protecting the basin’s 400 million residents from another devastating flood.
The Dam’s status as the world’s biggest has caused it to attract a lot of attention. That’s fine. It’s quite big. But we should all recognize that the Three Gorges Project is just one of the many dams holding back and controlling the Yangtze river.
In addition to the 40 billion cubic meters in the Three Gorges reservoir itself, there is another 52 billion cubic meters in upstream reservoirs. More dams are planned, with a total planned capacity of 100 billion. In total the Yangtze has over 45,000 reservoirs along its basin.
They all follow operating procedures similar to those of the Three Gorges. Thus, together, the system can prevent the type of floods that killed hundreds of thousands in previous years. Let us look at 2016.
2016 saw an extreme amount of rainfall in the Yangtze basin - the most since 1950. Some 950 mm of cumulative rainfall. The result was what would have been the biggest flood since 1954. It is comparable to the 1998 flood as well.
But the Yangtze flood plain has vastly changed. The system captured 34% of the floodwaters and so the flood peak topped at 71,000 cubic meters per second. As you recall, this still represents a major flood event but falls short of 2012 or 2020. The Three Gorges Dam by itself was estimated to be responsible for half of this stored water. The actual observed flooding in 2016 was greatly diminished.
2020 has been a challenging and rain-heavy year, but it remains within engineered parameters. Ye Jianchun, China's vice-minister of water resources, remarked that 64.7 billion cubic metres of floodwater has been stored in 2,297 reservoirs, including 2.9 billion cubic metres at Three Gorges. It remains within parameters.
Those parameters have been tested before in 2012 and 2016 without incident.
I don’t want to sit here and tell you that everything is fine and dandy. There are remain some serious issues.
For one thing, climate change is for sure disrupting the status quo. You had a time when you could expect that the dry season would be dry and the wet season would be wet. So as a result, the dam had worked on that predictable cycle.
What happens if 950mm of precipitation hits the Three Gorges Dam when the dam reservoir level is at its maximum?
And then additionally, there seems to be the fact that the engineering around the dam and the reservoir system could trigger feedback loops that exacerbate flooding elsewhere. For example, let’s go back to the 2016 floods.
There is no doubt that the Three Gorges helped blunt the impact of the rains and prevent a devastating flood. But the dam’s existence helped offset its own impact. The reservoir had trapped substantial amounts of sediment flowing down the Yangtze. Sediment in the waters that could have been deposited downstream on the river banks, building up natural levees and flood protection.
Thus, the dam’s presence ironically weakens some of the flood protection that it had been built for in the first place.
That situation reminds me of what I mentioned in my other video. The government reinforced the Yangtze’s levees in the 1950s. But in doing so, they dumped river silt into the nearby Dongting lake, reducing its ability to absorb flood waters and ironically making flooding even more likely.
Systems are complicated and hard to model. Unintended consequences like these happen all the time. More research and vigilance will be needed in the future to handle situations like these down the line.
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