The Three Gorges Dam

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Termites Dream
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The Three Gorges Dam

Post by Termites Dream » Fri Jul 31, 2020 1:20 pm

The Three Gorges Dam is apparently under a lot of pressure due to high rains and has suffered damage. There are several youtube videos on this. The concerns are the dam could fail.


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Devil
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Re: The Three Gorges Dam

Post by Devil » Fri Jul 31, 2020 3:40 pm

I visited the Three Gorges Dam while it was in construction in 2001. At the time, I wrote a bit of a blog about it but it has since disappeared. I must admit that I was not convinced that every precaution had been taken against an accident, even though the engineering appeared to be very sound. What worried me the most was that upstream from the dam there are very high overhanging cliffs of friable rock in a zone noted for earthquakes. Whereas it is true that the recorded seismic activity was fairly low and the dam was constructed to not cede at under magnitude 7.2 (if I remember correctly). What was less certain was that the weight of the water in the retention lake, at its full 180 m height, would introduce strain into the rocks of the gorges. I felt that a major rockfall into the lake could cause a "tidal" surge that would travel downstream to reach the dam at considerably higher levels than 180 m, causing it to overflow with a surge that could easily reach Wunan and possibly even Shanghai.

I took the photograph possibly about 30 km upstream from the dam in one of the less rugged areas. It shows a landslide actually occurring, estimated at about 200-250 m above the river level. Although this one was very small, there are signs along the whole of the steep gorges that rockfalls are not infrequent and some of these were obviously massive. This was not the only concern that I had about long-term safety of the retention lake.

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Last edited by Devil on Fri Jul 31, 2020 4:04 pm, edited 1 time in total.

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Devil
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Re: The Three Gorges Dam

Post by Devil » Fri Jul 31, 2020 4:02 pm

I wrote the following extra thoughts about 2006
How safe are existing large dams? It is not easy to answer, because many failures are due to freak weather or other conditions. For example, the Banqiao disaster has been described as a 1-in-1000-year freak. Even if this assessment were valid, there is nothing to stop a similar event occurring tomorrow and then nothing for 2000 years! In my opinion, the biggest potential catastrophe could occur on the Three Gorges Dam if something happened. The dam itself is built close to a seismic fault line but has been built to withstand a magnitude 7 earthquake, so the risk there is low. However, upstream the gorges even overhang the reservoir and an earthquake in known seismic zones could trigger a large rockfall, creating a tsunami. The last time I was in China, I witnessed a small landslide about 30 km west of the dam and this started me thinking about the safety of the project. This could be exacerbated by reservoir induced seismicity (RIS) and experts have published figures that the frequency of seismic events has increased since the reservoir was filled. The level of the reservoir has been dropped in anticipation of increased water inflow from rainstorms upstream of the reservoir; what would have happened if an upstream dam (not built to the same standards) fails? There would be no time to drop the water level. Whoosh! Let's just imagine what would happen if the dam overflowed from whatever cause. The first place to be hit would be Yichang, just 20-odd km downstream, with a population approaching 1 million. There would be no time to evacuate the population. The major city of Wuhan with over 8 million inhabitants, some 200-odd km downstream. In between Yichang and Wuhan, there are tens of smaller communities and much good agricultural land.
Environment

Again, I'll take the Three Gorges as my basis for this discussion, mainly because I've seen the effects myself. The problems started even before any concrete was laid. There was much rich agricultural land bordering the Yangtze and over 1,000,000 peasants were evacuated upwards some 200 m onto poor, steep, often unusable land. They left behind a rich biodiversity of species, now gone for ever. It is impossible to say how many species were endemic to the region, now definitively drowned. Several fish species, such as the Chinese Paddlefish and possibly the Yangtze Sturgeon, below the dam, became unable to travel upstream to spawn and are now extinct. There are two problems with silt, too much upstream from the dam and not enough downstream. Effectively, the comparatively still waters of the reservoir cause silt travelling downstream to drop to the bottom. They form an anaerobic organic-rich layer which decomposes into methane gas which is emitted from the surface. Sometimes, the methane collects in a large bubble under the silt layer and then erupts to the surface. Methane is a powerful greenhouse gas, but I don't know whether the effect on climate change is more or less than if an equivalent quantity of electricity were generated in a fossil fuel plant. The silt also bungs up the turbine intakes and dredging becomes essential. Downstream, the lack of silt in the Yangtze flood plains are rendering the land less productive and the peasants are obliged to use synthetic chemical fertilisers, themselves both polluting by nitrous oxide emissions (another greenhouse gas) and require polluting petrochemical plants for their production: a double-whammy! Overall, my feeling is that large dams are environmentally quite negative.

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