3 gorges dam at risk??

[fuzzy]

https://www.youtube.com/watch?v=n6QyfrS7ARI

Failure would take out more than the US population.

[kenneal - lagger]

And a load of factories and fossil fuelled energy plants. That would be good for global warming and is, unfortunately, the sort of thing, along with a few pandemics, that will be required to save the human population from itself.

[BritDownUnder]

Very interesting video. I doubt it would collapse but if quality control was somewhat lax it could be a possibility. The usual playbook hysterical Chinese response to any form of criticism is still being used. I am not aware of any other dams collapsing in China and they have built a lot in recent years.

Chinese engineers I encountered in China were very competent but rather unwilling to take responsibility for anything.

[fuzzy]

Try this video - it looks crazy:

https://www.youtube.com/watch?v=DAvokPdcq2I

[adam2]

I fear that a collapse is possible.
I have little faith in Chinese engineering and building skills.
The nation is known for forgery and faking of all sorts, therefore although the design may be competent I doubt the quality of the actual construction.
Rebar was likely replaced with smaller or less numerous metalwork than called for in the design. Concrete was likely made with a lower proportion of cement than was called for in the specification.

[adam2]

https://www.constructionmanagermagazine ... ng-safety/

Steel rebar is commonly "stretched" or made thinner than called for in order to divert a proportion of the supplied material to other purposes.

Add to that an original size that was probably in Chinese mm, or about 25% too small.
8mm steel rebar in the UK means 8mm, whereas in china it means about 25% less. due to accepted manufacturing tolerances of plus or minus 10%, plus one mm

[emordnilap]

It's able to withstand a 'worst flood in 100 years'. WTF does that even mean?

[UndercoverElephant]

It's able to withstand a 'worst flood in 100 years'. WTF does that even mean?

I don't think I'd find it very reassuring if I lived in the area myself.

[clv101]

It's able to withstand a 'worst flood in 100 years'. WTF does that even mean?

That's very poor if true. There's a distribution of different magnitude of 'flood event', looking back through history you can construct that probability density function. From that you can calculate the likelihood of a given magnitude event.

Given that the structure must be expected to have at least a hundred year life, building to the 'hundred year event' is hopeless. Additionally, climate change is significantly shifting these historical PDFs and not in a good way.

[adam2]

It's able to withstand a 'worst flood in 100 years'. WTF does that even mean?

What I SUSPECT that they mean, is that the overflows, spillways or outlets can allegedly cope with the water flows resulting from a "once in 100 years" rainfall event in the catchment area.

A greater flow could result in the dam filling right "to the brim" and the extra weight and pressure would increase the risk of failure.
Any overflowing the top of the dam could start a total collapse as the rush of the water might erode the downstream face or the foundations.

There might also be political pressure to fill the dam to a higher level than was intended.
A higher than planned for level would increase the stresses on the structure and reduce the safety margin before overtopping occurs.

[vtsnowedin]

If a dam is constructed to withstand a 100 year flood event with a safety factor of three which is the normal standard it will take a lot more then just a 100 year flood. These dams are called "gravity" dams as the weight of the dam is what resists the forces of the water behind it or flowing over it. How well it is notched into the underlying bedrock to avoid leakage is much more important then the amount of resteel or it's strength in the upper levels of the dam. If it has been shaped so water flowing over the top in excess of what the flood spillway can handle will not endanger the dam as long as the face has been built to not create a scour current at the toe of the dam.

[fuzzy]

If I can't stop my shed roof leaking, what chance have these guys got?

[kenneal - lagger]

...........These dams are called "gravity" dams as the weight of the dam is what resists the forces of the water behind it or flowing over it. ...

Not many people know this but the size of a dam is entirely related to its height and not to the capacity, the weight of water it is holding back or the length of the lake behind the dam.

[vtsnowedin]

...........These dams are called "gravity" dams as the weight of the dam is what resists the forces of the water behind it or flowing over it. ...

Not many people know this but the size of a dam is entirely related to its height and not to the capacity, the weight of water it is holding back or the length of the lake behind the dam.

Yes I remember solving the physics problem of the overturning moment on a dam. The back slope under the water took advantage of the weight of the water pressing down on it adding to the effective weight of the dam. The water pressure increases 0.433 P.S.I for each foot of water depth and pressed both down on the portion of the concrete under it and horizontally trying to push the dam down stream.
I have a picture of I dam failure I came across while hunting in North central Pennsylvania. It had not been built wide enough at its base or reinforced sufficiently. A major flood had pushed about a 100 foot long section completely off it's base leaving one end of it about fifty feet from its original position and completely draining the impoundment which ruined the factories it powered just down stream. The town has never really recovered from the disaster some ninety years ago.

[kenneal - lagger]

It's even more simple than that. The pressure on the dam is equal to half the square of the height of the water times the density of water and the maximum pressure is exerted at the base.