I visited the dam site a couple of years ago and seen a lot of what is there, both upstream and downstream. The first link above is partial fact. The second link is just male bovine excrement with most of the posts displaying total ignorance of the project.
So, what did I find?
1. There is a risk with this, as in any major energy project. In this case, the risk is aggravated because the Yangtze flows through the gorges which were formed by a seismic fault. No earthquakes of > Richter 5 have ever been measured in the region and the dam is built for Richter 6+, so there is theoretically an order of magnitude of safety built in.
2. Water is incompressible, so that the shockwave of an upstream earthquake hitting the dam may be more disastrous than an earthquake at the dam site itself.
3. It is impossible to calculate the effect of an extra 60 billion tonnes of water on the seismic balance and this weight may be sufficient to trigger an earthquake.
4. The gorges themselves are formed from mainly friable sedimentary rocks. There are many places where it is evident that enormous chunks of mountain have, in recent geological history, broken off and fallen into the river. In fact, I witnessed a small landslide (I have photos). A large one could cause a surge wave which could overflow the dam and probably cause it to rupture.
5. Two large HE dams about 100 km N of the site burst in the latter half of the 20th c. (I can't remember the exact date) causing the death of an estimated 250,000 persons (official figure is 75,000), due to weather conditions causing an overflow. These were also correctly built and engineered dams.
6. The city of Yichang is ~20 km downstream from the dam, with a population of ~ 2 million. If the dam burst or overflowed, for any reason, the population would be wiped out. Wuhan is a major city (>7 million) 100 km or so downstream. Some of the population there could be evacuated. Jingzhou (6 million) is halfway between these, but not on the Yangtze, although in its flood plain. My guess is that a burst would kill >10 million in Hubei Province alone.
7. There is another risk, totally unrelated to a dam burst. China's largest city is Chongqing with a population of 31 million. Its sewage flows, untreated, into the fast-flowing Yangtze, where it decomposes rapidly aerobically. It is at the head of the retention lake. As the river slows down, the sewage will fall to the bottom as organic silt and it is estimated that a layer some 10 m thick will form in the gorges within a decade. This will decompose anaerobically, forming vast amounts of methane gas. Some of this will bubble up harmlessly (except that methane is a bad greenhouse gas causing climate change). Experts have stated that the pressure of 180 m of water above it will cause it to crust before decomposition is complete, forming vast reservoirs of methane which will grow and eventually burst. If this happens near a town, such as Wushan, on a calm day, a gigantic explosion could occur, as the humid gas would be held in by the gorge walls, rather than spread out. Where you have habitations, you have sources of ignition. At the best, the river will become very smelly from the quantity of raw human waste stagnating in the dam lake, probably with disease-bearing organisms. It should be remembered that the cities like Wushan have been rebuilt from current river levels to form a new shoreline 180 m higher.
I'm not mentioning the catastrophic human, agricultural, cultural and archaeological issues resulting from this terrible construction. _________________ Devil
I had no idea about those other problems. They are serious, many could be overcome by dredging, better sweage management and inspection of the dam on a daily basis....
others are more fundemental:
Such as the sedimentary rocks you mentioned, and the fact that we only have a 1 magnitude of safety "built in"..while exponentialy this is a huge factor, how do we know that 1 or more above the 6.0 wont happen in the next 50 years or so... Granted 7.0 quakes are rare, but they do happen. So little of qauke history is actualy recorded acurately, especialy in rural areas. How can we be so sure thart a >6.0 has not happned in the last 500 years?
As for the "Freerepublic" article...
I will agree that most of the posters were just making fun of the Chinese, or not up to speed on exactly what is happening over there
Do you think the cracks that appeared on the dam are insignificant? My reason for posting about the three gorges was to compare to the Solar tower that is supposed to go 1KM high in NSW, Austrtalia...
Is it true that concrete makes its own heat and this can cause cracks, and that the Hoover dam has pipes w/ cold water still flowing through it? (or even had at one time?)
Where did China go "wrong" that there would be such cracks before the resivour was even filled? Or is this "normal".[/quote]
I had no idea about those other problems. They are serious, many could be overcome by dredging, better sweage management and inspection of the dam on a daily basis....
Dredging sewage 180 m deep ain't easy and must be mighty smelly! I agree that sewage treatment would alleviate some of the risk, but let's not forget that the catchment area has a population the triple or more of Chongqing itself. The dam is monitored constantly by laser metrology and satellite.
Anonymous wrote:
Such as the sedimentary rocks you mentioned, and the fact that we only have a 1 magnitude of safety "built in"..while exponentialy this is a huge factor, how do we know that 1 or more above the 6.0 wont happen in the next 50 years or so... Granted 7.0 quakes are rare, but they do happen. So little of qauke history is actualy recorded acurately, especialy in rural areas. How can we be so sure thart a >6.0 has not happned in the last 500 years?
I'm not over-worried about the direct effects of an earthquake, so much as the indirect effects, such as half a mountain falling into the lake: this could happen with a mild tremor acting as the last straw. Remember that dam burst in N. Italy a bit over 50 years ago when several thousand in a remote valley got killed with just this. The Mattmark disaster in Switzerland killed about 30-odd workers building the dam above Saas Almagell in 1965 or thereabouts. The lake was only about 1/5 full but a small chunk of glacier fell into it.
Anonymous wrote:
Do you think the cracks that appeared on the dam are insignificant? My reason for posting about the three gorges was to compare to the Solar tower that is supposed to go 1KM high in NSW, Austrtalia...
Is it true that concrete makes its own heat and this can cause cracks, and that the Hoover dam has pipes w/ cold water still flowing through it? (or even had at one time?)
Where did China go "wrong" that there would be such cracks before the resivour was even filled? Or is this "normal".
It is probable that the cracks are insignificant and, in any case, could be filled by high-pressure injection. The solar towe would be guyed steel, so cannot be compared. I'd imagine the section would be tapering to provide the required compressive and lateral strengths.
Yes, the chemical reaction as concrete hardens is decidely exothermic (never cast a concrete slab with rubber boots on while vibrating it???). A "strong concrete" (ie with too much cement in the mix) will always crack. A good engineer will always specify the weakest mix which will hold together to do the job and rely on the reinforcement for the strength. I don't know about the Hoover dam: it's possible. In very large masses, it may take years for the reaction to effectively finish; it's an exponential decay, asymptotic to zero so, theoretically, concrete never sets! _________________ Devil
Joined: May 21, 2004 Posts: 158 Location: Melbourne, Australia
Posted: Sat Aug 28, 2004 4:50 am Post subject:
gnm wrote:
concrete definitely makes its own heat as it cures but I couldn't speak to whether or not this would require cooling pipes on such a large volume...
While it cures. Not once it's set.
The Hoover Dam had to be built piece by piece for this reason, and yes, cooling pipes were used during construction. You can't pour one mass of concrete in one hit like that - it gets too hot and tends to fracture.
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