h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Vaiont/Vajont Dam: the dam did not fail, but the reservoir slope, resulting in the dam overtopping. Last I knew the dam was still there, but it will likely be never (or, not for a long time) filled, because of human reasons. Even if the upstream slope can be completely stabilised.

Arch dams are generally double curved, so a (flat) bowl shape, convex upstream.

“Virtually all modern dams that are of the 'Whoa, that's huge!' size are arch dams, and they are almost always very tall”. Not so. Some of the biggest dams, in USSR (former), Pakistan, Africa and S America are earth or rock fill. One of the biggest, Three Gorges in China, is Gravity Concrete, that is it stands freely by reason of its weight distribution, not relying on arch thrust at its ends. An arch dam needs steep sides of sound/competent rock to the valley, to provide support to the horizontal component of the arch thrust.

A simplified reason for the failure of the slope upstream of the dam was, everyone concentrates on “rapid drawdown” of the impounded water, a well known case to analyse for stability of slopes. In this case, as pointed out there was a weak layer near the base of the slope, sloping towards the reservoir. The “dry” weight of the base part of the slope was sufficient to support the steep upper face. The submerged or buoyant weight was not. Half full was the worst case scenario. An alternative to considering “buoyancy” is to consider the water pressure distribution, internally and externally, around the potential sliding masses or wedges of rock or soil.

I cannot open the “excellent discussion of the details” link at the end of this paragraph.

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lil x

I think the problem here is just the phrase "toppled over the dam itself ". The writer intended this to mean that the water went over the top of the dam, whereas it actually means that the dam fell over. You use the correct term "overtopping" in your posting above. If I replace "toppled over the dam itself" with "went right over the top of the dam itself", I think it will be OK.

I've made that change and removed the link that didn't work.

Many thanks Gnomon.

Excellent correction, I did indeed mean that the water itself had come over the dam, not that the dam had toppled. Thanks for the revision and clarification. Much better now.