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Some time ago I heard a comment on some National Geographic or Discovery Channel program that the earth rotation is slowed down due to water trapped in dams. I did not pay attention to this at the time as it was said to be a fraction of a second.

Planning my next vacation I came across this article and this made me wonder about the effects of a major dam failing at full capacity. This dam has $180km^3$ of water and will spill over and also take out this dam having another $55km^3$, combined this is a substantial mass of water that will run into the ocean.

I am not so much interested in the immediate effects but more on the long term effects. How will this devastation affect the world economy?

What will such devastation do to the environment? Certainly many coral reefs will be destroyed by this amount of fresh water.

Will the release of these amounts of water affect the rotation of the earth?

Surely this will cause a tidal wave that will wipe out some islands in the Indian ocean, tourism will be affected.

Electrical supply to many neighboring countries will stop suddenly.

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Locally, it would be a disaster. The rushing water will tear a pretty big and nasty scar on the way out along the river. The local economy will most likely be in shambles when they need to rebuild nearby towns and many, many people will be without electricity.

But globally? Not much will happen.

There is roughly 1.33$\times$10$^9$ km$^3$ salt water in the ocean, adding 235 km$^3$ from the two dams combined won't be noticeable at all.

There are some coral reefs along the south eastern coast of Africa, which is where I guess the flood will exit. These will likely be damaged by the flood, but probably not so much from the concentration changes as it will not take that long for the salt water to go back to normal salinity. Edit: I just found where the Zambezi river exists and it does not look like it will coincide with any coral reefs. The river wildlife will have a pretty big surprise, but no corals should be damaged.

No islands (except anything that happen to be just outside on the coast) will be affected by the rush of water. A tsunami is a rapid displacement of a large volume of water, and this is arguably a large volume of water. However, for a tsunami to occur, it has to come so that the forces gets concentrated as the water is pushed to shallower regions; the opposite is happening here. This means that the water will rush out into the ocean and there the forces will rapidly be spread out and diminished and will not carry any dangerous forces more than, maximum, a few kilometers out (if even that far).

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In 1889, the South Fork Dam, a few miles north of Johnstown, collapsed, releasing the entire reservoir into the town.

It released 20 million tons of water into the valley, which hit the town, killing 2209 people and causing damage worth around 450 million 2015 dollars.

Devastation, naturally, reigned, as the entire town was basically hit by a weight of many million tonnes travelling around 40 miles per hour. Writ large, this sort of devastation isn't all that important — the environment outside those areas where the water hit, won't be all that affected.

The rotation of the Earth has so much momentum that nothing much will happen to that. A tsunami is unlikely, as something a lot more forceful than this would have to move utterly enormous amounts of water (earthquakes are many orders of magnitude more powerful than dam failures).

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230-240 km³ of water is a lot of water, but it is nothing but a droplet in the ocean.

  • World economy:

    The world economy won't even notice. The contribution of Zimbabwe to the world economy is mostly in the area of examples of how not to run a country. Zambia is much better, but still.

  • Coral reefs:

    What coral reefs? Is there any notable coral reef at the mouth of the Zambezi?

  • Rotation of the earth:

    The rotation of the earth will absolutely be affected, but while the effect can be calculated it will be much too small to be measurable.

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In Operation Chastise during World War II British Lancaster bombers attacked several dams with bouncing bombs.

Möhne: 1600 people died, 0.135 km^3 water lost
Eder: 0.2 km^3 water lost

The global effect was neglible, even the local effect was small; it took only months to restore the dam and get full power and water capacity again.

You seem to overestimate the effects of fresh water, it won't cause tsunamis (because you need a vertical disruption like earthquakes or landslides), it won't affect tidal rotation noticably (yes, it will be measurable by precise instruments) and it has absolutely no effect on oceans because those contain 1 350 000 000 km^3 sea water.

The devastation behind the dam will be absolutely catastrophic for the local population (many, many lives lost, loss of hydropower and water), but Zambia and Zimbabwe are one of the poorest countries and even the complete collapse of the economy will not make a dent in world's economy.

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