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Comment by foota | Hacker News Reader

How could they drain it?

I would imagine siphoning might be a simple enough process and could be setup in multiple places along the dam to avoid the asymmetric draining issues mentioned further in the discussion here on hacker news.

You can syphon up about 30ft. The dam is 371 ft.

Can you explain why a syphon won't work over 30ft?

"This siphon effect relies on atmospheric pressure to allow the pressure and pressure potential energy to drop as the water travels upward inside the sealed pipe. But eventually the pressure of the rising water reaches zero and no further reductions in pressure and pressure potential energy are possible. That failure of the siphon effect occurs when the water is about 30 feet (10 meters) above the higher container. You can't use a siphon to lift water higher than 30 feet because above that height, an empty region will develop at the top of the pipe and stop the siphon process."

http://www.physicscentral.com/experiment/askaphysicist/physi...

What about running the tube over the dam and down to a lower level than it originates? Then gravity(?) does the trick?

It would only work if the difference between the water level and the top of the wall is less than rouhgly 30ft (as per xkcd). The bottom line is, the siphon will not work if it has a vertical water column taller than 30ft (water weight per unit area becomes greater than opposing atmospheric pressure)

The issue is that top of the dam is (way) over 30 feet above the water level behind the dam, which prevents the suction from overcoming gravity. A siphon won't work with that much of a rise in the middle no matter what the difference is between the elevations of the ends of the tube.

Also what you're describing is literally how all siphons must work--you can't siphon to a higher elevation.

It's not atmospheric pressure:

"Here we report an experiment of a siphon operating at sea level at a height of 15 m, well above 10 m. Prior degassing of the water prevented cavitation. This experiment provides conclusive evidence that siphons operate through gravity and molecular cohesion."

http://www.nature.com/articles/srep16790

Quote from the conclusion:

"""Extrapolating these results from even the most conservative experimental measurements of the tension under which cavitation occurs it is possible that the cohesive strength of fully degassed water is able to support a continuous vertical column greater than several hundred meters. While the experiment performed here did not reach anywhere near the absolute limit predicted it does shed light on the stability of flowing water under tensile stress and the possibility of constructing apparatus of suitable dimensions to test such a limit. (...) In view of the many anomalies of bulk water, it would be interesting to explore the physical properties of water in the negative pressure regime of a siphon above 10 m."""

So, as long as we don't break the water column by bubbles (or other impurities in the water that will lead to formations of bubbles on their surface) we could empty the Mosul dam via a siphon.

That's a fascinating result! We only have to remove the rocks, the fish and put the dam under high-vacuum for outgassing for a few weeks ;-). If we don't do that, the old atmospheric pressure model holds, and the siphon will be limited to 10 metres, 30 ft.

I wasn't implying that the result from the paper is applicable to draining the dam, just that we should stop explaining the siphon effect in terms of atmospheric pressure now that we know it's not the case.

I seriously was amazed about the result, with a "block" of water being able to withstand tension. Also the working principle of a siphon, I think, never was about atmospheric pressure. It's only the maximum height limit where pressure comes into place.

As long as you are below the limit, you can still claim that the "two sides" of your siphon are held together by the pressure extorted on both sides by our atmosphere. And only as you exceed this limit you'll require the water to resist actively to be "pulled apart".

Unfortunately this interesting property is pretty irrelevant in real-life situations where water you'd like to siphon contains dirt, rocks, crockodiles... And these impurities will make your "water cable" break as soon as (or probably just a little later when...) you reach vacuum on the top, at 10m.

https://www.wolframalpha.com/input/?i=(1+atmospheres)+%2F+(d...

http://what-if.xkcd.com/143/ has more details (and a rather more fantastical situation).

Putting aside the obvious detail that the dam would be empty already if we were trying to siphon water up a full 371 feet... Your correct, the entire thread this spawned was educational, and I learned something new today which was pretty damn interesting. Interesting enough I'm not bothered by the small karma loss my initial comment incurred.

Why not use pumps instead of syphoning?

Actually though, you could presumably cut holes in the dam as you go. So siphon off the top 30ft, then cut a hole at 341 feet and siphon through there.

Dig an alternate channel.

I'm not sure it would be quite so simple. The article mentions the reason that they can't just open one gate is because the asymmetric draining would be harmful to the dam. It's possible that any sort of adhoc draining would adversely affect the dam. Not to mention quickly building a secure tunnel to handle the water pressure at that depth sounds like no mean feat.

That's called a spillway BTW and the problem with spillways is erosion and the energy of the water. So spillways have to be made of reinforced concrete or similarly strong materials when the local rock isn't hard enough and they have to be shaped to dissipate the energy.

The spillway of the Mosul dam is made of concrete the problem with it appears to be that it allows too high a water level for the dam in its current shape even when fully open.

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