Not that destroying infrastructure and killing the power to a city of civilians, despite ISIS control, is necessarily a good idea, but could they not just cut the lines?
Do you know what happens when you run a turbine with nowhere to put the load? 750MW is enough to boil something like a kiloton of water every hour, that energy needs to go somewhere.
A dam should have something like that....
Drilling a second tunnel just to let water out is probably not considered economical.
I don't know if the control gates can drain the reservoir completely or they have some minimum level, but the article implies that there is some action that the Iraqi government could take:
> Last spring, the Iraqi government prepared by lowering the maximum water level in the reservoir, to ease severe pressure on the dam wall. This year, such a precaution could dramatically lessen the number of people at risk—to about three hundred and sixty-four thousand.
What isn't clear is whether the water level right now could be lowered immediately or the water level now is at a seasonal low point and the Iraqi government hasn't announced whether they'll spill water in the spring to keep it low.
Economical relative to what? The projected losses if half the populated area of the country is flooded?
When a generator is disconnected from the power system without first removing the power from the shaft then power in is greater than power out and the result is an acceleration of the generator. Generators are built to be able to withstand a certain maximum rotational speed without flying apart for a certain duration of time before the bearings get too hot. Of course it can operate at the normal speed forever, but the rated over speed might be twice as fast for 10 minutes.
So when the power lines are cut the turbines and generators speed up until the flow of water is stopped. Pictures of the Mosul dam show four surge tanks, one for each turbine, so they can probably turn off the water in 5-10s.
Pelton turbines are used for higher head and the flow can be maintained without turning the turbine by deflecting the water away from the runner.
I've commissioned about a dozen hydro electric power plants and opening the breaker to simulate cutting the transmission line while making 10, 25, 50,75,100% of the power is standard procedure.
Moreover, if the dam is indeed that easy to destabilize, letting ISIS control the load seems equivalent to handing them a big red "trigger flood" button for use at their discretion. Sure, right now it's in their interest to keep the dam intact - because they still control many parts of Mossul and need the electricity. But "scorched earth" is not exactly a new concept and if the operation in Mossul continues as planned and ISIS is driven out, they could decide to cut the lines on their end to deliberately cause mechanical failures.
The mosul dam is 113 m high and the article said they were operating the dam with the water level 30m below the normal full level. Probably the sill of the spillway is only 10 or less below the normal operating level, because if the dam isn't full why would you want to spill.
The control gates at the bottom of the dam could have been primarily for allowing sediment to be sluice out from behind the dam or to allow the reservoir to be drained.
There is no guarantee the two control gates could pass as much flow as the turbines. Who knows how they were sized.
As I tried to explain before, cutting the power lines shouldn't damage any equipment. It effectively happens all the time due to natural causes, be they tree branches falling on power lines, birds, squirrels, etc.
Flow through an orifice (sluice gate, control gate, turbine, etc) varies with the square root of the pressure of water (the head), so a side effect of operating the dam with 30 m less head (30% less!) is that the turbines would be operating outside of their normal range and the control gates would pass less flow than they would at the nominal head.
There was a terrible accident at a dam in russia that was caused by operating the turbine outside of the normal range of pressure, and poor maintenance (loose bolts holding the turbine down...):
https://en.wikipedia.org/wiki/2009_Sayano%E2%80%93Shushenska...
Now you've got a dam full of boiling water. And as you'll know from cleaning dishes, boiling water is far better at dissolving minerals than is cold water.
I couldn't find the formulas to calculate this myself, but given Tuna-Fish's comment above that it's enough to boil a kiloton of water (1000 m^3) ever hour that's 1K/8B =~ 0.00001%.
So it's going to be more on the order of dropping a single drop of boiling water into a backyard pool every hour, is it not? I.e. something that wouldn't change the overall temperature of the reservoir to any significant degree.
This is all assuming that you'd have to use the energy to heat the reservoir itself, you could just as easily use it to pump out some water and boil it until it evaporates, not impacting the temperature of the reservoir at all, just emitting a lot of steam into the air.
If this is a concern, why isn't there a worry that ISIS or just a stray bomb would kill the power lines when retreating from Mosul? Though I suppose they could just target the dam itself...
The load in the circuit acts as a brake. If you kill the circuit, the turbine spins faster and faster until it breaks into pieces. To prevent that, the control systems stop the water from flowing into the turbine, but stopping the water from draining is the opposite of what they want.
Next time you are near a dam look around and you will find that most have three paths for the water to leave the reservoir. The first is through the power generating turbines, second is the sluice gates which control water level, and third is the emergency spill way which is designed to fend off catastrophic failure.