I think the only equivalent is the floodgates, but, they only work when the water level is at a certain height, and I think I've read that if you open them to a true flood point you damage the dam.
Drilling a second tunnel just to let water out is probably not considered economical.
According to the article there are two control gates and a spillway. The control gates are used to regulate the water level and the spillway is used in an emergency.
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.
A dam should have something like that....