How do I revert the flood?

Question

I accidentally flooded levels 42, 41, and 40 of my Fortress, and I don't know how to stop this mess. No dwarf is going to build a wall, a floodgate, a channel, or a floor where there is water, even if it is just 1 level of water. So I don't see an obvious way of shutting off the stairs and limiting the flooding to the original level (42).

If I can manage to limit the flood to level 42, then it will be much, much easier to tackle. Which is a good way to stop this flooding?

Version: 0.50 (Steam).

Details:

One dumb Miner did not notice that the wall was damp, struck the Pickaxe there, and released all of one underground river's destructive power into my Fortress:

The water is going downstairs and flooding everything else below it:

Elevation 42

Elevation 41

Elevation 40

Update:

Elevation 43. I cannot channel on the border of the map, so CAVE-IN, is not an option.

Answer 1

As this is underground you should be able to put a plug close to where the initial breach of the underground river happened. You are doing a slightly simplified version of what is described in the wiki article "Aquifer", section "Exploiting Cave-Ins". You dig around a plug that consists of several fields of solid rock and then let these drop from above into the channel north of the river. This works because on the top level of the underground river, you can safely access the water from above.

If this is successful you only have a finite amount of water left in your fortress. You can either spread it out over a large enough area that it will eventually evaporate (note that this will leave soil where underground plants can grow) or you can drain it into some hole where it doesn't disturb you.

Edit: More details on the plug. The actual plug will be a chunck of rock in its original state. For a plug a 1x1x1 block is enough, for aquifier piercing you need something bigger. To make the plug fall into the river you need to dig out every block around it in all three dimensions. So you will dig out a 3x3x3 segment of rock minus the central cube which will form your actual plug. The central block needs to be directly above the river and will fall (at least) two levels when you set it loose. For the two higher levels you first mine them out and the channel into the ground to free the plug on all sides. You need to plan ahead a bit so no dwarf gets injured from falling, gets stuck in the hole or has the plug fall on their head.

Edit 2: per qazmlpok's comment it is probably enough to dig out only the directly adjacent blocks to your plug (in all 6 cardinal direction). This requires a less space but the general idea is the same.

Answer 2

Disclaimer: Much of this is theoretical, as I've never had to actually deal with a massive flooding event. Some of my knowledge is also out of date; I've never run into an underground river before. I'm assuming that they function like an overground river or an aquifer, i.e. they produce an infinite amount of unpressurized water.

The first step is to seal off the water as much as you can. Forbid any doors that are in its path - doors are waterproof until they're opened. Forbidden doors are locked and can't be opened, so they'll act as a seal. Just make sure you don't lock anyone in.

You should be able to place constructions up to a depth of 3 units. It looks like the water is slowly expanding, so you should be able to still add doors to 1-2 depth areas. If you have narrow corridors, place doors in them to block the water and forbid them. Make two layers, just in case something goes wrong. If not, then just use walls. Doors are suggested simply because they're lighter than raw stone and usually plentiful.

There are two ways to remove water:

1. Pump it back into a source of water (I can't guarantee this works for an underground river; it will not work for a light aquifer or for any finite source of water)
2. Let it evaporate. Water at 1/7 depth will eventually disappear on its own. If you seal off the breach, eventually all the water will settle and evaporate from the top. This won't remove any water that has pooled into lower areas, but you can use pumps to pull this water out into a wider area where it can evaporate.

To pump the water out:

1. Excavate the area directly above the breach: You need a channel over the breached area you want to reclaim, and another channel over the underground river.
2. For maximum safety, place floor grates over both channeled areas, and add doors to ensure water cannot escape. Water can travel through grates, even through a pump, but dwarves cannot. Just in case I'm wrong about how pressurized water works here.
3. Build a pump over the breached area. Have it set up so the pumped water follows a narrow corridor into the other channeled area - in other words, pump the water back into the river.
4. An infinite source of water also acts as an infinite reservoir, so this should result in the pumped water being removed. Pumps pump faster than unpressurized water travels, so this will prevent any further water coming out, and will pick up the water that had already escaped.
5. Once the water drains and goes under 3/7 past the pump, you should be able to install doors, actual flood gates, or walls just past the pumped area, and thus permanently seal it off, at the source.

This will not recover any flooded lower areas, but you can just install a pump on top of another channeled area and pump it into a larger area. This water will then settle to 1/7 and evaporate. Slowly, but it's going to be easier than trying to work it back into the underground river.

Answer 3

1. Do not load the most recent active save;
2. Load an older autosave that is prior to the flood;
3. Be happy.