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[betaking]

In my Game the first cavern layer is flooded.

this wouldn't be so bad in theory but in practice it's incredibly obnoxious, I've have at least 10-15 dwarves who've drowned through either forgotten beast attacks, or by misadventures with screw pumps, and I'm starting to both run out of legendary fish dissectors and get sick of the water.

how can I drain the water out of the cavern, it's only 7u deep but it's flowing from the other side of the map, and the screw pumps seem to create a lot of dangerous backflow that kills my dwarves or gets them to rather quickly stop pumping.

part of the problem is that the dwarves cannot be relied upon to know how to swim, or to get themselves out of 7/7 water even with a ramp.

[Starver]

I've never seen the situation you're describing (unless I engineered it myself, in which case I've probably prepared ahead of time), and draining what is clearly a refilling system is going to be a bit troublesome.

But what I'd try to do is to edge-drain (set up fortifications in edge-tiles, a number of them in small bunches, you can perfect the design for each new iteration - and always arrange this with all due flood-friendly pre-planning/control[1]) or perhaps find a lower aquifer to permasink into.

Hard to be definitive from what little I know, and it could be that your pump philosophy is the right idea, too, but not quite applied correctly.

(If it's not replenishing water, you could also try a massively multi-well draining project. Mandwarfpower-intensive, but visually would be stunning. Hopefully not dwarf-stunning, but I rarely have well-accidents so I'm not sure if I'm just naturally averting the usual reported problems.)


[1] Easier to show than explain, but I can't do it and screenshot it right now, so Explain it has to be...  Dig right up to 'breakthrough' but not yet, along a length. Set up floodgates along that final wall. One option is to have two levers, each controling alternate floodgates. Set up a 'chequerboard' of pillars in the space this side of them, for pressure-anulling purposes, open the gates to that continue that pattern onto the wet wall. Set to Dig/Ramp/Smooth-And-Fortify one of the gaps (whatever process you want to use to breach, but I recommend the SAF one for several reasons) and as the dwarf is starting the breach unpull the lever. The floodgate will not close until he moves back, either when taking a break (in which case re-pull to let the job restart), he's finished and moving away or he's finished and gets swept by water. The closed floodgate secures the area. Build a new retaining wall in his direct escape route(s) so you can repull that floodgates-lever to try the next(-but-one) along. Repeat-and-rinse... When you've got those done, clear the damp-dampening walls out, prep the wide-enough drain from this area, remove any remaining debris, get out, seal up, open both sets of floodgates... Experience FPS drop, but (with luck) also that of the water-level. No dwarves need to be drowned (or FBed) in the process. Always possible that errors or other adverse factors will (possibly because I forgot some detail of the process, which I've also found useful for magma-tapping but with a slight chance of burns if I'm in a rush where wet feet wouldn't be an issue in this case).

[PatrikLundell]

I've never seen or heard of anything like that, except for reclaimed fortresses in Ocean biomes.

However, if you've got a 7 Z level deep body of water that's replenished all along the way I'd use my standard recipe: Magma.
No, it's not a stupid meme iteration but a literally useful application of magma, although that much is going to result in a lot of work.

If you're lucky enough to have SOME air above the water along some of the open cavern edges, build a "floor" out of downstairs at the highest level, adjacent to the opening, and then another one out of Up stairs at the level of just above the water. Make one or more magma safe minecarts, and build track stops dumping towards the edge on each tile of the top level floor.  Build a magma filling station for mine carts, fill carts and assign the carts to the track stops that dump onto the tiles with the deepest water. When the magma hits the water it turns into obsidian and sinks to the bottom, fusing with the wall there, while, at the same time, causing the displaced water to splosh onto the lower "floor", possibly pushing whatever is there into the water. Repeating this again and again should allow you to slowly build a wall that closes the gap in the wall, blocking any water from entering through that opening. Once the water is blocked you can just build a regular wall to close out the rest, which I'd do by building up/down stairs between the upper and lower "floor" to reach the whole area. Once the water is blocked off, it's a matter of pumping the water away, which can be done either by legitimate edge drains or a "deployable drain", i.e. pump the water into a fully enclosed tile containing a minecart on a track stop set to dump into any of the walls (this will magically cause water to disappear to only leave a constant 5/7 amount of water in the tile).

[vp]

I recommend massive cave-ins along map edges.

[Eric Blank]

Caving in solid unmined walls from the roof would be the easy way to create a barrier to refilling the center of the map. Then you could drain the center down and off the map edge through fortifications. That's the easy route. No pumps, just tap a hole in the side of a pillar you hollow out once you cut off the map edge.

[betaking]

thanks everyone, I had built a massive dwarf-water-reactor+pump-wall and that kind of worked for a little bit and then I found a situation where it made the "number of drowned" worse such that I quit..

magma it is.

[PatrikLundell]

Controlled cave-ins can indeed be very useful, but if dealing with 7 z levels it might be hard to find that many untouched levels above the sites you want the walls.

[gchristopher]

When the magma hits the water it turns into obsidian and sinks to the bottom, fusing with the wall there,

This would also be the first thing I'd try, but when using that approach to filling in ocean areas, I have run into problems where the newly formed obsidian wall attaches to adjacent walls horizontally instead of sinking as desired. That might happen here.

Before embarking on this particular megaproject, try backing up your save and using the dfhack liquid command to drop 2/7 magma in the space above the water in a few places to get a feel for the right sequence of magma drops that will build the walls you want. That experimentation might save you many hours of frustration when going back and actually doing it using dwarves and minecarts.

On the other hand, there's all new wonderful kinds of chaos to experience with magma and water and cave-ins all in the same project! (Certainly a giant obsidian tower in the ocean felt more dwarven surrounded by the mangled burned drowned corpses of its builders!)

[PatrikLundell]

When the magma hits the water it turns into obsidian and sinks to the bottom, fusing with the wall there,

This would also be the first thing I'd try, but when using that approach to filling in ocean areas, I have run into problems where the newly formed obsidian wall attaches to adjacent walls horizontally instead of sinking as desired. That might happen here.
:

Yes, you're correct (and I forgot to bring that part up). This method only works on the tiles that are not adjacent to a wall.

However, even closing off the top level only is useful in that it allows you to pump/drain out the water from that level. You'd then get a new top level that you can close off. If you start with "proper" wall building everywhere except at the edges of an opening the amount of work you then have to finalize each level is limited.

It's also possible to reverse the logic to build a top water level wall along the edge by building from the sides (still using obsidianization, not "real" building), pump out the water from that level, repeat the process one level further in from the edge (as you need free space above the tile to be obsidianized), pump, repeat. Note that you'd have to remove the "floor" just above the water level to free up the space for magma dumping. I'd wall up the external above water section once the water level obsidianization is done to block entering monsters to spawn on the ledge, at the cost of having to build a new floor from which to dump magma. Once the second wall is built the floor again has to be removed, but for the subsequent levels the top of the obsidian wall can be used.

[betaking]

I think I made a mistake or mispoke, I meant there was 1 z level that had these wide mouths at map edge with water a full z level deep.

 I'm going to try a combination of things including magma if I can find it. I will post pics here soon

[PatrikLundell]

I think I made a mistake or mispoke, I meant there was 1 z level that had these wide mouths at map edge with water a full z level deep.

 I'm going to try a combination of things including magma if I can find it. I will post pics here soon

For the normal 1-2 Z level depth I use magma shuffled in mine carts. I've only encountered a cavern covered by water once, but since that was a pain to deal with I've changed the parameters for cavern water in my worlds.

[Starver]

I'd personally still go with my draining idea (maybe as many single-tile taps into the side of every edge-sourced channel as that edge-source is wide), the chamber preparing this then leading straight into as many edge-fortification drains).

Once all sources are matched with drains, then I could send in regular builders up to as close to the splaying edge-water as I can (prevent them from trying to wade through the 1..4ish-depth water) to put up normal walls to dam the normal flow and stop the drains (hitting your FPS) to leave mere edge-ponds.

With a bit of extra building (ensuring you build in the right order and with the usual "build this from here, not the other side!" precautions) you can make a (muddy-)floor-to-ceiling wall, with or without drawbridge doorways on every edge-opening and claim nearly all the caverns for various fortress uses such as you may desire.



...or, because this is DF... Yeah, magma.

[betaking]

maybe multi-Z-Level Drains will play a role.. who knows though.