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[Graebeard]

Ok, we could pump the water off the map or let it evaporate wastefully once you're done with it, but what's the environmentally-minded dorf to do?

Recycle!

If you have a drowning chamber and reservoir then all you really need to do is move water between them.  This can be done with minimal waste and evaporation while retaining maximal lethality.

Make your drowning chamber (level z=0) with lots of lovely floor grates.  Mine out the area beneath (z-1).  You're going for a 2 level effect here.

Dig a channel down a couple levels from the middle of z-1.  This will be the other end of a U bend.  Don't think that just because it's in the floor that it'll be the drain.  Au contraire!  This access tube will allow pressurized water to fly instantaneously into your drowning chamber from your reservoir several levels above.

Next, go ahead and make that reservoir.  If your drowning chamber is on the ground floor, do not fear!  Take this opportunity to consider your future with large above-ground projects.  Be sure your reservoir has a footprint at least as large as your drowning chamber, and make it at least 2 z levels high.  Remember: the bottom of this must be above your drowning chamber for the water to be properly pressurized.

Once you have your reservoir (or know where it will be) shove a tube into the bottom that goes down, goes horizontal, and then connects with the tube at the bottom of your 2-level drowning chamber.  Cover the hole in the bottom of the reservoir with a hatch linked to a conveniently placed lever.

OK, so now we've got a reservoir full of water that will fall and spout back up in your drowning chamber.  This will happen satisfyingly quickly.  But how do we get the water out?  Why, that's right!  PUMPS!

No need to expose your machinery to the guest-filled drowning chamber.  Just dig a few (only really need 1, but more is faster) diagonal holes to depressurize z-1, and put a stack of pumps tall enough to reach the top of your reservoir to draw from each one.  For extra points, have these powered, set to turn off when the switch is thrown to open the reservoir-hatch and to re-activate when you close the hatch back down.

Voila!  You now have an fast-acting murder device with no horizontal flow to worry about, no fortifications to push things through, and, best of all, it's ENVIRONMENTALLY FRIENDLY!  You can now continue to scour the landscape, defile the depths of the earth, and ruthlessly murder the creatures of the world knowing you've done your part for the environment.

[kujpat]

You can now continue to scour the landscape, defile the depths of the earth, and ruthlessly murder the creatures of the world knowing you've done your part for the environment.

I mayyy just have to sig that

[Graebeard]

You can now continue to scour the landscape, defile the depths of the earth, and ruthlessly murder the creatures of the world knowing you've done your part for the environment.

I mayyy just have to sig that

\o/

[Panando]

I've experimented with drowning spiral "staircases" (spiral ramps) - that is - a 1z wide path winding around an open interior. In the bottom, is a retracting bridge. Under the retracting bridge, are grates.

I think this is quite a good design. The trap is filled from above. Once everything is drowned, it is quickly drained by opening the bottom bridge, since water pressure teleports the water out. The water quickly flows down the drain, and as it flows off the spiral path, it takes items with it, and most of the items will end up caught in the grates. Then the drain is closed. Now here's the cool part, all those items are on the grates. The dwarves can now come in and grab the loot, even while the trap is drowning the next batch of victims.

The big benefit of a vertical spiral design, over a flat design, is that a flat design requires a lot of separate drains to drain quickly. With a vertical spiral path, all levels of the trap are serviced by a single drain. You can make the path in the spiral design very long, allowing whole sieges to be drowned, and still, all you need is that one bridge to drain the whole thing in moments. Quick cycle time, and easy recovery of loot. Being tall isn't a problem, particularly if the bottom level is right at your magma smelters for quick and easy recycling of loot .

Now this is all for a dedicated drowning trap. Obviously, if the idea is to have a section of the main entrance which can be flooded on command, the design has to be different, because you don't want to force your traffic along a long path. Still pretty easy though. Have the path dip down 1z, use ramps at each end - with the dip being the drowning chamber. Build retracting bridges in the space over those ramps. Dig downstairs into the floor of the chamber, and bridge over them (or bridge over open space along the edge of the path, if you want the system to flush items out of the trap to be caught on grates and recovered while the trap is re-used), the drains should stretch nearly the entire length of the chamber, because long bridges are cheap, so why not? All the bridges can be linked to one lever, and also a raising bridge holding back the water can be linked to the same lever. Pulling that one lever causes all the bridges to close, sealing the entrance, exit and drain, and the retracting bridge lets the water flood in. Pulling the lever again opens the drain, entrance and exit, and seals the water input, the water will drain very quickly. I like my traps to be 1-lever affairs when possible.

[flieroflight]

What about using pressure plates so that you can cause an automatic variant, with lever backup for when trapavoid enemies arrive?

I ussed a variant of the retracting bridge trap, with pressure plates that cause floodgates to open along one side of the pit, and the pressure plates are linked in such a manner that when no creatures are present it triggers the drainage sluice at the other side.

[HraTaika]

Very interesting suggestions everybody! Been designing vigoriously some chamber variations as I had to scrap most of the designs due to not taking into account the need for several z-levels of water, nor the actual time taken to drain the water, was under the impression that it would be relatively fast.

I prefer bridges to atom-smash the water in the drainage area if I don't have an aquifer to absorb the water. (Also, any square at lake level will absorb water dropped from above, if you're near a lake.) With a little planning, you can put a water-activated pressure plate in the draining area to control the water smashing bridge and it'll be fully automated with unlimited draining capacity.

The atomsmashing of the water seems like interesting idea, but does it work? Probably should cause water is no different from an item yes? Also the fact that it is automated is a big plus. How fast in your experience is the drainage this way?

I've experimented with drowning spiral "staircases" (spiral ramps) - that is - a 1z wide path winding around an open interior. In the bottom, is a retracting bridge. Under the retracting bridge, are grates.

Vertical trap design? .. this opens up A LOT of new ideas! Brilliant!

[Viking]

You can indeed atomsmash water and presumably lava. There is a repeater design based in part on this mechanic actually. Look at the fluid logic repeater: http://dwarffortresswiki.org/index.php/Repeater

[Reese]

for fast operation, you really need to take advantage of the natural properties of liquids

what I like to use is grates and deep pits., basically like this:

█      ╬_%% █
█######█ %%_█
█      █_%% █
█######█ %%_█
█╞════╡█_%% █
█      █ %%_█
█      █_%% █
█      █ %% █
█        ████
█████████████


Now, so far as I know, building destroyers don't deconstruct grates from above or below, so what you do is have the bridge below both sets of grates closed, activate the pumps, the area between the two layers of grates will fill with water from the lower reservoir.  When you need to drain the trap, turn off the pumps and open the bridge, gravity will cause all the water to fall down into the reservoir in a few tens of ticks (water seems to test for gravity every 3-4 ticks)

the only thing to watch for is that you need a supporting tile touching at least one edge of each grate, but you can tesselate the tiles to maximize the number of grates like so:

Spoiler (click to show/hide)

##█###█#
█###█###
##█###█#

or

█####█
##█###
####█#
#█####
###█##
█####█

or just make the drowning chamber 2 tiles wide...

[Snaake]

What about trolls? Recently, some trolls showed up with a goblin siege, proceeded to break the floodgates I was using for my water moat drainage control (the moat was only half-full), and then got flushed by the escaping water down the drain, which ended in a channel dug into an aquifer tile. The 4 trolls were stuck in the 7/7 water, surrounded by aquifer tiles, but didn't seem any worse for wear. As in, I waited for a while, but they didn't drown. They didn't path out, because there was no path out. In the end, I killed them by dropping some boulders on them from a couple z up, and am going to build a pump over each square to remove their items&corpses from my aquifer.

So basically, does drowning work on trolls? Do your plans take into account possible amphibious building destroyer mounts (giant toads?), since these presumably follow goblin pathing, not building destroyer pathing, and so they can't be lured away with juicy furniture? Do building destroyers destroy grates/bridges from above, and if so, are there any countermeasures?

I've been considering a drowning entrance, but would probably have a pump or few empty from above, since that at least should be safe from building destroyers (I do know they won't destroy grates from below, if they can't path to it).

[Urist McSpike]

Do your plans take into account possible amphibious building destroyer mounts (giant toads?), since these presumably follow goblin pathing, not building destroyer pathing, and so they can't be lured away with juicy furniture? Do building destroyers destroy grates/bridges from above, and if so, are there any countermeasures?

First, if you look at the first picture of my layout, I keep the floodgate behind a fortification, to keep anything from getting to (or through) it.  The bridges are safe, as building destroyers don't go after bridges - but they will rip up the grates.  As for amphibious critters, hmm...  I guess my design would limit them - the very bottom (below the grates) of my draining pit is walled off from the actual room by lots of fortifications, and the top portion is sealed with a bridge.  I honestly don't even need the grates, as they're just there to keep the debris out of the water.  I guess if I needed to, I'd let the water drain off, as things play with the grates, and I'd be able to position my military before opening that area up.

I don't think trolls are amphibious, at least from looking at the raws on the wiki - maybe it was something with the aquifer tile or something.  I'm thinking I've drowned trolls before, but I'm not positive about that.

[Panando]

What about trolls?

Okay, building destroyers.

They can't destroy bridges. Prefer a raising bridge to a floodgate [although it operates in an opposite state], or a retracting bridge to a hatch [although unlike a hatch it has a delay].
They also can't destroy constructions. Fortifications are a construction. If you need to use a floodgate, or if you want to turn on a pump to deliver liquid into the trap, then protect the floodgate or pump with a fortification.
They also generally can't destroy hatches from below, I think they can if they can path through it (i.e. it's unlocked and has a staircase or ramp), but a lever-operated hatch should be indestructible from below. Although bridges are a better grade of indestructible. Grates are also indestructible from below.

Basically if your design exclusively uses bridges (as mine do) you wont have a problem with building destroyers. (although I normally put a fortification in front of the raising bridge "floodgate" just in case it's ever opened while the water is off).

The other possible hiccup is creatures so large that they jam bridges. The trick here, is use retracting bridges over open space. A retracting bridge closing over open space cannot be jammed, because nothing can stand in the way. So retracting bridges over ramps are very very reliable. If something massive stands on the retracting bridge, I believe it wont be able to re-open. So if your trap relies on retracting bridges over open space, then massive invaders will be able to jam it in the "lethal" state, but wont be able to jam it in the "safe" state.