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[Boingboingsplat]

So, I'm making a dam underground to drain off part of the caverns. However, I don't know if my magma damming level will work for areas that are two z-levels deep. Basically, this:

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From side
. = open space
O = Wall
≈ = Water
~ = Magma
▓ = Obsidian

.......~~
≈≈≈≈≈OOOO
≈≈≈OOOOOO
I know that for the single z-level deep the magma will just form and create a nice little dam for me. However, once that's said and done...

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...~~~~~~
≈≈≈▓▓OOOO
≈≈≈OOOOOO
Will the next formed block of obsidian drop down and continue my dam as I would like:

Code: [Select]

...~~~~~~
≈≈≈▓▓OOOO
≈≈▓OOOOOO
...or would the adjacent block of obsidian prevent it from caving in and falling:

Code: [Select]

...~~~~~~
≈≈▓▓▓OOOO
≈≈≈OOOOOO
I want to know before I attempt to do it, since a dam with a giant hole in it won't work very well.

[Von Krieger]

I think it makes an obsidian wall, thus it would stick to any adjacent squares that also have a wall in them.

[Rockx]

I've successfully created a 1 z-level magma dam in my fortresses. However, with 2 z-levels, I think the obsidian created on the top-level would block the magma from flowing to the bottom level.

[Grumman]

The obsidian will stick. To solve the problem, I suggest the following:
First, build the guide walls for the magma as usual, but then build a floor inside the channel that extends 1 square past the second underwater layer of rock:

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.....___..~~
≈≈≈≈≈≈≈≈OOOO
≈≈≈≈≈≈OOOOOO
Then, fill the channel as normal, and remove the excess magma. This will create a wall of obsidian supported by the guide walls and floor of the channel:

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.....___..~~
OOOOO≈≈≈OOOO
≈≈≈≈≈≈OOOOOO
Then, remove the floor and retaining walls (or just a one-wide gap, and leave the rest to deconstruct on their own). This should allow your first obsidian wall to fall to the bottom:

Code: [Select]

......__..~~
≈≈≈≈≈≈≈≈OOOO
OOOOO≈OOOOOO
Then, build a one z-level dam as normal:

Code: [Select]

..........~~
OOOOOOOOOOOO
OOOOO≈OOOOOO
This should produce a dam with only 2 one square holes in it, which should let you drain the dry side faster than it refills through those two small holes, and then use more obsidian to plug the holes by dropping magma on the dry side of the dam, once the water there is only one z-level deep.

[BigD145]

You'll need to overhang your operation and have things up at least 1 whole z-level from the surface of the water. Just set up a slow drip, or one long line of magma on bridges at depth 3ish, and any obsidian blocks formed will drop to the bottom and begin to stack.

A big dam has been done in the past week or so, I just don't remember what the topic title was called.