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[Jerm]

I've got an aboveground river that I'm interested in doing one or both of the following to:

Dam.   How do I dam a river?  I can't build walls or floodgates in it..   is there a trick that I just don't know?  Does it  have to be redirected first?

Big trapdoor under the river.  I figure'd I would reroute the river to a chasm underground (another question, if I route water into an underground chasm that is open to the map edge, will the water run off?).  I dug tunnels to route the water downward, but I'm at a loss for how to build the lever operated trap door beneath it.

Again, is there a clever way to do this?

[LordSlowpoke]

Well, you can try to pull off the Moses effect with enough windmills and screw pumps (bonus points for making a perpetual motion machine with water wheels)
And yes, if you route water to an underground chasm that has an opening to the map edge, the water will disappear.

[Zaranthan]

A setup like this should work:

Code: [Select]

~~~~ River
.... Flat land
O==O Walls
< Stairs
SS Screw pumps

~~~~.O==
~~~~SS
~~~~SS
~~~~SS
~~~~.O==
~~~~<...
Note that you'll need to power those pumps, and using a few more might make things more reliable. You'll also need to make sure that the reservoir doesn't fill up. You can either drain it off the map, fill a truly massive cistern, or just be clever and dump it back into the river bed downstream.

Alternatively, you can collapse walls into the water, but make sure the bed is stable. If you've dug out the ground under the river, a cave-in will pierce your entire fortress, which will immediately start flooding. Fast.

[Andir]

Alternatively, you can collapse walls into the water, but make sure the bed is stable.

I thought all constructions were deconstructed when caved in...

[Satarus]

Pump up some magma.  Make an obsidian cast dam.  If you got the pumping down, it's easy to make the dam.  Just build walls along the top of the river along either side of where you want the dam.  Magma gets droped down into the river making the dam and then continues along following the walls.

[Urist McDepravity]

Since we now can dig into map edges, damming may be much easier now.
Just dig into river from below and let it flow into egde.

[gtmattz]

Since we now can dig into map edges

No we cant...


As for damming the river, My preferred method is to collapse a constructed floor through the riverbottom into a dug out chamber on the z level below which directs the water to a fortification edge drain with a raising bridge to block the drain.  When you collapse the floor it should punch through the river bottom and the water will then flow down through the edge drains and let the river dry out.  Then you can get into the riverbed and construct some walls or a raising bridge or whatever to dam the flow.  After that pull the lever to block the edge drain and your river will fill back up.

[Dave Mongoose]

To build walls on the riverbed you need to have a ramp to get down there and some pumps to reduce the water level to less than 4/7. For something small like a brook, one or two screw pumps is all you need; for bigger rivers you'll want a setup like Zaranthan suggests.

If the pumping isn't quite reliable enough you'll keep getting 'Dwarves suspended construction' announcements, but so long as you keep un-suspending them they will get built eventually. Make sure the pumps are machine-powered (windmill or waterwheel), because if a dwarf is pumping and gets tired your mason will be washed away.

For the chasm question, I'm guessing you mean a cavern from the new version, not the bottomless chasms from older versions?
A cavern layer will drain water if it has an opening to the edge of the map, but I think the opening would need to be the same width or wider than the river start point... otherwise more water will be coming in than going out and the water pressure will cause it to fill up the cavern. If the opening is multiple z-levels then this probably wouldn't be a problem.

My approach for the trapdoor thing would be to dig the plumbing first without connecting it to the water and set up some floodgates linked up to a lever somewhere (make sure to close up the access used to dig this out or water may flood through it). As others have mentioned: a cave-in is probably the best way to connect the river to your tunnels, but you could also use pumps as you would for building a dam to safely channel out a section of the riverbed.

[Psieye]

While we can't dig rock at the edge of the map, can't we make fortifications out of it still? That'll let water out.

[Andir]

While we can't dig rock at the edge of the map, can't we make fortifications out of it still? That'll let water out.

Yes, as far as I'm aware, fortifications still work, but those have to be in rock... so it may involve digging down a few layers if you are in sandy or heavy soil areas.