40d:Flood

Flooding generally refers to water getting loose and spreading out over an area, usually in an unregulated fashion. While this can be useful in creating plots of muddy ground for farming, deliberately setting any amount of water loose in an area is risky at best, and at worst, will result in lots of fun.

As in real life, water will seek to spread out given open space. In Dwarf Fortress, this means that water will spread until all tiles achieve a depth of 1/7, or it fills the space it is enclosed in. Water at a depth of 1/7 will eventually evaporate, leaving behind a muddy floor. Because of this, it would be nearly impossible to ruin a fortress with water from finite sources such as a murky pool. Note, however, that rivers, brooks, and oceans are considered to have offscreen water sources; their supply has been observed to be infinite, as has that of aquifers. Tapping into one of these without some means of regulating the flow is almost certain to result in fun.

Water movement is a major source of lag in the game.

Flood speed is counterintuitive. For example, if you build a ramp beneath the bottom of an underground pool connected to a large drainage shaft leading to a chasm, all the water in the upper levels of the pool will drain down into the drainage shaft leading all the way out into the chasm, filling it all nearly instantly 7/7 with water and submerging your hapless dorfs however far away in the shaft they may be. But once the space is filled, the flood slows down to the glacial pace at which water usually moves down tunnels, giving them plenty of time to drown.

How to flood a fortress

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So all your friends on the forums are recounting tales of the fortresses they've flooded, and you're feeling left out. Fret no more! In a few simple steps, you too can join the ranks of the hydrologically challenged.

The key is to tap into a renewable source of water, such as a river, and use its water pressure to overcome obstacles like distance or altitude. Remember that water always "wants" to rise back up to the same Z-level from which it originated!

Side view:

▒≈≈river≈≈ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒o <--well ▒▒▒▒▒▒▒▒▒▒ (well is optional)
▒▒▒▒▒▒▒▒▒▒   channel   ▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ your fortress ▒

In this design, water will travel from the river through the channel, then up through the well and across and down into the rest of the fortress. For convenience, the vertical openings don't even need to be completely hollow chambers--ramps and stairs will work just as well!

If some of the levels that you need to flood are higher than the surface of the river, you can use screw pumps to compensate.

                 ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
pumps-->  %%_  ▒▒▒▒▒▒▒o <--well ▒▒▒▒▒▒▒▒▒▒ (well is optional)
        ▒_%% ▒ ▒▒▒▒▒▒▒ ▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
          %% ▒ channel ▒▒▒ your fortress 
≈≈river≈≈ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒

Ordinarily, this above-river fortress would have been unfloodable, but with judicious application of pumps, the water can flow from a higher level than its original source, allowing it to engulf all of your precious bedrooms, workshops, and stockpiles.

Common pitfalls

Building the well above the level of the river:

                 ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
               ▒▒▒▒▒▒▒o <--well ▒▒▒▒▒▒▒▒▒▒ (well is optional)
▒≈≈river≈≈ ▒▒▒▒▒▒▒▒▒▒▒ ▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒ ▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒  channel    ▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ your fortress ▒

In this failed flooding attempt, the water moves through the channel but only rises to the level immediately below the well. This means that the well will be usable and the fortress remains safe and dry. Better luck next time!

Re-pressurization with pumps:

▒≈≈river≈≈ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒o <--well ▒▒▒▒▒▒▒▒▒▒ (well is optional)
▒▒▒▒▒▒▒▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒ ▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒ %% <--pump  ▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ your fortress ▒
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒               ▒

Here the reckless placement of a pump blocks water from flowing directly through; instead, the water level is again "reset" to the same level as the pump, and the well will be safe to use. Our condolences to this fort's pathetically dry dwarves.