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40d:Water pressure
Dwarf Fortress attempts to replicate hydrostatic water pressure.
There are, however, other mechanisms at work too that try to simulate water pressure, so never think you are on the safe side, especially when trying to trick the game.
Hydrostatic water pressure
In layman's terms, if you have a tall body of water (such as a lake or a tower you have pumped full of water) then the water at the lowest z-level is being 'squeezed' by the weight of the water above it. If it is released it will move with remarkable speed and can flow up stairs, ramps, and over channels. It will continue flowing until it runs out of space or it fills to a z-level one level lower than the source, as stated by Toady. This does not exactly match natural hydrostatic water pressure. Also, aditional mechanisms pick up from there and usually push the water up well back to the offspring level, especially when there is a continuous feed of water.
As another model, think of a pipe shaped like a "J". If you pour water in the taller end of the pipe, it will come "up" out of the lower end until the water levels on both sides are equal. Until all the water in the tall part is at the same level as the lower part, water will continue to come out that lower end. This is one part of Dwarf Fortress "water pressure". For economic reasons (CPU time saving) the game stops not when both ends of the pipe are on the same level, but already one level before.
Dwarf Fortress does not model surface friction nor air pressure, so the water will not slow in transit nor will 'trapped air bubbles' form. Magma does not have pressure (it cannot flow up, and doesn't appear to move at greater speeds).
Pumps do not create this type of pressure - they simply pull liquid from one end and dump it out at the other. However, the liquid dumped out at the other end does force any other liquids out of the way, potentially up additional Z-levels (though not past the top of the pump), whether the liquid is water or magma.
Dangers
It is easy to flood your fortress accidentally by not accounting for water pressure. For example:
- It is safe to dig out a cistern one level below a murky pool, and to channel above a few tiles of the cistern so that your dwarves can get water from it without having to go outside.
- It is safe to refill a murky pool with water from a pump or brook/river/etc on the same level.
- It is not safe to do both to the same pool! The water from the pump/brook/river/whatever will fill the pool to 7/7, and will then pressurize the water in the cistern, which will then flow up out of your channels and flood your fort.
Waterfalls
Waterfalls are of special concern. If you tap river section downstream from a waterfall, the water will be under additional pressure as it is coming from above the river's surface. It is absolutely critical to reduce the pressure in such a system if you do not wish flooding, the easiest way being diverting the water diagonally - although if used solely for a complex drowning trap or other purpose, flooding may be desirable.
Mitigating dangers
Diagonal Flow
Pressure cannot push water through diagonal gaps between tiles - instead, it will merely flow through if the water level on the other side is low enough.
Top View
###### #########≈≈≈≈######## Direction-> ≈≈≈≈#≈≈≈≈≈≈ -> of -> ≈≈≈≈≈#≈≈≈≈≈ -> Flow -> ≈≈≈≈≈≈#≈≈≈≈ -> ###########≈######### ###
# = wall, constructed or undug ≈ = pressurized water ≈ = neutral/normal water pressure
This does not work on a vertical basis - water only travels vertically to a different z-level, never diagonally.
If you wish to maintain the rate of flow after de-pressurizing, it's recommended that you have more diagonals than water tiles - that is, if the source is 3-tiles wide, you may wish 4 or more diagonal passages.
Hatches
Hatches can be placed over channels, stairs, ramps etc to prevent water moving vertically but still allow the tile to be used, even as a water source (and possibly still for fishing too).
Pumps
Water pressure does not propagate through pumps, so it is possible to fill a pool using a screw pump without it having the same pressure as its source. Of course, there is a downside - you still have to run the pumps and due to the source water's pressure, the pump must be powered instead of run by a dwarf, as the tile the dwarf needs to stand on is filled by water. Furthermore, the pump will likely need to be powered from above or below (as water would simply flow around a gear or axle placed next to the pump), though creative setups are still possible by using additional screw pumps to transmit power.
Your vertical axles or gear assemblies need to be placed above the unwalkable tile of the pump, and there must not be a channel over the walkable pump tile. (Water can only flow straight upward, not up and to the side at the same time.) Multiple adjacent pumps will also transfer power between themselves automatically.
Side view
Power Water Key
↓ ↓↓↓↓↓ # = Normal wall
######║####≈≈≈≈≈ # = Wall that pressurised water would flow into if it were to be dug out
######║#####≈≈≈≈ ≈ = Regular water
_ ___#║#######≈≈ ≈ = Pressurised water
#≈≈≈≈≈%%≈≈≈≈≈≈≈≈ %% = Pump
########≈####### ║ = Axle
################ _ = Floor
Do note that the screw pump will create additional water pressure in the destination area, but said pressure will be independent of the source and can be subsequently 'reset' by additional pumps or diagonal gaps.
Overall behavior
- If a tile contains water which is floating on top of another tile of 7/7 water (and apparently only if there is no water above it[Verify]), the water in the upper Z-level will be pushed downward and moved to the nearest orthogonal (not diagonal) tile on the lowest available Z-level, up to the Z-level just below the top. Each tile of liquid performs this check once every few steps. This type of pressure applies only to water, and is what causes large bodies of water multiple Z-levels deep to rapidly drain when opened.
- If a liquid source (river/brook source, underground river waterfall tile, map edge, or screw pump output) attempts to create liquid in its output tile but cannot due to it being full already, the liquid will be created in the nearest orthogonal (not diagonal) tile on the lowest available Z-level, up to the same Z-level as the source. This applies to both water and magma, and can be observed by damming a river.
- Liquids adjacent (both orthogonally and diagonally) to non-full tiles will flow into them and average their depths, pushing lightweight objects and creating flow (which will power water wheels) if the depth difference was 2 or more.
See [1] and [2] for more info from Toady.
Movies of pressure experiments
- [3] - Showing that pumps output 0-pressure water even from a high-pressure source, that water will not flow up and to the side at the same time (has to flow straight up), and a few other things
- [4] - Showing that pressure is not transmitted through non-7/7 tiles.
- [5] - Pump turned into infinite water generator, but still provided useful information on how overpressure causes upward flooding. The infinite water generation behavior has since been fixed.
- [6] - Uses three pumps connected to different tunnel layouts to test a few of these rules: One tunnel has three accessible z-levels. The second tunnel has one accessible z level and periodic shafts up. The third has only one accessible z level with no shafts. The bottom level of all three filled first, and the shafts did not fill until the bottom was filled. The second level of the three-high tunnel did not begin filling until the first was full. They did not all fill the bottom at once, but this is believed to be due to the order in which their pumps are placed on the river.
Possible future experiments
- Have a pump pumping water into a 3-wide tunnel with a 1-wide tunnel below it. Have another pump pumping water into a 1-wide tunnel with a 3-wide tunnel below it. Observe whether the bottom tunnel's water spreads faster in both cases or just in the smaller tunnel.
- From a pump, fill a cistern which is several levels lower. Shut off the pump and the higher level tiles with hatches once the whole thing is 7/7. Open other hatches above the cistern, combine water with unpressurized water, and see what happens.