Water even very fast under huge pressure water freezes basicly instantly when leaving the warm inside to the cold outside.
Picture if you will a mountain with a massive empty hole in the middle of it. Like any good dwarf would do, it was filled with water in case something needed to be watered. The size was on the order of 10 in radius and 15z levels, the bottom had a 16 (4x4) wide opening that fed into a 4 wide flood tunnel. Floodgates held the water back at the bottom and the inflow at the top was a presure plate controling a set of 15 flood gates that were holding back an aquifer that was oddly enough located at the top of the mountain. Secondardy flood gates prior to the pressure control gates were under lever control for safety and maintance.
Anyway the flood tunnel lead to the side of a rather sizable cliff face where it would dump into a open plain, normaly flooding goblins, because it's aways nice to wash goblins before picking them over for loot, and fighting while wet sucks.
The flood tunnel crossed a mine access tunnel a few squares back from the cliff face, I had floor grates installed in an 4x4 drain grid so I could flood at any time and not stop my mining operation. (The flood tunnel and mine access tunnel met, at the crossing the grates dropped the water down one Z level and spit it out the side of the cliff).
For safety I only use floodgates because I like the style of massive gates snapping shut trapping the slow on the other side, and because unlike doors stupid people can't open them and flood new areas. I also like to build a level of automation into my forts.
This fort had a multi level drainage system that drained different parts of the fort as pressure plates activated and started water wheels driving drainage pumps. Also the very lowest level drained into a massive dwarf made lake as a final back up if all other automation failed. I think the lake was 5 Z levels deep and about 1/4 of the map. This lake had floodgate controled drainage to a bottomless pit, but also a number of controled pump stacks that would pull water from the bottom level and transport it to 3 Z levels above the surface and just dump it. While not part of the story the massive lake was also connected to the same top of the mountain aquifer, but was ran through a set of controlable* pressure reducers to head off any flooding. (* controlable because what if I wanted to flood my fort from the bottom up?)
Anyway on the plain I had finished some walls ment to extend the water flow and incress the distance traveled to get to the fort's main gate. I wanted to test the flow rate of the water and I also wanted to even out the main flood chambers walls with some selective digging. So I was going to compleat a two part test, one to messure the flow rate of the water, two to test the output of water from the aquifer to see how much sustained water would flow.
I was worried about the 4x4 grid of floor grates as I had not done a full scale full flow test before.
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I open the safety and maintance flood gates at the top, the pressure plate controled filling gates are shut because the cistern is full. Next I pull the bottom flood tunnel gates, and water start to rush out.
Water flows, flows, flows turns to ice and starts to backup and flood. I can't stress enough how fast high pressure water flows between the first 'cancels dangerous ground' and when I paused it I had water in the main part of the fort.
So after taking a look at the problem I deside to:
A: close the flood tunnel gates
B: close the safety and maintance aquifer gates
C: activate the internal division floodgates
D: standby to bury our dead
Frist thing that happens, water races across the mine access level, drops a level and seals off the main crafting and dinning hall level. Automated systems seal areas and start draining water. Problem is, the drains all dump outside so they all freeze as soon as the water hits outside. So now I have half drained rooms that are floodgate locked, I do have manual levers for opening closed gates but that will have to wait because I need to shut the incoming flow off first.
The lever room is best discribed as a level dedicated to levers. It has a single access point that was sadly located beside the main stair stack but on the water side. I still had the one dwarf on the lever level that pulled the first 2 switches, however after a hard day of work he was going home and some dwarf halfway across the fort had taken up the job to pull the lever to stop the flow of water.
If that dwarf has turned around the instant I issued the order to pull the lever, I think he still migh have been too late. So water rushes into the lever level, trips a pressure plate and floodgates close around enterance; Other floodgates open to the drainage level below, still more floodgates open and water starts to power waterwheels, screw pumps start to drain the closed off lever level and the enterance were my lever man that was going home for the day is stuck. Water flows in, then gets pumped out, then freezes and the water rises.
So water slowly fills the fort, the main stair stack keeps water from flowing into the far side, but at least half the fort is under some water. The auto floodgate system sometimes closes off before a lot of water gets into an area, so that area is able to drain the water, get it backed up in the drainage system, reopen the floodgates which then pour in more water till the gates close for good with the screw pumps stopped.
All my waterwheels drain into my bottomless pit so they keep running. A few of the areas also drained into the pit, and I think those dwarfs had it the worst as areas flooded, closed, drained, opened and then reflooded.
I was thinking I was going to need my fortress in a box reboot kit, (A set number of dwarfs locked away in a self sustaining sub fort, able to repopulate a fort after any possible disaster). However when I started this test it was about mid winter.
My worry was the massive lake was going to fill up and start a bottom up flood, however it was able to handle the water and sring came while it was still 1 z level from full.
With the spring, water started pouring from every opening and pipe, frame rate crashed for a while. About halfway drained a dwarf was able to get to the lever room and stop the flow of water. It was touch and go for a while as a few dwarfs raged over dead friends, and lack of sleep/food/drink. More than a few learned to swim.
The flood tunnel was moved down a level and walled off, access control by lever flood gate. The enterance to the lever room was moved. Most of the drainage systems were reworked to create mushroom like drainage covers so the water wouldn't freeze instantly. Others were routed into the pit.
So remember water freezes, fast. Not like I thought it did. Face palm, tears of joy and fear.
Oh and I'm glad no tantruming dwarf set off the 'enemy' deep inside the fort, the fort must be lost, flood the place with magma pressure plate.