My hospital needed a water source. I decided to try and get a pool of water that would be exactly 3 deep (actually for a water source alone, that isn't too helpful; but at least I can re-use this design for a pool elsewhere where I want it to be 3 or 4 deep for bathing in!) The mechanism is: fill up 9 tiles with 7/7 water, then drop to water into a pit of 21 tiles. Water should even out to exactly 3/7 per tile. I will refer to drawbridges here as floodgates, as they are basically being used as floodgates.
Here's my tapping of the river:
The main floodgate is operated by a lever nearby to the northwest. The channel goes down a few levels until it gets alongside the hospital after being depressurized:
Below, a floodgate will keep the pool of water to 21 tiles exactly, and then let the water be flushed out off the west edge of the map:
The main floodgate is opened and the channel fills up nicely:
Now fully filled up to 7/7, we're ready to close up the 9x9 section with another floodgate:
Why does a dusting of mud have to look identical to blood?:
The moment of truth; we open the retractable drawbridge to fill the water pit below:
Yay, it works! All tiles have exactly level 3 water on them:
Only problem is the flushing mechanism didn't quite work as I'd hoped:
It took months for the remaining water to evaporate away so I could fill the pit again:
So... can anyone think of a good way to get the water flushed out of there quickly? Even a screw pump only sucks water out of one tile so I don't think it would work properly on a 21-tile area. My first thought is to make the base of the pool a big retractable bridge that overhangs a sewerage channel that flows from the river round the fortress, but that would be a major undertaking and I have tons of other stuff to do.