I've been playing DF for a while now, and I've handled water before in irrigation, artificial cisterns and wells. But I only recently got around to making a real, honest-to-Armok waterfall.
My current fort's entrance is a 3 tile wide bridge over a small river, then a road between two towers and into the fort proper. So a waterfall at the entrance seemed like an easy way to start. I built a pump stack in each tower and floor grates just behind the bridge.
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Underneath the grates and walls I channeled a path back to the natural river on both sides, but left a solid base for the drawbridge. The pump stacks expel water from 3 z levels up over the two outer grates.
So far so good, I thought. But it didn't occur to me to de-pressurize the output. The first time I turned a pump stack on, I got a 2-tile fan of solid water at the output z-level, and briefly standing blocks of 6/7 and 7/7 water down at ground level. And it moved some spare stone blocks in the roadway several tiles, which was impressive.
At this point I felt I was so close, the quickest fix would be a protective rail along the bridge to keep my dwarfs from being washed away. I went with fortifications rather than a solid wall so the water could drain away. I knew in a general way about creatures being pushed through fortifications, but I imagined that applied more to full submersion.
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After re-learning bridges don't support constructions, and you have to build in one block at a time for support, I was ready to go. (I lost a decent mason, too. I don't know how a cave-in down to water injures a dwarf left back at the top - maybe he tried to catch it when it fell?) I turned on both stacks and ordered all my military squads to move outdoors for their first annual shower, sun exposure and happy mist thought.
It should have been a warning to me that there was so much standing water at ground level the dwarfs couldn't path out past it. But I was so very close! I shut one pump stack off and waited, and the dwarfs tentatively, one by one, ventured outside. All was going well, and I was sitting there letting it run, proud of myself, until I noticed I was a few dwarfs short. One z-level down... one hauler and three weaponlords, hanging out in the river. Not yet drowned, just "unable to follow orders." Unfortunately they did drown before miners could reach to dig ramps. Normally I hate losing skilled dwarfs, but at this point I was just laughing.
Fortunately I have room in the towers to de-pressurize, I just have to de-construct and shift the pump stacks back. And I actually like the power-hose effect for defense if I can time it right, so I'll try using floodgates for the diagonal block so I can keep the option.
All that said, I have questions. First, can dwarfs be forced down through floor grates or only out through fortifications? I'm hoping fortifications only, but I didn't actually see it happen, and they were some ways away by the time I looked, so I can't tell from that. More generally, what does a good waterfall layout look like? (My dining room's next after I get this re-done, for instance.)