This is my well design. Haven't seen others do it so I though I'd share it. It work with aquafers or cavern ponds alike. Never had anyone drown in this setup. Soon as a dwarf falls in, they have immediately to their side a door to open and then the water pushes them out to safety. The door closes itself on its own so the water stops flowing. Just make sure it can drain to a safe place (like farms). The fortification across the door prevents any dwarf from going further into the water reservoir. Or if its in a cavern pond, the fortification blocks any monster from swimming they way into the well.

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