How do I set up a pond for efficient drainage?

Solution 1:

A wider pipe will cause it to drain faster, as will an altitude drop as Martin Sojka suggests. Not sure which will be more effective; if you do both it should drain pretty fast. I know when I was building a waterfall in my dining room, it tended to back up sometimes when the drainage to the caverns was only 1 square.

You can connect any number of things to a single lever or pressure plate. There is no limit (well, no meaningful limit, probably if you try to connect more than 65k things it gets confused). So feel free to go wild with that.

Since you're fiddling with drainage systems, I'll give you another bit of advice for free: especially if you build a wide drainage pipe, put in some vertical bars blocking the outflow, with some convenient stairs up right in front of them. Otherwise, Urist McMason will decide that he just absolutely needs to use a rock left behind when you were digging out the sewers, and if there's any way he can find to get there, he'll go charging down to grab it and get washed away by the current. Give him something to hit before he gets washed entirely off. I had this happen to me with my waterfall as well; the poor guy got washed out the sewers, down the 6 z-level drop to the floor of the first cavern.

Solution 2:

Drain the ponds to some place lower than their lowest point. The water pressure will make sure they'll empty into them almost instantly. If you're worried about the pressure overflowing and pushing the water back up, make a simple de-pressurizer somewhere on the pipe, like this:

                           ####################
                           ############         <- pressurized water in
depressurized water out <-             ########
                           ####################

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