Imagine your bathtub was a merry-go-round. No, a turnstile--
H'mm. That's not the place to begin. How does a road cross a canal? A bridge, of course. If it is a low bridge and the canal is actually used for boats, it might be a lift bridge or a swing bridge.
Okay, how does a canal cross a road? An aqueduct, right? Yep.
Fine, almost there. How does a canal cross a canal? Especially if the lower canal is for huge ships and the difference in height won't clear them?
As far as I know, this problem has only come up once, over a hundred years ago. The answer was a swing aqueduct -- the Barton Swing Aqueduct (and companion road bridge) and it's still in use. Follow the link and see it in action!
Simple and easy; you seal off a great long section of canal and gently pivot tonnes and tonnes of water out of the way, with no fuss or muss and very little noise. Why, anyone could do it...if they were clever enough. (Some details).
Living within a short bicycle ride of Indy's very own canal, I'm a bit jealous. Ours is only navigable by tiny boats, thanks to low bridges, and they chase you right off ahead of the system's only (non-moving) aqueduct.
3 months ago