Thank you Ben for an Interesting puzzle!
I'm not into hydrology, but these vertical shafts might be the equivalent of "safety valves" that can resist pressure up to a certain degree. These vertical shafts, presumably connected to the horizontal pipes (the "aqueduct") so as to make sure that the pressure in the aqueduct will remains within a preset limit. That limit equals the height of the shaft times the unit weight of water. Should pressure increase for whatever reason, water will flow out of the top of shafts.
That raises the question: what can raise water pressure that much? Is the aqueduct connected to a dam (with the potential if rising water level)?
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Neil Kazen, M.Eng., M.Sc., P.Eng.
FASCE, FCPCI, FEC,
Retired Structural Engineering Manager, Transportation Division, SNC-Lavalin
Toronto, Ontario, Canada
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