[Taniwha]

It's really more a pipe than a tunnel, the whole pipe needs to be able to support the weight of the boat and all the water to the top of each step

[amelius]

What is the difference between a pipe and a tunnel in this case?

[Taniwha]

a tunnel is supporting some downward pressure above it while a pip is resisting an outwards pressure from inside - a brick/stone arch might work for downwards pressure, but will just come apart if pushed from the inside - unlike the illustration they'd more likely need some reinforced concrete structure to support the weight of the water/boat