The ones we visited in Paris have 50cm thick solid steel doors and submarine hatch style wheels to wind them shut.

I would anticipate that even just the air pressure that is expelled during a surge is probably enough to cause serious damage to people.

On the scale of G-cans, it's hard to know if it would be more dangerous or less dangerous..

It's probably not safe for people when flooded; it's unlikely to be a static pond, but a cascading torrent.

... but yeah, now I want to see pictures of it in use :D