> A closed canal system (with proper tubes) would be safe, no dirt (street dirt, mostly dog shit) could overspill on heavy summer rains into the canals and river

There is a logic step I'm missing here. I fail to see how a closed canal system is safer. Will the dirty water not, by definition, end up in the lowest point anyway, i.e. the river? Whether the canals are open or closed? I mean, the dirty water is not just going to disappear, is it?