you definitely don't want a sewer system that is only pipes, open channel flow is essential to ensure that sewage doesn't become pressurized, unless you want ex

you definitely don't want a sewer system that is only pipes, open channel flow is essential to ensure that sewage doesn't become pressurized, unless you want exciting effects like toilet fountains. Basically every sewer system has video game style free flowing open air vaulted areas, although usually at smaller scale.