For those reading Annalee Newitz' "Automatic Noodle", please just skip past the generating-power-from-a-turbine-in-a -flowing-sewer idea.
One thing you don't ever ever want to do is obstruct or slow down flowing water (The turd herders call it "mixed liquor". No. Really.) in a sewer.
The big ones generally have *just* enough slope to keep everything moving gently along when flowing full. Slow it down further and the fluid level will rise above street level.
#Water #WaterEngineer