I think stormwater runoff concerns are generally unserious. Any large and dense city should have a robust drainage system, and if it doesn't, the concerns over street flooding or sewer backflow a few times per year pale in comparison to the environmental damage inflicted by sprawl. Especially auto-centric sprawl, which covers more area with impervious surfaces (roads) than simply building dense housing. There's a reason that everyone in Houston has a yard, but storm runoff is still a problem.