Sewage
doesn't have to be dumped. Simply separate your black water and grey water. Storm drains can go to rivers (if the rivers have capacity – if not, it's sometimes easier to give the river more capacity than to build more sewers), and the amount of water in sewage pipes will be independent of the amount of rain.
Sure, the sewers might not currently be designed that way, but that can be changed. (It's a logistical challenge, but it needs to be done.)
