I think it basically is a given. To segregate storm water from sewer would require a rebuild of the system. Pretty impractical at this point. It can be done in some cases. I live in the west suburbs of Chicago, and the storm water from my (less than 5 year old) complex drains into the Salt Creek. I don't think my city is connected to the big dig, but we do have the capability to dump into a local quarry. The quarry has also been used to dump snow into for large winter storms. We've also been through a large expansion in recent years of green areas and detention drainage areas to alleviate flooding. Living right on the Salt Creek, I routinely see the water level fluctuate by around 10 feet or so after even a decent storm. We're probably about 5-6 feet above normal right now after recent storms.