I read this earlier in the week, but I missed this interesting quote from Todd Rasmussen, a hydrologist with the University of Georgia ...
Rasmussen says there are two aspects of flood water: "There is more runoff because of greater volume. The second is water is conveyed more rapidly downstream. It accumulates faster." People upstream benefit from municipal storm drains, "but those downstream get flooded." In Northeast Georgia during the recent heavy rains, he says, "we came out of a long drought. A lot of it got held up in the watershed. The [relatively low] amount of urbanization in the region held that back, but if we had Atlanta north of us, we would have experienced a raging torrent," Rasmussen says.