The mountains are not a flood plain they're a watershed. Floodplains pertain to areas near rivers (like riparian zone) where water cannot flow, such as flatlands and/or close to sea level. Rivers begin in the mountains and flow to sea, generally speaking. The french broad of NC becomes the broad river in SC after a few reservoirs and dams, and then joins the congaree to become the saluda in columbia sc and flows out to sea.

The reason this helene is an issue is because it's one in a thousand plus year event where more water was dumped on the mountains at one time than is dropped in toto onto those mountains in 3 yrs