We don't normally get this much water off the Atlantic. It is rare. Most of our moisture comes off the Gulf after it has run through Louisiana, Mississippi, Alabama and Georgia. You can smell the difference in the air.
Columbia and the coast will get more rain between tonight and tomorrow evening so it isn't over for them. Then consider the pattern of every river in SC. The big lakes are in the NW. They flow into rivers and creeks that all travel SE. They go directly through Columbia on a direct path to just north of Charleston. The rain may end but the rivers will continue to rise.

NC and Virginia may see some more rain through Tuesday. I can't speak to the topography in VA but in the Western North Carolina mountains one would think that flooding wouldn't be an issue and that the water would all run down to the lower areas but there are the rivers to consider. They overflow and spill out on the mountain roads often wiping them out.