South Carolina Flooding Shifts to the Lowcountry

Dr. Matt Sitkowski, coordinating weather producer for The Weather Channel, calculated that if 12 inches of rain fell over the entire Palmetto State, that would equate to 6.68 trillion gallons of water. This is about 20,900 gallons of water for every American, or enough to fill the world's largest swimming pool 101,000 times over. 
Now all that water is draining downstream. 

South Carolina's elevation varies. The Blue Ridge Mountains touch South Carolina's far northwest, with elevations up to 3,560 feet.

The state's rolling Midlands, the transition between the relatively more rugged Upstate and the Lowcountry, contributed to the destructive power of the flash floods.

There is only a 272 foot elevation difference between Columbia – the state's capital in the Midlands – and Charleston on the Atlantic coast.

http://www.weather.com/storms/severe/news/south-carolina-flood-forecast-oct2015