NUMERICAL MODELING OF COMPOUND FLOODING IN CHARLESTON, SC DURING HURRICANE JOAQUIN

A stalled low pressure frontal system met with offshore tropical moisture from Hurricane Joaquin from 10/1/2015 through 10/5/2015, which set a record rainfall of widespread amounts of 25-30 inches in the Charleston tri-county, South Carolina. As a result, the flash flood from rivers combined with storm surge originating from Hurricane Joaquin offshore led to significant flooding along the coast, especially in downtown Charleston. This compound flooding is induced by coastal storm surge involving oceanic processes and maintained simultaneously by riverine flooding upstream of the watershed which involves hydrological processes. A 3D coastal ocean model SCHISM (Semi-implicit, Cross-scale, Hydroscience Integrated System Model) coupled with National Water Model (NWM) was used to simulate this event from 09/23/2015 to 10/15/2015. An unstructured grid was generated to cover the US East and Gulf coasts, with finer resolution along the South Carolina coast. The open boundary was set along 60 degrees west longitude in the open ocean and the land boundary was set at 10 m above MSL. Stream flows from NWM reanalysis are introduced as point sources and sinks at the intersections between NWM segments and SCHISM land boundaries. The 11 km grid resolution wind and pressure archived fields obtained from the European Center for Medium-range Weather Forecast was used as the atmospheric forcing. The SCHISM model results without upstream riverine forcing show acceptable results for the storm surge water level away from the coast. The water level inside the Charleston Harbor, however, underpredicts the NOAA gauge observation by about 0.5 meters (out of 1.5 m peak flood) from 10/1/2015 – 10/5/2015 and is severely depressed for another 1-2 weeks. After the stream flow discharge at Cooper, Ashley, and Wando Rivers and the non-point source flows from the watershed were fixed and implemented, the modeled water level compared favorably with the observation. This demonstrates the importance of the compounding roles storm surge and river discharge have on coastal flooding.