Hydrology-Based Coastal Risk Assessment in Charleston, South Carolina: Sea-Level Rise, Land Subsidence, Nuisance Flooding, and the Overlooked Role of Groundwater Attenuation

Charleston, South Carolina is among the most flood-exposed cities on the United States Atlantic coast. Tide-gauge records show mean sea level rising at 3.51 mm per year since 1921, while InSAR analyses identify localized subsidence exceeding 4 mm per year, producing effective relative rise of 7 to 8 mm per year. This acceleration explains the increase in nuisance flooding from fewer than 5 days annually in the 1950s to more than 70 days annually today, with NOAA projecting 90 to 120 days by 2030 and over 180 by 2050. Subsurface dynamics further amplify this trajectory. Rising groundwater reduces soil storage capacity, while saltwater intrusion degrades permeability and vegetation, compounding chronic inundation. FEMA floodplain maps, which designate 35 percent of the peninsula as 100-year floodplain, omit much of the area already subject to routine flooding. This study integrates oceanic, geodetic, groundwater, and regulatory data into a hydrology-based framework. Results indicate Charleston’s flood risk is systematically underestimated. Effective resilience requires accounting for all interacting drivers rather than treating them in isolation.