THE EFFECT OF EXTREME RAINFALL FROM HURRICANE FLORENCE ON GROUNDWATER-SURFACEWATER INTERACTIONS IN THE NORTH CAROLINA COASTAL PLAIN: OBSERVATIONS FROM BARRIER ISLANDS AND A CAROLINA BAY

Hurricane Florence dropped 8 trillion gallons of water in NC initiating record floods in rivers, wetlands, and coastal areas. This event also provided a window into the hydrological impact of extreme rainfall in the coastal Carolina plains. Surface water level gauges and piezometers along the barrier islands and a Carolina Bay recorded the change in water levels in both surface and shallow groundwater during Hurricane Florence and the immediate aftermath. Groundwater levels in wells adjacent to a flow-through lake (White Lake) rose to full saturation, 90 cm increase, within 6 hours whereas lake level, 80 cm increase, reached its peak after 3 days. Groundwater gradients were towards the lake in the first 36 hours before resuming flow-through conditions. Water level decrease during the receding phase in both lake and groundwater track each other suggesting equal rates of recovery in both systems. Groundwater levels on the beach of the barrier islands rose by 80 cm primarily during seawater inundation by the storm surge apparent from the spike in groundwater salinity. Water level in wells on the barrier island dunes increased by about 150 cm primarily from rainfall recharge. Peak water level was reached after 12 hours on the beach and 36 hours on the dunes. The rate of decline in water level is similar for both beach and dune wells with tidal fluctuations superimposed with the beach groundwater level. Groundwater in all wells remain higher than the pre-hurricane levels with beach groundwater salinity still orders of magnitude higher weeks after the event. This work documents the relatively rapid response in both surface and groundwater levels in the North Carolina coastal plain during Hurricane Florence and potential lingering effects weeks and potentially months after the event.