Coastal flooding generated by ocean wave- and surge-driven groundwater fluctuations on a sandy barrier island

Three years of observations of groundwater elevations, ocean tides, surge, and waves, and rainfall are used to study coastal groundwater-driven flooding along the ocean side of a barrier island. Increases in surge and wave-driven water levels (setup) during 26 ocean storms with little rainfall, including the passage of 3 hurricanes, caused O(1 m) increases in groundwater heads under the dunes on the ocean side of the island, nearly double previously reported magnitudes. The inland propagation of the resulting pulses in groundwater levels is consistent with an analytical model (without recharge) based on shallow aquifer theory (Nash Sutcliffe model efficiencies of >0.7, maximum water-table level estimates within 0.1 m of observations). Infiltration of precipitation results in approximately a threefold increase in the groundwater level relative to the amount of rainfall. The analytical model (with recharge) driven with estimated ocean shoreline water levels (based on the 36-hr-averaged offshore tide, surge, and wave height) and measured precipitation predicts the maximum water-table height within 0.15 m of that observed across the barrier island during Hurricane Matthew, which was the only wave event during the 3-yr data set with more than 0.1 m rainfall. Citizen-science reports from a smartphone app (iFlood) are used to evaluate the regional application of the model. Twenty-five ocean-side reports associated with 7 ocean storms (6 of which had minimal rainfall) between Sept 2019 and Feb 2020 showed flooding on natural (permeable) land surfaces along 70 km of the northern Outer Banks barrier island, from Corolla to Rodanthe, NC. The analytical model (with recharge) predicts flooding that is consistent with the timing and location for 19 of the 25 reports. Applying the model regionally suggests that more than 10% of the land area on the ocean side of the northern Outer Banks would be inundated by coastal groundwater even in the absence of rainfall for an ocean storm that generates a 2.25 m increase in the shoreline water level.