Groundwater flooding by storm surges on atolls

Water tables in atolls rise during storm surges, which leads to groundwater flooding. Storm surges will occur more frequently and become more intense owing to climate change, and assessing storm-surge impacts on groundwater flooding is urgent for communities on atolls. In a typical atoll, Holocene sediments unconformably overlie highly permeable Pleistocene limestone, creating an interface called "Thurber discontinuity". The influence of the discontinuity on groundwater flooding, however, is not well documented. This study investigates how atolls' aquifers with different elevations of Thurber discontinuity respond to storm surges. We ran surface-subsurface coupled numerical simulations with the code HydroGeoSphere, which can reproduce dynamic processes of groundwater flooding. In the simulations, we assumed a 6-h storm surge with a maximum sea level rise of 2 m and compared various elevations of the Thurber discontinuity. The results showed that the shallower the Thurber discontinuity, the faster the water table rose. If the Thurber discontinuity were shallower than 25 m below mean sea level, the water table became higher than the land surface, and groundwater flooding occurred. Our findings suggest that accurate information on the elevation of Thurber discontinuity might be necessary to evaluate the potential risk of groundwater flooding on atolls accompanying storm surges.