[A-HW21] Surface and subsurface hydrologic models: Technical advances and applications for water management

[AHW21-07] Groundwater flooding on atolls caused by storm surge: effects of the dual-aquifer configuration

*Satoshi Tajima^1,2, Philip Brunner³, Jiaqi Liu², Hugo Delottier³, Tomochika Tokunaga² (1.Spanish National Research Council (CSIC), 2.Graduate School of Frontier Sciences, The University of Tokyo, 3.Université de Neuchâtel)

Keywords:Storm surge, Groundwater flooding, Numerical simulation, Atoll, HydroGeoSphere

Storm surges associated with tropical cyclones can cause groundwater flooding, in which groundwater exfiltrates from the land surface, resulting in a longer duration of inundation than typical rainfall-driven floods. Groundwater flooding caused by storm surges can be particularly severe on atolls owing to their characteristic high permeability, small land surface area, and low elevation. Atolls are known to have “dual-aquifer” configurations where Holocene sediments unconformably overlie highly permeable Pleistocene limestone, creating an interface called “Thurber discontinuity”. This study quantitatively analyses how the dual-aquifer configuration of atolls controls the spatial and temporal dynamics of groundwater flooding caused by storm surges. We ran integrated surface-subsurface numerical simulations with the code HydroGeoSphere. The baseline simulation is assumed to be a 12-hour storm surge with a maximum sea level rise of 2 m. We compared various scenarios of the dual-aquifer configuration with different elevations of the Thurber discontinuity and hydraulic conductivities of the Pleistocene aquifer, assuming the same hydraulic conductivity of the Holocene aquifer throughout the scenarios. The results show that the shallower the Thurber discontinuity and the higher the hydraulic conductivity of the Pleistocene aquifer, the larger the maximum surface water depth and the higher the maximum salinity in the swamp at the atoll's centre during the storm surge. If the Thurber discontinuity is shallow and the hydraulic conductivity of the Pleistocene aquifer is high, a fast temporal change, including both rise and drop, is observed in the surface water depth because of the faster flow within the Pleistocene aquifer than the Holocene one. These conditions shorten the flooring duration and the time required for the surface water depth to reach the maximum. Moreover, in a quasi-steady state, the water table elevation and salinity distribution were found to be insensitive to the dual-aquifer configurations, i.e., the Thurber discontinuity elevation and the hydraulic conductivity of the Pleistocene aquifer. 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.