Simulating tsunami flooding and seawater infiltration using coupled surface-subsurface flow models

Tsunami disasters cause not only human casualties and economic losses by short-term seawater flooding but also long-term salinization issues to groundwater resources due to seawater infiltration into coastal unconfined aquifers. The two processes, seawater flooding and infiltration, have been commonly simulated in a separate manner using tsunami models and groundwater models, respectively. Thanks to many recent advances in fully integrated hydrological modeling techniques, simulating surface and subsurface flow processes across various time scales within one conceptual framework is now feasible. Here, we present numerical simulations of the coupled processes of seawater flooding and infiltration based on a coastal urban area of Niijima Island, Japan, under the future Nankai Trough earthquake and tsunami scenarios. The HydroGeoSphere code was used to solve 2-D surface flow by St Venant equation and 3-D subsurface flow by Richard’s equation. The baseline simulation showed that road networks acted as fast paths for seawater flooding, while bare lands and building areas were the primary locations for seawater infiltration into the subsurface. The occurrence of seawater ponding was found to be controlled by both topographic variations at the land surface and the saturation condition of the soil medium in the subsurface. Moreover, the type of topographic data used in the model (DEM or DSM) and the equivalent hydraulic conductivity applied to building restructures showed significant effects on the simulated intensity of surface flow and the amount of seawater infiltration. These findings indicated that surface-subsurface interactions and the properties of both surface and subsurface domains are important factors to be considered for improving infrastructure safety evaluation and water resources management in tsunami-prone areas.