Modeling seawater flooding, ponding, and infiltration processes under future tsunami scenarios: A case study at Niijima Island, Japan

Tsunami disasters can cause infiltration of seawater into coastal unconfined aquifers over large scales, which would induce long-term salinization of groundwater resources. According to the report by Cabinet Office of Japan (2011), an earthquake of 9.0 M_w will very possibly occur along the Nankai Trough during the years 2011-2030. Under the worst tsunami scenario, the coastal area of Niijima Island, Japan, will be inundated by seawater up to about 15 m a.m.s.l. (above mean sea-level) (Tokyo Disaster Management Council, 2013). As a result, groundwater, the only freshwater source for the island, will face severe salinization. In order to assess the risk of groundwater salinization under such scenario, a 3-D numerical model of Niijima Island was developed using the HydroGeoSphere code which can solve coupled surface-subsurface flow processes.  The results showed that the simulated early stage seawater ponding at the land surface was controlled by the type (DEM or DSM) and spatial resolution of topographic data  used in the model. This suggested that high-resolution topographic data considering the existence of artificial structures should be preferred for modeling seawater ponding and infiltration after tsunamis in urbanized areas. Furthermore, compared with the baseline case, the reduction in hydraulic conductivity and the Manning’s roughness coefficient in places of buildings and roads reduced the amount of seawater infiltration. The results highlighted the land surface conditions as an important indicator for the vulnerability of groundwater resources to tsunami-induced seawater infiltration.