[AHW31-P06] Modeling of tsunami-induced seawater intrusion into unconfined coastal aquifers: uncertainties analysis and water management
Tsunami-induced seawater inundation causes vertical seawater intrusion into coastal aquifers. Assessing future risks of seawater intrusion in tsunami-prone zones can provide essential information supporting disaster preparedness and water management. We investigated seawater intrusion and aquifer recovery processes under the future Nankai earthquake and tsunami scenarios at Niijima Island, Japan. Both 2D and 3D numerical groundwater models were developed to solve variable-density flow and mass transport in unsaturated-saturated porous media using the code, FEFLOW. Our analysis suggests that the tsunami inundation height, the rainfall recharge rate, hydraulic conductivity, and bedrock structures are the primary sources of uncertainties in the simulation results. The simulations indicated that the maximum amount of seawater intrusion during the tsunami was controlled by the total unsaturated void space of the soil beneath the inundation area. After the tsunami, directions of seawater movement and flushing time depended on the pre-tsunami groundwater flow conditions and bedrock structures. Some groundwater was found to be survived from seawater intrusion, and showed the potential to provide water supply in an equivalent amount of the pre-tsunami level without worsening the recovery process. The simulated attempt to remove the intruded seawater from a polluted well could accelerate aquifer recovery but might not be practical due to the cost of maintaining intensive pumping over years.