Hydraulic Response to Sea Level Rise in a Coastal Aquifer Extending under the Sea with a Cut-off Wall

Seawater intrusion can cause the freshwater-saltwater interface to move inland toward coastal freshwater aquifers. Sea level rise has become a significant driver of this phenomenon. Installing cut-off walls along coastal aquifers is an effective engineering measure to mitigate seawater intrusion. However, most analyses of groundwater flow under sea level rise, particularly with cut-off walls, primarily rely on numerical methods, with limited analytical approaches available. In this study, we developed mathematical models for groundwater flow induced by sea level rise, dividing the coastal aquifer into offshore and inland regions along the cut-off wall. An unknown flow function was introduced as a boundary condition at the shared boundary. Using homogenization and the finite Fourier transform method, we derived analytical solutions for the two regions separately. A global coupling solution, achieving hydraulic continuity between the two regions, was obtained by applying the collocation method at the shared boundary. The validity of the solution was confirmed through comparisons with finite difference numerical simulations. Furthermore, we analyzed the impacts of factors such as sea level rise amplitude and cut-off wall embedment depth on hydraulic changes. The results indicate that increases in the amplitude of sea level rise significantly amplify hydraulic head changes in the inland aquifers, while deeper embedment of the cut-off wall enhances its effectiveness in preventing seawater intrusion. However, the model does not consider density differences between freshwater and saltwater or the dynamics of the saltwater-freshwater interface.