Coupled Hydrodynamic–Geotechnical Analysis of Vertical Breakwater Stability under Solitary Wave Impact

When subjected to extreme wave loading, vertical breakwaters are prone to shear failure in their foundations, while the caisson body may experience sliding or overturning. Despite extensive research on breakwater stability, few studies have investigated the coupled mechanical response of vertical breakwaters under solitary wave impacts, which represent highly nonlinear long-wave conditions comparable to tsunami-like loads. In this study, a two-dimensional coupled hydrodynamic–geotechnical finite-element model is developed through one-way dynamic pressure coupling. Wave motion is described by using Reynolds-averaged Navier–Stokes equations, and the wave-induced pore pressure in the seabed is calculated based on Darcy's law. The strength reduction method is employed to quantify the shear stability factor of the rubble-mound foundation, while sliding and overturning safety factors are evaluated from the resultant forces and moments on the caisson. Model performance is validated against laboratory measurements of solitary wave propagation and impact pressures. Parametric analyses are conducted to examine the effects of wave height, water depth, wave–current interaction, and foundation thickness on overall stability. The results show that higher wave nonlinearity and codirectional currents significantly influence the risks of shear and overturning failure, whereas a thicker rubble-mound foundation enhances stability. These findings offer quantitative design insights and a validated framework for assessing the stability of vertical breakwaters under tsunami-like solitary waves.