Internal erosion generally poses a critical threat to river dikes constructed over paleo-valleys. Identifying the nature of the interactions between subsurface internal erosion and surface manifestations is crucial for understanding erosion mechanisms and developing mitigation strategies. This study investigates the mechanistic links between subsurface internal erosion and surface signatures of a river dike in southern France. By combining geophysical data with three-dimensional (3D) finite element modeling, we simulate the seepage and suffusion processes under realistic flood scenarios. The simulation results aligned closely with field observations, vividly revealing the primary mechanisms responsible for the formation of sinkholes, leaks and sand-boils. In addition, the interplay between subsurface suffusion and surface erosion signatures was highlighted. The findings highlight how stratigraphic complexity governs both seepage behavior and erosion progression, providing critical insights for dike risk assessment and mitigation strategy development.

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