Hybrid dunes are a coastal flood defense structure that combines the sandy and wave- dissipating capacity of natural dunes with the robustness of hard structures. Their increasing application in coastal environments highlights the need for a better understanding of how these systems respond to storm forcing. This study investigates the morphodynamic behavior of hybrid dunes under storm conditions through a large-scale field experiment at the Sand Engine (NL), aiming to improve predictive insight into erosion mechanisms and structural performance. High-resolution LiDAR, pressure sensors, and acoustic instruments were deployed across four engineered dune configurations, ranging from a fully sandy dune to hybrid setups with embedded sea dike and seawall cores, to monitor morphological changes and hydrodynamic forcing during five consecutive storms.
The main research question is: What are the dominant erosion mechanisms and structural influences of embedded hard elements in hybrid dunes during consecutive storm events?
The findings show that hybrid dunes transition through three consistent erosion phases: (1) initial sand-dominated retreat characterized by notching, slumping, and offshore sediment transport, (2) reduced erosion once hard elements become partially exposed, and (3) structure-dominated stability after full exposure, during which landward retreat halts. Core geometry strongly influenced behavior: The Dike-in-dune (S1) supported smoother energy dissipation and profile adjustment, while the wall-in-dune (S4) halted retreat more abruptly but caused localized vertical beach erosion. In contrast, the Sandy dune (S2) continued to retreat by over 7 m, with cumulative erosion volumes exceeding 11 m3 /m. Erosion volumes correlated significantly with 20 min time-averaged waterlevel, measured 80 m seaward of the dune foot. Field observation identified the total water level as the dominant driver of erosion. Once exposed, hard structures reduced wave run-up and changed energy distribution, influencing sediment mobility. Oblique wave conditions likely disrupted longshore sediment supply, especially toward unarmored sections, amplifying erosion near structural transitions. These interactions underscore the need to jointly assess cross- and longshore processes when evaluating hybrid dune behavior. Key design factors include sand cover thickness, core geometry, and especially transition zone reinforcement. Structural flanking and undermining were critical failure mechanisms, as seen in the collapse of the Dike-in-Dune and Dike sections during the fourth storm. These results highlight the importance of integrated design strategies addressing not only structural shape, but also lateral sediment continuity and toe stability.
Hybrid dunes shift erosion from retreat-driven to structure-constrained behavior, offering localized resistance under storm conditions. However, their long-term performance depends on robust, systemscale design, balancing sand volume, structural exposure, and sediment pathways to deliver adaptable and resilient coastal protection.