Performance of Hybrid Coastal Protection in Extreme Storm Conditions

Hybrid coastal protection systems combine hard and soft design elements and are increasingly utilized to address coastal erosion and flood risks. However, limited data are available on the performance of hybrid structures under extreme storm conditions, and the interactions at the transition between hard and soft elements are not well understood. This study investigates the performance of a hybrid system comprising a beach nourishment placed in front of a rock revetment and confined between a harbour mole and a groin, focusing on its morphological response to extreme events. Observations from the storm Babet in October 2023 revealed significant erosion at the revetment’s toe, longshore variability in sediment redistribution, and the formation of a bar at the lower beach during high water levels, resulting from the interaction between the nourishment and hard structures. The observed patterns were partially reproduced using the morphodynamic model XBeach Surfbeat 2DH. However, the model underestimated scour depth and erosion magnitudes, emphasizing the challenge in representing processes at the interface of hard and soft components in numerical models. Simulations were further used to explore the influence of initial nourishment configurations on wave dissipation, overtopping, and morphological response. The results showed that wider beach widths reduced wave impacts and maintained higher bed elevations at the structure’s toe but at cost of larger downstream losses during the event. The freeboard level of the rock revetment and storm surge levels emerged as critical for the hybrid system’s capability to reduce overtopping. These findings emphasize the importance of tailored designs, frequent maintenance, and detailed monitoring in managing hybrid systems to maximize their protective benefits.