Performance of Hybrid Coastal Protection in Extreme Storm Conditions

Abstract

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 solution comprising a beach nourishment placed in front of a rock revetment and confined between a harbor 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. Using the morphodynamic model XBeach Surfbeat two-dimensional horizontal, the observed patterns were mostly reproduced with some discrepancies in the estimation of scour depth at the toe of the revetment, with a maximum underestimation of 0.5 m. 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 the cost of larger absolute volume losses to areas downstream during the event. For design beach widths between 20 and 50 m, relative losses computed to approximately 20% of the initial added nourishment volumes. The maximum H_m0 at the toe of the structure reached 1.4 m for the case with an initial beach width of 5 m. While for the case with a 50 m initial beach width, maximum H_m0 was reduced to 0.5 m. The freeboard level of the rock revetment and storm surge levels emerged as critical for the capability of the hybrid system to reduce overtopping. These findings emphasize the importance of tailored designs, frequent maintenance, and detailed monitoring in managing hybrid solutions to maximize their protective benefits.