Characterizing vegetation effects on wave mitigation performance of resilient hybrid vegetation-seawall systems

Increasing coastal flood risks and disadvantages of hard structures have called for innovative technologies in coastal defense, such as hybrid flood mitigation systems that combine engineered structures, e.g., seawalls, with natural and nature-based elements, e.g., vegetation, for enhancing coastal resilience. However, the performance of hybrid systems is relatively less understood compared to engineered systems. This study utilizes resiliency concepts, including robustness and serviceability, to investigate the effects of vegetation with different characteristics on the flood performance of hybrid vegetation-seawall systems during extreme wave events. Vegetation characteristics include primarily stem height, diameter, density, and vegetated area. A numerical modeling approach, utilizing the XBeach Non-hydrostatic model, is employed to assess the performance of a vegetation-seawall system in terms of wave runup and overtopping reduction, and improvements to the system’s robustness and flood serviceability. The results reveal that the vegetation characteristics significantly influence the system’s capacity to mitigate wave runup and overtopping and consequently its robustness and flood serviceability. Among the vegetation attributes studied, stem diameter emerged as the most influential factor in reducing system failure risks. As flood management is shifting towards resilient-based strategies, the results of this study encourage the performance of flood mitigation systems to be assessed using not only the widely adopted hazard metrics such as wave runup and overtopping but also resiliency-based measures such as robustness and flood serviceability.