A trade-off approach to optimize nature-based flood defense designs: riparian willow forests as case study

Nature-based solutions are increasingly recognized as effective and multifunctional components of climate-resilient flood protection. While tropical mangroves have received substantial attention, temperate riparian forests, particularly willow systems, offer comparable wave attenuation and biodiversity benefits, yet remain understudied. This study assesses the ecological and protective value of three types of willow floodplain forests: a so-called wild-grown willow forest, a pollard willow forest, and a willow plantation. Using field data from the Biesbosch National Park (the Netherlands), we quantified forest structure, ground-dwelling invertebrate diversity, and modelled wave attenuation under storm scenarios. Structural complexity and biodiversity were highest in the wild-grown forest, with significantly greater invertebrate order richness, larger body sizes, and more heterogeneous canopy architecture. The pollard forest showed the highest wave attenuation efficiency due to their dense, low-lying crown structures. The plantation forest showed lower values across both axes. We integrated these findings into a trade-off model evaluating ecological value, flood protection efficiency, and a 50-year simple cost analysis of each forest type as a hybrid solution alongside traditional dikes. While the pollard forest is the most spatially efficient for flood attenuation, the wild-grown system provides greater ecological value at lower lifecycle cost. Our results underscore the importance of tailoring hybrid flood defense strategies to local priorities - balancing biodiversity, spatial constraints, and economic feasibility. The framework developed here can inform ecosystem-based design in delta regions worldwide, supporting integrated climate adaptation that aligns safety with ecological resilience.