Wave overtopping on eco-engineered seawalls: Does biodiversity enhancement conflict with coastal protection?

Topographic complexity is increasingly added to coastal infrastructure to enhance biodiversity and environmental sustainability, but potential trade-offs with coastal protection services due to altered wave dissipation remain underexplored. Using a hydrodynamic flume, we investigated how three commercially available, eco-engineered panel topographies ('Living Seawalls') at two scales (1:1 full-scale, 1:3 small-scale) affect wave overtopping volume and frequency on a vertical surface. We also tested whether topography-associated biogenic complexity, mimicked using artificial seaweeds, modifies these hydrodynamic effects. Lastly, we used small-scale panels to investigate spatial arrangement effects, comparing single panels to mosaic configurations. All experiments were conducted with the same JONSWAP-spectrum waves for direct comparability. Complex, full-scale panels increased overtopping volume by up to 16% compared to flat controls, but had little impact on overtopping frequency. Notably, artificial seaweed cover reduced overtopping volume and frequency by up to 12%, depending on topography, thus partially offsetting the negative hydrodynamic effects induced by blank topography. Comparison of small-scale mosaics to single panels further indicated that, on complex panels, arrangement size increases overtopping volume, but reduces frequency. Our results suggest that complex topographies may increase overtopping via flow channelling and turbulent splash-up, but subsequent seaweed establishment may dampen waves through form and friction drag. Topographic modification can thus influence hydrodynamic performance in both positive and negative ways, underscoring the necessity to understand and plan how eco-engineering affects the protective role of future marine infrastructure. Consideration of hydrodynamic mechanisms at the design stage may reduce trade-offs between biodiversity enhancement and coastal protection, and support infrastructure multifunctionality.