Quantifying the wave attenuation of hybrid seagrass-reef structures – An experimental study

Hybrid sea defences that incorporate both natural habitats and artificial structures (such as seagrass and artificial reefs) for coastal protection have gained traction because of their dual benefits of ecological enhancement and wave attenuation. However, while the hydrodynamic performance of individual components has been reported, the synergistic effects of such an innovative combination on the wave energy dissipation remain underexplored. This study, for the first time, investigates the hydrodynamic performance of a hybrid structure composed of seagrass and porous artificial reefs through a series of laboratory-scale experiments. Wave transmission, reflection, and dissipation were analysed under both swell and storm representative sea conditions. The developed seagrass mimics exhibit geometric and physical similarities to the real seagrass, Zostera marina . Two types of porous artificial reefs, cubic and trapezoidal, were considered in the current study. The results demonstrate that the presence of seagrass enhances wave attenuation, reducing wave transmission coefficients by up to 9.2% for cubic reefs and 8.6% for trapezoidal reefs compared to bare reef conditions. To quantify this engineering benefit more intuitively, an 'equivalent height enhancement index' is introduced, which represents the reef height increment required for a bare reef to achieve the same wave attenuation as the hybrid system. It was found that the seagrass canopy contributes an equivalent reef height enhancement of up to 39.0% for trapezoidal reefs and 22.2% for cubic reefs. Furthermore, a new empirical model was developed to predict the seagrass contribution to equivalent reef height in the hybrid system. The proposed model provides a foundation for developing practical tools for coastal engineers and stakeholders to design sustainable hybrid sea defences.