Wave attenuation by artificial reef balls with varying oyster coverage density for coastal protection: A laboratory study

Nature-based hybrid coastal defences that integrate functions of protection with ecological enhancement are increasingly recognised as sustainable strategies for dissipating wave energy and restoring marine habitats. In this study, a comprehensive series of controlled laboratory experiments was conducted to evaluate the wave attenuation performance of hemispherical reef ball modules, enhanced with varying densities of native oyster ( Ostrea edulis ) shell coverage. Five configurations were tested in a 20 m wave flume, including a baseline condition (no reef balls), plain reef balls, and three levels of oyster colonisation density (low, medium, and high density). Experiments were carried out under both swell and storm conditions across a range of incident wave heights (0.04 – 0.10 m), peak periods (0.72 – 2.07 s), and still water depths (0.30 – 0.40 m) were examined to replicate nearshore incident wave conditions. Results show that biogenic surface roughness significantly improves wave attenuation compared with plain reef ball modules. During storm conditions at a shallow water depth of 0.30 m, high-density oyster-covered modules reduced transmitted wave heights by up to 42% compared to 29% for plain reef balls. Wave attenuation increased with oyster density and reduced structural submergence, indicating stronger wave–structure interaction. Existing empirical formulae underpredicted wave transmission in biogenically enhanced configuration. A modified empirical equation that incorporates an effective structure width adjusted to accounts for additional roughness introduced by oyster colonisation shows strong agreement with experimental data (88-90% within ±5% error), especially for the case of high-density oyster reef balls, where the RMSE was 0.09. These findings confirm that oyster-covered reef modules can deliver both ecological and engineering benefits and contribute to an evidence base for the incorporation of nature-based solutions in coastal protection strategies for dynamic intertidal environments. • Laboratory experiments assess wave attenuation by oyster-covered artificial reef balls. • Biogenic roughness from oyster colonisation significantly enhances wave dissipation under swell and storm conditions. • High-density oyster coverage increases wave height reduction by up to 42% compared to plain reef modules. • A modified empirical wave transmission formulation accurately predicts attenuation for biogenically enhanced structures.