Wave attenuation across modular artificial reefs: effects of reef crest width, porosity and module spacing

Modular artificial reefs are increasingly used as a nature-based form of coastal protection, due to their ability to attenuate waves while providing ecosystem benefits. Compared to conventional submerged coastal structures (e.g., breakwaters), artificial reefs exhibit a wider range of length scales, porosities, and module configurations, reflecting diverse design objectives. Variations in reef geometry can substantially alter the relative importance of the hydrodynamic mechanisms governing wave attenuation; however, these geometry-dependent wave-reef interaction processes remain poorly understood. To address this, high-resolution computational fluid dynamics (CFD) modelling using a Smoothed Particle Hydrodynamics (SPH) approach was employed to investigate the hydrodynamics governing regular wave transformation across idealised cubic modular reefs, with emphasis on the mechanistic roles of reef crest width, module porosity, and module spacing. The results showed that variations in these geometric features control wave attenuation by distinct influences on wave reflection, breaking, and drag-induced dissipation. Reefs with wider crests provide larger dissipation zones than short-crested configurations. At shallow to moderate depths, lower porosity increases wave attenuation by increasing reflection and energy dissipation via breaking and drag forces. Increased module spacing reduces inter-module flow sheltering, promoting greater drag dissipation, while spacing approaching half the incident wavelength produces additional reflection due to Bragg resonance. These results informed the development of an improved predictive framework for wave transmission across modular artificial reefs under a broad range of hydrodynamic and geometric conditions. Overall, this study provides new insights into process-based controls on wave attenuation in modular artificial reef systems and develops an improved approach for evaluating wave transmission performance across diverse reef configurations, thereby supporting the effective design of artificial reefs for nature-based coastal protection.