Developing novel hybrid reefs to provide both wave energy reduction and opportunities for ecological restoration

Hybrid artificial–biological reefs have emerged as a promising strategy for achieving the longer-term goal of ecological coral restoration while providing immediate coastal defense benefits. Although there are a growing number of hybrid reef types and morphologies, there are nearly as many approaches for defining these structures’ hydrodynamic performance. This study presents a new type of hybrid reef structure composed of porous sub-elements that conduct water through the reefs via engineered flow pathways that are intended to (1) dissipate wave energy through breaking and internal friction, and (2) mimic the flow field of natural coral reefs. The aim of this work is to identify optimal reef configurations through rigorous testing, establishing a validated physical and numerical framework prior to construction and deployment. Here, we validate the multi-row reef designs at 1:10 scale in a physical wave tank under regular and irregular wave conditions. Regular wave conditions were then used to calibrate and validate a numerical wave tank (NWT) via the OpenFOAM computational fluid dynamics framework, to develop a deeper understanding of the hydrodynamics and forces acting on and being generated by these reefs. Results indicate that reducing the number of reef rows from three to two only decreases wave energy reduction by 13%, while reducing the total structure footprint by 33%, demonstrating a favorable cost-performance trade-off. The specific reef geometry also strongly influences internal flow, pressure gradients, and drag forces, suggesting minor changes to the reef structural design can meaningfully alter hydrodynamic performance. Yet, all designs are stable for sliding and overturning under the tested conditions. While the offshore reef experiences greater turbulence and surface shear stresses, a sheltering effect on the onshore reef creates a lower stress environment that may be more suitable for coral cultivation.