Designing modular, artificial reefs for both coastal defense and coral restoration

Coastal flooding and erosion are growing issues for coastal communities as their severity continues to worsen with climate change. As a result, there is increasing interest in the use of nature-based engineering as a sustainable and cost-effective strategy for protecting many coastlines globally. Among these approaches, reef engineering aims to integrate both the physical and biological aspects of reef communities to attenuate incident wave energy while still maintaining ecological values. However, few examples currently exist on reef engineering for coastal defense due to the multidisciplinary challenge of constraining physical and biological interactions with artificial reefs. Here, we present the first design iteration of a novel artificial hybrid reef system that intends to provide both coastal defense benefits as well as refugia for corals to enable their future growth. To balance these performance objectives, the pyramidal low-crested reef designs developed here combine two hexagonal sub-units: SEAHIVE® and lattice with tunable porosity. The hydrodynamic performance of these sub-units was tested using a numerical wave tank (NWT), based on the computational fluid dynamics (CFD) modeling suite OpenFOAM, to determine the best configuration of the sub-units for a given set of wave conditions, both as single reefs and as a three-row reef system. The goal was to produce a small subset of reef designs to be tested in a wave flume facility to support model calibration and future design iteration. The reef designs explored herein offer wave energy reduction values greater than 70%, consistent with natural coral reefs as well as other conventional submerged breakwater designs. Further, the highly porous sub-units provide further tunability of hydrodynamic performance when compared with traditional low-crested breakwaters. • A numerical wave tank based on computational fluid dynamics was used to test the wave energy reduction characteristics of novel artificial reefs composed of different hexagonal sub-units. • Designs feature a novel lattice structure with tunable porosity that directs water through tortuous pathways within the reef structure, reducing reflection and disrupting passing wave crests. • The reefs developed here can reduce up to 90% of incident wave energy, commensurate with existing low-crested structure designs and natural coral reef ecosystems.