Wave dissipation induced by flow interactions with porous artificial reefs

Porous artificial reefs can be used for coastal protection when they are effective at dissipating incident wave energy. Previous studies have used observations of wave interactions with porous reefs to develop empirical formulations to parameterize wave transmission as a function of reef geometry and hydrodynamic parameters. However, such approaches do not distinguish between the different processes that contribute to dissipation, namely wave breaking and drag-induced dissipation. While drag-induced dissipation can be more significant in porous reefs than in conventional rubble mound structures, the mechanisms that govern wave dissipation by drag forces within porous reefs are not well characterized. As a result, there is limited predictive capacity for describing wave-driven hydrodynamic processes in the interior of porous reefs and how these processes translate into wave dissipation. In this study, physical modelling experiments were conducted in a wave flume to investigate the detailed velocity structure, forces and wave dissipation within multi-row and single-row porous cubic artificial reefs that were exposed to a range of non-breaking regular wave conditions and submergence depths. The results reveal how the porous reef modifies the dynamics of the in-reef flows that are responsible for generating horizontal and vertical drag forces. Drag coefficients for different configurations of single- and multi-row reefs were similar and decreased with a reef Keulegan-Carpenter number (defined as the ratio of the wave orbital excursion to a structural hydraulic radius). Rates of wave dissipation derived from changes in wave energy fluxes across the reef could be explained primarily by the work done by horizontal drag forces, with vertical drag forces playing only a secondary role. Finally, the results from this study were used to develop an analytical model to predict drag-induced dissipation by porous reefs, which was shown to accurately predict wave attenuation across the reef as a function of reef, wave, and depth characteristics.