Fei Wu

Shell-based constructed oyster reefs (CORs) serve as nature-based coastal stabilization structures in living shoreline applications. This study investigates oyster colonization effects on the interactions of currents and waves with CORs on a slope. Using the computational fluid dynamics framework OpenFOAM®, a validated numerical model simulates the hydrodynamic performance over CORs with simple geometric surrogates for the oysters. Simulations are conducted considering: (1) unidirectional currents and regular wave conditions, (2) different oyster rugosities (R), and (3) still water depth variations on the flat seabed. Oyster colonization alters water velocity and turbulence distribution patterns. Under unidirectional currents, oysters expand wake regions and amplify upstream-propagating waves. Under pure waves, turbulent kinetic energy exhibits uneven distribution above oyster-colonized CORs, with maxima near wave-breaking zones. Oysters enhance wave nonlinearity, driving energy transfer from the primary harmonic to higher harmonics. Oyster-colonized CORs induce more wave height attenuation than bare CORs, though they produce an increase in wave shoaling and local energy. Along the slope, wave height attenuation varies significantly, with differences between R = 0 and R = 1.61 reaching up to 34% downstream of the landward COR. Lower oyster density may optimize wave dissipation through edge vortex formation and undertow intensification. Furthermore, increased still water depth reduces wave height attenuation by oyster-colonized CORs but enhances oyster-induced energy dissipation. These findings offer critical insights for designing oyster reef restoration strategies that balance ecological and coastal protection goals.

Oyster reef living shorelines are popular solutions to mitigate coastal hazards while providing ecosystem services, and their hydrodynamic characteristics deserve further examination. This paper reports an experimental and numerical study of the hydrodynamic behavior of a bagged oyster shell (BOS) reef under regular waves. The dependence of the pore pressure within the reef and the hydrodynamic coefficients on the reef height, length, porosity, and wave parameters are investigated. The results show that the normalized maximum pore pressure ps_m∗ decreases from the seaside of the BOS reef to its leeside and decreases with increasing dimensionless reef length. With increasing wave steepness, ps_m∗ fluctuates with an overall increasing trend. With increasing BOS reef porosity, ps_m∗ decreases at seaside locations and increases at leeside locations; simultaneously, the wave reflection coefficient Kr and the wave dissipation coefficient Kd decrease, and the wave transmission coefficient Kt increases. With increasing wave steepness for a given wave period, Kr and Kt tend to increase, but Kd exhibits the opposite trend. As the dimensionless reef height and length increase, Kt decreases and Kd increases. Multivariate nonlinear regression and genetic programming-based symbolic regression methods are individually used to derive two formulas for predicting the wave transmission coefficient, with the formula from the latter method being found to give a higher prediction accuracy.

Oyster reefs play a dual role in the ecological and economic sustainability of global estuarine resources. Due to human activity and climate change, the prevalence of cosmopolitan oyster reefs has noticeably declined in recent decades, triggering a global restoration movement. However, the hydrodynamic functions of oyster reefs during and after restoration, particularly the impacts of growth and morphology on the flow field, remain poorly understood. This study employs the lattice Boltzmann method coupled with large-eddy simulation to simulate unidirectional flow around restored oyster reef models using the open-source Palabos library. It examines the effects of unidirectional flow velocity and reef morphology on hydrodynamic characteristics. The research analyzes spatial and temporal variations in velocity, vorticity, and turbulence structure around the reef. The findings indicated significant flow field differences between the initially restored reefs and those post-restoration. The dimensionless wake region scale parameters of the initially restored reefs exhibit hysteresis effects, generating larger turbulence during the post-recruitment stage than in the initial stage. Areas of high turbulence in the wake are associated with above-canopy flow, bypass flow, and within-canopy flow. The presence of gaps and branches in the reef leads to complex turbulence structures and irregular vortex shedding in the reef's wake at the post-recruitment stage. These results are valuable for assessing oyster reef resilience and planning effective restoration interventions.