Gary Littler

Marine aquaculture structures alter local hydrodynamics through flow deflection, wake formation, and changes in near-bed shear stress, with potential implications for water exchange, benthic conditions, site selection, and environmental assessment. This study applies a three-dimensional unsteady Reynolds-averaged Navier–Stokes computational fluid dynamics framework to investigate how idealised fish-cage configurations influence flow fields, wake persistence, and kinematic bed shear stress. The numerical implementation was first benchmarked against analytical potential-flow solutions for single and multiple cylinders, with a mean relative velocity error of 1.47% for the benchmark cases. Farm-scale simulations represented circular cages using 38 m diameter discs and examined the effects of inlet velocity, cage diameter, layout, farm size, cage spacing, seabed roughness, and biofouling. Increased inlet velocity generated the largest single-cage bed-stress response, increasing peak kinematic bed shear stress by 139% relative to the medium-velocity baseline. Farm layout strongly affected wake persistence: a linear nine-cage arrangement increased wake length by 354% relative to the grid baseline, whereas staggered and honeycomb layouts reduced wake length. Biofouling increased peak bed shear stress by up to 24%, while larger cages and farms produced more persistent downstream disturbances. No scenario exceeded the adopted bed shear-stress threshold for sediment mobility. The results demonstrate that a computationally efficient, numerically verified, idealised Computational Fluid Dynamics (CFD)framework can support comparative assessment of relative trends across aquaculture farm layouts and operational conditions, informing sustainable farm design, environmental assessment, and marine spatial planning.

Shipwrecks are important cultural heritage sites offshore. In many instances, given their often long-term emplacement on the seafloor, they offer natural laboratories to study complex interactions between human-induced obstacles and seabed dynamics. Such interactions and induced sediment mobility also pose significant threats to offshore engineering infrastructure, such as turbine monopile foundations. Traditional methods can struggle to capture the nuance of these processes, with real-world surveys measuring effects only after installation, and laboratory models suffering from scale-down inaccuracies. Computational fluid dynamics (CFD) modelling offers an effective means of investigating the effects of obstacles on seabed dynamics, and by using shipwrecks as proxies for infrastructure, it can utilize long-term datasets to verify its predictions. In this study, high-resolution temporal bathymetric data were used in, and to verify, CFD modelling to investigate the interactions between hydro- and sediment dynamics at a shipwreck site in a tidally dominated wreck site. From this comparison, simulations of bed shear stress and scalar transport correlate well with known areas of erosion and deposition, serving as a basis for future scour prediction studies and creating effective tools in offshore renewable infrastructure planning and de-risking.