Turbulence and bed shear stress over oyster-bed roughness

Oysters protect the seabed from erosion and enable a rich habitat for other species. Direct human actions and climate change, however, have contributed to a decline in the population of Ostrea edulis, an indigenous species commonly known as the flat oyster. The species, which thrived just 150 years ago, is now classified as threatened, and nature-based solutions are being implemented to restore the population to its native environment. A viable nature-based approach is to make use of the material often employed to protect the foundations of large marine offshore structures, enabling the development of oyster reefs on these submerged structural elements.

Flat oyster reefs are characterised by sharp edges and highly varying roughness, which create complex flow patterns and alter oyster feeding efficiency. To understand the operational and optimal conditions for oyster reef stability and growth, an experimental study of flow turbulence on top and around oyster reefs is carried out.

A rigid ultra-rough bed representing oyster reefs over a scour-protection armour layer is installed in a wave flume, and tested under waves, current, and combined wave-current forcing conditions. Wave height time-series and high‑resolution vertical velocity profiles are collected using synchronised wave gauges and an acoustic doppler velocity profiler. The effects of regular wave forcing and steady flows on the near‐bed roughness are then measured.

Maximum bed shear stress, turbulent kinetic energy (TKE), hydraulic roughness and wave friction factors vary depending on the shape of the armor layer rocks and oyster reef elements, which lead to different frictional energy dissipations. To improve turbulence and bed shear stress estimates, near‑bed velocity fluctuations and vertical velocity profiles are measured. Results include phase‑resolved horizontal velocity profiles, Reynolds shear stresses, and TKE computed from resolved velocity fluctuations. They indicate that roughness increases near‑bed turbulence intensity and TKE production during wave crest passage. The bed shear stress exhibits phase dependence, and stress peaks occur slightly after maximum orbital velocities.

Future work will combine experiments with numerical simulations to refine bed shear stress and TKE parameterisations, and extend the analysis to more complex shapes and oyster cluster configurations.

Acknowledgements: this research is conducted within the project entitled REEFCOVERY funded by the Flemish Government VLAIO and supported by The Blue Cluster under project with reference number HBC.2023.0394.