Enhanced bed shear stress and mixing in the near wake of an offshore wind turbine monopile

Abstract. Tidal flow past offshore wind farm (OWF) infrastructure generates a turbulent vortex wake that is hypothesised to enhance seabed stress and water column turbulence mixing, and thereby affect seabed mobility, water column stratification, the transport of nutrients and oxygen, and result in ecological impact. We collect novel hydrodynamic data 40 m from an OWF monopile over a spring-neap cycle, and use high frequency velocity measurements to quantify turbulence. Outside of the wake we observe a classical depth-limited boundary layer, with strong turbulence production and dissipation forced by tidal shear at the seabed. Inside the wake, turbulence production, dissipation and stress are enhanced throughout the full water column and are maximised above mid-depth where they correspond to a strong mean velocity deficit. Our results show that the seabed drag coefficient is doubled from C_d = 3.5 x 10^-3 to 7.8 x 10^-3, suggesting greater seabed mobility, and the eddy viscosity is increased by an order of magnitude indicating enhanced water column mixing. This research provides some valuable insight as OWFs expand into deeper seasonally stratified waters using floating structures, where the addition of enhanced wake turbulence may have broad impacts as the additional mixing energy is added to regions with low rates of background mixing.