Turbulence drives seabed modification by offshore windfarms

The drive to cut carbon emissions and harness renewable energy has spurred rapid industrialization of the seas. Current estimates of seabed modification due to offshore windfarms (ca. 1% of a windfarm's area) do not account for enhanced seabed mobility due to monopile wakes. Here we demonstrate how turbulence emanating from windfarm infrastructure mobilizes seabed sediments. Whilst mean flow conditions in our laboratory experiment could mobilize fine sand, near-bed turbulence from monopile's wake could mobilize sand 17 monopile diameters (D) downstream. Related bedform formation coincided spatially with downwelling of the monopile's turbulent wake, indicating that the 3D wake flow structure both creates and defines enhanced sediment mobility. We introduce a method for predicting this region of enhanced transport capacity by combining bed shear stress from near-bed turbulence with that from mean flow; and our field evidence at an operational offshore windfarm validates this approach. Our results show bed stress amplification is expected cover 3-8% of a typical windfarm area. Enhanced seabed mobility can alter seabed habitats and should be factored into impact assessment and marine planning.