Offshore wind farms (OWFs) are a key component of our journey to Net Zero; however, this must be done in a sustainable manner, so understanding their impacts on the marine environment is important. Where tidal flows interact with OWF their flow field can become dramatically modified. Satellite data has shown disturbances such as kilometre-scale wake effects on the sea surface. Yet little attention has been paid to far field effects on the seabed. The ECOWind-ACCELERATE project focusses on how OWFs can modify seabed processes, and consequently, the habitats and the ecosystem services they provide. Here, we integrate large-scale physical laboratory and 3D numerical modelling to better understand these impacts. A 3 m wide, 35 m long flume was used to monitor flow, turbulence, and bed changes, assessing the influence of different monopile diameters representative of present-day and future OWFs. Experimental runs were varied to represent differing levels of bed mobility.
Under mean flow conditions that would otherwise fall below the threshold of motion, new bedform fields were generated from the turbulence produced in the wake of the monopile. These bedform fields extended up to 17x downstream and 7x as wide as the monopile diameter; and longer experimental runs would likely have likely extended them further. Under mobile bed conditions the effect was less than expected. Changes to the bed extended to 14x the monopile diameter downstream, suggesting widespread existence of bedforms can reduce the effect of the monopiles turbulent wakes on near bed dynamics. A suite of velocimeter data from the flume laboratory tests were used as the basis for a 3D Telemac model of the flume laboratory environment. We then assessed how much of this induced sediment mobility is due to the turbulent wake from the monopile and rock armour, compared to that generated by the bedforms themselves.

Our work indicates that installing offshore wind farms on seabeds that would otherwise be immobile can create newly active and dynamic seabeds with bedforms extending far beyond the monopile. This can modify existing habitats and potentially create new ones. Our findings provide a new evidence-base concerning the modification and recovery of the seabed around offshore wind farms, with implications for benthic biodiversity and opportunities for marine net gain.