Sediment Wakes Within Offshore Wind Farms Using Sentinel-2 and Landsat-8/9

Offshore wind energy has been widely accepted as a major component of renewable electricity to support Net Zero objectives and tackle climate change. This acceptance has led to an accelerated deployment of the offshore wind industry in the North Sea, with larger wind farm areas and bigger structures to support longer blades and more powerful turbines. As water flow encounters the foundations of structures, turbulence is generated, leading to the creation of visible downstream wakes. This redistribution of sediment may have an impact on water column processes, and near-field benthic communities.  Sediment wakes within the wind farm may also influence predator/prey interactions and could indicate areas to target or avoid for the location of aquaculture lines or other collocated activities.

The use of remote sensing techniques allows regular monitoring of turbidity in wind farm areas. A virtual constellation composed of Sentinel-2 (European Space Agency) and Landsat-8/9 (NASA) makes it possible to study sediment wake movements at high spatial and temporal resolution while considering tidal influence. Satellite images combined with current velocity, wave models, oceanography data and inherent properties of each turbine allow a better understanding of the parameters regulating the intensity of wakes.

Six offshore wind farms have been studied in two different sites: one in the UK (Lincolnshire) and the other in Belgium (Belgian EEZ) for a total of 269 turbines. A new method of pixel extraction has been developed to automatically extract turbid wakes around every turbine depending on the current direction. This was used to study the changes in suspended particulate matter (SPM) concentration within the wake. The British site, containing 3 wind farms, showed a strong turbine effect, whose intensity varied from 0 (absence of wake) up to 6 g.m^-3 of SPM concentration compared to the concentration upstream of the turbines, while the Belgian site, containing 3 wind farms, showed less variability and less intense wakes. As a preliminary result, and without isolating each parameter from the others, the type of foundation seems decisive for wake formation and SPM concentration at the surface: jacket foundations showed less intense wakes than monopiles and gravity-based structures (Mann-Whitney, p<0.001), showing that the shape and size of the foundation affect sediment resuspension. Also, and especially within the Belgian wind farms, wakes were more intense in winter/spring than during summer/autumn as storm events may create even more turbulence and enhance sediment resuspension (Mann-Whitney, p<0.01 and p<0.001).

A Generalised Linear Model incorporating current velocity, wind wave height, swell period and height, type of foundation, seabed morphology and seasonality was generated to explain the influence of physical and environmental parameters on sediment wake in wind farms and can help predict the SPM increase downstream of a structure.