Shelf seas comprise approximately 7% of the world’s oceans and are the focus of an enormous amount of economic activity; they hold around 20% of all life in the ocean, and sustain around 90% of the world’s fisheries.

Image of the sea around the Walney, Ormonde, West of Dudden Sands and Barrow wind farms west of Morecombe Bay from the Landsat 8/OLI satellite on the morning of 23rd June 2014. The image shows the color of ocean and the turbid wakes (darker areas of water) generated by the wind turbines extending south-east away from the farms due to the tidal flow. Credit: PLM

They are used for recreation, oil, gas and gravel extraction and more recently renewable energies in the form of wind, wave and tide. The development of Offshore Wind Farms (OWFs), in particular, has grown as demand for renewable energy has increased (in UK waters 1465 offshore wind turbines have already been installed). Overall the environmental impact of renewable energy is expected to be overwhelmingly positive, via the reduction of greenhouse gas emissions and the reduction of climate change. However to date there has been little evidence gathered of the local to regional impact from these installations; it is vital to understand such impacts as the numbers of farms increase worldwide.

A new study from Plymouth Marine Laboratory scientists sought to fill these gaps in our knowledge by using mathematical models to predict the impacts of offshore wind farms in coastal seas. Most previous studies have only focused on small-scale domains, for example a single wind farm. This study investigated impacts at scales from an individual turbine to the whole shelf sea based on seven existing Irish Sea OWFs with a total of 242 turbines.

The PML scientists found that although each individual turbine is small, cumulatively they can change the local flow of water, with the turbines creating more mixing of the water in the vicinity of the offshore wind farm. This changes the layers of water which form around the wind farms (stratification), potentially altering local ecosystems. As the majority of future offshore wind expansion in the UK happens coincidentally to be focused in regions which are seasonally stratified, this work is crucial in order to inform decision making on how those new developments are implemented, in order to best optimize their siting.

The model also shows that the OWFs alter the heights of tides. These changes (both increases and decreases) extend around the whole UK coastline. Unlike previous models, which have restricted the domains investigated to the immediate area around OWFs, this study expanded the domain and indeed found so-called far-field effects as far away as off the south-east coast of England. This showed increases in a component of the tide of 1%-2% (about 6cm-12cm). Small changes of a few centimeters might have consequences for coastal habitat. Also given the economic investment around parts of the UK coast, any potential change in tidal height may redistribute flooding risks. For example, the Thames Barrier protects parts of central London and is seeing increasing deployments since its introduction. This work suggests a potential contributing factor to that increase in addition to factors such as increased storminess, climate change and changes in river discharge.

Given that the OWFs used in the model are relatively modest yet still demonstrate both local and far-field impacts, this research, the scientists say, will help to inform developers of future, potentially larger, offshore wind farms and the regulators who decide on the locations of the developments. It is also of interest to the insurance industry and the public at large due to changes in the tides and possible impacts on flood risk.

Further Information

Cazenave, P.W., Torres, R., Icarus Allen, J., Unstructured grid modeling of offshore wind farm impacts on seasonally stratified shelf seas, Progress in Oceanography (2016)

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