Upscaling positive effects of scour protection in offshore wind farms

Abstract

The Dutch government has the ambition to realise 3,5 GW in offshore wind farms before 2023. Longer term outlooks estimate 35 to 75 GW for 2050. This would result in the construction of 5,000 wind turbines in the Dutch North Sea. The introduction of artificial hard substrates by placing wind turbine foundations and scour protection facilitates epibenthic species and associated benthic fish. The addition of rocks could be considered as adding H1170 habitat (‘Reefs of open sea’) and as such has a positive impact on hard substrate associated benthic macrofauna and fish. The significance of this effect, however, is unclear on a scale outside the scour protection. This report describes the result of a quick-scan in which the significance of this positive effect is quantified. Negative impacts and effects of the turbine foundation and other man-made structures were not included in the quick-scan.

The aim of this quick-scan was to answer the following questions:

  1. What is the biomass of benthic macrofauna on scour protection found in monitoring programmes to date?
  2. Which benthic species, including macrofauna and hard substrate associated fish, have been found to date on and associated with scour protection?
  3. When 5,000 wind turbines are installed in the Dutch Sea, each with 2,000 m2 scour protection, would this significantly change the benthic communities? a. If not, what surface area would be needed to attain a significant change?
  4. To what extent are benthic species’ populations on offshore wind turbine foundations interconnected?
  5. What knowledge gaps prevent answering questions 1-4 with acceptable scientific certainty?

The questions were addressed by desk study combining data from scour protections and soft sediment seabed monitoring (MWTL). Biomass was extrapolated to the 2050 scenario by multiplying single turbine data to 5,000 turbines.

Values of biomass on scour protections were compared with the benthic community of a sandy seabed, with the following results:

  • Total epibenthic species richness on scour protections may double when introducing scour protection at locations other than the current locations;
  • Epibenthic biomass in the area covered by scour protection directly around a turbine (2,000 m2) rises 24 times. On a wind farm scale (72 km2), this increase is 4.9%.
  • 5,000 offshore wind turbines, with a total of 10 km2 scour protection, will increase total benthic biomass in the Dutch North Sea (57,000 km2) with 3,400 tonnes (0.43%).
  • To attain an increase of 5% on this scale, a total of 106 km2 of scour protection should be added.
  • The Dutch edible crab population may increase with 50 million individuals, an increase of 880% of the population on the sandy seabed.
  • Fish may increase with hundreds of thousands of Atlantic cod, and many millions of smaller reef-species such as rock gunnel and goldsinny wrasse.
  • Connectivity between populations of benthic species rises after the construction of wind farms but quantification is challenging due to differences in reported larval durations and lack of reported travel distances.

In general, data availability for scour protection species was low. Biomass data was available from only 5 locations and only a single fish dataset was formatted in a manner that could be extrapolated. The quick scan approached ignored potential variation caused by different scour protection types, negative impact on fish or infaunal benthic species, environmental differences and presence of additional man-made structure. These relevant variables need to be addressed in future research.

To increase our understanding of the ecological importance of wind farms with their turbines and scour protection, future monitoring should focus on the following:

  • Understanding species ecology and system ecology, e.g. by starting a broad study of the impact (including turbines and scour protection) on the wider ecosystem and food web.
  • Obtaining more field data on epibenthic fauna and fish on scour protections and connectivity between locations.
  • Making industry data available to scientists to increase data availability.
  • Investigate different forms of scour protection as well as ecological limitations of species, including attraction and production effects (including epifauna on foundations).
  • For fish studies, more use of specific reef-fish sampling methods, such as baited video and fykes or traps, is recommended. All results should be considered against a background of sandy seabed covered with thousands of other man-made structures, including shipwrecks, buoys and platforms.

We advise TKI Wind op Zee and RVO to focus their funding scheme on the following topics:

  1. Assessment of the effects of currently used scour protection types on biodiversity, in particular epibenthic species and scour protection associated fish, including quantification of biomass, numbers and densities.
  2. Investigation of the impact new scour protection specifications in the current tenders for new offshore wind farms have on biodiversity, in particular epibenthic species and scour protection associated fish, including quantification of biomass, numbers and densities.
  3. To evaluate the ecological success of the new scour protection specifications, results from the study on currently used types of scour protection should be compared to the altered types.