As part of the Danish Governmental Plan for Renewable Energy, permission was given in 2005 to carry out an Environmental Impact Assessment for the establishment of a new offshore wind farm at Horns Rev.
Knowledge about potential impacts on the marine benthic communities from the establishment and operation of offshore wind farms is available due to the demonstration projects carried out at Horns Rev 1 Offshore Wind Farm. However, provision of supplementary information was found necessary regarding the existing habitats and benthic communities, which include benthic vegetation and invertebrate communities, in the designated wind farm area.
Two alternative sites are designated for the wind farm at Horns Rev 40 km west of Blåvands Huk. Both sites cover an area of approximately 35 km2 and the water depths range from 5-15 m. The sediment in the wind farm areas display large variability and surface sediments consist of pure medium to coarse sand that is constantly reworked by waves and currents. Along the top of the reef and in shallower parts that are strongly exposed to waves, the sediment is more sorted compared to deeper parts where the sediment is coarser due to exposure to strong currents. Bedforms of small and large sand riffles caused by wave action and evidence of sand transport are found all over in the area. In the northern part of the designated sites, the sediment is generally finer closer to the reef. No unambiguous relationship between the depth regimes and the sediment structure is found at the different sampling sites in the wind farm area.
No vegetation, and no rare and endangered species, is found within the designated wind farm areas. The variations of the benthic infauna composition and community structure reflect the heterogeneous sediment in the area. In general, the benthic infaunal community in the Horns Rev area can be characterised as the Goniadella-Spisula or the shallow Venus community. These two communities are commonly found at sandbank where the seabed consists of relatively coarse sand and hydrographical conditions are turbulent. In the northern part of the designated wind farm area, the sediment generally shows a more uniform character with finer sand. In such areas, a more typical Venus community is found. Even within short distances, differences can be found in the community structure resulting in subdivisions of the main communities inside the designated wind farm areas, which reflects the character species’ preferences for different sediment characteristics.
In the Horns Rev area and the wind farm areas, more epifaunal species can be found including the brown shrimp (Crangon crangon), which is object of commercial fishing. The benthic communities in the Horns Rev area are generally influenced by trawling and dredging activities. Dredging for the character clam species (Spisula solida) and trawling for sandeels are the main fishing activities in the area.
The wind turbines will be founded by use of either monopiles or gravitation foundations. The main impacts on benthic communities from the activities in the pre-construction, construction, operation and decommissioning phases are considered equal for the two foundation types. The sources of impact that are similar to both types of foundations include noise generated from piling activities. However, additional sources of impact from dredging activities related to the establishment of gravitation foundations include increased smothering and suspended sediments.
In the pre-construction and construction phases, it is expected that noise and vibrations from pile driving activities may have a temporary and negligible local impact on the benthic communities and a very local and negligible destructive effect on infaunal species.
Smothering and increased suspended sediment from dredging activities is expected to have a temporary local negligible effect on benthic communities due to the general loss of fine sand. Benthic communities generally show a high tolerance to smothering with a presumed high recovery rate. Loss of seabed with native benthic communities and change in substrate type during construction and operation is less than 0.2% of the total wind farm area. The change of habitat type and change from sandy infauna communities to epifouling communities are expected to be local and of minor significance. The deployed hard substrate will rapidly be colonised with algae and invertebrates, which is known to increase the biodiversity in the wind farm area. The succession will increase the diversity over a period of 5-6 years after deployment of the hard substrates, at which time the communities are expected to reach climax.
The physical presence of the wind turbine foundations will have a very local, minor, but permanent effect on the benthic community structure due to changes of the hydrodynamics near the turbines. During operation, significant effects from noise and vibration are not expected. Effects from electromagnetic fields are considered negligible, although migrating crabs, believed to be sensitive to the Earth’s magnetic fields, may be affected.
Effects during decommissioning are generally considered as the same during construction but in the reverse order.
In the operation phase, cumulative impacts are be expected as a consequence of reduced trawling activities inside the wind farm sites, which will be beneficial to benthic communities by enabling very sensitive species to establish and all species to mature more undisturbed. The introduction of more consolidated substrates from more offshore wind farms may generate a cumulative effect by introducing higher species richness and faster colonisation of specific and potentially vulnerable species to newly deployed foundations. No cumulative effects on benthic communities are expected from simultaneous sand and aggregation activities and construction activities.
No specific mitigation measures are necessary because rare or endangered species are not found and only minor impacts from construction, operation and decommissioning activities are expected on the benthic communities inside the designated wind farm areas.