Kyhn, L.A., Tougaard, J. & Mikaelsen, M.A. (2021). Kattegatt Syd Offshore Wind Farm. Effects of pile driving, gravity foundations and sediment spill on marine mammals. Aarhus University, DCE – Danish Centre for Environment and Energy, 167 pp. Scientific Report No. 447 http://dce2.au.dk/pub/SR447.pdf.
Construction and operation of a proposed offshore wind farm at the Swedish site Kattegatt Syd between the two Natura 2000 sites Lilla Middelgrund and Stora Middelgrund and Röda Bank has been assessed with respect to impacts on marine mammals and Natura 2000 sites.
Abundance of marine mammals
One cetacean, the harbour porpoise (Phocoena phocoena, tumlare) is common in the waters of the Kattegatt Syd offshore windfarm site. In the southern part of Kattegat, these porpoises belong to the Belt Sea population which is assessed on national red lists as Least Concern (LC).
Two species of seals, harbour seal (Phoca vitulina, knubbsäl) and grey seal (Halichoerus grypus, gråsäl) use the area. The harbour seal is common and red-listed as Least Concern (LC), whereas the grey seal appears in low numbers and red-listed as Vulnerable (VU).
Sensitivity to impact
Underwater noise is likely to be the main source of impact on marine mammals from wind farm construction, but the impact of sediment spill is also assessed. Unabated percussive pile driving is known to generate very high sound pressures, likely capable of inflicting permanent damage to the hearing of seals and porpoises and has been shown to cause behavioural disturbances at distances of tens of km from the pile driving site.
Various mitigation measures for pile driving are available, including use of deterring devices, soft-start and reduction of radiated noise by means of for example air bubble curtains and other noise abatement systems.
Magnitude of impact on harbour porpoises and seals was assessed for sediment spill, as well as for effects of underwater noise from installation of foundations by pile driving. The assessed effects are direct damage (acoustic trauma), hearing loss (permanent threshold shift, PTS), disturbance of behaviour and masking of other sounds. Hearing loss was assessed by considering total cumulated sound exposure levels (SELcum) over the duration expected for piling of one foundation (14 m diameter, app. 6 hours), taking movements of the animals into consideration and applying appropriate auditory frequency weighting to the acoustic measurements. Disturbance of behaviour was evaluated through assessing area and time exposed to levels above the reaction threshold.
Impact was also assessed for the nearby Natura 2000 sites. Here, the impact was assessed as the area over which the noise level exceeded the reaction threshold of harbour porpoises. The impact was assessed in accordance with the guidelines recently put forward by the British Joint Nature Conservation Committee (JNCC) (JNCC 2020b).
Impact from construction
For the noise exposure assessment, modelling was performed for three positions (north, middle and south) and two seasons (Summer/July and Winter/December). The two seasons were picked to represent the most extreme hydrographical conditions with respect to sound propagation, with December being the worst (upward-refracting conditions) and July the most favourable (downward-refracting conditions) due to a complete mixing of the water column at this time of the year, which leads to less favourable conditions for long-range sound propagation (iso-velocity, or downward-refracting conditions).
Modelling was performed for a monopile diameter of 14 m, which at the time of writing this report was the worst-case scenario for the monopiles to be installed.
Based on experience from similar projects it was assumed that construction will require substantial mitigation of noise impact, in the form of noise abatement systems. Scenarios without noise abatement was therefore judged as unrealistic and therefore not included in the assessment. Modelling was thus performed for pile driving with industry standard noise abatement (Big Bubble Curtains) and with best available technology (BAT) for noise abatement. Currently, the technology identified as best available is a system of Hydro Sound Dampeners and Double Big Bubble Curtain (HSD-DBBC system) deployed around the monopile to reduce radiated noise from the piling. The results of modelling using the HSD-DBBC system also pertains to piling with an equivalent, but other abatement system that mat be developed before the windfarm will be constructed.
Assuming use of the assessed noise abatement systems, the following can be stated from the assessment:
Impact from operation
There is a lack of long-term studies examining the effect on harbour porpoises of operating offshore windfarms, especially taking service vessels into account, as well as the increase in size of the turbines. The existing studies varies in effect from attraction (likely due to a lack of trawling) to a reduced number of animals as compared with reference stations. The impact is therefore assessed with some uncertainty to negligible for harbour porpoises, and it would be of benefit if the area were to be closed off for all fishing activities. Based on studies of effects from existing offshore wind farms in operation, no negative effects of the wind farm is predicted on seals once in operation and the effect is thus assessed as negligible. The cumulative effect of adding an additional offshore wind farm to already existing offshore wind farms in the area is likewise considered negligible for the seal population in Kattegat.
Impact on Natura 2000 sites
The Natura 2000 sites Lilla Middelgrund, Stora Middelgrund & Röda Bank, could be affected by construction of the offshore wind farm. Under worst case conditions, which is pile driving abated with Big Bubble Curtains in December, both sites will be affected by exposure to noise levels above the behavioural reaction thresholds for porpoises to an extent of more than 20 %, which is used as the recommended maximum disturbance threshold as put forward by JNCC and used in the absence of national guidelines from Sweden. This is according to the JNCC guidelines assessed as an unacceptable impact.
Application of Best Available Technology for noise abatement during pile driving, which would be Double Big Bubble Curtains in combination with hydro sound dampeners (or similar), and piling during a period with a downward refracting sound speed profile in July (Best Environmental Practice) will reduce the emitted noise considerably. This would reduce the fraction of the Natura 2000 sites Lilla Middelgrund and Stora Middelgrund & Röda Bank exposed to noise levels above the behavioural reaction threshold, to between 3 - 5 % for 14 m monopiles. The impact is according to the JNCC guidelines thus assessed to be acceptable.
Under worst-case conditions vessel noise from installation of gravity based foundations none of the nearby Natura 2000 sites will be affected by the vessel noise above the 20 % threshold and the effect is assessed as acceptable