Aarhus University Seal / Aarhus Universitets segl

No. 286: Effects of larger turbines for the offshore wind farm at Krieger’s Flak, Sweden. Assessment of impact on marine mammals

Tougaard, J. & Michaelsen, M. 2018. Effects of larger turbines for the offshore wind farm at Krieger’s Flak, Sweden. Assessment of impact on marine mammals. Aarhus University, DCE – Danish Centre for Environment and Energy, 112 pp. Scientific Report No. 286. http://dce2.au.dk/pub/SR286.pdf


Construction and operation of an offshore wind farm on the Swedish part of Kriegers Flak has been assessed with respect to impacts on marine mammals, in the context of an application to increase the size of turbines in the wind farm, compared to the original Environmental Impact Assessment from 2004.

Abundance of marine mammals

Two species of seals, harbour seal (Phoca vitulina, knubbsäl) and grey seal (Halichoerus grypus, gråsäl) are abundant in the waters around Kriegers Flak. Both populations are in favourable conservation status.

One cetacean, the harbour porpoise (Phocoena phocoena, tumlare) inhabits the waters around Kriegers Flak. The area is a mixing zone between two populations. The by far largest number of porpoises in the area are believed to belong to the Danish Belt Seas population, which is in favourable conservation status. A smaller number of porpoises, in particular during winter months, are believed to belong to the critically endangered population in the Baltic Proper.

Sensitivity to impact

Underwater noise is likely to be a main source of potential impact from windfarm construction, in particular percussive piling of foundations. 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 are available, including use of deterring devices, soft-start and reduction of radiated noise by means of for example air bubble curtains.

Magnitude of impact was assessed for direct damage (acoustic trauma), hearing loss (permanent threshold shift, PTS), disturbance of behaviour and masking. Hearing loss was assessed by considering total cumulated sound exposure levels (SELcum) over the duration expected for piling of one foundation (4.5 hours), taking movements of the animals into consideration and applying appropriate auditory frequency weighting to the acoustic measurements. Disturbance of behaviour was assessed by applying reaction distances from observational studies at other wind farm construction projects.

Impact from construction

Noise exposure from pile driving was modelled in a worst-case scenario. This scenario included the largest monopiles under investigation for the project (15 m pile diameter) and worst-case assumptions regarding location of the turbine foundation and sound propagation properties of the surrounding waters (upward-refracting sound speed profile). This scenario included standard practice of deployment of a deterring device and soft-start procedure as mitigation against damage to marine mammal hearing.

It is considered unlikely that marine mammals will be exposed to sound pressures likely to cause acoustic trauma.

Under this scenario, it is expected that seals and porpoises close to the pile-driving site will be exposed to levels capable of inflicting smaller amounts of permanent hearing loss. This hearing loss is assessed to have a minor impact on the populations of harbour seals, grey seals and porpoises from the Danish Belt Sea population. In a precautionary assessment, the impact on the critically endangered Baltic Proper population of porpoises is assessed to be moderate during winter months and minor during summer months. The difference in assessments is due to the seemingly very low likelihood of encountering porpoises from the Baltic Proper population around Kriegers Flak during summer.

Pile driving is also likely to cause disturbances to the natural behaviour of both seals and porpoises. The magnitude of this disturbance was assessed by relating the expected area of disturbance to the total area where the different species are regularly encountered. The largest impact from disturbance is expected for harbour seals and Baltic Proper porpoises in winter, where the impact is assessed to be moderate. The impact on grey seals and Belt Sea porpoises is assessed to be minor and for Baltic Proper porpoises in summer impact is assessed to be negligible, due to the very low likelihood of encountering porpoises from this population during summer.

Impacts can be reduced by reducing the radiated noise from the pile driving. This was investigated by modelling the noise impact by application of a bubble curtain, currently considered best available technology for reduction of noise radiation. The bubble curtain is expected to reduce the broadband source level of the piling by at least 5.5 dB and frequency weighted levels even more. Such a reduction in radiated noise means that permanent hearing loss is not expected to occur in neither seals, nor porpoises, and impact from hearing loss is thus assessed to be negligible on all populations.

Reduction of radiated noise from piling is also predicted to reduce both impact ranges and duration of the disturbance of behaviour. Impact from disturbance of behaviour is thus assessed to be minor for all populations, except for Baltic Proper porpoises in summer, where it is assessed as negligible, due to the low likelihood of encountering these animals.

It is considered unlikely that pile driving noise will be capable of masking sounds relevant to porpoises to any noticeable degree and the magnitude of this impact on porpoises was thus assessed as negligible

There is a possibility that communication sounds from both grey seals and harbour seals can be masked by pile driving noise, but as this communication is only expected to take place close to haul out sites (primarily at Falsterbo), the possible masking is considered to be small and impact assessed as minor.

Cumulative effects of simultaneous pile driving at one or more currently planned offshore wind farms in the area are considered negligible.  

Impact from operation

No negative effects of the wind farm is predicted once in operation and the effect is thus assessed as negligible, based on studies of effects of operating offshore wind farms on seals and porpoises. The cumulative effect of adding an additional offshore wind farm to already existing offshore wind farms in the area is likewise considered negligible.

Impact on Natura2000 areas

It is considered likely that the adjacent Natura2000 area Sydvästskånes Udsjövatten will be impacted during construction of the wind farm. Disturbance from pile driving is estimated to make 27% of the habitat area inaccessible to seals and porpoises (computed in both time and space) during the period where pile driving takes place, and is assessed to constitute a major impact. Application of mitigation measures to reduce the emitted noise during pile driving, such as bubble curtains, is estimated capable of reducing the habitat loss to 2.5%, and the disturbance in that case is assessed to constitute a minor impact.

Impact on Sydvästskånes Udsjövatten during operation of the wind farm is assessed as negligible, as are the impacts of both construction and operation on the remaining Natura2000 areas in Swedish, Danish and German waters.