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Figure 11: Overview of the ship traffic in the area around Kriegers Flak. Yel-low-red-blue lines indicate shipping lanes with increasing load.
Other sources of underwater noise
Several other sources of underwater noise of variable nature will be present throughout the whole or parts of the construction site. This includes side-scan sonars, echosound-ers, Doppler logs, Doppler current profilers and underwater communication with divers.
All of these, apart from underwater communication, include emission of very powerful sounds in various frequencies, of those <180 kHz will be detectable for porpoises and seals. Side-scan sonars are likely to constitute the biggest impact on harbour porpoises that are capable of hearing in the high frequency range up to around 150 kHz. Source levels are often very high and as signals are pointed forward and/or sideways, instead of downwards as in normal echo sounders, may affect both porpoises and seals in an area in front/sideways of the boat, particularly in narrow channels. A case of mass strandings and deaths of melon-headed whales (Peponocephala electra) in relation to the use of hydrographic survey sonar have been well documented (Southall, Rowles, Gullard, Baird,
& Jepson, 2013). The precise causal link between sonar and strandings is however not clear.
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appearance of the rotating wings has been suggested as a factor potentially affecting porpoises (Teilmann & Carstensen, 2012).
Noise from operating wind farms
Based on measurements of existing offshore wind turbines, the noise from the operat-ing wind turbines is expected to be of relatively low intensity and frequency. A number of measurements from different turbines exist and all share common features of low ab-solute sound levels and no significant energy at frequencies lower than 1000 Hz (Betke, 2006) (Madsen, Wahlberg, Tougaard, Lucke, & Tyack, 2006) (Tougaard, Henriksen, &
Teilmann, 2009). Apparently, there is little difference in the radiated underwater noise from monopile and gravitational foundations. One example from Horns Reef is shown in Figure 12. One measurement which stands out is from Utgrunden wind farm (Ingemans-son Technology AB 2003). Noise from these turbines is considerably higher in intensity (approx. 10 dB) and with considerably more energy at higher frequencies than emissions from the other wind farms. The reason why these turbines differ from the rest is un-known, but may have to do with the foundation on solid bedrock, in contrast to the hard sand at the other wind farms.
Figure 12: Measurements of noise from turbine in Horns Reef Offshore Wind Farm running close to maximum power rating (left) and at low level (right). Turbine noise consists of multiple peaks at discrete frequencies, which rise above the background noise. From: Betke (2006).
Noise from service and maintenance activities
Another potentially disturbing factor during operation of the wind farm is service and maintenance of the turbines, where small, fast boats commute between land and the wind farm, as well as between the wind turbines. Although activity levels will be much lower than during construction, the nature of this disturbance is nevertheless likely to be qualitatively similar to ship traffic during construction.
15.6 m/s 18.1 rev/s 2 kW output
5.9 m/s 12.1 rev/s 0.3 kW output
39 Electromagnetic fields
Any cable carrying current will be surrounded by an electromagnetic field. The magnetic part of this field adds to the natural magnetic field of the earth and thus has the poten-tial to interfere with magnetic and electric orientation in the vicinity of the cable.
The cables at Kriegers Flak will consist of three conductors carrying three phases of al-ternating current (AC). Each conductor generates its own alal-ternating field and in theory the three fields should cancel out each other. Due to the geometry of the cable, they do not cancel out completely, but the total field is nevertheless considerably weaker than from a single conductor cable. The size of the magnetic field from the same type of sea-cable connecting Nysted Offshore Wind Farm to land has been calculated to approxi-mately 5 μT on the sea bottom one meter above the cable when the wind farm runs at maximal capacity (cable carrying 600 A (Eltra, 2000)). The natural magnetic field in Den-mark is approximately 50 μT (EnergiNet.dk). The magnetic anomaly introduced by the cable is thus limited and local around the cable.
Visual appearance
The foundations below water and the turbines above water represent a change to the visual scene of the area and it could be hypothesized that this could deter seals and por-poises from the area. The visual impact underwater is likely to be minimal in the operat-ing wind farm. Underwater parts of the foundation and scour protection quickly become overgrown with algae and epifauna and will thus visually resemble other reef-like struc-tures in the sea. In air, the more than 100 m high turbines with their rotating wings rep-resent a major change to the visual scene and shadows cast by the wings in bright sun-shine will be visible in the water and hence perceptible to seals and porpoises.
Changes in the habitat
The construction of an offshore wind farm on sandy bottom will inevitably cause chang-es to the habitat. First of all is the direct loss of habitat to foundations and scour protec-tion. The absolute size of the area covered by foundations and scour protection is mar-ginal however, and any effects on the habitat are likely to be overshadowed by the changes that will occur as a consequence of introduction of hard substrates, that extend up into the water column. These will inevitably be colonised by algae and filter feeding epifauna and create an artificial reef (Petersen & Malm, 2006) and represent a perma-nent enrichment of biomass and biodiversity. Studies on colonisation of foundations at Nysted Offshore Wind Farm have shown that the species composition on the turbine foundations is identical to the species composition at a proximate natural stone reef (Schönheiders Pulle; (Birklund, 2005)).
Exclusion of fishery
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For reasons of safety (for fishermen and installations) restrictions on bottom trawl inside the wind farm are likely to be imposed. This will possibly increase fish diversity and den-sity and thus add to the improvement of the habitat, also because of the turbine foun-dations that are likely to attract fish to the area. Hence, wind farm areas may have a positive effect on the fish community and by extension, marine mammals. However, more fish may increase gillnetting activities which may result in an increase in by-catch of especially porpoises.
Permanent effects on seal haul-out
Permanent effects of the wind farm on seal haul-out can occur either through physical changes or through an increased level of disturbance to the seals (Figure 8). Physical changes to the haul-out sites, as a direct consequence of the wind farm, seem very un-likely as the wind farm is located more than 30 km away from the nearest known haul-out site at Falsterbo. Increased disturbance of the seals in the water may result in dis-placement of animals from the area and hence, from the haul-out site. No long term ef-fect was observed at the seal haul-out site Rødsand as close as 4 km from the Nysted Offshore Wind Farm (Edrén, et al., 2010).
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