Construction of an offshore wind farm is an operation of considerable magnitude and includes several components which may potentially affect seals and porpoises. Negative effects on the local abundance of harbour porpoises and to a lesser degree seals have been documented at previous construction works (see sections 7.1 and 7.2 below). A long-term negative effect of wind farm construction has been suggested for porpoises only. Specific effects of pile driving have been documented for both seals (Edrén, Wisz, Teilmann, Dietz, & Söderkvist, 2010) and porpoises (Tougaard, Carstensen, Teilmann, Skov, & Rasmussen, 2009) (Brandt, Diederich, Betke, & Nels, 2011) and this activity is likely to be the most disturbing and possibly injuring activity during construction. There-fore, pile driving will be assessed as the worst case scenario.
The seabed inside the wind farm area is inevitably disturbed during construction. This disturbance occurs by direct removal and redistribution of sediment in connection to es-tablishment of foundations and burying of cables. Suspension of bottom material is un-likely to affect seals and porpoises directly, but may have an indirect effect on local fish and bottom fauna on which these marine mammals feed.
No significant chemicals harmful or unpleasant to seals and porpoises are likely to be re-leased into the water during normal construction activities and thus will not constitute a risk to marine mammals. Therefore, effects of chemicals are not dealt with specifically in this assessment. However, accidental spills of oil or other substances released due to
er-34
rors or accidents during construction could potentially cause considerable damage to the local ecosystem and hence also seals and porpoises.
Noise from pile driving
Below, general descriptions of two types of pile driving are considered, however, for the actual impact assessment, only the worst case scenario of 10 MW monopiles is consid-ered. The two types are piling of steel monopile foundations and jacket foundations.
Even if gravitational foundations are used, some piling may also be needed in order to stabilise the seabed below the concrete foundations with a sheet pile wall or similar, as was the case for a single foundation out of 72 during construction of Nysted Offshore Wind Farm. The magnitude of sound emission of this type of piling is much lower com-pared to steel monopiles.
Pile driving, by which steel monopiles are driven into the seabed with a large hydraulic hammer, generates very high sound pressures. Measurements made at Horns Reef II Offshore Wind Farm during piling of one foundation; a 4m diameter steel monopile, shows that peak to peak sound pressure levels are over 190 dB re 1 μPa at 720 meters form the construction site (Figure 9).
Most energy of the pile driving sounds is at low frequencies, where especially porpoises and to a lesser degree seals have poor hearing. It is nevertheless evident from the spec-tra in Figure 9, that there is significant energy present in the signals well into the range of best hearing for porpoises and seals (see Figure 20 and Figure 48).
35
Figure 9: Left: Peak level and single-stroke Sound Exposure Level (SEL) for the whole pile driving operation of one monopile at Horn Reef II measured at 720 m distance. Also shown is the M-weighted (frequency weighting pro-cedure to take the hearing abilities of marine mammals into account) cu-mulative SEL (added energy of multiple exposures). The difference between the non-cumulative unweighted and M-weighted SEL varied from ~4 to 7 dB. Right: Spectra of pile driving noise at two measurement locations, aver-aged from 24 blows (locations can be seen at Figure 72). Source: Brandt, Diederich, Betke, & Nels, (2011).
Construction of jacket foundations will also require some piling, but the sound pressure level will be lower than for steel monopiles because of the smaller diameter of the pile (see also section 7.1). There is a general correlation between pile size and source sound pressures (Figure 10).
36
Figure 10: Measured peak levels and broadband SELs at 750 m versus pile diameter from various pile driving operations. Compiled by Matuschek &
Betke (2009).
Noise from ship traffic
During construction there will be an increased traffic of smaller and larger boats and ships to and from the construction site. The most significant impact from this traffic will be elevated levels of underwater noise. The effect of ship noise on marine mammals is not well studied, so no good estimates of the magnitude of impact can be given. How-ever, small and fast service vessels are likely more disturbing, due to the higher speed and frequencies emitted, than larger ships with slowly revolving propellers and lower frequency noise. In general, the faster the propellers are rotating, the higher the pitch of the noise (Richardson W. , Greene, Malme, & Thomson, 1995) (Erbe, 2002) and thus the more audible it is to seals and especially porpoises (see sections 6.1 and 6.5 below). The background noise in the Kriegers Flak area is generally dominated by low frequency shipping noise illustrated by the heavy traffic as seen in Figure 11.
37
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.