Noise from construction activities

The main sound source during construction will be noise emitted by pile driving. But in-creased ship traffic during the construction phase and dredging activity for sea bed prep-aration will add to the noise level in periods in-between pile driving. Increased sound lev-els from shipping traffic and dredging noise with source levlev-els of 155 to 181 dB re 1µPa at 1 m distance (Table 4.3) are likely to affect the behaviour and distribution of marine mammals at the construction site.

The underwater noise generated by pile driving during installation has been measured and assessed during construction of wind farms in a number of locations. Important fac-tors influencing the emitted noise levels are - among others -pile diameter and seabed conditions.

It was agreed for the Horns Rev 3 EIA to standardize ‘source’ level to a distance of 750 m as this is better comparable to other data as it avoids nearfield problems in calculating noise levels which have been measured in greater distance to 1 m from the source.

Therefore the noise model at Horns Rev 3 is based on a sound source of 181 dBSEL at 750 m distance (Table 4.10).

Table 4.10: Peak and SEL values at 750 m distance during pile driving at different pile driving positions (Mason & Barham 2013).

Level at 750 m dB_peak dB_SEL

North East 198.1 181.0

South 196.9 180.4

Figure 4.6 and Figure 4.7 present the impact ranges from installing a 10 m diameter pile by impact piling at both northeast and south modelling locations. The contours show where the noise levels are expected to fall between 180 to 145 dBSEL. The impact ranges for the two pile driving locations are also summarised in Table 4.11 and Table 4.12.

Figure 4.6: Noise contours from offshore pile driving of a 10 m monopile at a northeast position of Horns Rev 3 (Mason & Barham 2013). The noise emission is calculated for maximum effect from ramming at the end of piling.

Table 4.11: Estimated ranges for underwater noise transmission from installing a 10 m diameter pile at the Northwest (deep water) modelling location at Horns Rev 3 (Mason & Barham 2013).

Northeast 180 dB

SEL

165 dB SEL

150 dB SEL

145 dB SEL

Max Range 840 m

6.2 km 21.5 km 28.8 km

Min Range 810 m

6.0 km 16.7 km 20.3 km

Mean Range 830 m

6.1 km 19.0 km 24.4 km

Figure 4.7: Noise contours from offshore pile driving of a 10 m monopile at a southerly position of Horns Rev 3 (Mason & Barham 2013). he noise emission is calculated for maximum effect from ramming at the end of piling.

Table 4.12 Estimated ranges for underwater noise transmission from installing a 10 m diameter pile at the south (shallow water) modelling location at Horns Rev 3 (Mason & Barham 2013).

South 180 dB

SEL

165 dB SEL

150 dB SEL

145 dB SEL

Max Range 780 m

5.3 km 18.4 km 24.5 km

Min Range 750 m

4.9 km 12.3 km 15.5 km

Mean Range 770 m

5.1 km 15.7 km 20.5 km

4.4.2.1. Harbour porpoise

While comparing the sound propagation model (Figure 4.8 and Figure 4.9) with the mod-elled spatial distribution of harbour porpoises in the Horns Rev area (Figure 3.4 and Fig-ure 3.5) an overlap between areas of high harbour porpoise density and higher sound levels becomes visible.

The sound propagation model maps were combined with the modelled spatial distribution in Figure 4.8 to Figure 4.11 and the number of harbour porpoises in the area exposed to at least 145 dBSEL was extrapolated.

During the summer month the density of harbour porpoises in the Horns Rev area is very high. Large areas with densities of up to 20 porpoises/km² and more were modelled on the basis of aerial surveys with densities of up to 5 or more harbour porpoises inside the greater part of the Horns Rev 3 wind farm area.

Depending on the location of the pile driving activity the sound levels vary within different parts of the area. During pile driving close to the north-east corner of Horns Rev 3 por-poises in the high density area west of the wind farm Horns Rev 2 will partly be exposed to more than 145 dBSEL (Figure 4.8). In the high density area north-west of Horns Rev 3 porpoises will be exposed to sound levels of more than 150 dBSEL.

Figure 4.8: Summer spatial distribution model for harbour porpoise combined with sound propagation model during pile driving in the north-easterly part of the Horns Rev 3 offshore wind farm.

During winter the density of harbour porpoises in the area is much lower therefore the areas of highest density are smaller than in summer but more or less in the same posi-tions.

Only small parts of a harbour porpoise high density area west of the wind farm Horns Rev 2 will be affected by sound of 145 dBSEL or more during pile driving. Another smaller high density area north-west of Horns Rev 3 will be exposed to sound pressure levels of more than 150 dBSEL. Inside the wind farm Horns Rev 3 densities of 1 to 10 porpoises/km² were modelled and would be exposed to 150 to 180 dBSEL.

Figure 4.9: Winter spatial distribution model for harbour porpoise combined with sound propagation model during pile driving in the north-easterly part of the Horns Rev 3 offshore wind farm.

Pile driving at the south tip of Horns Rev 3 will move areas of high sound pressure levels toward the largest high density area of harbour porpoises. Therefore during the summer month the high density area west of Horns Rev 2 will be completely exposed to 145 and more dBSEL. The high density area north-west of Horns Rev 3 will partly be exposed to more than 145 dBSEL.

Harbour porpoise © Carline Höschle

Figure 4.10: Summer spatial distribution model for harbour porpoise combined with sound propagation model during pile driving at the south tip of the Horns Rev 3 offshore wind farm.

Figure 4.11: Winter spatial distribution model for harbour porpoise combined with sound propagation model during pile driving at the south tip of the Horns Rev 3 offshore wind farm.

In winter large parts of the harbour porpoise high density area west of Horns Rev 2 will be exposed to more than145 dBSEL. The same is true for the high density area north-west of Horns Rev 3.

Based on the spatial distribution models and the sound propagation models the number of harbour porpoises affected by ≥145 dBSEL was calculated. The number of harbour por-poises was calculated for every single grid cell in the model and was added up to calcu-late an average number of harbour porpoises per km². Depending on the position of the pile driving activity an area in the north or in the south that was affected by noise was not covered by the distribution model. The expected harbour porpoise density for this area was calculated from the overall average calculated for the modelled area. The number of harbour porpoises affected by pile driving in different positions and in different seasons is given in Table 4.13.

Table 4.13: Number of harbour porpoises affected by ≥145 dBSEL during pile driving (single strike) in the Horns Rev 3 area. The average number per km² was calculated on base of the modelled har-bour porpoise distribution and was used to estimate the numbers in areas not covered by the model.

Pile driv-ing posi-tion

season

Size of area affected by more than 145 dB_SEL in

A high number of harbour porpoises will be exposed to noise levels causing behavioural responses. Noise immissions in the northern part of the wind farm might cause behav-ioural reactions to nearly 700 harbour porpoises during the low density period in winter.

The same piling would affect about 3800 harbour porpoises in the high density summer month. The numbers are higher in the southern part of the wind farm, where more than 1000 harbour porpoises in winter and about 4900 in summer would be affected.

In the course of a piling operation about 60% of the porpoise within the 145 dBSEL contour are predicted to leave this area, thus total displacement from this area is estimated at maximal about 3000 porpoises for the southern location and 2300 for the northern loca-tion. Recovery time will be about 2 to 3 days in areas exposed to high noise levels but only last a few hours at the outer range of the impact area. Depending on the construc-tion schedule it is possible, that recovery time lasts longer than the interval between two piling activities and is thus assumed that porpoise densities in the in construction area will be reduced over the whole construction period.

It is not possible to assess precisely how many porpoises will be exposed to higher noise levels which may induce hearing impairment, as porpoises will be deterred from the con-struction area and move further away as noise immissions increase. As the 165 dB

SEL-radius of a single strike ranges between 5.1 and 6.1 km it is likely that porpoises will be exposed to noise levels causing TTS, though numbers may be small as piling will start with low energy of the hammer and thus with reduced noise levels.

The 180 dBSEL-contour for a single strike with full energy reaches a distance of about 800 m. If cumulation of high noise levels is taken into account the PTS-contour during the whole piling operation increases with the number of blows.

Ranges for cumulative received Sound Exposure Level (SEL) for a marine mammal over the entire piling operation have been modeled for a 6 h piling operation assuming that the receptor is fleeing from the noise at a speed of 1.5 m/s. The results of modelling a 10 m pile being installed with a maximum blow energy of 3000 kJ at Horns Rev 3 are summa-rised in Table 4.14. The propagation range is highly dependent on water depths and at-tenuation is stronger where the water is shallower. Therefore maximum and minimum and medium ranges were calculated with shorter ranges in the direction of shallow coastal waters and longer ranges in areas with greater water depths. Since harbour porpoises mostly avoid very shallow waters the calculated maximum range in deeper waters is more suitable to estimate the number of affected animals.

The model takes a soft-start with reduced piling energy and a gradual ramp-up procedure into account. It further assumes that porpoises flee from the noise source at a constant speed of 1.5 m/s. Even though harbor porpoises are certainly capable of swimming faster than 1,5 m/s, a moderate swimming is more appropriate for an impact assessment as it is not clear whether porpoise would swim directly away from the source. Further, mother-calve pairs may not reach a faster swimming speed as long as mother-calves are small. From the present data it thus cannot be excluded that harbour porpoises would be exposed to noise levels causing PTS from cumulative exposures. The result of the model is that por-poises being within a range of 8.7 km for the northern location and 6.6 km for the south-ern location will receive a cumulative noise level inducing PTS. Although the model has to be regarded as being very conservative as it allows equal cumulation for high and low noise levels, the size of the impact ranges make it likely that at least part of the porpoises being present within these ranges will receive PTS during the piling operation of a 10 m monopile.

Table 4.14: Predicted impact ranges from cumulative exposures using the PTS criteria for marine mam-mals (Mason & Barham 2013).

PTS

The harbour porpoise is listed in Annex II and IV of the EU-habitat directive as an endan-gered species. In the Horns Rev area the density of harbour porpoises is very high in summer exceeding the very high importance level of >2 harbour porpoises/km² defined in

Table 2.5 and the area is used during mating and nursing periods. Cumulative noise lev-els may induce PTS in harbour porpoise. Therefore the severity of the impact is ranked as very high.

4.4.2.2. Harbour seal and grey seal

Large parts of the Horns Rev area including the Horns Rev 3 offshore wind farm will be af-fected by sound of more than 145 dBSEL. This is the threshold for behavioural reactions in harbour porpoises and was defined as a precaution threshold for seals in this study (Table 4.5). The threshold defined for TTS in harbour seals is 171 dBSEL (Southall et al. 2007). De-pending on the water depth in which pile driving takes place the sound propagation model predicts the SEL to fall below 170 dB_SEL at a distance between 2,8 and 3.3 km from pile driv-ing. Though seal density in the area is low it must be assumed that seals may be exposed to such noise levels. The PTS threshold is 186 dBSEL. This level is exceeded at a distance of 1-2 km if cumulative exposure is considered. As the initial range of PTS-levels are very small it is considered unlikely that seals will be exposed to such levels during a piling operation.

Due to the low density or harbour seals in the area the number of seals that might be exposed to sound that could cause TTS or PTS will be small.

4.5. Impact of operation and structures

In document Horns Rev 3 Offshore Wind Farm Technical report no. 7 (Sider 90-98)