Secondary Mitigation and Residual Effect
  1. The PTS thresholds are not exceeded for most surveys and for most species. This is with the exception of cone penetration testing where the PTS range is so small (60 m predicted for harbour porpoise only) that it is considered that animals are likely to be deterred beyond this range (i.e. out to 300 m) by the vessel noise itself (see Table 10.53   Open ▸ ). Additionally, as a part of designed-in measures ( Table 10.21   Open ▸ ), Standard JNCC (2017) mitigation will be adhered to for the geophysical surveys which will involve the use of MMOs/PAM monitoring of a standard 500 m mitigation zone for a period of no < 30 minutes prior to the start of surveys ( Table 10.21   Open ▸ ). No secondary marine mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined above and in Table 10.21   Open ▸ ) is not significant in EIA terms.

Residual Effect – Auditory Injury

  1. Overall, the magnitude of the impact for all species is deemed to be low and the sensitivity of the receptors is considered to be high. The potential risk of injury will be reduced by appropriate designed-in measures and the scale of effect (injury radius and number of animals affected) was predicted to be very small. The effect on marine mammals will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Residual Effect – Behavioural Disturbance

  1. Overall, the magnitude of the impact for all species is deemed to be low and the sensitivity of the receptors is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Operation and Maintenance Phase

Magnitude of Impact

Auditory injury and behavioural disturbance

  1. Elevated underwater noise due to site investigation activities during the operation and maintenance phase of the Proposed Development may lead to injury and/or disturbance to marine mammals. The maximum design scenario comprises of routine geophysical surveys estimated to occur every six months for first two years and annually thereafter. This equates to up to 37 surveys over the 35-year life cycle of Proposed Development ( Table 10.16   Open ▸ ).
  2. An overview of potential impacts from auditory injury due to elevated underwater noise during geophysical site investigation surveys is described in paragraph 256 et seq. for the construction phase and has not been reiterated here for the operation and maintenance phase. Similarly, the magnitude of potential impacts for behavioural disturbance to marine mammals is described in paragraph 264 et seq. The magnitude of the impact of underwater noise from geophysical surveys during operation and maintenance phase could result in a negligible alteration to the distribution of marine mammals. Surveys are anticipated to be short-term in nature (weeks to a few months) and occur intermittently over the operation and maintenance phase. In addition, the proportion of the MU populations affected at any one time by disturbance is likely to be very small.
  3. The impact of site investigation surveys leading to PTS is predicted to be of very local spatial extent, short-term duration, intermittent and whilst the impact will occur during piling only, the effect of PTS will irreversible. It is predicted that the impact will affect the receptor directly. With designed-in measures in place, involving visual and/or acoustic monitoring, the risk is likely to be negligible, however, given the potential permanence of the effect (PTS) if it did occur, the magnitude is, conservatively, considered to be low.
Sensitivity of the Receptor
  1. The sensitivity of the receptors during the operation and maintenance phase is not expected to differ from the sensitivity of the receptors during the construction phase. Therefore, the sensitivity of marine mammal receptors to elevated underwater noise during site investigation surveys (PTS and behavioural disturbance) is as described previously in paragraph 270 et seq., where it has been assessed as high for PTS and medium for behavioural disturbance.
Significance of the Effect
  1. Overall, the magnitude of the impact of PTS is deemed to be low and the sensitivity of the receptor is considered to be high. The potential risk of injury will be reduced by appropriate designed-in measures and the scale of effect (injury radius and number of animals affected) was predicted to be very small. The effect on marine mammals will, therefore, be of minor adverse significance, which is not significant in EIA terms.
  2. Overall, the magnitude of the impact of disturbance is deemed to be low and the sensitivity of the receptor is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary Mitigation and Residual Effect
  1. As described above for the construction phase (paragraph 280 et seq.), the PTS thresholds are not exceeded for most surveys and for most species (with exception of CPT). Designed-in measures ( Table 10.21   Open ▸ ) in the form of standard JNCC (2017) mitigation will be adhered to for the geophysical surveys. This will involve the use of MMOs/PAM monitoring of a standard 500 m mitigation zone for a period of no < 30 minutes prior to the start of surveys ( Table 10.21   Open ▸ ). No secondary marine mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 10.10) is not significant in EIA terms

Residual Effect – Auditory Injury

  1. Overall, the magnitude of the impact for all species is deemed to be low and the sensitivity of the receptors is considered to be high. The potential risk of injury will be reduced by appropriate designed-in measures and the scale of effect (injury radius and number of animals affected) was predicted to be very small. The effect on marine mammals will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Residual Effect – Behavioural Disturbance

  1. Overall, the magnitude of the impact for all species is deemed to be low and the sensitivity of the receptors is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Injury and Disturbance to Marine Mammals from Elevated Underwater Noise During UXO Clearance

  1. The clearance of UXO prior to commencement of construction may result in detonation (high order) of a UXO. This activity has the potential to generate some of the highest peak sound pressures of all anthropogenic underwater sound sources (von Benda-Beckman et al., 2015), and are considered a high energy, impulsive sound source. The potential impacts of this activity will depend on noise source characteristics, the receptor species, distance from the sound source and noise attenuation within the environment.

Summary of Noise Modelling

Detonation
  1. Noise modelling for UXO clearance (both low order and high order detonation) has been undertaken using the methodology described in Soloway and Dahl (2014), which provides a simple relationship between distance from an explosion and the weight of the charge (or equivalent trinitrotoluene (TNT) weight). Since the charge is assumed to be freely standing in mid-water, unlike a UXO which would be resting on the seabed and could potentially be buried, degraded or subject to other significant attenuation, this estimation of the source level can be considered conservative. Marine mammal hearing weighted thresholds were compared by application of the frequency dependent weighting functions at each distance from the source. Based on findings presented in Robinson et al. (2020), noise modelling for low order techniques followed the same methodology as for high order detonation, with a smaller donor charge size.
  2. Further detail on noise modelling of UXO clearance are provided in volume 3, appendix 10.1.

Construction Phase

Magnitude of Impact
  1. Potential effects of underwater noise from high order UXO clearance on marine mammals include mortality, physical injury or auditory injury. The duration of effect for each UXO detonation is less than one second. Behavioural effects are therefore considered to be negligible in this context. TTS is presented as a temporary auditory injury but also represents a threshold for the onset of the fleeing response. Proposed Development specific noise modelling was carried out using published and peer-reviewed criteria to determine the potential magnitude (range) of effect on marine mammal receptors. A project specific MMMP will be developed in order to reduce the potential to experience injury (see Table 10.21   Open ▸ ).
  2. It is anticipated that up to 70 UXOs are likely to be found within the Proposed Development array area and the Proposed Development export cable corridor, however, only 14 of these will require clearance. The maximum design scenario is based on experience of UXO clearance at Seagreen offshore wind farm (in close proximity to the Proposed Development). For Seagreen, of the 20 UXOs estimated to be present for the purposes of the marine mammal risk assessment (Seagreen Wind Energy, 2021), only four (20%) were found to require clearance within the proposed development site, one of which was relocated rather than cleared by high order techniques (SSE pers. Comm.). The estimate of 70 UXOs for Berwick Bank Offshore Wind Farm was extrapolated from the same study carried out for Seagreen (Ordtek, 2017; Ordtek, 2019) and therefore it is considered likely that the number of UXOs requiring disposal will be significantly less than assessed here (i.e. based on the same proportion cleared for Seagreen, there may only be 14 UXOs requiring clearance for the Proposed Development). The precise details and locations of potential UXOs is unknown at this time. During the UXO clearance campaign at Seagreen Offshore Wind Farm the maximum UXO size identified was 250 kg NEQ. Given that Seagreen Offshore Wind Farm is located approximately 4 km from the Proposed Development array area, a similar maximum size of munition is expected to be encountered in the same region. Therefore, for the purposes of this assessment, it has been assumed that the maximum design scenario is UXO size up to 300 kg. The maximum frequency would be up to two detonations within 24 hours. The clearance activities will be tide and weather dependant. The aim is to enable clearance of at least one UXO per tide, during the hours of daylight and good visibility.
  3. Low order techniques will be applied as the intended methodology for clearance of UXO. The technique uses a single charge of up to 80 g NEQ which is placed in close proximity to the UXO to target a specific entry point. When detonated, a shaped charge penetrates the casing of the UXO to introduce a small, clinical plasma jet into the main explosive filling. The intention is to excite the explosive molecules within the main filling to generate enough pressure to burst the UXO casing, producing a deflagration of the main filling and neutralising the UXO. Recent controlled experiments showed low-order clearance using deflagration to result in a substantial reduction in acoustic output over traditional high-order methods, with SPLpk and SELcum being typically significantly lower for the low order techniques of the same size munition, and with the acoustic output being proportional to the size of the shaped charge, rather than the size of the UXO itself (Robinson et al., 2020). Using this low-order clearance method, the probability of a low-order outcome is high; however, there is a small inherent risk with these clearance methods that the UXO will detonate or deflagrate violently. It is also possible that there will be residual explosive material remaining on the seabed following low order clearance. In this case, recovery will be performed, including the potential need of a small (500 g NEQ) ‘clearing shot’.
  4. There is a small risk that a low order clearance could result in high order detonation of UXO. In addition, some UXOs may be deemed to be too unstable to warrant a low order approach and therefore for safety reasons would need to be cleared using high order methods. At Neart na Gaoithe Offshore Wind Farm in the Firth of Forth, a total of 53 items of UXO required detonation and four of the 37 (c. 10%) monitored UXO clearance events resulted in a high order detonation, largely as a result of the age, condition and type of munition (Seagreen Wind Energy, 2021).
  5. UXO clearance activities will also involve the use of up to seven vessels on site at any one time with up to 30 vessel movements in total. Noise impacts associated with vessel movements are identified in paragraph 385 et seq. as well as paragraph 404 et seq.

Auditory injury

  1. An explosive mass of 300 kg (maximum design scenario due to high order detonation) yielded the largest potential PTS ranges for all species, with the greatest effect ranges seen for harbour porpoise ( Table 10.42   Open ▸ ). As described in paragraph 298, there is just a small (10%) chance that low order detonation could result in a high order detonation event. Therefore, whilst this assessment considers the most likely scenario to be based on a detonation of 0.08 kg donor change (maximum size of donor charge used for low order techniques) and a detonation of 0.5 kg clearance shot (maximum size of clearing shot to neutralise any residual explosive material ( Table 10.43   Open ▸ ), the assessment will consider both high order and low order techniques for the purposes of secondary mitigation. With regard to UXO detonation (low order techniques as well as high order events), due to a combination of physical properties of high frequency energy, the sound is unlikely to still be impulsive in character once it has propagated more than a few kilometres (see volume 3, appendix 10.1). The NMFS (2018) guidance suggested an estimate of 3 km for transition from impulsive to continuous (although this was not subsequently presented in the later guidance (Southall et al., 2019). For other impulsive noise sources (pile driving and airguns) Hastie et al., (2019) suggests that some measures of impulsiveness change markedly within c. 10 km of the source. Therefore, great caution should be used when interpreting any results with predicted injury ranges in the order of tens of kilometres as the impact ranges are likely to be significantly lower than predicted.

 

Table 10.42:
Potential PTS Impact Ranges for Marine Mammals Due to UXO High Order Detonation

Table 10.42: Potential PTS Impact Ranges for Marine Mammals Due to UXO High Order Detonation

 

Table 10.43:
Potential PTS Impact Ranges for Marine Mammals Due to Low Order Techniques

Table 10.43: Potential PTS Impact Ranges for Marine Mammals Due to Low Order Techniques

 

  1. The subsea noise assessment found that the maximum injury (PTS) range estimated for harbour porpoise using the SPLpk metric is 685 m for the detonation of charge size of 0.08 kg and 1,260 m for the detonation of 0.5 kg clearance shot ( Table 10.43   Open ▸ ). Conservatively, the number of individuals that could be potentially injured, based on the peak seasonal densities from site-specific survey data, was estimated as one and four harbour porpoises for 685 m and 1,265 m respectively ( Table 10.44   Open ▸ ).
  2. The subsea noise assessment found that the maximum injury (PTS) range estimated for bottlenose dolphin and white-beaked dolphin using the SPLpk metric is 40 m for the detonation of charge size of 0.08 kg and 75 m for the detonation of 0.5 kg clearance shot ( Table 10.43   Open ▸ ). Conservatively, the number of bottlenose dolphins that could be potentially injured within the maximum range of 75 m, based on the peak densities in the outer Firth of Tay from the probability of occurrence model (Arso Civil et al., 2019), was estimated as less than one individual ( Table 10.44   Open ▸ ). In the case of white-beaked dolphin, the most conservative number of animals that could be potentially injured within that range (based on SCANS III densities) was also estimated as less than one individual ( Table 10.44   Open ▸ ).
  3. The subsea noise assessment found that the maximum injury (PTS) range estimated for minke whale using the SPLpk metric is 120 m for the detonation of charge size of 0.08 kg and 225 m for the detonation of 0.5 kg clearance shot ( Table 10.43   Open ▸ ). Conservatively, the number of minke whales that could be potentially injured within the maximum range of 225 m, based on the SCANS III densities, was estimated as less than one individual ( Table 10.44   Open ▸ ).
  4. Both seal species (harbour and grey seal) could experience potential injury at the maximum range of 135 m due to detonation of charge size of 0.08 kg and 250 m due to detonation of 0.5 kg clearance shot ( Table 10.43   Open ▸ ). Taking into account the most conservative scenario, maximum density for both species (based on mean at-sea seal usage from Carter et al. (2020)), there will be less than one animal of each species that could be potentially injured within the maximum range of 250 m.

 

Table 10.44:
Number of Animals with the Potential to Experience PTS due to Low Order Techniques

Table 10.44: Number of Animals with the Potential to Experience PTS due to Low Order Techniques

 

  1. As discussed previously, whilst the preferred approach is to clear UXOs using low order techniques, this assessment also presents the number of animals potentially injured by high order detonation ( Table 10.45   Open ▸ ).
  2. Harbour porpoise is likely to be the most sensitive species to potential injury from high order UXO clearance. The subsea noise assessment found that the maximum injury (PTS) range estimated for harbour porpoise using the SPLpk metric is 10,630 m for the high order detonation of charge size of 300 kg ( Table 10.42   Open ▸ ). Conservatively, the number of harbour porpoise that could be potentially injured during each high order detonation of UXO is greater (up to 293 individuals) compared with other species. A maximum of 0.08% of the NS MU population and 0.76% of SCANS III Block R could be potentially injured during each high order detonation of the UXO. The second most sensitive marine mammal that could be affected by the high order UXO clearance event is grey seal with up to 16 animals with the potential to be injured during each high order detonation of the UXO (0.04% of the ES plus NE Mus). Less than one individual has the potential to be injured for all other species considered in the assessment ( Table 10.42   Open ▸ ).
  3. To reduce the potential of experiencing injury, designed-in measures will be adopted as part of a MMMP (see Table 10.21   Open ▸ ). However, mitigation zones of c. 10 km are considerably larger than the standard 1,000 m mitigation zone recommended for UXO clearance (JNCC, 2010b) and there are often difficulties in detecting marine mammals (particularly harbour porpoise) over such large ranges (McGarry et al., 2017). Visual surveys note that there is often a significant decline in detection rate with increasing sea state (Embling et al., 2010; Leaper et al., 2015). Therefore, additional mitigation will be applied in the form of soft start charges and ADDs to minimise residual risk of injury and the assessment of effects therefore considers the deployment of these as a secondary mitigation measure (see paragraph 338 et seq. for further details).

 

Table 10.45:
Number of Animals with the Potential to Experience PTS due to High Order Detonation

Table 10.45: Number of Animals with the Potential to Experience PTS due to High Order Detonation

 

  1. Due to the small numbers of marine mammals potentially injured from low order techniques ( Table 10.44   Open ▸ ) the magnitude of the impact could result in a negligible alteration to the distribution of marine mammals. In addition, the proportion of the MU populations affected at any one time by PTS is likely to be very small. For low order techniques the impact of PTS is predicted to be of local spatial extent, very short-term duration, intermittent and of low reversibility. It is predicted that the impact will affect the receptor directly. The magnitude for low order techniques is therefore considered to be low.
  2. In comparison, larger numbers of marine mammal could potentially be injured by high order detonation which could lead to a minor alteration in the distribution of marine mammals with up to 0.08% of the NS MU harbour porpoise population affected for each high order detonation of the UXO. Grey seals could also be affected with a maximum of 0.04% of the ES plus NE Mus potentially injured for each high order detonation of the UXO. For high order detonation the impact of PTS is predicted to be of local to regional spatial extent, very short-term duration, intermittent and the effect of injury is of low reversibility. It is predicted that the impact will affect the receptor directly. Only a small proportion (c. 10% of the UXO) are considered likely to result in high order detonation. The magnitude is therefore considered to be low (bottlenose dolphin, white-beaked dolphin, minke whale, harbour seal) to medium (harbour porpoise and grey seal).

Temporary threshold shift

  1. A second threshold assessed was the onset of TTS where the resulting effect would be a potential temporary loss in hearing. Whilst similar ecological functions would be inhibited in the short term due to TTS, these are reversible on recovery of the animal’s hearing and therefore not considered likely to lead to any long-term effects on the individual. The onset of TTS also corresponds to a ‘fleeing response’ as this is the threshold at which animals are likely to flee from the ensonified area. Thus, the onset of TTS reflects the threshold at which behavioural displacement could occur. As previously described in paragraph 300, the sound is unlikely to be impulsive in character once it has propagated more than a few kilometres. It is particularly important when interpreting results for TTS with impact ranges of up to 51 km as these are likely to be significantly lower than predicted. As before, the assessment of TTS will consider a most likely scenario of the detonation of a 0.08 kg donor change (maximum size of donor charge used for low order techniques) and the detonation of a 0.5 kg clearance shot (maximum size of clearing shot to neutralise any residual explosive material), as presented in Table 10.46   Open ▸ . Due to the potential for a low order detonation technique to result in a high order detonation (as per paragraph 298) the assessment also considers high order detonation of 300 kg UXO munition size.

 

Table 10.46:
Potential TTS Impact Ranges for Marine Mammals Due to Low Order Techniques

Table 10.46: Potential TTS Impact Ranges for Marine Mammals Due to Low Order Techniques

 

  1. The subsea noise assessment found that temporary hearing impairment and behavioural displacement from the area (TTS) may affect harbour porpoise at a maximum range of 2,015 m for the detonation of charge size of 0.08 kg and 3,110 m for the detonation of 0.5 kg clearance shot. Up to 11 animals (0.003% of the MU population) have the potential to be affected by TTS due to the low order techniques (charge size of 0.08 kg) and up to 25 animals (0.01% of the MU population) have the potential to experience TTS from the detonation of 0.5 kg clearance shot ( Table 10.47   Open ▸ ).
  2. The subsea noise assessment found that temporary hearing impairment and behavioural displacement from the area (TTS) may affect bottlenose dolphin and white-beaked dolphin at a maximum range of 75 m for the detonation of charge size of 0.08 kg and 135 m for the detonation of 0.5 kg clearance shot. The maximum range of 135 m is only slightly larger when compared to PTS (75 m) and therefore less than one animal of each species has the potential to be affected by TTS.
  3. The subsea noise assessment found that temporary hearing impairment and behavioural displacement from the area (TTS) may affect minke whale at a maximum range of 1,110 m for the detonation of charge size of 0.08 kg and 2,645 m for the detonation of 0.5 kg clearance shot. Up to one animal (0.004% of the MU population) have the potential to be affected by TTS due to the detonation of charge size of 0.08 kg and less than one animal (0.01% of the MU population) has the potential to experience TTS from the detonation of 0.5 kg clearance shot.
  4. The subsea noise assessment found that temporary hearing impairment and behavioural displacement from the area (TTS) may affect harbour and grey seal at a maximum range of 250 m for the low order techniques (charge size of 0.08 kg) and 505 m for the detonation of 0.5 kg clearance shot. Less than one harbour seal and one grey seal have the potential to be affected by TTS due to the detonation of charge size of 0.08 kg as well as the detonation of 0.5 kg clearance shot.

 

Table 10.47:
Number of Animals with the Potential to Experience TTS due to Low Order Techniques

Table 10.47: Number of Animals with the Potential to Experience TTS due to Low Order Techniques

 

  1. High order detonation has the potential to impact animals over larger ranges when compared to low order techniques. The maximum range for TTS across all species was for minke whale where the potential for TTS was predicted to occur out to 34,135 m for detonation of charge size of 300 kg ( Table 10.48   Open ▸ ). Second largest ranges were modelled for harbour porpoise with the maximum range of 19,590 m due to high order detonation of charge size of 300 kg. Seals are also anticipated to experience TTS across relatively large range of up to 6,430 m as a result of detonation of charge size of 300 kg.

 

Table 10.48:
TTS Impact Ranges for Marine Mammals Due to High Order Detonation

Table 10.48: TTS Impact Ranges for Marine Mammals Due to High Order Detonation

 

  1. Due to relatively large ranges of potential impacts presented in Table 10.48   Open ▸ , up to 995 harbour porpoises (0.29% of the NS MU population) have the potential to be affected by TTS due to detonation of the 300 kg charge size ( Table 10.54   Open ▸ ). Up to 142 minke whales (0.07% of the CGNS MU population) have the potential to be affected by TTS due to the high order detonation of 300 kg charge. Taking into account the most conservative scenario, up to 156 grey seals could potentially experience TTS due to the high order detonation of charge size of 300 kg. As described previously in paragraph 71 et seq. the duration of effect is very short-lived and since TTS is a temporary hearing impairment, animals are likely to fully recover from the effects (reversible).

 

Table 10.49:
Number of Animals with the Potential to Experience TTS due to High Order Detonation

Table 10.49: Number of Animals with the Potential to Experience TTS due to High Order Detonation

 

  1. The impact of TTS for low order techniques is predicted to be of local spatial extent, very short-term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be negligible.
  2. The impact of TTS high order detonation is predicted to be of regional spatial extent, very short-term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
Sensitivity of the Receptor

Auditory injury (PTS)

  1. The acoustical properties of explosives are characterised by a short shock wave, comprising a sharp rise in pressure followed by an exponential decay with a time constant of a few hundred microseconds (volume 3, appendix 10.1). The interactions of the shock and acoustic waves create a complex pattern in shallow water, and this was investigated further by Von Benda-Beckmann et al. (2015). As harbour porpoises have high sensitivity to noise, impacts on these species are most often assessed in a scientific literature.
  2. Von Benda-Beckmann et al. (2015) investigated the range of effects of explosives on harbour porpoise in the southern North Sea. The study measured SEL and peak overpressure (in kPa) at distances up to 2 km from the explosions of seven aerial bombs detonated at approximately 26 m to 28 m depth, on a sandy substrate. Six bombs had a charge mass of 263 kg (580 lb) and one had a charge mass of 121 kg (267 lb). The study looked at the potential for injury to occur as an ear trauma caused by the blast wave at a peak overpressure of 172 kPa (190 dB re. 1 µPa). Furthermore, the potential for noise-induced PTS to occur was based on a threshold of 190 dB re. 1 µPa2s (PTS ‘very likely to occur’) and an onset threshold of 179 dB re. 1 µPa2s (SEL) (PTS ‘increasingly likely to occur’) (Lucke et al., 2009 criteria). The results suggested that the largest distance at which a risk of ear trauma could occur was at 500 m and that noise-induced PTS was likely to occur greater than the 2 km range that was measured during the study since the SEL recorded at this distance was 191 dB re. 1 µPa2s (i.e. 1 dB above the ‘very likely to occur’ threshold).
  3. In the same study Von Benda-Beckmann et al. (2015) modelled possible effect ranges for 210 explosions (of up to 1,000 kg charge mass) that had been logged by the Royal Netherland Navy (RNLN) and the Royal Netherlands Meteorological Institute (RNMI) over a two year period (2010 and 2011). Using the empirical measurements of SEL out to 2 km to validate the model (described above in paragraph 320), the authors found that the effect distances ranged between hundreds of metres to just over 10 km (for charges ranging from 10 kg up to 1,000 kg). Near the surface, where porpoises are known to spend a large proportion of time (e.g. 55% based on Teilmann et al., 2007) the SELs were predicted to be lower with effect distances for the onset of PTS just below 5 km. The authors caveat these results as, whilst the model could provide a reasonable estimate of the SEL within 2 km (since the empirical measurements were made out to this point), estimates above this distance required further validation since the uncorrected model systematically overestimated SEL. Salomons et al. (2021) analysed the sound measurements performed near two detonations of UXO (charge masses of 325 kg and 140 kg). From the weighted SEL values and threshold levels from Southall et al. (2019), a PTS effect distance in the range 2.5 km – 4 km has been derived (Salomons et al., 2021).
  4. By comparing experimental data and model predictions, Salomons et al. (2021) found thar harbour porpoises are at risk of permanent hearing loss at distances of several kilometres from large explosives, i.e. distance between 2 km and 6 km based on 140 kg and 325 kg charge masses. Following clearance of ground mines in the Baltic Sea in 2019, 24 harbour porpoises were found dead in the period after those clearing events along the coastline (Siebert et al., 2022). The post-mortem examination found that in ten cases the cause of death was associated with a blast injury, however the charge masses of the explosives in this study are unknown (Siebert et al., 2022).
  5. Not much is known about sensitivity of bottlenose dolphin, white-beaked dolphin and minke whale to blasting. However, during a clearance of relatively small explosive (35 kg charge) at an important feeding area for a resident community of bottlenose dolphin in Portugal, acoustic pressure levels in excess of 170 dB e 1 µPa were measured. Despite pressure levels being 60 dB higher than ambient noise, no adverse effects were recorded in the behaviour or appearance of resident community (Santos et al., 2010). Nonetheless, other studies reported that external injuries consistent with inner ear damage have been found in dolphins subjected to explosives, with little change in surface animal behaviour near blast areas (Ketten, 1993).
  6. Robinson et al. (2020) described a controlled field experiment and compared the sound produced by high-order detonations with a low-order disposal method, i.e. deflagration. He found that using low order techniques offers a substantial reduction in acoustic output over traditional high-order methods, with the peak SPLpk and SELcum observed being typically > 20 dB lower for the deflagration of the same sized munition (a reduction factor of just over ten in SPLpk and 100 in acoustic energy). The study also reported that the acoustic output depends on the size of the shaped charge, rather than the size of the UXO itself. Considering the above, compared to high-order methods, Robinson et al. (2020) provided the evidence that low order techniques offers the potential for greatly reduced acoustic noise exposure of marine mammals.
  7. The sensitivity of the receptors to the injury from impulsive underwater noise has been described previously for piling and is presented in paragraphs 197 to 211.
  8. All marine mammals, which are IEFs of international value, are deemed to be of high vulnerability and low recoverability. The sensitivity of the receptor to PTS is therefore considered to be high.

Temporary threshold shift

Harbour porpoise

  1. Explosions during UXO clearance activities and associated underwater noise have the potential to produce behavioural disturbance, however there are no agreed thresholds for the onset of a behavioural response generated as a result of explosion. Given different nature of the sound, using noise levels and probability of a response to pile driving would not be appropriate. Southall et al. (2007) suggests that the use of TTS onset as a auditory effect may be most appropriate for single pulses (such as UXO detonation) and therefore it has been used in other assessments where the impacts of UXO clearance on marine mammals have been investigated. TTS is a temporary and reversible hearing impairment and therefore, it is anticipated that any animals experiencing this shift in hearing would recover after they are no longer exposed to elevated noise levels (i.e. they may have moved beyond the injury zone or piling has ceased). The implication of animals experiencing TTS, leading to potential displacement, is not fully understood, but it is likely that aversive responses to anthropogenic noise could temporarily affect life functions as described for PTS. However, due to the reversible nature of TTS, this is less likely to lead to acute effects and will largely depend on recoverability. The degree and speed of hearing recovery will depend on the characteristics of the sound the animal is exposed to, and on the degree of shift in hearing experienced. A study measuring recovery rates of harbour porpoise following exposure to sound source of 75 db re 1 μPa (SEL) over 120 minutes found that recovery to the pre-exposure threshold was estimated to be complete within 48 minutes following exposure (the higher the hearing threshold shift, the longer the recovery) (SEAMARCO, 2011).
  2. Finneran et al. (2000) investigated the behavioural and auditory responses of two captive bottlenose dolphins to sounds that simulated distant underwater explosions. The animals were exposed to an intense sound once per day and no auditory shift (i.e. TTS) greater than 6 dB in response to levels up to 221 dB re 1 µPa p-p (peak-peak) was observed. Behavioural shifts, such as delaying approach to the test station and avoiding the ‘start’ station, were recorded at 196 dB and 209 dB re 1 µPa p-p for the two dolphins and continued at higher levels. There are several caveats to this study (discussed in Nowacek et al. (2007)), (i.e. the signals used in this study were distant and the study measured masked-hearing signals). The animals used in the experiment were also trained and rewarded for tolerating high levels of noise and subsequently, it can be anticipated that behavioural disruption would likely be observed at lower levels in other contexts.
  3. Susceptibility to TTS depends on the frequency of the fatiguing sound causing the shift and the greatest TTS depends on the SPL (and related SEL) (Kastelein et al., 2021). In a series of studies measuring TTS occurrence in harbour porpoise at a range of frequencies typical of high amplitude anthropogenic sounds (0.5 kHz to 88.4 kHz) the greatest shift in mean TTS occurred at 0.5 kHz, which is very close to the lower bound of porpoise hearing (Kastelein et al., 2021). Hearing always recovered within 60 minutes after the fatiguing sound stopped. Scientific understanding of the biological effects of TTS is limited to the results of controlled exposure studies on small numbers of captive animals (reviewed in Finneran, 2015). Extrapolating these results to how animals may respond in the natural environment should be treated with caution as it is not possible to exactly replicate natural environmental conditions, and the small number of test subjects would not account for intraspecific differences (i.e. differences between individuals) or interspecific differences (i.e. extrapolating to other species) in response.

Bottlenose dolphin and white-beaked dolphin

  1. Whilst there are no available species-specific recovery rates for mid-frequency cetaceans to TTS, there is no evidence to suggest that recovery will be significantly different to harbour porpoise recovery rates therefore animals can recover their hearing after they are no longer exposed to elevated noise levels (i.e. they may have moved beyond the injury zone or piling has ceased). The assessment considered that both white-beaked dolphin and bottlenose dolphin would be able to tolerate the effect without any impact on reproduction or survival rates and would be able to return to previous behavioural states or activities once the impacts had ceased.

Minke whale

  1. As above for high-frequency cetaceans (paragraph 330), whilst there are no available species-specific recovery rates for minke whale to TTS, there is no evidence to suggest that recovery will be significantly different to harbour porpoise recovery rates. There is evidence that minke whales avoiding a 15 kHz ADD and clearly react to signals at the likely upper limit of their hearing sensitivity (Boisseau et al., 2021). In addition, minke whale exhibit a temporal distribution, with most sightings in continental shelf waters occurring between May and September. The assessment considered that minke whale would be able to tolerate the effect without any impact on reproduction or survival rates and would be able to return to previous behavioural states or activities once the impacts had ceased.

Harbour seal and grey seal

  1. A study measuring recovery rates of harbour seal following exposure to a sound source of 193 dB re 1 μPa2s (SELcum) over 360 minutes found that recovery from TTS to the pre-exposure baseline was estimated to be complete within 72 minutes following exposure (Kastelein et al., 2018a). These results are similar to recovery rates found in SEAMARCO (2011), which showed that for small TTS values, recovery in seals was very fast (around 30 minutes) and the higher the hearing threshold shift, the longer the recovery. Kastelein et al. (2019a) also demonstrated recovery was rapid, with hearing recovered fully within two hours. Therefore, in most cases, reduced hearing for such a short time probably has little effect on the total foraging period of a seal. If hearing is impaired for longer periods (hours or days) the impact is likely to be ecologically significant (SEAMARCO, 2011). The results indicate that harbour seal (and therefore grey seal, using harbour seal as a proxy) are less vulnerable to TTS than harbour porpoise for the noise bands tested. In addition, it is expected that animals would move beyond the injury range prior to the onset of TTS. The assessment considered that both grey seal and harbour seal are likely to be able to tolerate the effect without any impact on both reproduction and survival rates and would be able to return to previous behavioural states or activities once the impacts had ceased.
  2. All marine mammals, which are IEFs of international value, are deemed to be of medium vulnerability and high recoverability. The sensitivity of the receptor to TTS is therefore considered to be low.
Significance of the Effect

Auditory injury

  1. Although the preferred approach is the use of low order techniques to clear UXO, in the case that a low order technique results in a high order detonation (as per paragraph 298, approximately 10% of the total number of UXOs could result in high order detonation) conclusions presented in paragraph 335 et seq. are based on the assessment for high order clearance.
  2. For bottlenose dolphin, white-beaked dolphin, minke whale and harbour seal, the magnitude of the impact is deemed to be low and the sensitivity of the receptors is considered to be high. The potential risk of injury will be reduced by appropriate designed-in measures, including visual and/or acoustic monitoring, and the scale of effect (injury radius and number of animals affected) was predicted to be very small. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
  3. For harbour porpoise and grey seal, the magnitude of the impact is deemed to be medium and the sensitivity of the receptors is considered to be high. Given that the injury zone is too large to be mitigated by designed-in measures (visual and/or acoustic monitoring) and the proportion of respective MU populations potentially injured is moderate, the effect will, therefore, be of moderate adverse significance, which is significant in EIA terms. Secondary mitigation and residual significance is discussed in paragraph 338 et seq.

Temporary threshold shift

  1. As described for PTS in paragraph 334, the preferred approach is the use of low order techniques to clear UXOs, however in the case that a low order technique results in a high order detonation, the magnitude of the impact for all species is deemed to be negligible to low and the sensitivity of the receptor is considered to be low., The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary Mitigation and Residual Effect
  1. Secondary mitigation will be applied to reduce the potential for injury occurring during UXO clearance. As previously described in paragraph 297 et seq., low order techniques will be applied as the intended methodology for clearance of UXO, however there is a small risk that a low order clearance could result in high order detonation of UXO (as per paragraph 298, approximately 10% of the total number of UXOs could result in high order detonation). The secondary mitigation has been therefore tailored based on the size of the UXO and high order detonation scenario. A range of UXO munitions sizes have been considered for purpose of determining effective mitigation measure, up to a maximum scenario of a UXO size of 300 kg. This approach follows a similar strategy to what was done for Seagreen EPS Risk Assessment and MMMP (Seagreen Wind Energy Ltd, 2021).
  2. A MMMP will be developed for the purpose of mitigating the risk of auditory injury (PTS) to marine mammals from the proposed UXO clearance activities at the Proposed Development. As previously mentioned, an approach used in Seagreen EPS Risk Assessment and MMMP (Seagreen Wind Energy Ltd, 2021) has been followed for the Proposed Development. The MMMP will be provided as a stand-alone document, however this section provides an overview of the procedures prior to making conclusions on the potential for residual effects.
  3. The designed-in measures included as a part of the MMMP ( Table 10.21   Open ▸ ) are in line with JNCC guidelines for minimising the risk of injury to marine mammals from using explosives (JNCC, 2010b). Details of ADD use and soft-start charges application are specific for each of the anticipated UXO sizes. A flow-chart, originally presented in Figure 2 of Seagreen EPS Risk Assessment and MMMP (Seagreen Wind Energy Ltd, 2021), has been used to inform the mitigation procedures. Prior to the commencement of UXO clearance works, a more detailed assessment will be produced as a part of the EPS licence supporting information, including an evaluation of the most appropriate measures to employ particularly with respect to emerging evidence on the use of scare charges as the most widely applied approach alongside ADDs. During Road Map Meeting 4 stakeholders were informed that appropriate mitigation measures will be agreed via consultation as a part of a UXO specific MMMP and this will include consideration of the efficacy of noise abatement measures ( Table 10.9   Open ▸ ).
  4. The approach to mitigating injury to marine mammals involves the monitoring of a 1 km radius mitigation zone. Monitoring will be carried out by suitably qualified and experienced personnel within a mitigation team, comprising two dedicated MMOs and one dedicated PAM operator. The purpose of this monitoring is to ensure that the mitigation zone is clear of marine mammals prior to detonation.
  5. Given that there is a potential to experience auditory injury by harbour porpoise and minke whale at a greater range than can be mitigated by monitoring of the 1 km mitigation zone alone ( Table 10.42   Open ▸ ), an ADD will be deployed for a pre-determined length of time to deter marine mammals to a greater distance prior to any detonation. The assessment of effects provided above in paragraph 297 et seq. determine the auditory injury range based on high order detonation of a 300 kg UXO ( Table 10.42   Open ▸ ). At the time of writing, the number and size of the UXOs within the Proposed Development array area and the Proposed Development export cable corridor are unknown and therefore, the secondary mitigation has been designed for a range of UXO munitions sizes so that the most appropriate approach can be applied to balance the risk of injury from UXO detonation with any additional noise introduced into the marine environment as deterrent measures ( Table 10.50   Open ▸ ). The assumption is that the animals swim in a straight line away from the ADD at a speed agreed in consultation with NatureScot and MSS for the Proposed Development. Swim speeds are summarised in Table 10.24   Open ▸ along with the source papers for the assumptions. Therefore, the duration of the application of the ADD prior to UXO detonation will determine whether the animal can move out of the injury zone prior to UXO detonation ( Table 10.50   Open ▸ ).
  6. Activation of an ADD will commence within the 60 minutes pre-detonation search, providing no marine mammals have been observed within the mitigation zone for a minimum of 20 minutes. Summaries provided in this paragraph refer to harbour porpoise and minke whale only, however, deterrence distances are provided for all marine mammal IEFs in Table 10.50   Open ▸ . Based on the UXO clearance flow chart ( Figure 10.26   Open ▸ ; informed by Seagreen Wind Energy Ltd, 2021), for UXO size up to 3 kg, the required time of ADD activation is 22 minutes and this is expected to displace harbour porpoise and minke whale to 1,980 m and 3,036 m range, respectively ( Table 10.50   Open ▸ ). If UXO size of up 6.5 kg is identified during the survey, then ADD will be activated for 30 minutes and this is expected to deter harbour porpoise and minke whale to 2,700 m and 4,140 m, respectively. For UXO mass charge of up to 15 kg, the required time of ADD activation is 40 minutes and this is expected to displace harbour porpoise and minke whale to 3,600 m and 5,520 m range, respectively. For larger UXO sizes up to 50 kg, an ADD will be activated for 60 minutes and this is expected to deter harbour porpoise and minke whale to 5,400 m and 8,280 m, respectively.
  7. For UXO sizes up to 300 kg, to reduce the risk of PTS, there is a need to deter animals from larger ranges that cannot be achieved using an ADD alone. Therefore, following an ADD activation period of 60 minutes, a ‘soft start’ will be undertaken, using a sequence of small explosive charges, detonated at five minutes intervals, over a total of maximum 20 minutes ( Table 10.50   Open ▸ ). It is expected that 80 minutes of combined ADD/soft start procedure will displace harbour porpoise and minke whale to ranges of 7,200 m and 11,040 m, respectively. Whilst this secondary mitigation is considered to be sufficient to deter most animals, there may be a residual effect for harbour porpoise for this largest UXO size, as the maximum predicted PTS impact range for this species was 10,630 m ( Table 10.42   Open ▸ ).

Figure 10.26:
Proposed Development UXO Clearance Mitigation Flow Chart

Figure 10.26: Proposed Development UXO Clearance Mitigation Flow Chart

 

Table 10.50:
Recommended ADD Duration for High Order UXO Clearance and Sizes, and Associated Displacement Distance

Table 10.50: Recommended ADD Duration for High Order UXO Clearance and Sizes, and Associated Displacement Distance

 

  1. The analysis presented in Table 10.50   Open ▸ suggests that for UXO sizes of up to 300 kg, pre-detonation search and use of ADD will be sufficient to reduce the potential of experiencing PTS by bottlenose dolphin, white-beaked dolphin, minke whale, harbour seal and grey seal to negligible magnitude. As presented in paragraph 344, it has been estimated that harbour porpoises could potentially experience an auditory injury at distances that cannot be fully mitigated by application of ADD and soft start charges. The maximum mitigation zone has been assessed as 7,200 m and PTS range for this species has been modelled as 10,630 m. To assess the residual effect, the average and maximum number of animals that may potentially be present within an area of 192 km2 (difference between the area across which effects could be mitigated and area of effect) could be calculated using harbour porpoise density range ( Table 10.13   Open ▸ ). However, this approach is considered likely to lead to an overestimate and may result in unrealistic predictions for the numbers of animals potentially injured. For example, for highly impulsive sounds such as piling, at ranges from the source in the order of tens of kilometres, the sound changes from being impulsive in character to being non-impulsive. At even greater ranges, the sound will not only be non-impulsive but can be characterised as being continuous (i.e. each pulse will merge into the next one). As presented in volume 3, appendix 10.1, annex D, assessment of transition range is an area of ongoing research but it is considered that any predicted injury ranges in the tens of kilometres are almost certainly an overly precautionary interpretation of existing criteria (Southall et al., 2021).
  2. There is also a likelihood that the range over which the animals are anticipated to be displaced during 60 minutes of ADD plus application of soft start charges ( Table 10.50   Open ▸ ) is underestimated. Firstly, strong and far-reaching responses to an ADD have been recorded by Thompson et al. (2020) at approximately 10 km to the ADD source. Moreover, to assess the range of 7,200 m, an average harbour porpoise swim speed has been applied (i.e. 1.5 m/s). Various scientific papers provided significantly faster speeds with a maximum speed of 4.3 m/s and 6.2 m/s cited by Otani et al. (2000) and Leatherwood et al. (1988), respectively.
  3. For harbour porpoise, it is expected that small numbers of animals could be exposed to potential PTS. Given that details about UXO clearance technique to be used and charge sizes will not be available until after the consent is granted (pre-construction period, following UXO survey), it is not possible to quantify the effects of UXO detonations and therefore the residual number of animals is not presented within this chapter. At a later stage, when details about UXO sizes and specific clearance techniques to be used become available, it will be possible to provide detailed assessment and tailor the secondary mitigation to specific UXO sizes and species to reduce the risk of injury. Therefore, prior to the commencement of UXO clearance works, a more detailed assessment will be produced as a part of the EPS licence supporting information for the UXO clearance works. Appropriate secondary mitigation measures will be agreed with stakeholders as a part of a UXO specific MMMP. It is therefore anticipated that following the application of secondary mitigation measures following receipt of more detail regarding size and number of UXO, the magnitude of this impact will be reduced to low.

Residual Effect – Auditory Injury

  1. Overall, following secondary mitigation, the magnitude of the impact for all species, except harbour porpoise, is deemed to be negligible and the sensitivity of the receptors is considered to be high. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
  2. For harbour porpoise, following secondary mitigation, the magnitude of the impact is deemed to be low and the sensitivity of the receptors is considered to be high. Given that only a small proportion of population could be potentially injured (PTS), the effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Residual Effect – Temporary threshold shift

  1. Overall, following secondary mitigation, the magnitude of the impact for all species is deemed to be negligible to low and the sensitivity of the receptors is considered to be low. Given that temporary loss in hearing is reversible and therefore not considered likely to lead to any long-term effects on the individual and only small proportion of respective populations could be potentially injured (TTS), the effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Injury and Disturbance to Marine Mammals from Elevated Underwater Noise Due to Vessel Use and Other Activities

  1. Increased vessel movements during the construction, operation and maintenance, and decommissioning phases have the potential to result in a range of impacts on marine mammals such as avoidance behaviour or displacement and masking of vocalisations or changes in vocalisation rate.
  2. The assessment of impacts from elevated underwater noise due to vessel use and other activities is based on vessel and/or activity basis, considering the maximum injury/disturbance range as assessed in volume 3, appendix 10.1. However, several activities could be potentially occurring at the same time and therefore ranges of effects may extend from several vessels/locations where the activity is carried out and potentially overlap.