6.3.2. Underwater Noise Generated during UXO Clearance

  1. The LSE2 assessment during the HRA Stage One process identified that during the construction phase, LSE2 could not be ruled out for underwater noise generated during UXO clearance. This relates to the following sites and relevant Annex II marine mammal features:
  • Berwickshire and North Northumberland Coast SAC;

–           grey seal.

  • Southern North Sea SAC; and

–           harbour porpoise.

  • Moray Firth SAC;

–           bottlenose dolphin.

  1. The MDS and designed in measures considered for the assessment of underwater noise generated during UXO clearance are shown in Table 6.18   Open ▸ and Table 6.19   Open ▸ , respectively.

 

Table 6.18:
MDS Considered for the Assessment of Potential Impacts to Annex II Marine Mammals due to Underwater Noise Generated During UXO Clearance in the Construction Phase

Table 6.18: MDS Considered for the Assessment of Potential Impacts to Annex II Marine Mammals due to Underwater Noise Generated During UXO Clearance in the Construction Phase

Table 6.19:
Designed In Measures Considered for the Assessment of Potential Impacts to Annex II Marine Underwater Noise Generated During UXO Clearance in the Construction Phase

Table 6.19: Designed In Measures Considered for the Assessment of Potential Impacts to Annex II Marine Underwater Noise Generated During UXO Clearance in the Construction Phase

                        Information to support the assessment

                        Overview of underwater noise modelling conducted for the Array
  1. Clearance of UXOs before construction begins could lead to effects from high order detonation of UXO ( Table 6.18   Open ▸ ). This action has the capacity to produce some of the most elevated peak sound pressures among all human-made underwater noise sources and is recognised as a high-energy, impulsive noise source (von Benda-Beckmann et al., 2015). The potential effects of this impact will vary based on the characteristics of the source, the species affected, proximity to the source and the degree of noise attenuation within the surrounding environment.
  2. Further detail on underwater noise modelling of UXO clearance is provided in volume 3, appendix 10.1 of the Array EIA Report. In the case of high order detonation, acoustic modelling was conducted following the approach outlined in (Soloway et al., 2014). The estimates are conservative, assuming the charge is freely positioned in mid-water, unlike a UXO resting on the seabed, which could experience burial, degradation, or significant attenuation. Additionally, the explosive material is likely to have deteriorated over time, making maximum noise levels probable overestimations of actual noise levels. Frequency-dependent weighting functions were applied to facilitate comparison with marine mammal hearing weighted thresholds.
  3. As per Robinson et al. (2020), low order deflagration yields a considerably lower amplitude of peak sound pressure compared to high order detonations. Therefore, for low order clearance, underwater noise modelling has been based on the methodology outlined in paragraph 558, but with a smaller donor charge size.
  4. Potential impacts of underwater noise resulting from UXO clearance on marine mammals could include mortality, physical injury, or auditory injury. The duration of potential impact (elevated noise) for each UXO detonation is very short (seconds) therefore behavioural effects are considered to be negligible in this context. As such, TTS represents a temporary auditory injury but can be also considered as a threshold for strong behavioural disturbance (for the onset of a moving away response) (see Table 6.6   Open ▸ ). A detailed underwater noise modelling assessment was carried out to investigate the potential PTS and TTS to occur, using the latest assessment criteria (volume 3, appendix 10.1 of the Array EIA Report). A project specific MMMP will be developed to mitigate the potential for injury ( Table 6.19   Open ▸ ).
  5. It is anticipated that up to 15 UXOs within the site boundary may require clearance. The maximum UXO size is assumed to be 698 kg NEQ and the most realistic maximum size is 227 kg NEQ ( Table 6.18   Open ▸ ). A low order clearance donor charge of 0.25 kg NEQ is assumed for each clearance event and up to 0.5 kg NEQ clearance shot may be required for neutralisation of residual explosive material at each location. The clearance activities will be tide and weather dependent. The aim is to enable clearance of at least one UXO per tide, during the hours of daylight and good visibility.
  6. Whilst the clearance of UXO can result in the high order detonation, in line with the UK Government et al. (2022) joint interim position statement, the Applicant commits to prioritise low order clearance techniques ( Table 6.19   Open ▸ ). To ensure a precautionary approach, the assessment for auditory injury (PTS, paragraph 563 et seq.) and strong behavioural disturbance (using TTS onset as a proxy, paragraph 574 et seq.) is based on the high order clearance of maximum UXO (698 kg NEQ), however noting that the realistic maximum case NEQ of 227 kg is considered the more likely scenario ( Table 6.18   Open ▸ ).
                        Injury (PTS)
  1. It is considered that there is a small risk that a low order clearance could result in high order detonation of UXO and therefore the assessment considered both high order and low order techniques. 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 noise is unlikely to still be impulsive in character once it has propagated more than a few kilometres (see volume 3, appendix 10.1 of the Array EIA Report for more details). The precise range at which this transition occurs is unknown, however the NMFS (2018) guidance suggests an estimate of 3 km for transition from impulsive to continuous. Hastie et al. (2019) suggest that some measures of impulsiveness change markedly within approximately 10 km of the source (for seismic surveys and piling). as such, caution should be used when interpreting any results with predicted injury ranges in the order of tens of kilometres as the PTS ranges are likely to be significantly lower than those predicted.
  2. PTS ranges for low order clearance donor charge and clearance shot are presented in Table 6.20   Open ▸ and high order clearance of UXO presented in Table 6.21   Open ▸ . The number of animals predicted to potentially experience PTS due to low order clearance donor charge and clearance shot is presented in Table 6.22   Open ▸ and high order clearance in Table 6.23   Open ▸ .
  3. A high order clearance of 698 kg NEQ yielded the largest PTS ranges for all species, with the greatest injury range (14,540 m) seen for harbour porpoise (SPLpk) ( Table 6.21   Open ▸ ). The PTS range as a result of the high order detonation of the realistic maximum case (227 kg NEQ) is reduced to 10,000 m for harbour porpoise (SPLpk). Conservatively, the number of harbour porpoise that could be potentially injured, based on the site-specific seasonal peak density of 0.651 animals per km2, was estimated as 433 animals for 698 kg NEQ UXO high order explosion (SPLpk) equating to 0.12% of the North Sea MU ( Table 6.23   Open ▸ ). Predicted numbers are smaller for the realistic maximum case UXO (227 kg NEQ) with up to 205 animals potentially experiencing PTS (SPLpk) equating to 0.06 % of the North Sea MU Table 6.23   Open ▸ ). For low order clearance donor charge (0.25 kg NEQ) and clearance shot (0.5 kg NEQ), the PTS ranges of 1,050 m and 1,320 m were predicted ( Table 6.20   Open ▸ ), which could injure up to three and four harbour porpoises, respectively ( Table 6.22   Open ▸ ).
  4. The underwater noise assessment found that the maximum injury (PTS) range estimated for bottlenose dolphin using the SPLpk metric is 840 m for the high order detonation of 698 kg NEQ, but this is reduced to 577 m for the realistic maximum case (227 kg NEQ) ( Table 6.21   Open ▸ ). Given relatively low densities of bottlenose dolphin within the Array marine mammal study area, the high order detonation of 698 kg and 227 kg could result in injury for no more than one individual ( Table 6.23   Open ▸ ). With reference to the wider population, this equated to small proportions of the relevant MU (less than 0.01%). For low order clearance donor charge (0.25 kg NEQ) and clearance shot (0.5 kg  NEQ), the injury ranges were considerably lower with a maximum of 61 m and 77 m respectively ( Table 6.20   Open ▸ ), and there would be no more than one animal potentially injured within these ranges ( Table 6.22   Open ▸ ).
  5. The maximum injury (PTS) range estimated for grey seal was 2,850 m using the SPLpk metric, for the high order detonation of 698 kg NEQ, but this was reduced to 1,960 m for 227 kg NEQ ( Table 6.21   Open ▸ ). The number of individuals that could be potentially injured, based on average densities within the Array marine mammal study area from Carter et al. (2022), was estimated as up to five animals for 698 kg NEQ ( Table 6.23   Open ▸ ), which equates to 0.01% of the East Scotland plus North-east England SMUs, and up to three animals for the realistic maximum design scenario (227 kg NEQ). For low order clearance donor charge (0.25 kg NEQ) and clearance shot (0.5 kg NEQ), the injury ranges were considerably lower with a maximum of 50 m and 259 m (SPLpk), respectively ( Table 6.20   Open ▸ ) and there would be no more than one animal potentially injured within these ranges ( Table 6.22   Open ▸ ).
  6. The auditory injury (PTS) ranges do not overlap with the Berwickshire and North Northumberland Coast SAC, Southern North Sea SAC, or the Moray Firth SAC.
Table 6.20:
Maximum PTS Ranges For Low Order Clearance Donor Charge and Clearance Shot (N/E = Threshold Not Exceeded)

Table 6.20: Maximum PTS Ranges For Low Order Clearance Donor Charge and Clearance Shot (N/E = Threshold Not Exceeded)

 

Table 6.21:
Maximum PTS Ranges for High Order Detonation of Maximum and Realistic Maximum Case

Table 6.21: Maximum PTS Ranges for High Order Detonation of Maximum and Realistic Maximum Case

 

Table 6.22:
Maximum Number of Animals With the Potential to Experience PTS Due to Low Order Clearance Donor Charge and Clearance Shot (N/A = Not Applicable As the Threshold Was Not Exceeded)

Table 6.22: Maximum Number of Animals With the Potential to Experience PTS Due to Low Order Clearance Donor Charge and Clearance Shot (N/A = Not Applicable As the Threshold Was Not Exceeded)

 

Table 6.23:
Maximum Number of Animals With the Potential to Experience PTS Due to High Order Detonation of Maximum and Realistic Maximum Case (Prior to Any Mitigation)

Table 6.23: Maximum Number of Animals With the Potential to Experience PTS Due to High Order Detonation of Maximum and Realistic Maximum Case (Prior to Any Mitigation)

 

  1. With primary mitigation (i.e. using low order techniques, Table 6.19   Open ▸ ) in place the assessment found that there would be a risk of injury over a range of 1,050 m (for harbour porpoise using the SPLpk metric ( Table 6.20   Open ▸ ). The injury range for clearance shot of 0.5 kg NEQ was predicted across a range of 1,320 m ( Table 6.20   Open ▸ ).
  2. However, if low order clearance is not feasible or accidentally results in high order detonation, there is a maximum risk of injury (predicted for harbour porpoise) out to 14,540 m during detonation of 698 kg NEQ and 10,000 km for a 227 kg NEQ. Therefore, in line with standard industry practice (JNCC, 2010a), tertiary mitigation will be applied as a part of the MMMP ( Table 6.19   Open ▸ ). In line with stakeholder advice provided in response to Marine Mammal Consultation Note 2 (volume 2, chapter 10 of the Array EIA Report), the assessment with respect to PTS from UXO clearance will be based on both SPLpk and SELcum injury ranges.
  3. The maximum injury ranges presented in Table 6.20   Open ▸ and Table 6.21   Open ▸ are larger than the standard 1,000 m mitigation zone recommended for UXO clearance (JNCC, 2010a). The mitigation zone cannot be excessively large (e.g. a few km) as there may be difficulties in detecting marine mammals (particularly harbour porpoise) over large ranges (McGarry et al., 2017) with a significant decline in visual detection rate with increasing sea state (Embling et al., 2010, Leaper et al., 2015).
  4. Tertiary mitigation will therefore include the use of ADDs and scare charges to deter animals from the injury zone ( Table 6.19   Open ▸ ). The efficacy of such deterrence will depend upon the device selected and reported ranges of effective deterrence vary. The reported effective deterrence range for harbour porpoise vary from 2.5 km out to 12 km (Brandt et al., 2013, Dähne et al., 2017, Kyhn et al., 2015, Olesiuk et al., 2002). A full review of available devices is provided in McGarry et al. (2022). In addition to the ADD use, deterrence can also be achieved through the use of soft start charges. Details of appropriate tertiary mitigation are discussed in the outline MMMP and will be discussed and agreed with consultees post-consent when further details of the size and type of potential UXOs are understood.
  5. For harbour porpoise, the ranges of effect are large for high order clearance, and it is likely that following tertiary mitigation measures there will be a residual risk of PTS to a number of individuals ( Table 6.23   Open ▸ ). To illustrate what this may entail for high order clearance of the realistic maximum case (227 kg NEQ), based on a conservative swim speed of 1.5 m/s for harbour porpoise ( Table 6.5   Open ▸ ), a total of 112 minutes of deterrence activities would be required to allow animals to flee the injury range. Secondary mitigation is discussed in paragraphs 580 et seq which address the potential residual risk from a high order detonation.
Behavioural disturbance (TTS as a proxy)
  1. As discussed in paragraph 560, the duration of effect for each UXO detonation is less than one second and therefore behavioural effects are considered to be negligible in this context. The assessment for behavioural disturbance uses the onset of TTS as a proxy. Although the effect would be a potential temporary loss in hearing and some 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 corresponds to a moving away or ‘fleeing response’ as this is the threshold at which animals experience disturbance and are likely to move away from the ensonified area. The onset of TTS is also considered to represent the boundary between the most severe disturbance levels and the start of physical auditory impacts on animals. Considering the above, the results of underwater noise modelling based on TTS onset as a proxy, will be hereinafter referred to as ‘strong behavioural disturbance’.
  2. Strong behavioural disturbance ranges for low order clearance donor charge and clearance shot are presented in Table 6.24   Open ▸ and high order clearance of UXO presented in Table 6.25   Open ▸ . The largest ranges using SPLpk metric were predicted for clearance of the 698 kg NEQ with potential strong disturbance over a distance of up to 26,790 m for harbour porpoise ( Table 6.25   Open ▸ ). Ranges predicted for other species using SPLpk only slightly exceeded 5 km for grey seal, with the largest strong behavioural disturbance range predicted at 5,250 m ( Table 6.25   Open ▸ ). For harbour porpoise and bottlenose dolphin, the SELcum­ metric yielded lower strong disturbance ranges during high order detonation than the SPLpk metric ( Table 6.25   Open ▸ ). However, for grey seal, the maximum strong disturbance range of 6,120 m was modelled using the SELcum­ metric for high order disposal, in contrast to the 5,250 m modelled using the SPLpk metric ( Table 6.25   Open ▸ ). It should be noted that impulsive noise thresholds (TTS onset) were used in the underwater noise modelling for strong behavioural disturbance as a result of UXO clearance. As previously described in paragraph 563, the noise is unlikely to be impulsive in character once it has propagated more than a few kilometres and it is particularly important when interpreting results for disturbance within ranges larger than 10 km as these are likely to be significantly lower than predicted see (Hastie et al., 2019) (see volume 3, appendix 10.1 of the Array EIA Report for more details).

 

Table 6.24:
Maximum Strong Behavioural Disturbance Ranges (TTS Used As a Proxy) For Low Order Clearance Donor Charge and Clearance Shot (N/E = Threshold Not Exceeded)

Table 6.24: Maximum Strong Behavioural Disturbance Ranges (TTS Used As a Proxy) For Low Order Clearance Donor Charge and Clearance Shot (N/E = Threshold Not Exceeded)

 

Table 6.25:
Maximum Strong Behavioural Disturbance Ranges (TTS Used As a Proxy) for High Order Detonation of Maximum and Realistic Maximum Case

Table 6.25: Maximum Strong Behavioural Disturbance Ranges (TTS Used As a Proxy) for High Order Detonation of Maximum and Realistic Maximum Case

 

  1. The number of animals predicted to experience strong behavioural disturbance due to low order clearance donor charge and clearance shot is presented in Table 6.26   Open ▸ and high order clearance in Table 6.27   Open ▸ . Given the largest strong behavioural disturbance ranges ( Table 6.25   Open ▸ ) and precautionary peak seasonal site-specific densities ( Table 6.3   Open ▸ ), the largest number of animals affected was found for harbour porpoise where up to 1,467 animals could experience strong disturbance as a result of high order detonation of a 698 kg NEQ (based on SPLpk metric, 0.42% of the North Sea MU population). Based on SELcum, the number of grey seal at risk of experiencing strong behavioural disturbance within a predicted 6,120 m disturbance range was estimated as 22 animals (0.06% of the East Scotland SMU plus the North-east England SMU). For bottlenose dolphin, the number of animals predicted to be disturbed was very small with no more than one animal within the predicted effect zones ( Table 6.26   Open ▸ , Table 6.27   Open ▸ ).
  2. The strong behavioural disturbance ranges will not overlap with the Berwickshire and North Northumberland Coast SAC, Southern North Sea SAC, or the Moray Firth SAC.

 

Table 6.26:
Maximum Number of Animals With the Potential to Experience Strong Disturbance (TTS Used as a Proxy) Due to Low Order Clearance Donor Charge and Clearance Shot

Table 6.26: Maximum Number of Animals With the Potential to Experience Strong Disturbance (TTS Used as a Proxy) Due to Low Order Clearance Donor Charge and Clearance Shot

 

Table 6.27:
Maximum Number of Animals With the Potential to Experience Strong Disturbance (TTS Used as a Proxy) Due to High Order Detonation of Maximum and Realistic Maximum Case

Table 6.27: Maximum Number of Animals With the Potential to Experience Strong Disturbance (TTS Used as a Proxy) Due to High Order Detonation of Maximum and Realistic Maximum Case

 

  1. Strong behavioural effects are reversible and therefore animals are anticipated to fully recover following cessation of the activity. It is, however, recognised that where tertiary mitigation applies to reduce the risk of auditory injury (PTS), the deterrence measures (i.e. ADD and soft start charges) by their nature would contribute to, rather than reduce, the moving away response.
  2. As previously described in paragraph 562, the assessment considered the magnitude of a high order detonation for the MDS of 698 kg NEQ. The magnitude of disturbance resulting from a high order detonation is predicted to be of regional spatial extent, very short-term duration, intermittent and both the impact itself (i.e. the elevation in underwater noise during detonation event) and effect of disturbance is reversible (TTS represents a non-trivial disturbance but not permanent injury). It is predicted that the potential impact will affect the receptor directly, however, for all species a small proportion of the relevant MUs is predicted to be affected by strong behavioural disturbance. As such, whilst there may be effects at an individual level, these are not predicted to be at a scale that would lead to any population-level effects.
                        Secondary mitigation and residual effect
  1. If required, secondary mitigation (i.e. ADD with a duration over 30 minutes) will be applied to further reduce the potential for injury to harbour porpoise occurring during UXO clearance (detailed in Table 6.19   Open ▸ ). Final mitigation required will be addressed post consent, in consultation with stakeholders, following more detailed information such as the size, number and quality of UXOs to be cleared (following site-investigation surveys), noting that it may be possible to reduce the ADD activation period and soft start procedure depending on the size and number of UXOs located within the Array. Paragraph 582 et seq. therefore details a worked example for mitigation based on the most significant predicted effect, and focused on harbour porpoise (as this is the species with a potential residual risk of injury), which considers the different timescales that would be required to clear the injury zone if ADD and soft-start is required.
  2. As described in paragraph 562 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 563 et seq.563). The secondary mitigation has been therefore tailored based on the size of the UXO and high order detonation scenario.
  3. A range of UXO munitions sizes have been considered for the purpose of determining effective mitigation measures, up to a maximum scenario of a UXO size of 698 kg. This approach follows a similar strategy to that which was taken for Seagreen 1 Offshore Wind Farm EPS Risk Assessment and outline MMMP (volume 4, appendix 22) (Seagreen Wind Energy Ltd, 2021).
  4. An outline MMMP (volume 4, appendix 22 of the Array EIA Report) has been developed for the purpose of mitigating the risk of auditory injury (PTS) to marine mammals from the proposed UXO clearance activities at the Array. This has been provided as a stand-alone document; however, this section provides an overview of the procedures for ADD and soft start, prior to making conclusions on the potential for residual effects and requirement for secondary mitigation.
  5. The designed in measures included as a part of the outline MMMP (volume 4, appendix 22 of the Array EIA Report) ( Table 6.19   Open ▸ ) are in line with JNCC guidelines for minimising the risk of injury to marine mammals from using explosives (JNCC, 2010a). Details of ADD use and soft start charge application are specific for each of the anticipated UXO sizes. As discussed in paragraph 580, prior to the commencement of UXO clearance works, a more detailed assessment will be produced 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. The approach to mitigating injury to marine mammals involves the monitoring of a 1 km radius mitigation zone in line with current guidance (JNCC, 2010a). Monitoring will be carried out by suitably qualified and experienced personnel within a mitigation team, comprising of two dedicated MMO2 and one dedicated PAM operator. The purpose of this monitoring is to clear the mitigation zone of marine mammals prior to detonation.
  6. Given the potential for auditory injury from high-order detonations for harbour porpoise and grey seal is at a greater range than can be mitigated by monitoring the 1 km zone ( Table 6.21   Open ▸ ), an ADD will be deployed to deter marine mammals to a greater distance before any detonation. The assessment of effects provided in paragraph 563 et seq. determines the auditory injury range based on high order detonation of a 698 kg NEQ UXO ( Table 6.21   Open ▸ ). At the time of writing, the actual number and size of the UXOs within the site boundary are unknown and therefore, the example 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.
  7. Swim speeds are summarised in Table 6.5   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. 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. Example deterrence distances are provided for all Annex II marine mammals in Table 6.28   Open ▸ .
  8. Based on the UXO clearance flow chart ( Figure 6.11   Open ▸ ; informed by Seagreen Wind Energy Ltd (2021)), for low order UXO size up to 0.25 kg NEQ, the required time of ADD activation is 12 minutes and this is expected to displace harbour porpoise to 1,080 m (exceeding the PTS distance of 1,050 m). If UXO size of up 0.5 kg NEQ is identified during the survey, then ADD will be activated for 15 minutes and this is expected to deter harbour porpoise to 1,350 m. For all other species, three minutes of ADD would be sufficient to deter the animals from the injury zone.
  9. However, for high order UXO clearance, injury ranges are larger. Assuming the ADD is activated for an indicative 60 minutes ( Table 6.28   Open ▸ ), the displacement distance for harbour porpoise would be 5,400 m, meaning there is a need to deter harbour porpoise from larger ranges that cannot be achieved using an ADD for 60 minutes duration alone (i.e. the injury zone exceeds 5,400 m). However, for all other species, a duration of 60 minutes ADD activation will be sufficient to deter animals from the injury zone up to the 698 kg NEQ ( Table 6.28   Open ▸ ).
  10. For high order UXO, to reduce the risk of PTS, there is a need to deter animals from larger ranges than can achieved using an ADD alone. Therefore, following an ADD activation period of up to 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 6.28   Open ▸ , Figure 6.11   Open ▸ ). It is expected that up to 80 minutes of combined ADD/soft start procedure (up to 60 minutes of ADD and 20 minutes of soft start) will displace harbour porpoise to ranges of 7,200 m. Whilst this secondary mitigation is considered to be sufficient to deter most animals (noting that use of ADD alone deterred all other species from the injury zone), 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 10,000 for the 227 kg NEQ and 14,580 m for 698 kg NEQ ( Table 6.28   Open ▸ ).
Table 6.28:
Recommended ADD Duration for Low Order and High Order UXO Clearance and Sizes, and Associated Displacement Distance

Table 6.28: Recommended ADD Duration for Low Order and High Order UXO Clearance and Sizes, and Associated Displacement Distance

Figure 6.11:
High Order UXO Clearance Mitigation Flow Chart for the Array (1Assuming UXO is not Suitable for Low Order Techniques)

Figure 6.11: High Order UXO Clearance Mitigation Flow Chart for the Array (1Assuming UXO is not Suitable for Low Order Techniques)


  1. The analysis presented in Table 6.28   Open ▸ suggests that for UXO sizes of up to 698 kg, pre-detonation search and use of ADD will be sufficient to reduce the potential of experiencing PTS by bottlenose dolphin and grey seal to negligible magnitude. However, it has been estimated that harbour porpoise could potentially experience an auditory injury at distances that cannot be fully mitigated by application of ADD and soft start charges. The maximum displacement distance has been assessed as 7,200 m and PTS range for this species has been modelled as 14,580 m.
  2. To assess the residual effect, the average and maximum number of animals that may potentially be present within an area of 501 km2 (difference between the area across which effects could be mitigated and area of effect) could be calculated using harbour porpoise density range ( Table 6.21   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 due to a combination of factors (e.g. dispersion of the waveform, multiple reflections from sea surface and seafloor, and molecular absorption of high frequency energy). Empirical evidence has suggested such shifts in impulsivity could occur markedly within 10 km from the source (Hastie et al., 2019). Since the precise range at which this transition occurs is unknown (not least because the transition also depends on the response of the marine mammals’ ear), models still adopt the impulsive thresholds at all ranges, and this is likely to lead to an overly precautionary estimate of injury ranges at larger distances (tens of kilometres) from the source. It is noted defining this transition range is an active area of research and scientific debate, with a number of other potential methods being investigated. Furthermore, at even greater ranges, the noise will not only be non-impulsive but can be characterised as being continuous (i.e. each pulse will merge into the next one and therefore 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)
  3. 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 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). However 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.
  4. For harbour porpoise, it is expected that small numbers of animals could potentially be exposed to 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 phase, following a UXO survey), it is not possible to quantify the effects of UXO detonations and therefore a residual number of animals potentially impacted is not presented within this Part of the RIAA. At a later stage, when details about UXO sizes and specific clearance techniques to be used become available, it will be possible to tailor the secondary mitigation to specific UXO sizes following the UXO survey and species to reduce the risk of injury.
  5. Therefore, prior to the commencement of UXO clearance works, appropriate secondary mitigation measures will be discussed with stakeholders and proposed as a part the final MMMP for UXO clearance works. It is therefore anticipated that following the application of secondary mitigation measures following receipt of more detail regarding size and number of UXO (and tailoring of secondary mitigation measures as described), there will be no adverse effect on integrity of the SACs assessed with Annex II marine mammal features, particularly for harbour porpoise.

                        Construction phase

                        Berwickshire and North Northumberland Coast SAC
Grey seal

                        Injury (PTS)

  1. As presented in paragraph 567, the maximum injury (PTS) range estimated for grey seal was 2,850 m using the SPLpk metric for the high order detonation of 698 kg NEQ. However, this was reduced to 1,960 m for 227 kg NEQ ( Table 6.21   Open ▸ ). The number of individuals that could be potentially injured, based on average densities within the Array marine mammal study area from Carter et al. (2022), was estimated as up to five animals for 698 kg NEQ ( Table 6.23   Open ▸ ), which equates to 0.01% of the East Scotland plus North-east England SMUs, and up to three animals for the realistic maximum design scenario  (227 kg NEQ). For low order clearance donor charge (0.25 kg NEQ) and clearance shot (0.5 kg NEQ), the injury ranges were considerably lower with a maximum of 50 m and 259 m (SPLpk), respectively ( Table 6.20   Open ▸ ) and there would be no more than one animal potentially injured within these ranges ( Table 6.22   Open ▸ ). The maximum auditory injury (PTS) range for grey seal (2,850 m) does not overlap with the Berwickshire and North Northumberland Coast SAC, which is a minimum of 113.95 km south-west from the site boundary.
  2. Based on the maximum injury (PTS) range (estimated using the SPLpk metric) this potential impact would be localised within several kilometres of the detonation. UXO clearance would occur intermittently throughout the construction phase of the Array and be very short term. Although the potential impact itself is reversible (i.e. the elevation in underwater noise only occurs during the detonation event), the effect of PTS on grey seal is permanent. With tertiary mitigation applied ( Table 6.19   Open ▸ ), it is anticipated that grey seal would be deterred from the injury zone and therefore the likelihood of PTS and population-level effects would be unlikely (paragraph 590).

                        Behavioural disturbance (TTS as a proxy)

  1. As presented in paragraphs 574 to 579, the largest range of strong behavioural disturbance to grey seal (using TTS as a proxy) was predicted for clearance of the 698 kg NEQ using the SELcum metric: 6,120 m ( Table 6.25   Open ▸ ). The SELcum­ metric yielded higher strong disturbance ranges during high order detonation than the SPLpk metric: 5,250 m ( Table 6.25   Open ▸ ). It should be noted that impulsive noise thresholds (TTS onset) were used in the underwater noise modelling for strong behavioural disturbance as a result of UXO clearance. Based on SELcum, the number of grey seal at risk of experiencing strong behavioural disturbance within a predicted 6,120 m disturbance range was estimated as 22 animals (0.06% of the East Scotland SMU plus the North-east England SMU) ( Table 6.26   Open ▸ , Table 6.27   Open ▸ ). As previously described in paragraph 563, the noise is unlikely to be impulsive in character once it has propagated more than a few kilometres (Hastie et al., 2019) (see volume 3, appendix 10.1 of the Array EIA Report for more details). The strong behavioural disturbance ranges will not overlap with the Berwickshire and North Northumberland Coast SAC (which is a minimum of 113.95 km south-west from the site boundary).
  2. Kastelein et al. (2018b) measured recovery rates of harbour seal following exposure to a noise source of 193 dB re 1 μPa2s (SELcum) over 360 minutes and found that recovery from TTS to the pre-exposure baseline was estimated to be complete within 72 minutes following exposure. These results are in line with findings reported in SEAMARCO (2011), which showed that for small TTS values, recovery in seal species was very fast (around 30 minutes) and the higher the hearing threshold shift, the longer the recovery.
  3. Considering the above, in most cases, impaired hearing for a short time is anticipated to have little effect on the total foraging period of a seal. If hearing is impaired for longer periods (hours or days) the potential impact has the potential to be ecologically significant (SEAMARCO, 2011). Nevertheless, the findings of studies presented in this section indicate that seal species are less vulnerable to TTS than harbour porpoise for the noise bands tested. It is also expected that grey seals would move beyond the injury range prior to the onset of TTS. The assessment considered that grey seal is likely to be able to tolerate the effect without any potential impact on either reproduction or survival rates and would be able to return to previous behavioural states or activities once the impacts had ceased.
                        Conclusion
  1. Adverse effects on the qualifying Annex II marine mammal features of the Berwickshire and North Northumberland Coast SAC which undermine the conservation objectives of the SAC will not occur as a result of underwater noise generated during UXO clearance in the construction phase. Potential effects from this activity on the relevant conservation objectives (as presented in section 6.2.1) are discussed in turn below in Table 6.29   Open ▸ .

 

Table 6.29:
Conclusions Against the Conservation Objectives of the Berwickshire and North Northumberland Coast SAC from Underwater Noise Generated During UXO Clearance in the Construction Phase of the Array Alone

Table 6.29: Conclusions Against the Conservation Objectives of the Berwickshire and North Northumberland Coast SAC from Underwater Noise Generated During UXO Clearance in the Construction Phase of the Array Alone

 

  1. It can be concluded, beyond reasonable scientific doubt, that there is no risk of an adverse effect on the integrity of the Berwickshire and North Northumberland Coast SAC as a result of underwater noise generated during UXO clearance in the construction phase of the Array alone.
                        Southern North Sea SAC
Harbour porpoise

                        Injury (PTS)

  1. Scientific literature surrounding sensitivities to UXO clearance often focuses on harbour porpoise due to their very high sensitivity to noise. A study by von Benda-Beckmann et al. (2015) presented the range of effects of explosives on harbour porpoise in the southern North Sea; measures of SEL and peak overpressure (in kPa) were taken 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). von Benda-Beckmann et al. (2015) investigated 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). In addition, 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). Results demonstrated the largest distance at which a risk of ear trauma could occur was at 500 m. They also found 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, therefore 1 dB above the ‘very likely to occur’ threshold.
  2. The study also modelled possible effect ranges for 210 explosions (of up to 1,000 kg charge mass) that had been logged by the Royal Netherland Navy and the Royal Netherlands Meteorological Institute over a two year period (2010 and 2011) (von Benda-Beckmann et al., 2015). Validating the model using the empirical measurements of SEL out to 2 km, von Benda-Beckmann et al. (2015) 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). Harbour porpoises are known to spend a large proportion of time near the surface (e.g. 55% based on Teilmann et al. (2007)) where the SEL was 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 (given empirical measurements were made out to this point), estimates above this distance required further validation since the uncorrected model systematically overestimates SEL. More recently, Salomons et al. (2021) analysed sound measurements performed near two detonations of UXO (with charge masses of 325 kg and 140 kg). Subsequently a PTS effect distance in the range 2.5 km to 4 km was derived (Salomons et al., 2021), using the weighted SEL values and threshold levels from Southall et al. (2019). When comparing the experimental data and model predictions, the same study concluded thar harbour porpoise are at risk of permanent hearing loss at distances of several kilometres, i.e. distance between 2 km and 6 km based on 140 kg and 325 kg charge masses, respectively (Salomons et al., 2021). In 2019, 24 harbour porpoise were found dead following clearance of ground mines in the Baltic Sea in along the German 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.
  3. As presented in paragraph 565, a high order clearance of 698 kg NEQ yielded the largest PTS ranges for harbour porpoise using the SPLpk­ metric (14,540 m) ( Table 6.21   Open ▸ ). The PTS range from high order detonation of the realistic maximum case (227 kg NEQ) was reduced to 10,000 m for harbour porpoise (SPLpk). Conservatively, the number of harbour porpoise that could be potentially injured, based on the site-specific seasonal peak density of 0.651 animals per km2, was estimated as 433 animals for 698 kg NEQ UXO high order explosion (SPLpk) equating to 0.12% of the North Sea MU ( Table 6.23   Open ▸ ). Predicted numbers are smaller for the realistic maximum design scenario UXO (227 kg NEQ) with up to 205 animals potentially experiencing PTS (SPLpk) equating to 0.06 % of the North Sea MU Table 6.23   Open ▸ ). For low order clearance donor charge (0.25 kg NEQ) and clearance shot (0.5 kg NEQ), the PTS ranges of 1,050 m and 1,320 m were predicted ( Table 6.20   Open ▸ ), which could injure up to three and four harbour porpoises, respectively ( Table 6.22   Open ▸ ). The maximum auditory injury (PTS) range for harbour porpoise (14,540 m) does not overlap with the Southern North Sea SAC, which is a minimum of 129.86 km south-east from the site boundary.
  4. The maximum injury ranges for harbour porpoise presented in Table 6.20   Open ▸ and Table 6.21   Open ▸ are larger than the standard 1,000 m mitigation zone recommended for UXO clearance (JNCC, 2010a). Therefore, tertiary mitigation will be applied as a part of the MMMP and has been discussed in paragraphs 569 to 573. Tertiary mitigation will include the use of ADDs and scare charges to deter animals from the injury zone ( Table 6.19   Open ▸ ). In addition to the ADD use, deterrence can also be achieved through the use of soft start charges. However, given the large ranges of effect for harbour porpoise during high order clearance, it is likely that there will be a residual risk of PTS to a number of individuals after application of tertiary mitigation measures. Therefore, secondary mitigation has been discussed in paragraphs 580 et seq., which address the potential residual risk from a high order detonation. Whilst it is complex to quantify the residual risk, it is anticipated that there may be a measurable change at an individual level. An MDS approach has been applied comprising clearance of up to 15 UXOs with low order techniques being prioritised, it is expected that UXO clearance would not manifest to population-level effects due to the small proportion of harbour porpoise within the North Sea MU potentially affected. 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 phase, following a UXO survey), it is not possible to quantify the effects of UXO detonations and therefore a residual number of animals potentially impacted is not presented within this Part of the RIAA. The Array EIA Report anticipated that following the application of secondary mitigation measures upon receipt of more detail regarding size and number of UXO, the magnitude of this impact will be reduced to low for harbour porpoise. With the application of the secondary mitigation presented in paragraphs 580 et seq., and given that only a small proportion of the North Sea MU population could potentially experience PTS or TTS, this residual impact was concluded to result in no adverse effect on integrity of the harbour porpoise feature of the Southern North Sea SAC.

                        Behavioural disturbance (TTS as a proxy)

  1. Recovery rates of harbour porpoise were measured following exposure to a piling playback noise source of 175 dB re 1 μPa2s (SEL) over 120 minutes (SEAMARCO, 2011). SEAMARCO (2011) found that recovery to the pre-exposure threshold was estimated to be complete within 48 minutes following exposure and the higher the hearing threshold shift, the longer the recovery. Further, Kastelein et al. (2021) found that the susceptibility to TTS depends on the frequency of the fatiguing noise causing the shift and the greatest TTS depends on the SPL (and related SEL).
  2. In a series of studies reviewed in Finneran (2015), which measured TTS occurrence in harbour porpoise at a range of frequencies typical of high-amplitude anthropogenic sounds, the greatest shift in mean TTS occurred at 0.5 kHz with hearing recovery within 60 minutes after the fatiguing noise stopped. Scientific understanding of the biological effects of TTS is limited to the results of controlled exposure studies on small numbers of captive animals. 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.
  3. As presented in paragraphs 574 to 579, the largest range of strong behavioural disturbance to harbour porpoise (using TTS as a proxy) was predicted for clearance of the 698 kg NEQ using the SPLpk metric: 26,790 m ( Table 6.25   Open ▸ ). The SELcum­ metric yielded lower strong disturbance ranges during high order detonation than the SPLpk metric: 8,720 m ( Table 6.25   Open ▸ ). It should be noted that impulsive noise thresholds (TTS onset) were used in the underwater noise modelling for strong behavioural disturbance as a result of UXO clearance. As previously described in paragraph 563, the noise is unlikely to be impulsive in character once it has propagated more than a few kilometres (Hastie et al., 2019) (see volume 3, appendix 10.1 of the Array EIA Report for more details). Given the largest strong behavioural disturbance ranges ( Table 6.25   Open ▸ ) and precautionary peak seasonal site-specific densities ( Table 6.3   Open ▸ ), up to 1,467 harbour porpoise could experience strong disturbance as a result of high order detonation of a 698 kg NEQ (based on SPLpk metric, 0.42% of the North Sea MU population) ( Table 6.26   Open ▸ , Table 6.27   Open ▸ ). As per JNCC (2020) guidance, a 26 km EDR for UXO clearance is presented in Figure 6.12   Open ▸ . This EDR is comparable to the disturbance range of 26,790 m modelled using TTS as a proxy ( Table 6.25   Open ▸ ). The modelled strong behavioural disturbance ranges and the 26 km EDR will not overlap with the Southern North Sea SAC.

                        Conclusion

  1. Adverse effects on the qualifying Annex II marine mammal features of the Southern North Sea SAC which undermine the conservation objectives of the SAC will not occur as a result of underwater noise generated during UXO clearance in the construction phase. Potential effects from this activity on the relevant conservation objectives (as presented in section 6.2.2) are discussed in turn below in Table 6.30   Open ▸ .

Figure 6.12:
EDR of 26 km for harbour porpoise of the Southern North Sea SAC in relation to UXO Clearance (based on JNCC (2020)

Figure 6.12: EDR of 26 km for harbour porpoise of the Southern North Sea SAC in relation to UXO Clearance (based on JNCC (2020)


Table 6.30:
Conclusions Against the Conservation Objectives of the Southern North Sea SAC from Underwater Noise Generated During UXO Clearance in the Construction Phase of the Array Alone

Table 6.30: Conclusions Against the Conservation Objectives of the Southern North Sea SAC from Underwater Noise Generated During UXO Clearance in the Construction Phase of the Array Alone

 

  1. It can be concluded, beyond reasonable scientific doubt, that there is no risk of an adverse effect on the integrity of the Southern North Sea SAC as a result of underwater noise generated during UXO clearance in the construction phase of the Array alone.