Secondary Mitigation and Residual Effect
941 Due to the potential to experience injury to marine mammals, secondary mitigation will be applied in the form of an ADD to deter animals from the area of impact. This is due to the inherent uncertainties in applying the standard measures (visual and acoustic approaches), for example, problems with detecting animals in high sea states or low visibility due to adverse weather conditions.
942 ADDs have commonly been used in marine mammal mitigation at UK offshore wind farms to deter animals from potential injury zones prior to the start of piling. The JNCC (2010a) draft guidance for piling mitigation recommends their use, particularly in respect of periods of low visibility or at night to allow 24-hour working. With a number of research projects on ADDs commissioned via the Offshore Renewables Joint Industry Programme (ORJIP), the use of ADDs for mitigation at offshore wind farms has gained momentum. Indeed, for the BOWL, the use of ADDs was accepted by the regulators (Marine Scotland) to be applied pre-piling as it was thought to be more effective at reducing the potential for injury to marine mammals compared to standard designed-in measures (marine mammal observers and PAM) which, as mentioned previously, have limitations with respect to effective detection over distance (Parsons et al., 2009; Wright and Cosentino, 2015).
943 There are various ADDs available with different sound source characteristics (see McGarry et al., 2020) and a suitable device will be selected based on the key species requiring mitigation. The selected device will typically be deployed from the piling vessel and activated for a pre-determined duration to allow animals sufficient time to move away from the sound source whilst also minimising the additional noise introduced into the marine environment. The type of ADD and approach to mitigation (including activation time and procedure) are included in the MMMP (see volume 4, appendix 23 of the Offshore EIA Report).
944 Noise modelling was carried out to determine the potential efficacy of using ADDs to deter marine mammals from the injury zone (see volume 3, appendix 10.1 of the Offshore EIA Report). The results suggest that the use of an ADD for a duration of 30 minutes before the piling commences would further reduce the potential to experience injury to marine mammal receptors. For example, the maximum injury zones for species based on SPLpk metric for piling of the wind turbines and OSP/Offshore convertor station platform foundations at a maximum hammer energy of 4,000 kJ using 1% constant conversion factor are shown in Table 13.19 Open ▸ . Assuming conservative swim speeds, it was demonstrated that activation of an ADD for 30 minutes would deter all animals beyond the maximum injury zone ( Table 13.19 Open ▸ ). This corroborates findings of other studies that reported that ADDs deter different marine mammals over several hundreds of metres or indeed several kilometres from the source (reviewed in McGarry et al., 2020).
Table 13.19: Summary of Peak Pressure Injury Ranges for Marine Mammals Due to Single Piling of Wind turbine and OSP/Offshore Convertor Station Platform at 4,000 kJ Hammer Energy Using 1% Constant Conversion Factor, Showing Whether the Individual Can Flee the Injury Range During the 30 Minutes of ADD Activation
Injury and disturbance to marine mammals from elevated underwater noise during site investigation surveys
946 Source levels for borehole drilling ahead of standard penetration testing are in a range of 142 dB to 145 dB re 1 µPa re 1 m (rms). SEL measurements conducted during core penetration tests (CPTs) showed that it is characterised by broadband sound with levels measured generally 20 dB above the acoustic ocean noise floor (volume 3, appendix 10.1 of the Offshore EIA Report). For the purpose of this assessment, these sources are considered as impulsive sounds. Measurements of a vibro-core test (Reiser, 2011) show underwater source sound pressure levels of approximately 187 dB re 1 µPa re 1 m (rms). The vibro-core sound is considered to be continuous (non-impulsive).
947 Full description of the source noise levels for geophysical and geotechnical survey activities is provided in volume 3, appendix 10.1 of the Offshore EIA Report.
Auditory injury
948 Potential impacts of site investigation surveys will depend on the characteristic of the activity, frequency bands and water depth. The impact ranges presented in this section are rounded to the nearest 5 m. It should be noted that, for the sonar-based surveys, many of the injury ranges are limited to approximately 65 m as this is the approximate water depth in the area. Sonar based systems have very strong directivity which effectively means that there is only potential for injury when a marine mammal is directly underneath the sound source. Once the animal moves outside of the main beam, there is no potential for injury. This section provides estimated ranges for injury of Annex II marine mammals in the construction phase of the Proposed Development.
949 The noise modelling assessment showed that ranges within which there is a risk of PTS occurring to Annex II marine mammals as a result of geophysical investigation activities (based on comparison to Southall et al. (2019) SEL thresholds) are relatively low ( Table 13.20 Open ▸ ). For harbour porpoise PTS could occur out to 360 m during sub-bottom profiler surveys. However, impact ranges within which PTS could occur are smaller for other marine mammal species at maximum of 65 m.
Table 13.20 Potential PTS Impact Ranges (m) for Annex II Marine Mammals During the Geophysical Site Investigation Surveys
1 N/E = Threshold Not Exceeded
950 With respect to the ranges within which there is a risk of PTS occurring to marine mammals as a result of geotechnical investigation activities, PTS threshold was not exceeded for almost all Annex II marine mammal species, except harbour porpoise ( Table 13.21 Open ▸ ). PTS is only expected to occur during cone penetration test, out to a maximum of 60 m for harbour porpoise.
Table 13.21: Potential PTS Impact Ranges (m) for Annex II Marine Mammals During the Geotechnical Site Investigation Surveys
1 N/E = Threshold Not Exceeded
951 The number of Annex II marine mammals potentially affected within the modelled ranges for PTS were estimated using the most up to date species-specific density estimates ( Table 13.4 Open ▸ ). Where ranges for density estimates have been applied (harbour porpoise, bottlenose dolphin, grey seal and harbour seal), numbers of animals affected have been based on the maximum density value as a precautionary approach. It should be noted that since sonar-based systems have strong directivity, there is only potential for injury when marine mammal is directly underneath the sound source.
952 Due to low impact ranges, for all marine mammal species, there is the potential for less than one animal to experience PTS (and no animals where the threshold is not exceeded) as a result of geophysical and geotechnical site investigation surveys. The site-investigation surveys are considered to be short term as they will take place over up to a period of up to three months. Standard designed-in mitigation measures to reduce the risk of injury to marine mammals will be implemented for the geophysical surveys (JNCC, 2017). With such measures in place the risk during construction and decommissioning is deemed to be negligible.
953 Site investigation surveys will also involve the use of up to two geophysical/geotechnical survey vessels with up to 70 round trips. Noise impacts associated with vessel movements are identified in paragraph 988 et seq.
954 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. The assessment of the effect of injury from elevated underwater noise during site investigation surveys on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6.
Behavioural disturbance
955 The estimated maximum ranges for onset of disturbance are based on exceeding the 120 dB re 1 μPa (rms) threshold applicable for all marine mammals, noting that this threshold is for ‘mild disturbance’ and therefore is not likely to result in displacement of animals. Additionally, Russell et al. (2017) study reported the highest received level at which a response was detected by seals at 135 dB SELss with a zero probability of response measured at 130 dB SELss. The disturbance ranges as a result of geophysical and geotechnical site-investigation surveys ( Table 13.22 Open ▸ ) will be higher than those presented for PTS. Most of the predicted ranges are within 100s of metres, however, the largest distance over which the disturbance could occur is out to approximately 7.5 km during vibro-coring.
Table 13.22: Potential Disturbance Ranges for Marine Mammals During the Geophysical and Geotechnical Site Investigation Surveys
956 The number of marine mammals potentially affected within the modelled ranges for behavioural response (disturbance) are estimated using the most up to date species specific density estimates (see Table 13.4 Open ▸ ) and are presented in Table 13.23 Open ▸ . Where ranges for density estimates have been applied (harbour porpoise, grey seal and harbour seal), numbers of animals affected have been based on the maximum density value as a precautionary approach. Number of bottlenose dolphins potentially disturbed has been assessed based on the density for offshore populations.
Table 13.23: Number of Animals Potentially Disturbed Due to the Geophysical and Geotechnical Site Investigation Surveys
957 The data presented in Table 13.23 Open ▸ are considered to be conservative, especially for harbour porpoise as the number of animals likely to be disturbed is based on the peak seasonal density estimates from the Proposed Development aerial digital survey data during spring months. If these numbers were compared with estimates of the number of harbour porpoise potentially affected using the mean monthly density derived from the Proposed Development aerial digital survey data (0.299 animals per km2) or using the modelled density estimate for SCANS III for this area (0.599 animals per km2) these estimates would be shown to be highly precautionary. For example, based on the mean monthly density from aerial data or SCANS III data, the number of harbour porpoise affected by possible disturbance during vibro-core testing, would be 52 animals or 105 animals, respectively, compared to 144 animals estimated for peak seasonal density estimates. The same applies to the grey seal, where the numbers of potentially disturbed animals (based on Carter et al., 2020) were shown to be precautionary compared with estimates of the number of grey seal using the mean monthly or seasonal peak densities derived from the Proposed Development aerial digital survey data (0.276 animals per km2 and 0.321 animals per km2). For example, based on the mean monthly and seasonal peak density from aerial data, the number of grey seal affected by possible disturbance during vibro-core testing, would be 48 animals and 56 animals, respectively, compared to 210 animals estimated Carter et al. (2020) mean at sea usage.
958 Number of bottlenose dolphins that could be exposed to potential disturbance ( Table 13.23 Open ▸ ) relate to their offshore populations. Given that the vibro-core sampling locations are currently unknown and coastal distribution of bottlenose dolphin is spatially limited, any quantitative assessment of the disturbance to coastal populations would be an overestimation. All geotechnical and geophysical surveys will be very short duration (up to three months) and animals are expected to recover quickly after cessation of the survey activities. The magnitude of the impact could result in a negligible alteration to the distribution of marine mammals.
959 The impact of site investigation surveys leading to behavioural effects is predicted to be of local spatial extent, medium term duration, intermittent and the effect of behavioural disturbance is of high reversibility (animals returning to baseline levels soon after surveys have ceased). It is predicted that the impact will affect the receptor directly. The assessment of the effect of disturbance from elevated underwater noise during site investigation surveys on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6.
Secondary Mitigation and Residual Effect
960 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 13.35 Open ▸ ). Additionally, as a part of designed-in measures ( Table 13.11 Open ▸ ), Standard JNCC (2017) mitigation will be adhered to for the geophysical surveys which will involve the use of marine mammal observers/PAM monitoring of a standard 500 m mitigation zone for a period of no < 30 minutes prior to the start of surveys ( Table 13.11 Open ▸ ). No secondary marine mammal mitigation is considered necessary because there is no residual risk of injury in the absence of further mitigation (beyond the designed-in measures outlined above and in Table 13.11 Open ▸ ).
Injury and disturbance to marine mammals from elevated underwater noise during UXO clearance
Auditory injury
961 An explosive mass of 300 kg (maximum design scenario due to high order detonation; see Table 13.10 Open ▸ ) yielded the largest potential PTS ranges for all species, with the greatest effect ranges seen for harbour porpoise ( Table 13.24 Open ▸ ). As described in paragraph 873, 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 13.25 Open ▸ ), the assessment considers both high order and low order techniques for the purposes of 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 of the Offshore EIA Report). 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 13.24: Potential PTS Impact Ranges for Marine Mammals Due to UXO High Order Detonation
Table 13.25: Potential PTS Impact Ranges for Marine Mammals Due to Low Order Techniques
962 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 13.25 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 13.26 Open ▸ ).
963 The subsea noise assessment found that the maximum injury (PTS) range estimated for bottlenose 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 13.25 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 13.26 Open ▸ ).
964 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 13.25 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 13.26 Open ▸ ).
Table 13.26: Number of Animals with the Potential to Experience PTS due to Low Order Techniques
965 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 13.27).
966 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 13.24 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. 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. Less than one individual has the potential to be injured for all other species considered in the assessment (Table 13.27).
967 To reduce the potential of experiencing injury, designed-in measures will be adopted as part of a MMMP (see Table 13.11 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.
Table 13.2710: Number of Animals with the Potential to Experience PTS due to High Order Detonation
968 Due to the small numbers of marine mammals potentially injured from low order techniques ( Table 13.26 Open ▸ ) the magnitude of the impact could result in a negligible alteration to the distribution of marine mammals. 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.
969 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. 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.
970 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. Mitigation suggested in volume 2, chapter 10 of the Offshore EIA Report is considered to be sufficient to deter most animals ( Table 13.28 Open ▸ ), however there may be a residual effect for harbour porpoise for the 300 kg UXO size, as the maximum predicted PTS impact range for this species is larger than deterrence distance. It is expected that small, nominal number of animals could be exposed to PTS threshold. More information about secondary mitigation measures and residual effect is provided in paragraph 978 et seq. The assessment of the effect of PTS from elevated underwater noise during UXO clearance on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 7.6
Table 13.28: Recommended ADD Duration for High Order UXO Clearance and Sizes, and Associated Displacement Distance
TTS
971 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 961, 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). Due to the potential for a low order detonation technique to result in a high order detonation (as per paragraph 873) the assessment also considers high order detonation of 300 kg UXO munition size.
Table 13.29: Potential TTS Impact Ranges for Marine Mammals Due to Low Order Techniques
Table 13.30: Number of Animals with the Potential to Experience TTS Due to Low Order Techniques
972 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 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 have the potential to experience TTS from the detonation of 0.5 kg clearance shot ( Table 13.31 Open ▸ ).
973 The subsea noise assessment found that temporary hearing impairment and behavioural displacement from the area (TTS) may affect bottlenose 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 has the potential to be affected by TTS ( Table 13.31 Open ▸ ).
974 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 13.31 Open ▸ ).
Table 13.31: Number of Animals with the Potential to Experience TTS due to Low Order Techniques
975 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 Annex II marine mammals was for harbour porpoise with the maximum range of 19,590 m due to high order detonation of charge size of 300 kg ( Table 13.32 Open ▸ ). 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 13.32: Potential TTS Impact Ranges (m) for Marine Mammals Due to High Order Detonation
976 Due to relatively large ranges of potential impacts presented in, up to 995 harbour porpoises have the potential to be affected by TTS due to detonation of the 300 kg charge size ( Table 13.33 Open ▸ ). 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, 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.
Table 13.33: Number of Animals with the Potential to Experience TTS due to High Order Detonation
977 The impact of TTS for low order techniques is predicted to be of local spatial extent, very short term duration, intermittent and high reversibility. 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 assessment of the effect of TTS from elevated underwater noise during UXO clearance on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6
Secondary Mitigation and Residual Effect
978 Additional mitigation will be applied to reduce the potential for injury occurring during UXO clearance. As previously described in paragraph 873 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 873, approximately 10% of the total number of UXOs could result in high order detonation). The 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).
979 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.
980 The designed-in measures included as a part of the MMMP ( Table 13.11 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.
981 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 marine mammal observers 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.
982 Given that there is a potential to experience auditory injury by harbour porpoise at a greater range than can be mitigated by monitoring of the 1 km mitigation zone alone ( Table 13.24 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 961 et seq. determine the auditory injury range based on high order detonation of a 300 kg UXO. 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 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. 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 13.8 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 13.34 Open ▸ ).
983 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 only, however, deterrence distances are provided for all marine mammal IEFs in Table 13.34 Open ▸ . Based on the UXO clearance flow chart (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 to 1,980 m range ( Table 13.34 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 to 2,700 m. 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 to 3,600 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 to 5,400 m.
984 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 20 minutes ( Table 13.34 Open ▸ ). It is expected that 80 minutes of combined ADD/soft start procedure will displace harbour porpoise to range of 7,200 m. Whilst this 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 13.24 Open ▸ ).
Table 13.34: Recommended ADD Duration for High Order UXO Clearance and Sizes, and Associated Displacement Distance
985 The analysis presented in Table 13.34 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, harbour seal and grey seal to negligible. 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 13.4 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 of the Offshore EIA Report, 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).
986 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 13.34 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.
987 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 phase, 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 document. 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 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 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 risk of injury will be reduced to low.
Injury and disturbance to marine mammals from elevated underwater noise due to vessel use and other activities
Auditory injury
988 During the construction phase of the Proposed Development, the increased levels of vessel activity will contribute to the total underwater noise levels. The maximum design scenario for construction activities associated with site preparation and inter-array and offshore export cables is up to 316 return trips of up to nine boulder clearance vessels and 104 return trips of up to three sand wave clearance vessels, throughout the construction phase. Additionally, vessel movements associated with other activities such as foundation and OSP/Offshore convertor station platform installation, will contribute to a maximum scenario of up to 11,484 vessel round trips over the construction phase. Vessel types will include main installation vessels, cargo barges, support vessels, tug/anchor handlers, guard vessels and others (see Table 13.10 Open ▸ for full list of construction vessels and volume 3, appendix 10.1 of the Offshore EIA Report for SPLs associated with each vessel type). Whilst this will lead to an uplift in vessel activity, the movements will be limited to within the Proposed Development array area and Proposed Development export cable corridor and will follow existing shipping routes to/from the ports.
989 The main drivers influencing the magnitude of the impact are vessel type, speed and ambient noise levels (Wilson et al., 2007). Based on information presented in volume 2, chapter 13 of the Offshore EIA Report, baseline levels of vessel traffic in the Proposed Development marine mammal study area are relatively high. An average of 14 vessels per day were recorded within a 10 nm buffer around the Proposed Development array area (hereinafter Proposed Development shipping and navigation study area) over a 14-day survey period in August 2022. The vessel traffic surveys also showed an average of three to four vessels intersecting the Proposed Development array area per day, over summer. Throughout the season, a maximum of 25 vessels were recorded within the Proposed Development array shipping and navigation study area over one day. For the winter survey period (January 2021), there was an average of 16 unique vessels per day recorded within the Proposed Development array area shipping and navigation study area. As described in the Navigational Risk Assessment (NRA) (volume 3, appendix 13.1 of the Offshore EIA Report), occasional vessel traffic movements associated with jack-ups, semi-submersibles and other platforms also occur in the region.
990 Other noise generating activities for the Proposed Development will include drilled piling, with a maximum of 176 piles over the period of 135 days ( Table 13.10 Open ▸ ). Rotatory drilling is non-impulsive in character and the source sound levels associated with this activity have been based on pile drilling for the Oyster 800 project. The other noise sources potentially active during the construction phase are related to cable installation (i.e., trenching and cable laying activities), and their related operations such as the jack-up rigs. See volume 3, appendix 10.1 of the Offshore EIA Report for more information about SELs associated with above construction activities.
991 The noise modelling results indicate that ranges (within which there is a risk of PTS occurring to marine mammals as a result of elevated underwater noise due to vessel use) are either not exceeded or relatively low ( Table 13.35 Open ▸ ). The maximum range within which the PTS could occur across all species has been estimated for harbour porpoise at 525 m for a rock placement vessel ( Table 13.35 Open ▸ ).
Table 13.35: Vessels Involved in the Construction of the Proposed Development and Estimated Potential PTS Ranges for Marine Mammals
N/E = Not Exceeded
992 Of the other noise-producing activities cable laying is most likely to result in PTS compared to drilling, trenching and jack-up rigging ( Table 13.36 Open ▸ ). As before, the modelled effect ranges for cable laying suggest that harbour porpoise is the most sensitive species with PTS predicted up to 525 m of the source ( Table 13.36 Open ▸ ). The same activity is likely to result in a PTS to bottlenose dolphin within 15 m from the source and to seal species within only 5 m from the source. The jack-up rig has the potential to result in PTS to harbour porpoise within 5 m from the source. For all other activities and for all other species, the thresholds for PTS will not be exceeded as a result of underwater noise during construction activities.
Table 13.36: Estimated PTS Ranges for Marine Mammals During Other Activities
993 The number of marine mammals potentially affected within the modelled ranges for PTS from vessels ( Table 13.35 Open ▸ ) and other activities ( Table 13.36 Open ▸ ), were calculated and found to be less than one individual for all species. Whist the numbers of animals likely to be affected at any one time are extremely low, the maximum duration of the offshore construction phase is up to six years.
994 The impact is predicted to be of local spatial extent, medium term duration, intermittent and the effect of PTS on sensitive receptors is of low reversibility. It is predicted that the impact will affect the receptor directly. The assessment of the effects of injury from elevated underwater noise due to vessel use and other activities on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6.
Behavioural disturbance
995 Disturbance from vessel noise is likely to occur only where vessel noise associated with the construction of the Proposed Development exceeds the background ambient noise level. As discussed in paragraph 988 et seq., the Proposed Development is located in a relatively busy shipping area and therefore background noise levels are likely to be relatively high.
996 A detailed underwater noise modelling assessment has been carried out to investigate the potential for behavioural effects on marine mammals resulting from increased vessel noise and other activities. The estimated ranges within which there is a potential for disturbance to marine mammals along with number of animals with the potential to be disturbed are presented in Table 13.37 Open ▸ based on the most precautionary species-specific density estimates ( Table 13.3 Open ▸ ) with offshore density estimates applied for bottlenose dolphin. Estimated impact ranges and numbers of animals potentially are presented for different vessel types in isolation. It is likely that during construction, operation and maintenance and decommissioning phases, there will be a number of different types of vessels present within the Proposed Development marine mammal study area at the same time. However, given that the exact type, numbers and distances between vessels are unknown at this stage, the cumulative ranges were not quantified. Therefore the discussion presented in this section is based on the maximum scenario for a single vessel at any given time. Installation and construction vessels as well as rock placement vessels result in the greatest modelled disturbance out to 4,320 m for all marine mammal species. Similar ranges for behavioural effects are predicted to occur due to underwater noise from cable laying activities with disturbance ranges of 4,389 m. In comparison, vessels such as excavator, backhoe dredger, pipe laying, geophysical survey vessel and jack up vessel as well as jack-up rig were predicted to result in disturbance ranges out to 300 m.
Table 13.37: Estimated Disturbance Ranges for Marine Mammals and Number of Animals Potentially Disturbed as a Result of Vessels and Other Activities
997 As discussed previously in paragraph 874, there is likely to be a proportionate response of animals within the modelled contours (i.e. not all animals will be disturbed to the same extent). The life history of an individual and the context will also influence the likelihood of an individual to exhibit an aversive response to noise.
998 Grey seal is likely to be the most sensitive species to disturbance from vessel traffic. The second most sensitive marine mammal is harbour porpoise. The numbers of animals with the potential to be disturbed (as presented in Table 13.37 Open ▸ ) are considered to be highly conservative, especially for harbour porpoise and grey seal, as these estimates were based on the peak seasonal densities from the Proposed Development aerial digital survey data during spring months and maximum density based on at-sea mean usage maps (Carter et al., 2020), respectively.
999 Activities with the largest disturbance ranges, including installation, construction, rock placement and cable laying vessels, will be operating at distances from the outer Firth of Tay (the highest bottlenose dolphin densities) and are unlikely to affect coastal bottlenose dolphin populations. Therefore, bottlenose dolphins that could be exposed to potential disturbance ( Table 13.37 Open ▸ ) would belong to offshore population.
1000 The impact is predicted to be of local spatial extent, medium term duration, intermittent and the effect of behavioural disturbance is of high reversibility. It is predicted that the impact will affect the receptor directly. The assessment of the effect of behavioural disturbance from elevated underwater noise due to vessel use and other activities on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6.