8.10. Measures Adopted as Part of the Array

  1. As part of the Array design process, a number of designed in measures have been proposed to reduce the potential for impacts on benthic subtidal ecology (see Table 8.17   Open ▸ ). They are considered inherently part of the design of the Array and, as there is a commitment to implementing these measures, these have been considered in the assessment presented in section 8.11 (i.e. the determination of magnitude and therefore significance assumes implementation of these measures). These designed in measures are considered standard industry practice for this type of development.

 

Table 8.17:
Designed In Measures Adopted as Part of the Array

Table 8.17: Designed In Measures Adopted as Part of the Array

 

8.11. Assessment of Significance

  1. Table 8.12   Open ▸ summarises the potential impacts arising from the construction, operation and maintenance and decommissioning phases of the Array, as well as the MDS against which each impact has been assessed. An assessment of the likely significance of the effects of the Array on the benthic subtidal ecology receptors caused by each identified impact is given below.

Temporary habitat loss and disturbance

  1. Temporary habitat loss and disturbance will occur during the construction, operation and maintenance, and decommissioning phases of the Array. The MDS for this impact is summarised in Table 8.12   Open ▸ . The relevant MarESA pressures and their benchmarks which have used to inform this impact assessment are:
  • Habitat structure changes – removal of substratum (extraction): the benchmark for which is the extraction of substratum to 30 cm. This pressure is considered to be analogous to the impacts associated with sand wave and boulder clearance/relocation and UXO clearance.
  • Abrasion/disturbance at the surface of the substratum or seabed: the benchmark for which is damage to surface features (e.g. species and physical structures within the habitat). This pressure corresponds to the impacts associated with jack-up vessel operations, cable installation, and any infrastructure temporarily placed on the seabed.
  • Penetration and/or disturbance of the substratum subsurface: the benchmark for which is damage to sub-surface features (e.g. species and physical structures within the habitat). This pressure corresponds to the impacts associated with cable and DEA installation, sand wave clearance, UXO clearance, and jack-up vessel operations.
  • Smothering and siltation rate changes (heavy): the benchmark for which is heavy deposition of up to 30 cm of fine material added to the habitat in a single discrete event. This pressure corresponds to impacts associated with sand wave clearance and cable installation.

                        Construction phase

                        Magnitude of impact
  1. The MDS accounts for up to a total of 49.95 km2 of temporary habitat loss and disturbance during the construction phase ( Table 8.12   Open ▸ ). The represents 5.82 % of the total Array benthic subtidal ecology study area. The MDS has been based on the total temporary habitat loss and disturbance as a result of the following activities in the site preparation and construction phases:
  • sand wave and boulder clearance/relocation and UXO clearance;
  • installation of inter-array and interconnector cables;
  • footprint of temporary offshore wet storage;
  • footprint of jack up vessels used for OSP installation; and
  • installation of DEAs.
  1. Seabed preparation activities (sand wave and boulder clearance and relocation) will account for up to 14.72 km2 of temporary habitat loss and disturbance ( Table 8.12   Open ▸ ). Any mounds of cleared material will erode over time and displaced material will re-join the natural sedimentary environment, gradually reducing the size of the mounds. As the sediment type deposited on the seabed will be similar to that of the surrounding areas (and largely sandy, see section 8.7.1), displaced benthic communities would be expected to recolonise these areas (see ‘Sensitivity of the receptors’ below). Further, based on the site-specific bathymetry data, broadscale sand wave clearance is unlikely to be required. Bedforms recorded in the site-specific surveys were relatively low (volume 2, chapter 7). In addition, up to 15 UXOs may require clearance during site preparation activities, which could result in the formation of craters ( Table 8.12   Open ▸ ). Information on potential crater dimensions is challenging to predict for the Array at this stage, and there is limited information on this impact available in the literature. However, two recent studies assessed seabed disturbance from UXO clearance at southern North Sea wind farms (Ordtek, 2018, Royal HaskoningDHV, 2022). Modelling suggested that craters of up to 21 m in diameter could be created from UXO detonation, giving an area of approximately 346 m2 per crater (Ordtek, 2018, Royal HaskoningDHV, 2022). Using these calculations, if 15 UXOs require clearance during site preparation for the Array, a total of up to 5,190 m2 of temporary habitat loss could occur due to crater formation (i.e. 346 m2 x 15 craters). However, it should be noted that this is only a precautionary estimation, and these parameters and calculations will be refined at a later stage. As previously mentioned for sand wave and boulder clearance and relocation, it is expected that the craters will erode and infill overtime, and displaced material will re-join the natural sedimentary environment.
  2. Inter-array and interconnector cable installation will result in 25.39 km2 of temporary habitat loss and disturbance within the construction phase ( Table 8.12   Open ▸ ). This will include the installation of 1,261 km of inter-array cables (of which 116 km will be dynamic) and 236 km of interconnector cables on the seabed, with a 20 m width of disturbance from the installation tool. For the purposes the MDS, the total footprint of affected seabed has been calculated, assuming a mound of uniform thickness of 0.5 m height. However, it should be noted that, mounds may be taller and more unevenly distributed. Any mounds of cleared material will, however, erode over time and displaced material will re-join the natural sedimentary environment, gradually reducing the size of the mounds and so this estimate can be considered suitably precautionary.
  3. A recent study by RPS (2019) reviewed the effects of cable installation on subtidal sediments and habitats, drawing on monitoring reports from over 20 UK offshore wind farms. Following cable installation, sandy sediments were shown to recover quickly, with little to no evidence of disturbance in the years following cable installation (RPS, 2019). Although there was some evidence that remnant cable trenches in coarse and mixed sediments were conspicuous for several years after installation, these shallow depressions were of limited depth (i.e. tens of centimetres) relative to the surrounding seabed, and spread over a horizontal distance of several metres and therefore did not represent a large shift from the baseline environment (RPS, 2019). In muddy and muddy sand seabed habitats, remnant trenches were observed years following cable installation, although these were relatively shallow (i.e. a few tens of centimetres) (RPS, 2019). Given that the seabed sediments within the Array benthic subtidal ecology study area are dominated by sands and sandy gravels (see section 8.7.1), the results of the RPS (2019) study suggest that disturbance to these sediments is likely to be reversible. In addition, post-construction monitoring of the Block Island Offshore Wind Farm (off the coast of Rhode Island, United States of America (USA)) demonstrated that 62% of the trench formed during export cable installation had recovered within four months, and the remainder was partially recovered (Bureau of Ocean Energy Management (BOEM), 2020), further highlighting the reversibility of this impact.
  4. Temporary habitat loss and disturbance will occur as a result of depressions formed by jack up vessels used to install OSPs. The MDS accounts for a total area of up to 43,200 m2 ( Table 8.12   Open ▸ ) across the Array benthic subtidal ecology study area due to jack up vessel footprints. The MDS is derived from up to three large OSPs and 12 small OSPs requiring installation using jack up vessels, with up to two jack up events per OSP, and up to 1,440 m2 of disturbance per jack up usage. Depressions in the seabed caused by jack up vessel usage could last for up to a year or more. For example, monitoring studies at Barrow Offshore Wind Farm (Irish Sea) demonstrated that depressions were almost entirely infilled 12 months post construction (Barrow Offshore Windfarm Ltd, 2008). Similarly, post-construction seafloor disturbance monitoring at the Block Island Wind Farm suggested that depressions from the spud cans of jack up vessels were expected to fully recover (BOEM, 2020).
  5. The maximum footprint of temporary offshore wet storage is up to 250,000 m2 ( Table 8.12   Open ▸ ). At this stage of the Application, the wet storage requirements of the Array are uncertain, however they are temporary in nature, and benthic habitats are expected to recover in the same manner as described in paragraphs 58 to 61. Wet storage may be used to optimise delivery schedules during installation of mooring and anchors. Anchors or mooring components may be offloaded from the delivery vessel at or close to the final installation location to allow the delivery vessel to leave site. The installation vessels will then complete the final installation of the anchors and mooring lines at the turbine location. It is not anticipated that wet storage will be required for prolonged periods of time. Mooring lines and dynamic cables following installation may also be left on the seabed at the final locations, whilst awaiting hook up of the floating turbine.
  6. Finally, if DEAs are selected as an anchoring method for floating foundations (see Anchoring Option 2 and 3 in the Project Description, volume 1, chapter 3), these will be lifted from the installation vessel using a crane and positioned on the seabed. The DEAs will then be pulled using an anchor handling tug, or similar, in order to embed the anchor in the seabed. It is anticipated that based on the ground conditions at site, that the anchor will be pulled between 30 to 60 m during the installation process, subject to further ground investigations and anchor design. This process will be undertaken in a controlled manner to ensure that DEAs are installed at the correct position and to appropriate depth. There will be up to 1,590 DEAs installed in this manner in total, resulting in a maximum footprint of up to 9,540,000 m2 ( Table 8.12   Open ▸ ). 
  7. The maximum duration of the offshore construction phase for the Array is up to eight years (2031 to 2038 inclusive). Within this maximum construction phase, construction activities are anticipated to occur intermittently. They will be spread out across the full allotted timeframe with only a small proportion of the MDS footprint for this impact being affected at any one time.
  8. The impact is predicted to be of local spatial extent (5.82% of the Array benthic subtidal ecology study area), medium term duration (up to eight years), intermittent, and of high reversibility. It is predicted that the impact will affect the receptors directly. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. The sensitivity of the IEFs to temporary habitat loss and disturbance are presented in Table 8.18   Open ▸ . These sensitivities are based on the MarESA (where available).
  2. The two representative biotopes for the Offshore subtidal sands and gravels and Subtidal sands and gravels IEFs were both concluded, overall, to be of medium sensitivity to this impact based on the four MarESA pressures ( Table 8.18   Open ▸ ) (Tillin, 2016a; Tillin, 2016b). They have medium to high vulnerability to the pressure ‘Habitat structure changes – removal of substratum (extraction)’ as the benthic species associated with these biotopes are shallowly buried and extraction will remove them. However, these biotopes have medium resilience as they are characterised by opportunistic species that can rapidly colonise disturbed habitats or species that are larger and longer living and may present in established and mature assemblages ( Table 8.18   Open ▸ ) (Tillin, 2016a; Tillin, 2016b). These two biotopes also have medium sensitivity to the pressure ‘Smothering and siltation rate changes (heavy)’ as the characteristic species may not be able to migrate through heavy smothering (up to 30 cm of sediment in a single event) (Tillin, 2016a, Tillin, 2016b). The biotopes have low sensitivity to the pressures ‘Abrasion/disturbance of the surface of the substratum or seabed’ and ‘Penetration or disturbance of the substratum subsurface’ ( Table 8.18   Open ▸ ). While abrasion and penetration of the subsurface are likely to damage a proportion of the characteristic species at the surface of the substratum, resilience and recovery is high due to their opportunistic nature, resulting in a low sensitivity (Tillin, 2016a; Tillin, 2016b). Overall, the Offshore subtidal sands and gravels and Subtidal sands and gravels IEFs are deemed to be of medium vulnerability, medium to high recoverability and regional value. The sensitivities of the receptors are, therefore, considered to be medium.
  3. The ocean quahog IEF was concluded to be of high sensitivity to this impact based on the MarESA pressures ( Table 8.18   Open ▸ ) (Tyler-Walters and Sabatini, 2017). They have low to no resistance to the removal of substratum or abrasion and penetration of the substratum surface ( Table 8.18   Open ▸ ) (Tyler-Walters and Sabatini, 2017). This is because the ocean quahog feeds at the surface of the substratum, and burrows to several centimetres and down to depths of 14 cm periodically (Morton, 2011, Strahl et al., 2011). Therefore, these pressures would result in removal and/or damage to the substratum that individuals occupy alongside removal and damage to any individuals present (Tyler-Walters and Sabatini, 2017). Furthermore, ocean quahog are vulnerable to disturbance given their long lifespan (hundreds of years), slow growth rate, and high age of sexual maturity (from ten years old) (Thorarinsdóttir et al., 2010; Thorarinsdóttir and Jacobson, 2005). For example, the effects of hydraulic dredging on the benthic community in a bay in Iceland reported a 93% decreased in ocean quahog abundance, with recovery in only 7% to 26% of dredge channels within five years (Ragnarsson et al., 2015). Although ocean quahog have low vulnerability and high resistance to smothering (therefore a low sensitivity) ( Table 8.18   Open ▸ ), this species is still assessed as high sensitivity to the impact of temporary habitat loss and disturbance, due to its high sensitivity to the other three MarESA pressures. Overall, the ocean quahog IEF is deemed to be of high vulnerability, low recoverability and national value. The sensitivities of the receptors are, therefore, considered to be high.
  4. The dead man’s fingers IEFs was concluded to be of medium sensitivity to this impact based on the MarLIN assessment ( Table 8.18   Open ▸ ) (Budd, 2008). Sensitivity was assessed as medium to the pressure of ‘substratum loss’ (named differently in the MarLIN than the MarESA pressures for the other IEFs) ( Table 8.18   Open ▸ ) (Budd, 2008). This is because the species is permanently attached to the substratum, and therefore has a high vulnerability to removal. However, given suitable substrate remains or is deposited (which would be the case for the construction activities associated with this impact), potential for recovery is high (Budd, 2008). Recoverability for dead man’s fingers is high in general, given its broadcast reproduction strategy of long-lived larvae, which increases population resilience (Budd, 2008). Dead man’s fingers has a low overall sensitivity to the pressures of ‘Abrasion and physical disturbance’ and ‘Smothering’ associated with temporary habitat loss and disturbance ( Table 8.18   Open ▸ ) (Budd, 2008). The species has a medium vulnerability to abrasion and physical disturbance, which has been inferred from consistent reports of damage caused by abrasive fishing gear (Hartnoll, 1999, Hinz et al., 2011), and medium to high recoverability (Budd, 2008). The species has a medium vulnerability to smothering as it is permanently attached to the substratum, however, colonies can be large (up to 20 cm in height), and thus recovery is likely (Budd, 2008). Overall, the dead man’s fingers IEF is deemed to be of medium to high vulnerability, high recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be medium.
  5. As stated in paragraph 48, no MarESA was available for phosphorescent sea pen or sea tamarisk IEFs, so they have been assessed using the best available literature. Both of these IEFs are colonial species, which live attached to their substrate. The phosphorescent sea pen is embedded into the seabed by its peduncle (i.e. stalk, or anchoring structure), while the sea tamarisk is attached to bedrock, stable rocks, or shells (Jones, 2008, Wilson, 2002). Therefore, they are likely to be highly vulnerable to removal of substratum, and abrasion, penetration, and disturbance of the substratum subsurface. For phosphorescent sea pen, this has been inferred from observations of fishing gear and dredging removing entire sea pens from the substratum (Kenchington et al., 2011; Tuck et al., 1998). However, the phosphorescent sea pen can retreat into its burrow, which can be up to 25 cm deep (Greathead et al., 2007; Jones, 2008), and may be able to avoid removal, abrasion, disturbance or penetration of the first few centimetres of substratum as a result (Hill and Tyler-Walters, 2018). Furthermore, sea pens have been observed to be able to reinsert themselves into the sediment if the peduncle is in contact with the seabed (Eno et al., 2001). The phosphorescent sea pen is not likely to be vulnerable to heavy smothering and siltation rate changes given that it can readily retreat deep into its burrow until the sediments have been redispersed (Hill and Tyler-Walters, 2018). Recoverability is therefore considered to be high. Although the sea tamarisk cannot retreat into the sediment, it forms large colonies (over 15 cm) and its stem is supported by a non-living, protein-chitinous structure (Wilson, 2002). As this species naturally lives in turbid environments with moderate to strong tidal streams, it is likely to be adapted to smothering and surface disturbance. Further, colonial hydroids in general are known to be resilient taxa, given their modular body organisation and high environmental plasticity (Di Camillo et al., 2017; Gili and Hughes, 1995). Overall, the phosphorescent sea pen and sea tamarisk IEFs are deemed to be of medium to high vulnerability, high recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be medium.
Table 8.18:
Sensitivity of the IEFs to Temporary Habitat Loss and Disturbance

Table 8.18: Sensitivity of the IEFs to Temporary Habitat Loss and Disturbance

                        Significance of the effect
  1. Overall, for the ocean quahog IEF, the magnitude of the impact is deemed to be low, and the sensitivity of the receptor is considered to be high. As per Table 8.16   Open ▸ , the effect will, therefore, be of minor to moderate adverse significance. Given the low footprint of temporary habitat loss and disturbance with respect to both the Array benthic subtidal ecology study area and the North Sea as a whole, and the widespread availability of alternative suitable habitat, the effect is concluded to be of minor adverse significance, which is not significant in EIA terms.
  2. For all other IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

                        Operation and maintenance phase

                        Magnitude of impact
  1. The MDS accounts for up to a total of 51.41 km2 of temporary habitat loss and disturbance during the 35 year operation and maintenance phase ( Table 8.12   Open ▸ ). This represents 5.99% of the total Array benthic subtidal ecology study area. However, it should be noted that only a small proportion of the total temporary habitat loss and disturbance is likely to occur at any one time, with the MDS for this impact calculated over the 35 year lifecycle of the Array. There may be up to 1.47 km2 of temporary habitat loss and disturbance per year. Therefore, individual maintenance activities will be small scale and intermittent events. The MDS has been based on the total temporary habitat loss and disturbance as a result of the following activities in the operation and maintenance phase:
  • footprint of jack up vessels used for operation and maintenance activities; and
  • disturbance caused by reburial of inter-array and interconnector cables.
  1. The impacts of jack up vessel activities will be similar to those identified for the construction phase above and will be restricted to the immediate area where the spud cans are placed on the seabed, with recovery occurring following removal of spud cans. The footprint of temporary habitat loss and disturbance due to jack up vessel use has been calculated as up to 10,500 m2 per year. The impacts of inter-array and interconnector cable reburial will be similar to those identified for cable installation in the construction phase above but will only impact up to 1.22 km2 and 0.23 km2, respectively, per year.
  2. The spatial extent of this impact in the operation and maintenance phase is small in relation to the whole Array benthic subtidal ecology study area, although there is the potential for repeated disturbance to the habitats in the immediate vicinity of the infrastructure because of these activities. However, these effects are expected to be similar to the construction phase, but of a much lower magnitude.
  3. The impact is predicted to be of local spatial extent (5.99% of the Array benthic subtidal ecology study area), long term duration, intermittent, and of high reversibility. It is predicted that the impact will affect the receptors directly. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. The sensitivities of all IEFs are considered to be as previously described for the site preparation and construction phase (see Table 8.18   Open ▸ and paragraphs 66 to 70) and have not been repeated here.
                        Significance of the effect
  1. Overall, for the ocean quahog IEF, the magnitude of the impact is deemed to be low, and the sensitivity of the receptor is considered to be high. As per Table 8.16   Open ▸ , the effect will, therefore, be of minor to moderate adverse significance. Given the low footprint of temporary habitat loss and disturbance with respect to both the Array benthic subtidal ecology study area and the North Sea as a whole, and the widespread availability of alternative suitable habitat, the effect is concluded to be of minor adverse significance, which is not significant in EIA terms.
  2. For all other IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

                        Decommissioning phase

                        Magnitude of impact
  1. The MDS accounts for up to a total of 43,200 m2 of temporary habitat loss and disturbance during the decommissioning phase ( Table 8.12   Open ▸ ). The represents 0.01% of the total Array benthic subtidal ecology study area. The MDS has been based on the total temporary habitat loss and disturbance as a result of the footprint of jack up vessels used for decommissioning activities. The impacts of these jack up vessel activities will be similar to those identified for the construction phase above and will be restricted to the immediate area where the spud cans are placed on the seabed, with recovery occurring following removal of spud cans.
  2. The impact is predicted to be of local spatial extent (0.01% of the Array benthic subtidal ecology study area), short term duration, intermittent, and of high reversibility. It is predicted that the impact will affect the receptors directly. The magnitude is therefore considered to be negligible.
                        Sensitivity of the receptor
  1. The sensitivities of all IEFs are considered to be as previously described for the site preparation and construction phase (see Table 8.18   Open ▸ and paragraphs 66 to 70) and have not been repeated here.
                        Significance of the effect
  1. Overall, for the ocean quahog IEF, the magnitude of the impact is deemed to be negligible, and the sensitivity of the receptor is considered to be high. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
  2. For all other IEFs, the magnitude of the impact is deemed to be negligible, and the sensitivities of the receptors are considered to be medium. As per Table 8.16   Open ▸ , the effect will, therefore, be of negligible to minor adverse significance. Based on expert judgement and adopting a precautionary approach, the effect has been concluded to be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Long Term habitat loss and disturbance

  1. Long term habitat loss and disturbance will occur during the construction, operation and maintenance, and decommissioning phases of the Array. The MDS for this impact is summarised in Table 8.12   Open ▸ . The impact of long term habitat loss does not represent a complete removal of habitat, but rather a physical change from a predominantly sandy sedimentary habitat to a hard, artificial substratum. The relevant MarESA pressure and its benchmark which has used to inform this impact assessment is:
  • Physical change (to another seabed type): the benchmark for which is change in sediment type from sedimentary or soft rock substrata to hard rock or artificial substrate or vice-versa.
  1. The effects of long term habitat loss and disturbance are assessed here, however the potential for colonisation of the hard structures installed has been assessed below in ‘Colonisation of hard structures’. Further, while the long term habitat loss and disturbance will occur in the decommissioning phase through infrastructure that is left in situ, the potential effects to benthic subtidal ecology due to the removal of other infrastructure in the decommissioning phase has been assessed separately in ‘Effects to benthic subtidal ecology due to removal of hard substrates’ below.

                        Construction and operation and maintenance phases

                        Magnitude of impact
  1. The MDS accounts for up to a total of 19.27 km2 of long term habitat loss and disturbance as infrastructure is installed during the construction phase, which will persist into the operation and maintenance phase ( Table 8.12   Open ▸ ). The represents 2.25% of the total Array benthic subtidal ecology study area. The MDS has been based on the total long term habitat loss and disturbance as a result of the installation of the following infrastructure:
  • mooring lines and anchors on the seabed;
  • OSP foundations;
  • inter-array and interconnector cable protection and cable crossing protection;
  • subsea junction boxes; and
  • scour protection for mooring lines, anchors, OSP foundations, and subsea junction boxes.
  1. In addition, the MDS accounts for up to 778,464 m2 of long term seabed disturbance due to the persistent frequent and intermittent movement of dynamic cabling and mooring lines on the seabed ( Table 8.12   Open ▸ ). The represents 0.09% of the total Array benthic subtidal ecology study area over the 35 year lifecycle of the Array. Finally, the MDS includes drilling at up to 10% of piles, with up to 636 m3 of drill arisings associated with each.
  2. The installation of mooring lines and anchors on the seabed will result in up to 12.41 km2 and 25,288 m2 of long term habitat loss, respectively, with their associated scour protection accounting for up to 632,196 m2 ( Table 8.12   Open ▸ ). The installation of up to three large OSPs and 12 small OSPs will result in a maximum footprint of 2,163 m2, with up to 94,814 m2 of associated scour protection ( Table 8.12   Open ▸ ). Up to 4.89 km2 and 0.94 km2 of cable protection will be installed on the seabed for the inter-array and interconnector cables, respectively, with up to 24,000 m2 of cable protection in addition ( Table 8.12   Open ▸ ). Finally, the installation of up to 228 junction boxes and their scour protection accounts for 41,040 m2 and 201,552 m2 of long term habitat loss, respectfully ( Table 8.12   Open ▸ ).
  3. The impact is predicted to be of local spatial extent (2.25% of the Array benthic subtidal ecology study area), long term duration, continuous, and of low reversibility during the construction and operation and maintenance phases. It is predicted that the impact will affect the receptors directly. This impact presents some measurable, but minor long term loss of and alteration to areas of seabed within the Array benthic subtidal ecology study area, but not in the regional benthic subtidal ecology study area. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. The sensitivity of the IEFs to long term habitat loss and disturbance are presented in Table 8.19   Open ▸ . These sensitivities are based on the MarESA (where available).
  2. The two representative biotopes of the Offshore subtidal sands and gravels IEF and the Subtidal sands and gravels IEF are both characterised by their sedimentary habitats (circalittoral fine sand) (Tillin, 2016a, Tillin, 2016b). Therefore, a change to an artificial or rock substratum would alter the characteristics of the biotopes, and result in a loss of the characteristic species (such as E. pusillus, O. borealis, A. prismatica, B. elegans, and other polychaetes and bivalves) that live buried within sandy sediments (Tillin, 2016a, Tillin, 2016b). Overall, the Offshore subtidal sands and gravels and Subtidal sands and gravels IEFs are deemed to be of high vulnerability, low recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be high ( Table 8.19   Open ▸ ).
  3. Similarly, the ocean quahog IEF and phosphorescent sea pen IEF also require a soft sedimentary habitat, and physical change to hard artificial or rock substratum would represent habitat loss for individuals affected. These species are therefore highly vulnerable to this impact (Hill and Tyler-Walters, 2018, Tyler-Walters and Sabatini, 2017). Overall, the ocean quahog and phosphorescent sea pen IEFs are deemed to be of high vulnerability, low recoverability, and national and regional value, respectively. The sensitivities of the receptors are, therefore, considered to be high ( Table 8.19   Open ▸ ).
  4. In contrast however, dead man’s fingers and sea tamarisk naturally live on hard substrates, including bedrock, rocks, boulders, shells, and man-made artificial hard structures (Budd, 2008, Wilson, 2002). Therefore, this impact does not represent a change from a preferred habitat to an unsuitable one for these IEFs, in comparison to the others. In addition, hydroids (such as sea tamarisk) are typically one of the first taxa to colonise new substrates (Boero, 1984). Therefore, the dead man’s fingers IEF and the sea tamarisk IEF are deemed to be of low vulnerability, high recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be low ( Table 8.19   Open ▸ ).

Table 8.19:
Sensitivity of the IEFs to Long Term Habitat Loss and Disturbance

Table 8.19: Sensitivity of the IEFs to Long Term Habitat Loss and Disturbance

 

                        Significance of the effect
  1. Overall, for the dead man’s fingers IEF and sea tamarisk IEF, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be low. Based on Table 8.16   Open ▸ , the effect will, therefore, be of negligible to minor adverse significance. Based on expert judgement and adopting a precautionary approach, the effect has been concluded to be of minor adverse significance, which is not significant in EIA terms.
  2. For all other IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be high. As per Table 8.16   Open ▸ , the effect will, therefore, be of minor to moderate adverse significance. Given the low footprint of long term habitat loss and disturbance with respect to both the Array benthic subtidal ecology study area and the North Sea as a whole, and the widespread availability of alternative suitable habitat, the effect is concluded to be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

                        Decommissioning phase

                        Magnitude of impact
  1. The MDS accounts for up to a total of 6.79 km2 of long term habitat loss and disturbance in the decommissioning phase ( Table 8.12   Open ▸ ). The represents 0.79% of the total Array benthic subtidal ecology study area. The MDS has been based on the total long term habitat loss and disturbance as a result of the following infrastructure remaining in situ during the decommissioning of the Array:
  • anchors where they are embedded deep in the seabed;
  • inter-array and interconnector cable protection and cable crossing protection; and
  • scour protection for moorings, anchors, subsea junction boxes and OSP foundations.
  1. All other infrastructure on the seabed mentioned in paragraph 90, will be removed, with only cable protection (5.83 km2), cable crossing protection (24,000 m2), and scour protection remaining (928,562 m2) ( Table 8.12   Open ▸ ). Anchors will be removed where they can be easily recovered without major excavation of the seabed and where it is safe to do so. For piles that are embedded deep in the seabed, e.g. piles, or DAEs that are installed at depth within the seabed these will be cut at or below the seabed and left in-situ.
  2. The impact is predicted to be of local spatial extent (0.79% of the Array benthic subtidal ecology study area), long term duration, continuous, and of low reversibility during the decommissioning phase. It is predicted that the impact will affect the receptors directly. This impact presents some measurable and minor long term loss of and alteration to the affected areas of seabed within the entire Array benthic subtidal ecology study area but less so within the regional benthic subtidal ecology study area. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. The sensitivities of all IEFs are considered to be as previously described for the construction and operation and maintenance phases (see Table 8.19   Open ▸ and paragraphs 94 to 97) and have not been repeated here.
                        Significance of the effect
  1. Overall, for the dead man’s fingers IEF and the sea tamarisk IEF, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be low. Based on Table 8.16   Open ▸ , the effect will, therefore, be of negligible to minor adverse significance. Based on expert judgement and adopting a precautionary approach, the effect has been concluded to be of minor adverse significance, which is not significant in EIA terms.
  2. For all other IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be high. As per Table 8.16   Open ▸ , the effect will, therefore, be of minor to moderate adverse significance. Given the low footprint of long term habitat loss and disturbance with respect to both the Array benthic subtidal ecology study area and the North Sea as a whole, and the widespread availability of alternative suitable habitat, the effect is concluded to be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Increased SSCs and assocIated deposition

  1. Increased SSCs and associated deposition may arise in all phases of the Array. Indirect impacts of this on benthic subtidal ecology include increased turbidity and smothering. In the operation and maintenance phase, this impact has been informed by the qualitative assessments undertaken for physical processes (see volume 3, appendix 7.1, and volume 2, chapter 7). In the construction and decommissioning phases, this impact has been assessed qualitatively, however not informed by the physical processes assessment, as it was not scoped in for these phases for physical processes. The benchmarks for the relevant MarESA pressures which have been used to inform this impact assessment are:
  • Changes in suspended solids (water clarity): the benchmark for which is a change in one rank on the Water Framework Directive (WFD) scale (e.g. from clear to intermediate for one year, caused by activities disturbing sediment or organic particulate material and mobilising it into the water column).
  • Smothering and siltation rate changes (light): the benchmark for light sediment deposition is up to 5 cm of fine material added to the habitat in a single discrete event.

                        Site preparation and construction phase

                        Magnitude of impact
  1. The site preparation activities and installation of infrastructure associated with the Array may lead to increases in SSCs and associated deposition. The following activities have been considered:
  • seabed preparation activities: boulder and sand wave clearance;
  • DEA installation; and
  • inter-array and interconnector cable installation and burial ( Table 8.12   Open ▸ ).
  1. A qualitative assessment of increased SSC has been undertaken with reference to other quantitative modelling undertaken on other offshore wind projects in the regional benthic subtidal ecology study area to inform the assessment.
  2. Boulder clearance may be required for up to 25% of the length of inter-array and interconnector cables, a total of 315.25 km and 59 km respectively. A clearance width of up to 24 m will be required. Similarly, sand wave clearance will also require a clearance width of up to 24 m, with a total of 11,841,602 m3 of cleared material presented in the Project Description (volume 1, chapter 3) ( Table 8.12   Open ▸ ). The modelling conducted for Berwick Bank Wind Farm considered a clearance width of 25 m for site preparation activities, such as sand wave clearance (SSE Renewables, 2022). This modelling showed that the resulting sediment plume would be very small, with SSCs of <100 mg/l. SSCs were predicted to peak during the deposition of cleared material, with concentrations reaching 2,500 mg/l at the release site, but the plume was predicted to be at its most extensive during the redistribution of the deposited material on successive tides (SSE Renewables, 2022). Under these circumstances, concentrations of 100 mg/l to 250 mg/l were predicted with average values <100 mg/l extending out to one tidal excursion (SSE Renewables, 2022). Sedimentation of deposited material was focussed within 100 m of the site of release with a maximum depth 0.5 m to 0.75 m, whilst the finer sediment fractions were distributed in the vicinity at much smaller depths (circa 5 mm to 10 mm) over a maximum distance of one tidal excursion (SSE Renewables, 2022). As the seabed sediments at Berwick Bank are coarser than those of the Array benthic subtidal ecology study area (which comprises largely deep circalittoral sand; Figure 8.2   Open ▸ ), the smaller sedimentation depths associated with finer sediment fractions (5 mm to 10 mm; (SSE Renewables, 2022)) are more likely to be associated with site preparation activities for the Array.
  3. Furthermore, modelling conducted for the Seagreen 1 and 1A Wind Farms suggested that material released from seabed preparation activities will mostly fall to the seabed as part of a dynamic plume or a passive plume (Seagreen Wind Energy Limited, 2012). Any material released as a passive plume will be in low concentrations and remain for a relatively short duration, before becoming widely dispersed in the area of tidal currents (Seagreen Wind Energy Limited, 2012). Whilst the overall total of potentially released sediments at Seagreen 1 and 1A was considered to be high, it will take place on a foundation by foundation basis over the course of the construction phase, with a maximum of two foundations being installed at any one time (Seagreen Wind Energy Limited, 2012). The modelling concluded that dispersal of sediment is likely to occur along the main axis of tidal current flow with elevated SSCs being relatively low compared to background values, and of a short term duration (Seagreen Wind Energy Limited, 2012). A low magnitude of impact was concluded for site preparation activities at Berwick Bank and Seagreen 1 and 1A (Seagreen Wind Energy Limited, 2012, SSE Renewables, 2022).
  4. As described in paragraph 63, up to 1,590 DEAs may be pulled up to 60 m along the seabed during the construction phase. This process will be undertaken in a controlled manner to ensure that DEAs are installed at the correct position and to appropriate depth. DEAs were not assessed in any publicly available EIAs for projects within the regional benthic subtidal ecology study area, however they are discussed in a recent study on the environmental effects of wind turbine foundations (Horwath et al., 2020). This study concluded that floating foundations that use embedded anchors may have similar bottom-disturbing activities during installation when compared to monopiles, depending on the size of the anchors and method of installation (Horwath et al., 2020). The study noted that the extent that anchors drag along the seabed due to the forces on floating foundations is unknown but is likely to produce some additional SSCs (Horwath et al., 2020). Therefore, the low magnitude of impact associated with foundation installation at Berwick Bank Wind Farm, could be applied to the use of DEAs at the Array. Modelling of SSCs associated with foundation installation at Berwick Bank Wind Farm predicted plumes to have peak concentrations of <5 mg/l, with average values typically less than one fifth of this, and dropping to 1 mg/l to 2 mg/l within a very short distance, typically less than 500 m of the installation activity (SSE Renewables, 2022). The sediment plumes were expected to be temporary, returning to background levels within a few tidal cycles (SSE Renewables, 2022). The average sedimentation depth was predicted to be typically 0.05 mm to 0.1 mm during pile installation, with that maximum dropping to <0.003 mm one day following cessation of operations (SSE Renewables, 2022). This suggests that associated deposition would be imperceptible from the background sediment transport activity, with plotted sediment depths less than typical grain diameters (SSE Renewables, 2022). As per the Array, drill arisings will result from foundation installation at Berwick Bank Wind Farm. The assessment for these however, is considered under long term habitat loss and disturbance (paragraphs 90 et seq.) as this material will be deposited on the seabed in the same area which will be occupied by scour protection and is unlikely to be redistributed as a result of hydrodynamic processes.
  5. Finally, cable installation and burial have the potential to result in increased SSCs and associated deposition. The MDS considers up to 1,261 km of inter-array cables and 236 km of interconnector cables (noting that up to 116 km of the total inter-array cables will be dynamic, and not buried at the seabed) ( Table 8.12   Open ▸ ). As described in the Project Description (volume 1, chapter 3), cable installation methods are not currently defined, and will be identified at the final design stage (post-consent), however cable plough, jet trencher, mass flow excavator, and mechanical cutter are potential options. At the Berwick Bank Wind Farm, jet trenching was assumed for the modelling, which predicted peak increases in SSCs of 100 mg/l in the immediate vicinity of the cable installation, with the sediment subsequently re-suspended and dispersed on subsequent tides, giving rise to concentrations of up to 500 mg/l (SSE Renewables, 2022). The material was predicted to settle during slack water and then be resuspended to form an amalgamated plume. Sedimentation was predicted to be greatest at the location of the trenching and up to 30 mm in depth one day following cessation of inter-array cable installation (SSE Renewables, 2022). Levels of sedimentation were predicted to reduce significantly, down to single figures, within close proximity (i.e. 100 m) of the trench (SSE Renewables, 2022).
  6. Similarly, modelling conducted for the Seagreen 1 and 1A Wind Farms considered jetting as the installation technique, which will fluidise or liquefy the seabed sediments, which will therefore remain near to the bed (Seagreen Wind Energy Limited, 2012). The assessment noted that much of the sediment released by jetting was likely to settle back in the immediate vicinity of its release due to its relatively coarse grain size at Seagreen 1 and 1A (Seagreen Wind Energy Limited, 2012). Any sediment that remained suspended would become dispersed by the prevailing tidal currents in low concentrations, and the magnitude of impact was concluded to be low for this project (Seagreen Wind Energy Limited, 2012).
  7. Based on the qualitative assessment provided here and drawing on the low magnitudes of impact assessed for similar offshore wind projects in the regional benthic subtidal ecology study area, this impact is predicted to be of local spatial extent, short term duration, intermittent, and of high reversibility. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. The sensitivity of the IEFs to increased SSCs and associated deposition are presented in Table 8.20   Open ▸ . These sensitivities are based on the MarESA (where available).
  2. The two representative biotopes of the Offshore subtidal sands and gravels IEF and the Subtidal sands and gravels IEF were both assessed as having a medium vulnerability and high recoverability to both the MarESA pressures associated with this impact (Tillin, 2016a; Tillin, 2016b) ( Table 8.20   Open ▸ ). This is because the characteristic species, such as bivalves, are expected to be regularly exposed to, and tolerant of, short term increases in SSCs (Tillin, 2016a, Tillin, 2016b). Overall, the Offshore subtidal sands and gravels and Subtidal sands and gravels IEFs are deemed to be of medium vulnerability, high recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be low ( Table 8.20   Open ▸ ).
  3. Similarly, ocean quahog and phosphorescent sea pen both inhabit silty and sandy substrates, where the surface of the sediment is naturally regularly mobilised (Hill and Tyler-Walters, 2018, Tyler-Walters and Sabatini, 2017). Furthermore, the ocean quahog can burrow through deposited sediments to allow its feeding siphons to reach the surface; this has been demonstrated in several studies which illustrated the species could burrow through approximately 40 cm of sediment, at speeds of up to 3.89 cm per day (Powilleit et al., 2009, Powilleit et al., 2006). As detailed above in paragraph 70, the phosphorescent sea pen can retreat into its burrow, which can be up to 25 cm deep (Greathead et al., 2007, Jones, 2008). The phosphorescent sea pen is not likely to be vulnerable to light smothering and siltation rate changes given that it can readily retreat deep into its burrow until the sediments have been redispersed (Hill and Tyler-Walters, 2018). Overall, the ocean quahog and phosphorescent sea pen IEFs are deemed to be of low vulnerability, high recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be low ( Table 8.20   Open ▸ ).
  4. Dead man’s fingers has a medium vulnerability to smothering as it is permanently attached to the substratum, however, colonies can be large (up to 20 cm in height), and thus recovery is likely (Budd, 2008). The species has been shown to be tolerant of high levels of suspended sediment, for example, Hill et al. (1997) demonstrated that the species sloughed off settled particles with a large amount of mucous. Siltation is normally only a problem in sheltered areas, and the slope of the rock that the species is attached to is also important as little silt will settle on vertical surfaces and overhangs (Budd, 2008). Similarly, sea tamarisk colonies can be over 15 cm in height, and have widely spaced branches (Wilson, 2002). Given the colony size and that hydroids are adapted to live in turbid waters with medium to strong tidal streams (Boero, 1984, Wilson, 2002), it is likely that they will be tolerant of changes to suspended solids, light smothering and siltation rate changes. Overall, the dead man’s fingers IEF and sea tamarisk IEF are deemed to be of low vulnerability, high recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be low.

 

Table 8.20:
Sensitivity of the IEFs to Increased SSCs and Associated Deposition

Table 8.20: Sensitivity of the IEFs to Increased SSCs and Associated Deposition