7. Physical Processes

7.1. Introduction

1. This chapter of the Offshore Environmental Impact Assessment (EIA) Report presents the assessment of the likely significant effects (as per the “EIA Regulations”) on the environment of the Berwick Bank Wind Farm offshore infrastructure which is the subject of this application (hereafter referred to as “the Proposed Development”) on physical processes. Specifically, this chapter considers the potential impacts of the Proposed Development seaward of Mean High Water Springs (MHWS) during the construction, operation and maintenance, and decommissioning phases.
2. Likely significant effect is a term used in both the “EIA Regulations” and the Habitat Regulations. Reference to likely significant effect in this Offshore EIA Report refers to “likely significant effect” as used by the “EIA Regulations”. This Offshore EIA Report is accompanied by a Report to Inform Appropriate Assessment (RIAA) (SSER, 2022c) which uses the term as defined by the Habitats Regulations Appraisal (HRA) Regulations.
3. The assessment presented informs the following technical chapters and reports:

• volume 2, chapter 8: Benthic Subtidal and Intertidal Ecology;
• volume 2, chapter 9: Fish and Shellfish Ecology;
• volume 2, chapter 10: Marine Mammals;
• volume 2, chapter 17: Infrastructure and Other Users;
• volume 2, chapter 19: Water Quality; and
• volume 3, appendix 19.1: Water Framework Directive Report.
  4. The assessment further informs the Marine Protected Area (MPA) Assessment (SSER, 2022b) and the RIAA (SSER, 2022c) for the Proposed Development, which accompany the Application.
  5. This chapter summarises information contained within volume 3, appendix 7.1.

7.2. Purpose of this Chapter

6. The primary purpose of the Offshore EIA Report is outlined in volume 1, chapter 1. ‘Likely significant effect’ is a term used in both the “EIA Regulations” and the “Habitat Regulations”. Reference to likely significant effects’ in this Offshore EIA Report refers to ‘likely significant effect’ as used by the “EIA Regulations”. This Offshore EIA report is accompanied by a Report to Inform Appropriate Assessment (RIAA) (SSER, 2022c) which uses the term as defined by the Habitats Regulations Appraisal (HRA) Regulations.

7. It is intended that the Offshore EIA Report will provide the Scottish Ministers, statutory and non-statutory stakeholders with sufficient information to determine the likely significant effects of the Proposed Development on the receiving environment.
8. In particular, this Physical Processes Offshore EIA Report chapter:

• presents the existing environmental baseline established from desk studies, site-specific surveys, numerical modelling studies, and consultation with stakeholders;
• identifies any assumptions and limitations encountered in compiling the environmental information;
• presents the likely significant environmental impacts on physical processes arising from the Proposed Development and reaches a conclusion on the likely significant effects, based on the information gathered and the analysis and assessments undertaken;
• refers to the design aspect of the assessment of the Proposed Development as described in volume 1, chapter 3 of the Offshore EIA Report which prescribes the provision of cable and scour protection; and
• highlights any necessary monitoring and/or mitigation measures which are recommended to prevent, minimise, reduce or offset the likely significant effects of the Proposed Development on physical processes.
  9. The physical processes modelling that has been undertaken to support this chapter, presented in volume 3, appendix 7.1, is based on the previous Berwick Bank Wind Farm boundary (as detailed in SSER, 2021a) and as illustrated in Figure 7.1   Open ▸ (and referred to as the Previous Proposed Development array and export cable areas). The Proposed Development array area (also presented in Figure 7.1   Open ▸ ) has been reduced from the previous boundary with the same proposed wind turbine infrastructure, as described in volume 1, chapter 3. Due to the boundary change, in some cases modelling of construction activities extended beyond the current Proposed Development array area. These areas do however have bathymetry, tidal currents and sediment classifications consistent with those within the Proposed Development array area due to the close proximity. It is considered that, given these similarities and that the revised layout represents a modest change in terms of the physical processes assessment, the modelling undertaken for the previous Berwick Bank Wind Farm Boundary remains valid and has therefore been used to inform the physical processes assessment presented for the Proposed Development. In addition, the physical processes study area, as illustrated in Figure 7.1   Open ▸ , has been retained to provide additional context to the physical processes assessment for the Proposed Development. The assessment presented with this chapter is therefore based on the physical processes modelling undertaken within the same modelled study area as applied to the Proposed Development array area.

7.3. Study Area

10. The physical processes study area for the Proposed Development is illustrated in Figure 7.1   Open ▸ and encompasses the:

• Proposed Development array area (i.e. the area in which the wind turbines will be located);
• Proposed Development export cable corridor;
• intertidal area at landfall; and
• seabed and coastal areas that may be influenced by changes to physical processes due to the Proposed Development, based on the outputs of the physical processes modelling which will encompass a wider domain including the Firth of Forth Banks Complex.
  11. The physical processes study area was principally defined as one tidal excursion from the Proposed Development array area and would therefore encapsulate the distance suspended sediment is transported prior to being carried back on the returning tide. The area was then extended to include the banks within the Firth of Forth Banks Complex Nature Conservation Marine Protected Area (ncMPA) namely, Berwick Bank, Marr Banks, Montrose Bank and Scalp Banks along with Wee Bankie. It is however noted that the physical processes study area forms the focus for the assessment and that the numerical model extent was not limited to this region and would therefore also identify potential impacts beyond the physical processes study area.

7.3.1.    Intertidal Area

12. The offshore topic of physical processes study area includes the intertidal area. This intertidal area overlaps with the onshore topic of Geology, Hydrology, Soils and Flood Risk (landward of Mean Low Water Springs (MLWS)).

Figure 7.1: Physical Processes Study Area

7.4. Policy and Legislative Context

13. Policy and legislation on renewable energy infrastructure is presented in volume 1, chapter 2 of the Offshore EIA Report. Policy specifically in relation to physical processes, is contained in the Sectoral Marine Plan for Offshore Wind Energy (SMP) (Scottish Government, 2020), the Scottish National Marine Plan (NMP) (Scottish Government, 2015) and the United Kingdom (UK) Marine Policy Statement (MPS) (HM Government, 2011). A summary of the policy provisions relevant to physical processes are provided in Table 7.1   Open ▸ , with other relevant policy provisions set out in Table 7.2   Open ▸ and Table 7.3   Open ▸ .
14. These are summarised here with further detail presented in volume 1, chapter 2.
15. All the policy and legislation provided in Table 7.1   Open ▸ , Table 7.2   Open ▸ and Table 7.3   Open ▸ is also relevant to the intertidal area.

Table 7.1: Summary of SMP Policies Relevant to Physical Processes

Table 7.2: Summary of NMP Policies Relevant to Physical Processes

Table 7.3: Summary of Other Policies Relevant to Physical Processes

7.5. Consultation

16. The physical processes Road Map was a ‘live’ document which has been used as a tool to facilitate early engagement with stakeholders and subsequent engagement throughout the pre-application phase of the Proposed Development including on agreeing to scope impacts out of the assessment, and/or agreeing the level of assessment which will be presented for impacts, so that the focus in the EIA submission documents is on likely significant environmental effects as required by the EIA Regulations.
17. The physical processes Road Map (up to date at the point of Application) is presented as volume 3, appendix 8.2 and documents meetings and discussion points. At the request of MS-LOT[1], an audit document (the Audit Document for Post-Scoping Discussions (volume 3, appendix 5.1) has been produced and submitted alongside the application to document discussions on key issues, post-receipt of the Berwick Bank Wind Farm Scoping Opinion (MS-LOT, 2021).
18. A summary of the key issues raised during consultation activities undertaken to date specific to physical processes for the Proposed Development is presented in Table 7.4   Open ▸ . Further relevant consultation feedback is also presented together with how these issues have been considered in the production of this Physical Processes Offshore EIA Report chapter. Further detail is presented within volume 1, chapter 5.

Table 7.4: Summary of Key Consultation of Relevance to Physical Processes

7.6. Methodology to Inform Baseline

7.6.1.    Desktop Study

19. As described in paragraphs 10 and 11, the Proposed Development array area and physical processes study area are shown in Figure 7.1   Open ▸ .
20. Information on physical processes within the physical processes study area was collected through a detailed desktop review of existing studies and datasets. These are summarised in Table 7.5   Open ▸ and Table 7.6   Open ▸ respectively. The baseline was characterised by a combination of literature review of the reports and numerical modelling using the datasets. Full details of the analysis undertaken to develop the physical processes baseline is provided in the physical processes technical report, volume 3, appendix 7.1.

Table 7.5: Summary of Key Desktop Reports

Table 7.6:  Summary of Key Resources

7.6.2.    Identification of Designated Sites

21. All designated sites within the physical processes study area and qualifying interest features that could be affected by the construction, operation and maintenance, and decommissioning phases of the Proposed Development were identified using the three-step process described below:

• Step 1: All designated sites of international, national, and local importance within the physical processes study area were identified using a number of sources. These included the Marine Scotland website (http://marine.gov.scot/), the Atlas of Marine Protection website (https://mpatlas.org/) and JNCC resources (https://jncc.gov.uk/mpa-mapper/).
• Step 2: Information was compiled on the relevant geomorphological/coastal features for each of these sites.
• Step 3: Using the above information and expert judgement, sites were included for further consideration if:

–      a designated site directly overlaps with the Proposed Development array area or Proposed Development export cable corridor and therefore has the potential to be directly affected by the Project; or

–      sites and associated features were located within the physical processes study area for impacts associated with the Project and therefore have the potential to be indirectly affected by the Proposed Development.

7.6.3.    Site-Specific Surveys

22. To inform the Offshore EIA Report for the Proposed Development, site-specific surveys were undertaken. A summary of the surveys undertaken used to inform the physical processes assessment of effects is outlined in Table 7.7   Open ▸ .

Table 7.7: Summary of Site-Specific Survey Data

7.7. Baseline Environment

7.7.1.    Overview of Baseline Environment

23. A summary of the physical processes baseline environment is provided in the following sections. Full details of the analysis undertaken to develop the physical processes baseline is provided in the physical processes technical report, volume 3, appendix 7.1, which includes information on model development, resolution, calibration, and the modelling techniques implemented to develop the baseline characteristics.

Bathymetry

24. The Proposed Development array area lies within the Firth of Forth Banks Complex ncMPA; in particular the bathymetry is influenced by two bank features. The large-scale morphological bank features of Marr Banks and the northern extent of the Berwick Bank, whilst the physical processes study area also includes Montrose Bank, Scalp Bank and Wee Bankie as shown in Figure 7.1   Open ▸ .
25. Seabed levels across the Proposed Development array area vary from a minimum depth of 32.8 m below Lowest Astronomical Tide (LAT) to the north of the western central part of the Proposed Development array area to a maximum depth of circa 68.5 m below LAT in the east of the banks. The Proposed Development export cable corridor has a relatively variable bathymetry ranging from the low water mark to a depth of 69.8 m below LAT. Along the Proposed Development export cable corridor which extends to the western margin of the Berwick Bank, the bathymetry is influenced by a gently sloping seafloor topography to a depth of 60 m below LAT, as illustrated in Figure 7.2   Open ▸ .

Hydrography

26. The Proposed Development array area has an average tidal range of 3.25 m as published by Admiralty (UKHO) at Dunbar. This port is one of a number in the proximity of the physical processes study area and was used as a calibration point alongside several other reference points taken across the model domain, as detailed in volume 3, appendix 7.1.

Figure 7.2: Bathymetric Survey Fugro 2020 and XOCEAN 2021

27. Across the offshore wind farm area, the tidal current floods to the south and ebbs to the north. The flows are relatively weak with tidal current speeds typically between 0.5 m/s and 0.6 m/s during peak flood: with ebb currents being of a similar magnitude.

Wave climate

28. Characteristic of the northern North Sea, waves are generated by either local winds or from remote winds (swell waves). At the centre of the Proposed Development array area, the largest proportion of waves approach from the northerly sectors, typically combined wind and swell for the North Sea. However, a wave field can also develop from the east of the Proposed Development array area as there is a sufficient fetch length.
29. The largest waves approaching the Proposed Development array area are from the north; through the north-east and easterly sectors the associated waves become less common with lower wave heights for the same return periods. Further detail on the wave climate analysis is provided in volume 3, appendix 7.1.

Littoral currents

30. Littoral currents are driven by tides, waves, and meteorological events. The littoral currents were modelled from the northerly sector during a one in one year storm event, resulting in the increase of currents on the peak flood tide to 0.5 m/s to 0.7 m/s and reducing to 0.4 m/s to 0.6 m/s during the peak ebb within the Proposed Development array area. With the largest and most prevalent waves approaching from the north, these waves cause an increase in currents during the flood tide and decrease on the ebb tide.

Sedimentology

31. The seafloor morphology of the Proposed Development array area includes large scale banks (known as the Marr Banks and the Berwick Bank), arcuate ridges, incised valleys, relic glacial lakes, channels and bedforms. Seabed substrate within the Proposed Development array area ranged from coarse gravel, shelly gravel with boulder, mix sediments with patchy coarse material or boulders and muddy sand. Including several distinct features such as isolated/clustered boulders, areas of sand waves, ripples/mega ripples and trawl marks, however the majority of the area is featureless.
32. The underlying sediments identified in the Proposed Development export cable corridor were coarse sediments with cobbles, boulders, rock outcroppings, sand, fine sand and muddy sand as illustrated in Figure 7.3   Open ▸ . The Proposed Development export cable corridor recorded very few morphological features such as high rises or ridges and is described as smooth.
33. The landfall location for the Proposed Development export cable corridor is on the East Lothian coast, at Skateraw. The Skateraw landfall area is a 1.7 km stretch bordering the intertidal area comprised of a foreshore which is mostly made up of rock with areas of sand deposits, where the top of the beach is lined with a mixture of sand, pebbles, and small boulders. The seabed morphology comprises a rocky undulating Carboniferous platform with patches of megarippled sands where sediment has accumulated within larger channels. Extending offshore, the undulating rocky seabed becomes flatter, with areas of sediment with boulders interpreted as sediment overlaying rock.

Sediment transport

34. Within the Proposed Development array area, the residual current speeds are low resulting in low sediment transport rates. Residual currents are the net flow over a full tidal cycle and drive the sediment transport. Along the coastline the sediment transport rates are several orders of magnitude higher than in areas of offshore banks. During the tidal cycle, the bed levels are reduced along ripples but increase to previous levels on the return tide. At peak currents, changes in bed level can be in the order of a fraction of a millimetre per day which signifies that the bed area is mobile however it is considered stable. During storms approaching from the north, the residual current and subsequent sediment transport increases during flood tides.
35. The physical processes study area largely coincides with Scottish Coastal Sub-cell l a - St Abb's Head to North Berwick (Ramsay and Brampton, 2000). There are two main sources of beach material, those resulting from erosion of sandstone cliffs and glacially derived sands and gravels. The Ramsay and Brampton (2000) study states that in general most of the beach systems are largely self-contained in terms of sediment movements and there is little interaction or movement of beach sediment along this coast; hence no significant present day longshore drift gives rise to long-term erosion or accretion. Periodic storm damage will occur on most of the ‘soft’ coastal edges due to the exposed nature of the coastline, but this sediment is generally retained within the immediate beach system. This is corroborated by more recent Dynamic Coast 2 results (https://www.dynamiccoast.com/) which predict isolated pockets of erosion along the coastline of the physical processes study area under the Future Erosion 2050 High Emissions Scenario associated with soft cliff embayments.

Figure 7.3: Sediment Classification Fugro 2020 and XOCEAN 2021

Suspended sediments

36. SSC are regulated by tidal currents and intensify during wind-driven storm events throughout the water column. SSC levels have a seasonal pattern due to the seasonality of storm events. Monitoring nearshore and just to the south of the Skateraw landfall site, recorded typical SSC levels of <5 mg/l, however as expected during storm events this increased to above 100 mg/l corresponding with increased wave heights.
37. Within the Proposed Development array area, the non-algal SPM was estimated to be on average 0 mg/l to 1 mg/l between 1998 and 2015 (Cefas, 2016). As for the SSC, the SPM levels display a seasonal pattern with heightened levels during winter months and are regulated by tidal currents.

Designated sites

38. Designated sites and relevant qualifying interest features identified for the Physical Processes Offshore EIA Report chapter are described in Table 7.8   Open ▸ and presented in Figure 7.4   Open ▸ . This includes sites and features for which physical processes are examined within the Physical Processes Offshore EIA Report chapter.

Table 7.8: Designated Sites and Relevant Qualifying Interest Features for the Physical Processes Offshore EIA Report Chapter

Figure 7.4: Physical Processes Designated Sites

7.7.2.    Future Baseline Scenario

39. The EIA Regulations require that a “a description of the relevant aspects of the current state of the environment (baseline scenario) and an outline of the likely evolution thereof without implementation of the project as far as natural changes from the baseline scenario can be assessed with reasonable effort, on the basis of the availability of environmental information and scientific knowledge” is included within the Offshore EIA Report.
40. If the Proposed Development does not come forward, an assessment of the future baseline conditions has also been carried out and is described within this section.
41. The baseline environment for physical processes is not static and will exhibit a degree of natural change over time. Such changes will occur with or without the Proposed Development in place due to natural variability. Future baseline conditions would be altered by climate change resulting in sea level rise and increased storminess. This is unlikely to have the effect of significantly altering tidal patterns and sediment transport regimes offshore at the Proposed Development array area. The return period of the wave climates would be altered (e.g. what is defined as a 1 in 50 year event may become a 1 in 20 year event) as deeper water would allow larger waves to develop. There is, however, a notable degree of uncertainty regarding how future climate change will impact prevailing wave climates within the North Sea and beyond.

7.7.3.    Data Limitations

42. The physical processes study area has been the focus of study for both academic and government institutions. Additionally, considerable data collection campaigns have been undertaken by the Applicant of both the Proposed Development and other offshore wind farms in the locality. Although some physical processes are complex and inter-related, there is a considerable amount of data available. It is therefore considered that the data employed are robust and sufficient for the purposes of the assessment of effects presented.

7.8. Key Parameters for Assessment

7.8.1.    Maximum Design Scenario

43. The maximum design scenarios identified in Table 7.9   Open ▸ have been selected as those having the potential to result in the greatest effect on an identified receptor or receptor group. These scenarios have been selected from the details provided in volume 1, chapter 3 of the Offshore EIA Report. Effects of greater adverse significance are not predicted to arise should any other development scenario, based on details within the Project Design Envelope (PDE) (e.g. different infrastructure layout), to that assessed here, be taken forward in the final design scheme.
44. The results of the physical processes study, particularly the numerical modelling output detailed in volume 3, appendix 7.1, will be used to support and inform the following Offshore EIA Report chapters:

• volume 2, chapter 8: Benthic Subtidal and Intertidal Ecology;
• volume 2, chapter 9: Fish and Shellfish Ecology;
• volume 2, chapter 10: Marine Mammals;
• volume 2, chapter 17: Infrastructure and Other Users;
• volume 2, chapter 19: Water Quality;
• volume 3, appendix 19.1: Water Framework Directive Report; and
• the Marine Protected Area Assessment (SSER, 2022b).

Table 7.9: Maximum Design Scenario Considered for Each Impact as Part of the Assessment of Likely Significant Effects on Physical Processes

7.8.2.    Impacts Scoped out of the Assessment

45. The physical processes Road Map (see volume 3, appendix 8.2) has been used to facilitate stakeholder engagement on topics to be scoped out of the assessment.
46. On the basis of the baseline environment and the Project Description outlined in volume 1, chapter 3 of the Offshore EIA Report, one impact is proposed to be scoped out of the assessment for physical processes. This was either agreed with key stakeholders through consultation as discussed in volume 1, chapter 5, or otherwise, the impact was proposed to be scoped out in the Berwick Bank Wind Farm Offshore Scoping Report (SSER, 2021a) and no concerns were raised by key consultees. Where discussions with consultees took place after the publication of the Berwick Bank Wind Farm Scoping Opinion (MS-LOT, 2022), these are audited in the Audit Document for Post-Scoping Discussions (volume 3, appendix 5.1).
47. This impact is outlined, together with a justification for scoping it out, in Table 7.10   Open ▸ .

Table 7.10: Impact Scoped Out of the Assessment for Physical Processes (Tick Confirms the Impact is Scoped Out)

7.9. Methodology for Assessment of Effects

7.9.1.    Overview

48. The physical processes assessment of effects has followed the methodology set out in volume 1, chapter 6 of the Offshore EIA Report. Specific to the physical processes EIA, the following guidance documents have also been considered:

• Guidelines in the use of metocean data through the lifecycle of a marine renewables development, Construction Industry Research and Information Association (CIRIA) C666, ABPmer Ltd et al., (2008);
• Guidance on Environmental Impact Statement (EIS) and Natura Impact Statement (NIS) Preparation for Offshore Renewable Energy Projects, Department of Communications, Climate Action and Environment, (2017);
• Guidance on Marine Baseline Ecological Assessments and Monitoring Activities for Offshore Renewable Energy Projects Parts 1 and 2 (April 2018);
• Collaborative Offshore Wind Energy Research into the Environment (COWRIE) - Coastal Process Modelling for Offshore Wind Farm Environmental Impact Assessment, Lambkin et al. (2009);
• Advice to Inform Development of Guidance on Marine, Coastal and Estuarine Physical Processes Numerical Modelling Assessments. Natural Resources Wales (NRW) Report No 208, 139pp, NRW, Pye, K., Blott, S.J. and Brown, J. (2017); and
• Guidance on Best Practice for Marine and Coastal Physical Processes Baseline Survey and Monitoring Requirements to inform EIA of Major Development Projects, NRW Report No: 243, 119 pp, NRW, Cardiff, Brooks, AJ., Whitehead, PA., Lambkin, DO. (2018).

7.9.2.    Criteria for Assessment of Effects

49. Physical processes are not generally receptors in themselves; they may be a pathway by which coastal features may be impacted or form a pathway for indirect impacts on other receptors. For example, increases in suspended sediments during the construction phase may lead to the deposit of these sediments and smothering of benthic habitats. For this impact, the magnitude of the potential changes has been assessed, with the sensitivity of the receptors to these changes and the significance of effects assessed within volume 2, chapter 8, chapter 9, chapter 10 and chapter 17.
50. A full assessment of effects has however been provided for the hydrodynamic regime and the sediment transport regime, which have been identified as potentially sensitive physical processes receptors. Sediment transport is driven by a combination of tidal flow and wave climate, therefore, as each of these processes are intrinsically linked, the assessment was undertaken collectively.
51. The process for determining the significance of effects is a two stage process that involves defining the magnitude of the potential impacts and the sensitivity of the receptors. This section describes the criteria applied in this chapter to assign values to the magnitude of potential impacts and the sensitivity of the receptors. The terms used to define magnitude and sensitivity are based on those which are described in further detail in volume 1, chapter 6 of the Offshore EIA Report.
52. The criteria for defining magnitude in this chapter are outlined in Table 7.11   Open ▸ . In determining magnitude within this chapter, each assessment considered the spatial extent, duration, frequency and reversibility of impact and these are outlined within the magnitude section of each impact assessment (e.g. a duration of hours or days would be considered for most receptors to be of short term duration, which is likely to result in a low magnitude of impact).

Table 7.11: Definition of Terms Relating to the Magnitude of an Impact

53. The criteria for defining sensitivity in this chapter are outlined in Table 7.12   Open ▸ .

Table 7.12: Definition of Terms Relating to the Sensitivity of the Receptor

54. The significance of the effect upon physical processes is determined by correlating the magnitude of the impact and the sensitivity of the receptor. The particular method employed for this assessment is presented in Table 7.13   Open ▸ .
55. In cases where a range is suggested for the significance of effect, there remains the possibility that this may span the significance threshold (i.e. the range is given as minor to moderate). In such cases the final significance conclusion is based upon the author's professional judgement as to which outcome delineates the most likely effect. Where professional judgement is applied to quantify final significance from a range, the assessment will set out the factors that result in the final assessment of significance. These factors may include the likelihood that an effect will occur, data certainty and relevant information about the wider environmental context.
56. For the purposes of this assessment:

• a level of residual effect of moderate or more will be considered a ‘significant’ effect in terms of the EIA Regulations; and
• a level of residual effect of minor or less will be considered ‘not significant’ in terms of the EIA Regulations.
  57. Effects of moderate significance or above are therefore considered important in the decision-making process, whilst effects of minor significance or less warrant little, if any, weight in the decision-making process.

Table 7.13: Matrix Used for the Assessment of the Significance of the Effect

7.10. Measures Adopted as part of the Proposed Development

58. As part of the Project design process, a number of measures have been proposed to reduce the potential for impacts on physical processes (see Table 7.14   Open ▸ ). As there is a commitment to implementing these measures, they are considered inherently part of the design of the Proposed Development and have therefore been considered in the assessment presented in section 7.11 (i.e. the determination of magnitude and therefore significance assumes implementation of these measures). These measures are considered standard industry practice for this type of development.

Table 7.14: Designed In Measures Adopted as Part of the Proposed Development

7.11. Assessment of Significance

59. The potential effects arising from the construction, operation and maintenance and decommissioning phases of the Proposed Development are listed in Table 7.9   Open ▸ , along with the maximum design scenario against which each impact has been assessed. An assessment of the likely significance of the effects of the Proposed Development on the physical processes receptors caused by each identified impact is given below.

Increased Suspended Sediment Concentrations and Associated Deposition on Physical Features as a Result of Seabed Preparation, Foundation Installation and Cable Installation

60. Increased SSCs and associated deposition may arise due to the installation of the wind turbines and OSP/Offshore convertor station platform foundations, the installation and/or maintenance of inter-array cables and the offshore export cables. This impact is relevant to the construction, operation and maintenance phases of the Proposed Development and may cause indirect impacts to receptors.
61. The following scenarios were investigated:

• site preparation activities – sand wave clearance to facilitate cable installation;
• drilled pile installation – across the range of hydrodynamic conditions;
• inter-array/ interconnector cable installation (with the same characteristics) – for a zone of sandy seabed sediment; and
• offshore export cables installation – through sandy seabed sediment.
  62. Modelling was undertaken related to the maximum design scenario as outlined in Table 7.9   Open ▸ with the detail of the assessment provided in volume 3, appendix 7.1.

Construction Phase

Magnitude of Impact

63. The installation of infrastructure within the offshore wind farm and Proposed Development export cable corridor may lead to increased SSCs and associated deposition. The maximum design scenario is for the drilled installation of up to 179 x 24 MW with two 5.5 m piles per leg and four legs per foundation. Drilling may be required for 10% of the piles to an estimated depth of 16 m in each case. Included is the installation of seven OSPs/Offshore convertor station platform comprising of five High Voltage Alternating Current (HVAC) with six legs supported by up to four piles of 3.5 m diameter per leg and two HVDC platforms with eight legs supported on up to four 4 m diameters piles. The drilling depth for OSPs/Offshore convertor station platforms may be up to 12 m or 20% depth, not all piles will require drilling. Four drillings may be required per foundation for HVAC and HVDC OSPs/Offshore convertor station platforms. For the installation of inter-array cables (1,225 km) and offshore export cables (1,272 km) a trench of up to 2 m in width and 3 m in depth may be excavated.
64. The modelled scenarios used a drilling depth 20% greater than the maximum design scenario for piled jacket foundations of wind turbines to provide a worst case scenario and examined a range of locations across the Proposed Development array area with two concurrent drilling operations at adjacent locations. The drilled pile installations are anticipated to generate plumes with a suspended sediment level of <10 mg/l. These levels would be localised and only persist for short period, a couple of tidal cycles. Concentrations within the plume envelope are much lower, typically <1 mg/l a short distance (<1 km) from the discharge locations. Following the cessation of drilling the turbidity levels reduce within a few hours as tidal currents reduce. Some of the finer material associated with the drilling process is re-suspended during periods of increased tidal currents on successive tides as it is redistributed but turbidity levels remain low. The sedimentation beyond the immediate drilling location is indiscernible. This is due to the relatively slow drilling rate (0.5 m/hour), allowing the fine sediment to be widely dispersed while the larger material settles at the release point due to the limited current speed.
65. For the inter-array cable installation, the sediment plumes are much larger than those for the drilled pile installation. The reason for this is twofold, firstly there is a large amount of sediment initially mobilised (582,000 m3 of material from the trench); and secondly when there was elevated tidal currents on successive tides there was more available material to be remobilised over the extended period of installation. Peak plume concentrations are highest at around 500 mg/l with the sediment settling during slack water becoming resuspended in the form of an amalgamated plume. The greatest sedimentation of 30 mm depth occurs at the trench site, with sediment depths reducing moving away from the trench but remaining in the sediment cell and retained in the sediment transport system.
66. Following the completion of drilled foundations the turbidity levels will return to baseline within a couple of tidal cycles. It would however be anticipated that spring tides following the works may mobilise and redistribute unconsolidated seabed material deposited at the end of the construction phase; this material will therefore be incorporated into the existing transport regime. Following installation, the native seabed material settles close to where it is mobilised and remains in situ. This would be expected as the baseline modelling indicated that sediment transport potential is limited across the Proposed Development array area. The sedimentation is concentrated along the installation route as material effectively returns to the site from where it was disturbed. Sedimentation depths of <0.001 mm arise beyond the immediate vicinity of the trench the day after drilling cessation and therefore would be indiscernible from the existing seabed sediment.
67. Modelling was undertaken to quantify sediment plumes associated with offshore export cables installation to the trenchless technique (e.g. HDD) transition, where circa 400,000 m3 of material may be mobilised. It is noted that trenchless (e.g. HDD) punch out excavation will also occur with the volume of material mobilised being 250 m3, therefore as an independent activity is not significant and in the context of the assessment is encompassed in the final 20 m of the trenching activity. Offshore export cable installation shows a higher variability in suspended sediment concentration due to the change in hydrography along the Proposed Development export cable corridor. Average levels of SSC range between 50 mg/l and 500 mg/l with the level dropping to background levels on the slack tide. At the selected Skateraw landfall site some material migrates into the sediment cell however it would be insufficient to impact the beach morphology, increasing baseline levels of sediment by <3 mm along the coast off Torness Point and typically far less along the shoreline which is redistributed on successive tides flowing cable installation.
68. The PDE includes the provision of site preparation/sand wave clearance activities which have the potential to increase SSCs in the construction phase with associated sedimentation. The clearance width would be 25 m wide corridor to facilitate cable installation with an average depth of 5 m for the offshore cable corridor and a depth of 1.3 m for the inter-array/interconnector cables, with a clearance dredging rate of 10,000 m3/h and a 3% spill of material.
69. In practice, plough dredging which mobilises a much smaller amount of sediment into suspension at the seabed and has reduced sediment plume concentrations and extents compared to other types of dredging activities may be undertaken. However, the modelling simulated the use of a suction hopper dredger to remove material from the crest of sand waves and deposit on material in a trough, resulting in higher quantification of sedimentation compared to the plough dredging.
70. The impact of increased suspended sediment levels and associated sedimentation is predicted to be of local spatial extent, short term duration and intermittent and of high reversibility. It would not be of sufficient magnitude to alter the hydrodynamic regime or offshore bank or beach morphology. It is predicted that the impact will affect the Firth of Forth Banks Complex ncMPA directly whilst affecting the remaining receptors indirectly. The magnitude is therefore, considered to be low for the receptors within the ncMPA and negligible for other receptor groups.

Sensitivity of the Receptor

71. The Proposed Development partially overlaps with the Firth of Forth Banks Complex ncMPA ( Figure 7.4   Open ▸ ). The Firth of Forth Banks Complex ncMPA is a composite site with Berwick and Marr Banks lying within the Proposed Development array area, whilst Scalp and Montrose Banks, and the Wee Bankie lie within the wider physical processes study area. These banks are comprised of the following designated features; offshore subtidal sands and gravels, shelf banks and moulds and habitat to aggregations of ocean quahog Arctica islandica and moraine formations. Both offshore subtidal sands, gravels and ocean quahog are Priority Marine Features (PMFs) in Scotland’s seas and considered of conservation importance. The sedimentation identified is localised and composed of native material therefore the structure and function of the designated features is of low vulnerability and recoverable. The sensitivity of the receptor to changes as a result of seabed preparation, foundation installation and cable installation is therefore considered to be low.
72. Much of the Berwickshire and North Northumberland Coast SAC lies seaward of MHWS with designated features such as reefs, submerged/partially submerged sea caves, intertidal mudflats/sandflats and shallow inlets and bays. The Berwickshire Coast SSSI is comprised of rocky shore and sea cave features. These areas are extensive and would recover from the low magnitude of impact from sedimentation as no material reaches the intertidal zone from nearshore cabling. The sensitivity of the receptor to changes as a result of seabed preparation, foundation installation and cable installation is therefore considered to be negligible.
73. St Abb's Head to Fast Castle SSSI is characterised for its geomorphological coastal interests in particular the spectacular assemblage of rock coast landforms including clefts, gullies, geos, caves, stacks, reefs and skerries. These rock landforms would recover from the low magnitude of impact from sedimentation as no material reaches the intertidal zone from nearshore cabling. The sensitivity of the receptor to changes as a result of seabed preparation, foundation installation and cable installation is therefore considered to be negligible.
74. Pease Bay SSSI is designated on sea cliffs which provide exposures of a continuous succession of Upper Devonian and Lower Carboniferous strata which is of national and international importance. These rocky outcrop areas would recover from the low magnitude of impact from sedimentation as typically no material reaches the intertidal zone from nearshore cabling. The sensitivity of the receptor to changes as a result of seabed preparation, foundation installation and cable installation is therefore considered to be negligible.
75. The Firth of Forth SSSI is comprised of features such as mudflat, sand dune, saltmarsh and sea cliffs. The area is expansive and would recover from the low magnitude of impact from sedimentation as typically no material reaches the intertidal zone from nearshore cabling. The sensitivity of the receptor to changes as a result of seabed preparation, foundation installation and cable installation is therefore considered to be negligible.
76. Barns Ness Coast SSSI contains a variety of coastal features such as saltmarsh, sand dunes and shingle. Geologically comprised of lower carboniferous limestone of interest because it is rich in fossils and due to the succession between Scottish and Northumberland carboniferous limestone. The Skateraw landfall site for the offshore export cables borders this SSSI, however, as the trenchless technique has been selected and typically <3 mm of sedimentation reaches the coastline from nearshore cabling to the south of the SSSI off Torness Point. The sensitivity of the receptor to changes as a result of seabed preparation, foundation installation and cable installation is therefore considered to be negligible.
77. Bathing water quality is measured in terms of biological levels and due to the low potential influx of native sediment into the bathing waters of the intertidal zone the level of vulnerability would be low and recoverable. It is expected that the sensitivity of the receptor to changes because of seabed preparation, foundation installation and cable installation is therefore considered to be negligible.