Offshore Physical Environment

5. Offshore Physical Environment

5.1. Physical Processes

5.1.1. Introduction

  1. This section of this Scoping Report presents the relevant physical processes aspects to the Array and considers the scope of assessment on physical processes from the construction, operation and maintenance, and decommissioning of the Array.
  2. Physical processes are defined as encompassing the following elements for the purposes of this Scoping Report and subsequent Array EIA Report:
  • bathymetry;
  • wind and waves;
  • tidal currents and elevation;
  • seabed substrate and geology;
  • suspended sediments; and
  • sediment transport (which is influenced by the elements listed above).
  1. Throughout the remainder of this Scoping Report, the elements listed above are collectively referred to as ‘physical processes’.

5.1.2. Study Area

  1. The physical processes study area encompasses the site boundary (i.e. the area within which the Array will be located), including the seabed that may be influenced by changes to physical processes due to the Array to the extent of one spring tidal excursion. One spring tidal excursion is defined as the distance suspended sediment is transported prior to being carried back on the returning tide, this will be identified using numerical modelling techniques. This modelled tidal excursion will also be used to inform other chapters of the Array EIA Report (e.g. benthic subtidal ecology, fish and shellfish ecology, marine mammals, etc.).
  2. From published Admiralty data and preliminary modelling results, a tidal excursion of 8 km has been determined, which will not extend to the nearest designated area which is over 25 km away.

5.1.3. Baseline Environment

  1. The physical processes baseline has been compiled through a thorough review of desktop sources and site-specific survey data. A concise summary of the baseline environment is presented here, reference should be made to Appendix 5 for further detail.


  1. Geophysical data collected in 2022 suggests that the water depth within the site boundary ranges between 63.82 m and 88.66 m relative to LAT. The seafloor consists of gentle slopes and generally deepens towards the east (Appendix 5, Apx Figure 5.1   Open ▸ ). These gentle seafloor gradients range from 0 to 5, with numerous localised steeper areas observed within ripple areas and flanks of rippled scour depressions. Larger sediment features generally run in a direction from north to south, while smaller sediment features run in a more east to west direction.

                        Wind and waves

  1. Metocean and Light Detection and Ranging (LiDAR) buoys were deployed within the site boundary in August 2022. The Metocean buoy will and the LiDAR buoys  will collect physical processes data until autumn 2023 and Autumn 2024 respectively. In the absence of site-specific data, metocean data collected from the Round 3 Firth of Forth Zone, in the vicinity of the site boundary, has been used to characterise the physical processes baseline for the wave regime and tidal currents and elevation. These eight metocean buoys were deployed within the Round 3 Firth of Forth Zone, with the nearest buoys approximately 57 km from the site boundary, with others further inshore towards the Firth of Forth. In the Round 3 Firth of Forth Zone during the stormiest event over the 18-month wave buoy deployment, in January 2012, a significant wave height of 6.7 m was recorded which correlates with a 1 in 1 year sea wave climate return period event (Fugro, 2012). Peak spectral wave periods of up to 20 s were recorded, both associated with strong winds and storms that are characteristic of the North Sea, (Royal Haskoning DHV, 2012). Although this data is now somewhat dated, it is considered sufficient at characterising the wind and wave regime across the Array due to the absence of more recent data and the site-specific data. It is unlikely that the site-specific data will vary significantly in the intervening period. Similarly, the Marine Scotland National Marine Plan Interactive (NMPi) maps presented annual mean wave power ranges from 14 kW/m to 19 kW/m and annual mean significant wave height ranges from 1.81 m to 2.10 m within the site boundary (Marine Scotland, 2022a).

                        Tidal currents and elevation

  1. Metocean surveys conducted across the nearby Round 3 Firth of Forth zone provided an overview of tidal current flows in the vicinity of the site boundary, in the absence of site-specific data, which was unavailable at the time of writing. A maximum current of 0.91 m/s was recorded in April 2011 within the Seagreen 1 (formerly known as Segreen Alpha and Bravo) Offshore Wind Farm (50.72 km from the site boundary). Elsewhere in the Round 3 Firth of Forth Zone, the current speed ranged from 0.68 m/s to 0.88 m/s (Fugro, 2012, Royal Haskoning DHV, 2012).
  2. The Marine Scotland NMPi maps provided an overview of the tidal flows within the site boundary, illustrating that the mean spring tidal range varies from 1.1 m to 3 m (Marine Scotland, 2022a).


  1. The site boundary is part of a complex glacial system, in which the subsequent sedimentary depositions in the Quaternary sediments are affected by the alternating glacial and interglacial stages that affected the northern hemisphere. The ground model was defined from 2D Ultra High Resolution Seismic (UHRS) and Sub-Bottom Profiler (SBP) data integrated with bathymetric, backscatter and Sidescan Sonar (SSS) data that was collected in 2022. Ocean Infinity (2022a) states that “A total of five geological units were identified, with a total of five interpreted horizons, aided interpretation through the delineation of localised geological features”.
  2. Within the site boundary, the published British Geological Survey (BGS) mapping (Stoker and Bent, 1985; Gatliff et al., 1994) indicates that the Quaternary geology will comprise of the Forth Formation (late Weichselian to Holocene, fluviomarine mud and sand), cutting into the Marr Bank Formation (Pleistocene, glaciomarine sandy silty clay) and the Aberdeen Ground Formation (Pleistocene, hard clay).
  3. The geological morphology within the site boundary is varied and includes recent marine soft sediment deposits and deep channel structures (down to 60 m) with sedimentary infill.

                        Seabed substrate

  1. The recent geophysical surveys identified that the seabed within the site boundary consists primarily of sand, with some areas of gravel and occasional diamicton (poorly sorted mixed sediments; Appendix 5, Apx Figure 5.2   Open ▸ ). Gravel areas are more frequent in the north-west, with occasional diamicton also observed in this area. The seabed within the site boundary is relatively flat, with a general slope towards the east. The presence of megaripples and sand waves across the site boundary indicates mobile sediments. The presence of furrows indicates sedimental erosion ( Figure 5.1   Open ▸ ).
  2. Occasional boulder fields (5 to 20 boulders within a maximum area of 2,500 m2) and numerous boulder fields (≥ 20 boulders within a maximum area of 2,500 m2) are distributed across the site boundary, most frequently in the west, within areas of gravel and diamicton (Appendix 5, Apx Figure 5.2   Open ▸ ). Manmade seafloor features are present across the site boundary, with linear debris (such as wire and rope) observed most frequently, and occasional linear scars or trawl marks are present (due to anthropogenic activity, such as trawling) as illustrated in Figure 5.1   Open ▸ .
  3. Due to the presence of large homogenous areas of sand, and the absence of rapid variation between substrates and pockets of sand, the substrate within the site boundary is not classified as a sensitive receptor. Receptor in this Scoping Report refers to physical features that are sensitive to potential impacts of the Array and/or are qualifying features of designated sites. The closest physical processes receptor to the Array is the Firth of Forth Banks Complex Marine Protected Area (MPA), which is detailed further in paragraph 234 below.

                        Suspended sediment and sediment transport

  1. The spatial distribution of average non-algal Suspended Particulate Matter (SPM) for the majority of the UK continental shelf is presented in the Cefas Climatology Report (Cefas, 2016). Based on the data provided within this study, the average SPM associated with the site boundary area has been estimated at between 0 mg/l and 1 mg/l between 1998 and 2015. SPM levels are generally higher in the winter months (up to 3 mg/l in January and December) than the remainder of the year (Cefas, 2016).
  2. There was no site-specific sampling undertaken for the Array, however site-specific surveys were conducted for the Seagreen 1 Offshore Wind Farm in March and June 2011, albeit at least 50.72 km away from the site boundary, and situated within shallower water (39.77 m to 64.82 m). Nonetheless, these samples suggested Total Suspended Sediments (TSSs) to be low (<5 mg/l) with a maximum value of 10 mg/l recorded in March 2011 (Fugro, 2012).
  3. Suspended Sediment Concentrations (SSCs) are primarily influenced by tidal currents, with fluctuations occurring across the spring-neap cycle and the different tidal stages (high water, peak ebb, low water, peak flood) observed throughout the March and June datasets. Wave-driven currents during storms can temporarily elevate SSCs and can cause levels to rise significantly, which then gradually decrease to baseline conditions following storm events. Due to the seasonal nature and frequency of storms, SSC levels demonstrate a broadly seasonal pattern. These effects on SSCs during storm events are less significant in deeper waters, which have a lower degree of wave penetration than in shallower waters. Therefore, it can be inferred that the TSSs will be lower at the Array site than at the Seagreen 1 Offshore Wind Farm and therefore likely below a maximum value of 10 mg/l for a winter storm.
  4. Based on the physical processes modelling undertaken for the Berwick Bank Offshore Wind Farm, within the site boundary the tide moves in an approximate north to south direction, with the flood tide going to 190o and the ebb tide to 15o, and peak spring currents of around 0.5 m/s. Residual currents are minimal in the order of 0.008 m/s in a south south-west direction of approximately 190o. Therefore, the net sediment transport in the region is limited to below 0.003 m3/d/m. The seabed material at the site boundary is primarily gravelly sand (paragraph 227), therefore using the Wentworth scale, movement would only occur for a small proportion of the tidal cycle (typically less than half). Sediment transport would increase during storm conditions, with the largest and most frequent waves approaching from the northerly sector, therefore, net sediment transport under storm conditions would be in a southerly direction.

                        Designated sites

  1. The closest site designated with physical processes qualifying interest features is the Firth of Forth Banks Complex MPA, which is located a minimum of 25 km from the site boundary. This site includes the Berwick, Scalp, and Montrose Banks, and the Wee Bankie shelf banks and mounds. Although also designated for ocean quahog Arctica islandica aggregations, this MPA is designated for offshore subtidal sands, shelf banks and mounds and moraines representative of the Wee Bankie Key Geodiversity Area (Joint Nature Conservation Committee (JNCC), 2021a).
  2. Due to the distance of this designated site to the Array (at least 25 km) and the tidal excursion of 8 km (see paragraph 218), it is concluded that there are no designated sites with the potential to be impacted by the Array.

Figure 5.1:
Seabed Features within the Site Boundary

Figure 5.1: Seabed Features within the Site Boundary


5.1.4. Potential Array Impacts

  1. A list of all potential impacts on physical processes which may occur during the construction, operation and maintenance, and decommissioning phases of the Array in the absence of designed in measures is included in Table 5.1   Open ▸ .


Table 5.1:
Potential Impacts Identified for Physical Processes in the Absence of Designed in Measures

Table 5.1: Potential Impacts Identified for Physical Processes in the Absence of Designed in Measures


5.1.5. Designed in Measures

  1. The following designed in measures have been considered in the identification of potential impacts that have been scoped out of the Array assessment, including how these can reduce potential for impact ( Table 5.3   Open ▸ ):
  • spacing between wind turbines within the Array will be sufficiently distant (at least 1,000 m) that wake effects or changes to the wave field will be mitigated;
  • scour protection around Array structures and foundations will be deployed if required and may consist of concrete mattresses, rock placement or artificial fronds);
  • cable protection around Array cables (which is not yet finalised, and will be refined in the Project Description section of the Array EIA Report);
  • implementation of monitoring Array cable protection in the operation and maintenance phase; and
  • development and adherence to a Cable Plan (CaP, which is not yet finalised).
  1. The significance of the effects of the Array on physical processes will determine the requirement for any further mitigation requirements to be adopted and will be consulted upon with statutory consultees throughout the EIA process.

5.1.6. Relevant Consultations

  1. A summary of the details of the consultations with relevant stakeholders and Statutory Nature Conservation Bodies (SNCBs) undertaken to date is presented in Table 5.2   Open ▸ .


Table 5.2:
Summary of Key Consultations on the Scoping Assessment for the Array

Table 5.2: Summary of Key Consultations on the Scoping Assessment for the Array


5.1.7. Potential Impacts After the Implementation of Designed in Measures

  1. Table 5.3   Open ▸ describes the potential impacts to physical processes that have been scoped out of the assessment presently.


Table 5.3:
Impacts Proposed to be Scoped Out of the Array Environmental Impact Assessment for Physical Processes

Table 5.3: Impacts Proposed to be Scoped Out of the Array Environmental Impact Assessment for Physical Processes


5.1.8. Proposed Approach to the Environmental Impact Assessment

  1. As all impacts to physical processes arising from the Array are proposed to be scoped out of further assessment, there is no proposed approach to the environmental impact assessment for this topic and no further modelling  for physical processes proposed. 

5.1.9. Potential Cumulative Effects

  1. Based on the evidence provided in Table 5.3   Open ▸ , there are no likely significant impacts to physical processes as a result of the Array. Potential impacts to physical processes at a local scale are considered to be highly localised adjacent to any infrastructure and undetectable in close proximity of the installed infrastructure. Therefore, potential cumulative effects on physical processes are not predicted to occur and therefore no cumulative impact assessment will be undertaken. This assumption is well established, based on the expertise of both technical experts writing the physical processes chapter of this Scoping Report and from their previous experience on similar projects.

5.1.10. Potential Transboundary Impacts

  1. Appendix 3 presents the transboundary impacts screening which has been carried out for the Array. This screening exercise identified that there is no potential for transboundary impacts upon physical processes to their insignificant and highly localised nature. 

5.1.11. Scoping Questions to Consultees

  • Do you agree that the existing data available to describe the physical processes baseline remains sufficient to describe the physical environment in relation to the Array? Are there any other data sources that you would recommend?
  • Do you agree that all receptors and impacts have been identified for physical processes?
  • Do you agree that the designed in measures described provide a suitable means for managing and mitigating the potential effects of the Array on the physical processes?
  • Do you agree with the impacts that are proposed to be scoped out of the Array EIA ( Table 5.3   Open ▸ )?

5.1.12. Next Steps

  1. The next step is to seek agreement on the proposal to scope physical processes out of further assessment within the Array EIA Report. The over-arching next steps are outlined in section 4.3.4.