1. Introduction

1. This Physical Processes Technical Report provides information relating to the physical environment and coastal processes for the offshore components of the Berwick Bank Wind Farm (hereafter referred to as the “Proposed Development”). It describes the current baseline conditions and quantifies the potential changes due to the installation and presence of the Proposed Development. This report is divided into two main sections:

• baseline conditions – describing current hydrography and sedimentology; and
• environmental variations – describing changes to baseline arising from the installation and presence of the Proposed Development.
  2. For the purposes of this Physical Processes Technical Report physical processes are defined as encompassing the following elements:
• tidal elevations and currents;
• waves;
• bathymetry;
• seabed sediments;
• suspended sediments; and
• sediment transport.
  3. The physical processes modelling presented in this technical report relates to the Proposed Development site boundary as described in volume 1, chapter 3 of the Offshore Environmental Impact Assessment (EIA) Report, which supports an Application for development consent submitted in December 2022. In June 2022, this (‘the current Proposed Development site boundary’) was reduced from the previous Berwick Bank Wind Farm boundary (as detailed in SSER, 2021a), but contains the same proposed infrastructure.

2. Study Area

4. The physical processes study area for the Proposed Development is illustrated in Figure 2.1   Open ▸ and defined as the:

• Proposed Development array area (i.e. the area in which the wind turbines will be located);
• Proposed Development export cable corridor;
• landfall area; 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.
  5. It is however noted that the physical processes study area forms the focus for the assessment and that the numerical model extent is not limited to this region. The physical processes study area extends in excess of one tidal excursion from the Proposed Development boundary and would therefore encapsulate the distance suspended sediment is transported prior to being carried back on the returning tide. It includes all banks within the Firth of Forth Bank Complex namely, Berwick Bank, Marr Banks, Montrose and Scalp Banks along with Wee Bankie. The modelling study would therefore also identify potential impacts beyond the physical processes study area.

Figure 2.1:  Physical Processes Study Area

3. Methodology

3.1. Numerical Modelling

6. Numerical modelling techniques were used to describe tide, wave and sediment transport regimes. The MIKE suite of software was employed, as a single model mesh could be used to simulate these processes both individually and in combination. The model domain is shown in Figure 3.1   Open ▸ . The MIKE suite of models is a widely used industry standard modelling suite developed by the Danish Hydraulic Institute (DHI). It has been approved for use by industry and government bodies including the Scottish Environmental Protection Agency (SEPA). The MIKE suite is a modular system that contains a number of different but complementary modules encompassing different physical processes: these are summarised in Table 3.1   Open ▸ and described in further detail within the relevant sections.

Table 3.1:  MIKE Suite of Models

7. Following the establishment of baseline conditions, a number of scenarios were modelled to determine the environmental variations arising from the installation and presence of the Proposed Development. These are outlined in Table 3.2   Open ▸ .

Table 3.2:  Summary of Modelled Environmental Variation Scenarios

Figure 3.1:  Model Domain (Green Outline)

3.2. Data Sources

8. Information on the physical environment within the physical processes study area and beyond to the model domain was collected through a detailed desktop review of existing studies and datasets. These are summarised in Table 3.3   Open ▸ .

Table 3.3:  Summary of Key Resources

3.3. Site-Specific Surveys

9. A summary of the surveys undertaken of relevance to physical processes is outlined in Table 3.4   Open ▸ .

Table 3.4: Summary of Surveys Undertaken to Inform Physical Processes

4. Baseline Environment

10. This section outlines the numerical modelling undertaken in order to determine the baseline conditions. It describes the physical processes in terms of sea state and sediment transport regimes.

4.1. Bathymetery

11. The bathymetry of the Proposed Development array area is influenced by the presence of large scale morphological bank features, including the Marr Bank and the northern extent of the Berwick Bank. These two bank features are defined as Shelf Banks and Mounds and are part of the Firth of Forth Banks Complex.
12. Geophysical data collected in 2019 suggests the water depth within the Proposed Development array area varies between 32.8 m and 68.5 m relative to Lowest Astronomical Tide (LAT), and average depths of generally 51 m below LAT. Minimum water depths of approximately 38 m below LAT are found on top of the western central part of the Proposed Development array area and maximum depth around 68 m below LAT in the east of the banks. Figure 4.1   Open ▸ illustrates the bathymetry recorded across the Proposed Development array area during the 2019 geophysical survey.
13. The bathymetry of the Proposed Development export cable corridor is relatively variable, from the intertidal zone to 69.8 m below LAT at the time of geophysical investigation. This variance in depth is influenced by the seafloor typography which slopes gently, reaching 60 m depth below LAT approximately 20 km from landfall, before decreasing to circa 40 m below LAT in the area of the Proposed Development export cable corridor which extends to the western margin of Berwick Bank. Water depths in the far east extent of the route extends to 64 m below LAT.
14. The model domain had full bathymetry data coverage and was populated using a combination of data sources. The site-specific geophysical survey undertaken in 2019 and the resulting bathymetry data was used to populate the model. The survey data provided to LAT vertical datum was converted to model mean sea level datum using reference values published by Admiralty. Where additional data was required for the model extent beyond the survey area, bathymetry data was sourced from the EMODnet as illustrated in Figure 4.2   Open ▸ . This database is available under the European Inspire Directive and provides access to data in a variety of formats, datums and resolutions based on a combination of survey datasets. Data was extracted to mean sea level datum at the finest resolution in the vicinity of the physical processes study area with a resolution of at least three times the mesh resolution to ensure that coastal features were represented within the numerical modelling.

Figure 4.1:  Bathymetric Data Fugro Survey 2019 and XOCEAN 2021

Figure 4.2:  Bathymetric Data EMODnet Data Portal

15. The resolution of the model bathymetry was designed to reflect variations in water depth and bed forms for the accurate simulation of tidal currents as shown in Figure 4.3   Open ▸ . Additional model resolution was also included to incorporate the installation of the Proposed Development. This enabled the same cell arrangement to be used for the baseline and post-construction assessment, thereby avoiding the introduction of any numerical mesh effects into the assessment. Across the Proposed Development array area, the resolution varied between circa 50 m down to 5 m in order that the influence of scour protection on the tidal flow and sediment transport for the Proposed Development could be quantified. With increasing distance from the physical processes study area, the cell size was increased but maintained at a level which retained model accuracy. Figure 4.4   Open ▸ illustrates the mesh resolution with an inset of the mesh within the Proposed Development array area.
16. The extent of the domain was designed to provide the basis for a model which could be utilised for tide, wave and sediment transport modelling. The focus of the study is a tidal excursion from the Proposed Development to quantify any changes due to the installation however a larger domain is required to develop wave fields and ensure that tidal currents are simulated. The model extends north, south and offshore beyond the adjacent banks to ensure that any influence on the Firth of Forth Banks Complex is included in the detailed model.

Figure 4.3:  Model Bathymetry

Figure 4.4:  Model Mesh with Section of Proposed Development Array Area Inset

4.2. Hydrography

17. The UKHO states that the mean tidal range at the Standard Port of Leith is approximately 3.6 m whilst the Standard Port at Montrose is 3 m. The two sites have the tidal characteristics shown in Table 4.1   Open ▸ in metres referenced to Chart Datum (CD):

Table 4.1: Tidal Levels at Standard Ports

18. Tidal elevations across the outer Firth of Forth are governed by a southerly directed flood tide which moves along the eastern coastline of Scotland into the Firth of Forth and around Fife Ness (HR Wallingford, 2009). Across the mouth of the Firth, the flood tidal stream has a general east-southeast pattern, whilst the ebb tidal stream runs in a west-north-west direction. The main peak flood tide occurs approximately two hours before high water (HW), with the main peak ebb tide occurring approximately four hours after HW (HR Wallingford, 2009). Tidal processes are often characterised by the natural tidal elevation of an area. The Firth of Forth Zone is characterised by a tidal regime which is semi-diurnal with variable mean spring tidal ranges, based on the metocean data collated within the 2011 survey campaign (HR Wallingford, 2012).
19. The tidal flow simulations which form the basis of the study were undertaken using the MIKE21 FM modelling system. The FM Module is a 2-dimensional, depth averaged hydrodynamic model which simulates the water level variations and flows in response to a variety of forcing functions in lakes, estuaries and coastal areas. The water levels and flows are resolved on a mesh covering the area of interest when provided with bathymetry, bed resistance coefficient, wind field, hydrodynamic boundary conditions, etc.
20. The tidal model was driven using boundary conditions extracted from the DHI global tidal model which is based on the DTU10 ocean model developed by the National Space Institute of Denmark. These boundaries were fully defined ‘flather’ boundaries for which both surface elevation and current vectors are specified. The model was calibrated using the following data sources:

• metocean data collected for Seagreen;
• Admiralty tidal harmonics;
• Admiralty tidal diamonds; and
• data sourced from the British Oceanographic Data Centre (BODC).
  21. A large amount of hydrometric data was available across the model domain, a selection of the principal resources is illustrated in Figure 4.5   Open ▸ . A sample of this calibration data is presented in this report, Figure 4.6   Open ▸ presents the location of the selected data. The site 1407R relates to the tidal diamond “R” from the Admiralty chart number 1407 which is located to the south of the Proposed Development export cable corridor. The labelling of the other site references the locations as reported (Fugro, 2011 and Partrac 2020).
  22. Figure 4.7   Open ▸ shows the comparison of the modelled (blue) and Admiralty tidal levels predicted from harmonic analysis (red). The model correlated well through both spring and neap tidal phases. The comparative study undertaken to quantify the potential changes in tidal currents was undertaken during spring tides to ensure a wide range of tidal conditions were applied in the modelling. The validation data presented is therefore focussed on specific spring tidal periods. Short periods are presented for clarity along with representative data records to demonstrate the range and variability of monitored data.
  23. Figure 4.8   Open ▸ through to Figure 4.18   Open ▸ show the measured data during the Seagreen 1 campaign at a number of locations across the physical processes study area for both spring and neap tidal states. Figure 4.8   Open ▸ to Figure 4.11   Open ▸ show comparison of current speed and direction for spring and neap tides respectively at monitoring point B in the Fugro campaign. In each case three traces are included from the monitored data, these being representative of near bed, mid depth and near surface flow conditions whilst the depth averaged model has a single trace (shown in red). For completeness, further data is presented for a spring-neap cycle at this location in Figure 4.12   Open ▸ . The average of the value recorded through the water column is presented (red trace) along with the range of values from which this has been calculated (envelope formed between the green traces), whist the modelled value is shown by the blue trace. This plot illustrates that whilst there is a wide range of current speeds within the water column the modelled depth average value is representative of the average currents, particularly for spring tidal conditions during which the model was implemented.
  24. Further locations are presented in Figure 4.13   Open ▸ to Figure 4.18   Open ▸ and at each location the modelled data lies within the range of the measured data indicating that tidal currents are well represented particularly in terms of timing which is important in the area which exhibits tidal skew (i.e. where slack water does not correspond with high and/or low tide). It should be noted the measured data will also include the influence of meteorological conditions whilst the numerical tidal model calibration was driven by astronomical forces alone.    
  25. Figure 4.19   Open ▸ to Figure 4.21   Open ▸ relate to Admiralty tidal diamonds for locations within the domain. This data is published in a generalised format (i.e. there are 14 sets of hourly current speed data each referenced to high water Leith for spring and neap tides and a single set of current direction values). These values therefore do not relate to a known time period or specific tidal range. The Admiralty data (shown by points) correlate well with the modelled current directions and current speeds. These datasets are generally used as a cross reference as current speeds are not consistent across all neap or spring tides due to the varying tidal range. Generally, the field data supporting the diamonds was collected using drogues which often measure the higher surface current speeds than those simulated in a depth averaged model.
  26. The calibration data demonstrates that the numerical model simulates the tidal currents in the region. This includes the representation of the skew tide where peak flood and ebb flows are typically one to two hours prior to high and low water respectively. Figure 4.22   Open ▸ illustrates tidal patterns during peak flood on a spring tide whilst Figure 4.23   Open ▸ illustrates the ebb tide. These points in the tidal cycle are used as reference for the assessment of potential impacts and changes to tidal flows due to the Proposed Development. The period selected for the comparative study represents a spring tide on the upper end of the range experienced in the region; this was to ensure the study included the greatest variation in tidal conditions (i.e. water depth and current speed). Residual tidal flows and how they drive sediment transport regimes are examined in section 4.6.

Figure 4.5: Location of Selected Calibration Datasets Available

Figure 4.6: Location of Calibration Data Presented

Figure 4.7: Comparison of Model and Admiralty Harmonic Tide Data for Dunbar

Figure 4.8: Comparison of Model and Recorded Data Fugro Location B – Current Speed Spring

Figure 4.9: Comparison of Model and Recorded Data Fugro Location B – Current Direction Spring

Figure 4.10: Comparison of Model and Recorded Data Fugro Location B – Current Speed Neap

Figure 4.11: Comparison of Model and Recorded Data Fugro Location B – Current Direction Neap

Figure 4.12: Comparison of Model and Recorded Data Fugro Location B – Current Speed Spring - Neap Cycle

Figure 4.13: Comparison of Model and Recorded Data Fugro Location F – Current Speed Spring

Figure 4.14: Comparison of Model and Recorded Data Fugro Location F – Current Speed Neap

Figure 4.15: Comparison of Model and Recorded Data Fugro Location G – Current Speed Spring

Figure 4.16: Comparison of Model and Recorded Data Fugro Location G – Current Direction Spring

Figure 4.17: Comparison of Model and Recorded Data Fugro Location G – Current Speed Neap

Figure 4.18: Comparison of Model and Recorded Data Fugro Location G – Current Direction Neap

Figure 4.19: Comparison of Model and Recorded Data Admiralty Diamond Location 1407H (Spring Left, Neap Right)

Figure 4.20: Comparison of Model and Recorded Data Admiralty Diamond Location 1407L (Spring Left, Neap Right)

Figure 4.21: Comparison of Model and Recorded Data Admiralty Diamond Location 1407R (Spring Left, Neap Right)

Figure 4.22:  Tidal Flow Patterns – Peak Flood (HW-1 Hour)

Figure 4.23:  Tidal Flow Patterns – Peak Ebb (LW-1 Hour)

4.3. Wave Climate

27. Throughout the North Sea, strong winds can occur with wave heights varying greatly due to fetch limitations and water depth effects. Waves in the northern North Sea can be generated either by local winds or from remote wind systems (swell waves). East of the mouth of the River Tay, the dominant wave conditions approach from between 20⁰N and 60⁰N. However, extreme wave conditions (> 4 m) can be experienced from the entire eastern sector (0⁰ to 180⁰) (HR Wallingford, 2012).
28. Metocean surveys conducted across the former Firth of Forth Zone to characterise the zone provide an overview of the wave regime within the physical processes study area. During the stormiest event over the 18-month wave buoy deployment, a significant wave height of 6.7 m was recorded in January 2012, which correlated with a one in one year sea wave climate return period event (Fugro, 2012).
29. As offshore waves transfer from the deep offshore water to shallower coastal areas (e.g. Proposed Development export cable corridor to landfall), a number of important modifications may result due to interactions of offshore deep water waves with the seabed, with the resultant modifications producing shallow water waves. These physical ‘wave transformation’ interactions include:

• shoaling and refraction (due to both depth and current interactions with the wave);
• energy loss due to breaking;
• energy loss due to bottom friction; and
• momentum and mass transport effect.
  30. In addition to the data collected during the Seagreen 1 field study, longer term data was used in order to evaluate the baseline wave climate for more extreme events to enable assessment of the Proposed Development over a wider range of conditions. For this assessment the 22 year ECMWF Operational Wave model database was used. Figure 4.24   Open ▸ shows the wave rose from this dataset for a location at the centre of the Proposed Development array area. As noted from the field data even though the largest proportion of the waves are combined wind and swell from northerly sectors, there is sufficient fetch in the east to enable a wave field to develop.

Figure 4.24:  Wave Rose for Proposed Development Array Area – 22 Year ECMWF

31. In order to evaluate the potential changes in wave climate due to the Proposed Development, a comparative study was carried out. This meant that baseline wave climate was required; due to the comparative nature of the assessment, a full metocean study was not essential however representative sea-states were required.
32. An analysis was undertaken to determine the offshore conditions for which waves reach the site from all directions. Twenty-two years of data were obtained from the ECMWF’s operational dataset for multiple locations on the north, east and southern boundaries of the model domain. The wave roses for these sites are presented in Figure 4.25   Open ▸ overlain on the model domain at the locations to which they relate. Extreme value analysis using peak over threshold was undertaken for the principal sectors to determine the one in one and 1 in 20 year offshore wave climate. These were then used as boundary conditions within the wave modelling to determine the resultant wave climate at the site and across the physical processes study area.
33. In addition to boundary wave data, it was necessary to analyse the wind field to include the contribution of local wind seas. For this, a representative point for each of the key directions, was identified and utilised from the ECMWF ten year dataset, as for the relevant sectors, the variation in wind speed was found to have consistency across the domain. The wind rose for this period is presented in Figure 4.26   Open ▸ . This was analysed on the same sectoral basis as the wave data to give an indication of the return period wind speed.

Figure 4.25:  Wave Roses for Model Boundaries – 22 Year ECMWF Dataset

Figure 4.26:  Wind Rose for Proposed Development Array Area – Ten Year ECMWF

34. The wave modelling was undertaken using the spectral wave model, MIKE21 SW, to provide a full wave climate and wave breaking across the physical processes study area. The model used a quasi stationary formulation which meant that for each event the wave field fully established over a number of numerical iterations until convergence was reached. The waves were computed on the same grid as the tidal flows. The model resolves the wave field by simulating wind generation of waves within the model domain and the propagation of externally generated swell waves through the domain. The model setup ensured that the detail of both locally generated wind waves and swell conditions from further afield were captured.
35. The following set of figures show the wave climate for four one in one year return period events; from approximately a northerly (000°), north-easterly (045°), easterly (090⁰) and south-easterly (135°) direction. These sectors were selected to be representative of the different characteristics of the wave climate and also for sectors for which the Proposed Development may potentially affect marine processes along the coastline. Although the modelling was undertaken throughout the tidal cycle the variation in wave climate was limited therefore the figures presented relate to mean sea level mid flood tide.
36. Figure 4.27   Open ▸ shows the waves approaching from the north and demonstrates, as anticipated, the largest waves approach from this sector. Figure 4.28   Open ▸ and Figure 4.29   Open ▸ show the climates from 045⁰ and 090⁰ respectively and demonstrates that although significant waves may approach from the north-east sector, they are less common, giving rise to lower wave heights for the same return period. Figure 4.30   Open ▸ indicates that there is sufficient fetch to develop a wind sea. The resulting magnitude of the waves transformed from offshore correlate with those recorded during the measurement campaign. Three Waverider Buoy datasets captured during 2020 were used for comparison with data produced from the model, during a key northerly storm event in June. The location of the three buoys SG1, SG2 and SG3 are shown in Figure 4.6   Open ▸ and a number of model validation plots for Significant Wave Height, Peak Wave Period and Mean Wave Direction are presented in Figure 4.35   Open ▸ to Figure 4.39   Open ▸ . As can be seen the modelled storm event correlates well with the recorded data, taking account of the spatial and temporal resolution of atmospheric and wave data input to the model.
37. A second set of figures are presented relating to the 1 in 20 year return period. Figure 4.31   Open ▸ to Figure 4.34   Open ▸ present the data for the same sectors and tidal height as the one in one year return period. They have been introduced to ensure that the baseline for a more arduous conditions is established for assessment of the potential effect of the Proposed Development on wave climate.

Figure 4.27:  Wave Climate 1:1 Year Storm from 000° at Mid-Tide

Figure 4.28:  Wave Climate 1:1 Year Storm from 045° at Mid-Tide

Figure 4.29:  Wave Climate 1:1 Year Storm from 090° at Mid-Tide

Figure 4.30:  Wave Climate 1:1 Year Storm from 135° at Mid-Tide

Figure 4.31:  Wave Climate 1:20 Year Storm from 000° at Mid-Tide

Figure 4.32:  Wave Climate 1:20 Year Storm from 045° at Mid-Tide

Figure 4.33:  Wave Climate 1:20 Year Storm from 090° at Mid-Tide

Figure 4.34:  Wave Climate 1:20 Year Storm from 135° at Mid-Tide

Figure 4.35: Validation of Modelled Significant Wave Height with Measured Data at SG1

Figure 4.36: Validation of Modelled Significant Wave Height with Measured Data at SG2

Figure 4.37: Validation of Modelled Significant Wave Height with Measured Data at SG3

Figure 4.38: Validation of Modelled Peak Wave Period with Measured Data at SG1

Figure 4.39: Validation of Modelled Mean Wave Direction with Measured Data at SG1

4.4. Littoral Currents

38. The MIKE suite facilitates the coupling of models. The depth averaged hydrodynamic model, used for the tidal modelling, coupled with the spectral wave model, provides a full wave climate incorporating the impact of water levels and currents on waves and wave breaking. Using this, the littoral currents (i.e. those currents driven by tidal, wave and meteorological forces) were examined.
39. The one in one year storm from 000° sector was simulated with the inclusion of large spring tides to include a wide range of tidal conditions and the resulting peak flood and peak ebb currents are presented in Figure 4.40   Open ▸ and Figure 4.41   Open ▸ respectively. These correspond with the (calm) tidal plots presented in Figure 4.22   Open ▸ and Figure 4.23   Open ▸ . As expected, the presence of the south going waves increase the currents on the flood tide whilst reducing them on the ebb.

Figure 4.40:  Littoral Current 1:1 Year Storm from 000° - Flood Tide (HW -1 Hour)

Figure 4.41:  Littoral Current 1:1 Year Storm from 000° - Ebb Tide (LW – 1 Hour)