7.7. Baseline Environment

7.7.1. Overview of Baseline Environment

  1. The following sections provide a summary of the physical processes baseline environment. The physical processes technical report, volume 3, appendix 7.1, includes full details of the analysis undertaken to develop the physical processes baseline and information on hydrodynamics, wind and waves, as well as seabed morphology and suspended sediments.

                        Bathymetry

  1. Site-specific geophysical data collected by Ocean Infinity in 2022 (volume 3, appendix 8.1, annex A) were supplemented with Digital Terrain Model (DTM) data available from MEDIN (MEDIN, 2023) to describe the bathymetry within the physical processes study area, where water depths range between 59 m and 154 m relative to Lowest Astronomical Tide (LAT). An average depth within the physical processes study area was determined as circa 74.5 m, with the shallowest depths to the north-west and deepest to the south of the site boundary.
  2. The bathymetry of the physical processes study area consists of gentle slopes and generally deepens towards the east. 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 (volume 3, appendix 8.1, annex A).

                        Wind and waves

  1. The baseline wind conditions within the physical processes study area were assessed by examining a long term modelled datapoint extracted from the CFSR dataset by the NCEP, part of NOAA. This central point (56°41.6911’N, 0°25.3224’W) located within the physical processes study area demonstrates that the dominant wind direction is from the south-west, with mean hourly wind speeds, 10 m above the sea surface, of up to 31.5 m/s during the 1979 to 2023 period. Further statistical analysis has shown that a 1 in 10 year return period wind speed from the directional sector 225° to 255° is estimated to be 28.97 m/s, increasing to 34.39 m/s for a 1 in 200 year wind speed from that sector. These data were corroborated by site specific measurements undertaken by a Floating Light Detection and Ranging (FLiDAR) campaign, alongside the metocean measurements by Partrac (volume 3, appendix 7.1, annex A). The FLiDAR data provided a range of horizontal wind speeds at two locations for a series of heights above mean sea level (MSL). The twelve month dataset, captured from August 2022 to August 2023, also showed a dominance from the south-west and southerly sectors, with wind speeds commenserate with the NOAA data for a 1 in 1 year return period. Furthermore, the Partrac data have shown a circa 18% reduction in horizontal wind speed from a height of 150 m above MSL to 12 m above MSL at both survey locations. The data available at 12 m above sea level were the closest available measured data to the water surface, however, there will be a further reduction in horizontal wind speeds between 12 m above MSL and the water surface.
  2. Waves in the northern North Sea, where the Array will be located, can be generated either by local winds or from remote wind systems (swell waves) (Royal HaskoningDHV, 2012b). To inform the baseline wave regime within the physical processes study area, data from three metocean buoys within the site boundary were analysed. The buoys were deployed over a 12 month period commencing August 2022, recording maximum significant wave heights (Hs) up to 8.96 m and peak wave periods (Tp) up to 20 s towards the south of the site boundary in November 2022. Dominant wave directions were shown to be from the north and north-north-east. ECMWF wave model data also showed a dominant northerly sector within the physical processes study area, with northerly wave heights up to 8.2 m and peak wave peiords up to 24 s modelled during the 2000 to 2021 period.

                        Tidal currents and elevation

  1. To inform the baseline tidal regime within the physical processes study area, data from three site-specific metocean buoys were analysed, which were deployed within the site boundary from August 2022 for a period of 12 months. Maximum current speeds were recorded in July at each location, with maximum depth-averaged current speeds during the autumn or winter months when the water column was well mixed. Currents were dominated by semidiurnal tidal flow and surface currents were strongly rectilinear.
  2. The mean spring tidal range varied from 2.41 m in the north of the site boundary to 2.34 m in the south, with currents typically flowing in a south-south-westerly direction near the seabed and a southerly direction near the surface. Mean current speeds of 0.21 m/s and 0.27 m/s were captured near the seabed and surface respectively at Site 1 in the north of the site boundary, with smaller mean differences at Site 2 (central) and Site 3 (in the south) between the surface and seabed values. The maximum recorded current speed occurred at Site 1 in July 2023, where a value of 0.91 m/s was reached near the surface, whilst the maximum depth-averaged speed of 0.68 m/s occurred in October 2022 at Site 1. Maximum depth averaged current speeds of 0.66 m/s and 0.62 m/s occurred in January 2023 at Sites 2 and 3, respectively.
  3. The Atlas of UK Marine Renewable Energy Resources showed mean spring current speeds (in the absence of any meteorological influences) of up to 0.55 m/s in the north of the physical processes study area and 0.4 m/s in the south (ABPmer, 2008). Tidal levels at the standard ports of Leith and Montrose show a MHWS ranging from 4.9 m to 5.6 m, whilst a Mean Low Water Springs (MLWS) is 0.8 m at both ports (UKHO, 2023).

                        Water column processes

  1. North Sea salinity and temperature data are available from the ‘Climatology of Surface and Near-bed Temperature and Salinity on the North-West European Continental Shelf for 1971-2000’ (Berx and Hughes, 2009); these data include nearbed and surface salinity and temperature monthly mean values within the physical processes study area. The datapoints available within the physical processes study area shows that the largest surface and seabed salinity and temperature differences occur in August, with May and June also showing some of the larger differences. For example, at a point towards the centre of physical processes study area (56° 39.0000’ N, 0° 25.0020’ W), salinity differences between the surface and the seabed are 0.085 Practical Salinity Units (PSU) for August, whilst in April the difference is 0.016 PSU. Salinity values at the seabed are reported to be less than 0.1 PSU different from the salinity values at the surface for all months and across the physical processes study area, therefore the physical processes study area can be considered as being subject to weak seasonal stratification, with evidence of relatively thorough mixing, even in the summer months.
  2. The site-specific metocean survey campaign by Partrac between 2022 and 2023 confirmed patterns expected with seasonal stratification within the site boundary. Through the summer months, temperatures slowly increase, however most of the heat is retained in the upper stratified layer. Current shear was observed within the site boundary with strongest currents in the upper mixed layer during the summer months. Maximum nearbed temperatures were recorded in October, when the surface waters that were subject to increased temperatures in the summer months have become fully mixed with the deeper layers. This mixing occurs relatively quickly, with the seabed temperatures then cooling slowly until March, when temperatures reach a minimum.
  3. The month of August was identified as the most critical for seasonal stratification within the Berx and Hughes (2009) Climatology dataset. Further data have been extracted from the SSW-RS (Barton et al., 2022) which provides a 27 year reanalysis dataset (1993 to 2019) of the Scottish Shelf Model (De Dominicis et al., 2018). Conditions within the physical processes study area were reviewed under both calm and storm conditions during August, within a sample year of 2016.
  4. During both the sample storm and calm conditions, the salinity data shows that the stratified layer is within the top 30 m of the water column, or circa 35% of the depth, implying that the effects of wind on the weak stratification in this area are very low. Surface layers for both calm and storm conditions reflect very similar salinity patterns, underpinning the limited effect of the wind on the water column mixing processes in the physical processes study area. With regard to temperature, both storm and calm conditions portray a similar pattern to the salinity data. The differences between surface and seabed salinity within the physical processes study area are less than 0.1 PSU, therefore any stratification even under the most extreme conditions is classified as weak.

                        Geology

  1. The physical processes study area 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 geophysical data collected in 2022 during site-specific surveys by Ocean Infinity (volume 3, appendix 8.1, annex A). A total of five geological units were identified, with a total of five interpreted horizons, aided in interpretation through the delineation of localised geological features (volume 3, appendix 8.1, annex A).
  2. Offshore marine bedrock data (scale 1:250,000) provided by the BGS illustrate that the physical processes study area is dominated by chalk and palaeocene rocks (mudstone, sandstone and lignite) (Marine Directorate, 2017).
  3. The 2022 Ocean Infinity surveys (volume 3, appendix 8.1, annex A) confirmed the geological morphology within the site boundary is varied and includes the following features:
  • megaripples;
  • sand waves;
  • boulders (primarily in the north-west);
  • recent marine soft sediment deposits; and
  • deep channel structures (down to 60 m) with sedimentary infill (south-eastern corner).

                        Seabed substrate

  1. Particle Size Analysis (PSA) conducted for the site-specific benthic studies showed that sediment composition had limited variation across the site boundary (volume 3, appendix 8.1, annex A). Sand comprised the dominant sediment fraction with mean content of 86.4%, while mud content was low overall with a mean content of 9.1% (comprising 8.0% silt and 1.1% clay). The gravel content was the lowest with a mean, but variable, content of 4.5% (volume 3, appendix 8.1, annex A).
  2. 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). 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. Furthermore, the furrows are the most recent mobile sediment feature as they were observed to cut into the megaripples and sand waves (volume 3, appendix 8.1, annex A).
  3. Occasional boulder fields (five 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 (volume 3, appendix 8.1, annex A).

                        Suspended sediment and sediment transport

  1. The Centre for Environment Fisheries and Aquaculture Science (Cefas) Climatology Report presents the spatial distribution of average non-algal SPM for the majority of the United Kingdom Continental Shelf (UKCS) (Cefas, 2016). These data estimate that the average SPM associated with the physical processes study area was between 0 mg/l and 1 mg/l between 1998 and 2015, with higher levels during the winter months (up to 3 mg/l in January and December) than the remainder of the year (Cefas, 2016). Baseline SPM conditions within the physical processes study area can be described as very low in the context of the UKCS, where plumes associated with large rivers which discharge into, for example, the Thames Estuary or the Bristol Channel show mean values of SPM above 30 mg/l (Cefas, 2016).
  2. Site-specific surveys conducted for the Seagreen 1 Offshore Wind Farm in March and June 2011 recorded low Total Suspended Solids (TSS) across four sampling stations, with TSS levels of <5 mg/l in most samples and a maximum value of 18 mg/l (Royal HaskoningDHV, 2012b). The survey area is noted to be situated in shallower water than the physical processes study area.
  3. Wave-driven currents during seasonal storms can temporarily elevate SSCs and can cause levels to rise significantly, which then gradually decrease to baseline conditions following storm events. These effects are less significant in deeper waters; therefore, it can be inferred that the TSS will be lower within the physical processes study area than at the Seagreen 1 Offshore Wind Farm and therefore likely below a maximum value of 10 mg/l during a winter storm.
  4. Low sediment transport rates due to low residual current speeds were reported within the Berwick Bank Offshore Wind Farm array area (SSER, 2022). Modelled residual currents were minimal, in the order of 0.008 m/s in a south-south-west direction of approximately 190°, with net sediment transport limited to below 0.003 m3/d/m during a small proportion of the tidal cycle (SSER, 2022). It is anticipated that low rates of sediment transport would exist across the physical processes study area, to the similar tidal regime and wave climate.

7.7.2. Designated Sites

  1. A screening of designated sites in the vicinity of the Array has been carried out and has identified that there were no designated sites relevant to physical processes.
  2. The closest site designated with physical processes qualifying interest features is the Firth of Forth Banks Complex MPA, which is located a minimum of 20 km to the west of the physical processes study area. Relevant to physical processes, this site is designated for offshore subtidal sands, shelf banks and mounds and moraines representative of the Wee Bankie Key Geodiversity Area. However, as outlined in the Array EIA Scoping Report (Ossian OWFL, 2023), the distance of this MPA from the physical processes study area allows it to be screened out from the assessment, as there is no potential for impacts due to the construction, operation and maintenance or decommissioning of the Array to reach beyond the physical processes study area.

7.7.3. Future Baseline Scenario

  1. 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 Array EIA Report.
  2. If the Array does not come forward, the ‘without development’ future baseline conditions are described within this section.
  3. 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 Array due to natural variability. Future baseline conditions would be altered by climate change resulting in sea level rise and potential increased storminess (Met Office, 2018) (refer to volume 3, appendix 17.2 for further detail). This is unlikely to have the effect of significantly altering tidal patterns and sediment transport regimes offshore within the physical processes study 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 may allow larger waves to develop. There is, however, a notable degree of uncertainty regarding how future climate change will impact prevailing wave climates in the North Sea and beyond. Seasonal stratification may also increase in magnitude and be prevalent through more months of the year, due to a rise in ocean temperatures. This may result in increased impacts to tidal fronts, should infrastructure be developed above or below the sea surface.

7.7.4. Data Limitations and Assumptions

  1. Following stakeholder consultation, a wide range of reports and datasets have been collated for the purpose of establishing the baseline environment within the physical processes study area. All sources are listed under section 7.6.1 and volume 3, appendix 7.1. Although some physical processes are complex and inter-related, there are a considerable amount of data available. There are limitations associated with any modelled datasets analysed in the interpretation of the baseline, for example tidal, wind, wave, salinity, temperature and suspended sediment data, however as far as practicable, the most current and reliable information has been assessed and underpinned by comparison with measured data where available. Limitations in modelled datasets may include uncertainties or inaccuracies within input data and assumptions and approximations within the modelling in representing physical reality. Any uncertainties within statistical methods used, for example extreme value analysis have been included as confidence limits within volume 3, appendix 7.1. Data limitations and tolerances for site-specific survey campaigns within the site boundary are discussed within the relevant reports (volume 3, appendix 8.1, annex A; volume 3, appendix 7.1, annex A).
  2. Due to the quantity, coverage and quality of available data covering the physical processes study area, it is considered that the data employed are sufficient for the purposes of the assessment of effects presented. Any limitations within the datasets and reports are not considered to have any implications for the conclusions of the assessment.