1.7. Physical Processes

  1. Physical processes refer to the marine processes and the relationship with the physical environment including tidal currents, wave climate, water column characteristics, suspended sediments and sediment transport. The physical processes of the Array were assessed using datasets collected from a series of site-specific surveys, along with a detailed desktop review of existing studies and datasets.
  2. The geophysical survey carried out by Ocean Infinity in 2022 (volume 3, appendix 8.1, annex A) indicated that water depths range between 59 m and 154 m relative to Lowest Astronomical Tide (LAT) over the Array, with an average depth of 74.5 m. Seafloor gradients are gentle within the site boundary, with some localised steeper ripple areas. Megaripples and sand waves across the site boundary are indicative of mobile sediments which are comprised mainly of sand, with some poorly mixed sediments and gravel deposits. Offshore marine bedrock data provided by the British Geological Survey illustrates that the physical processes study area is dominated by chalk, mudstone, sandstone and lignite (Marine Directorate, 2017).
  3. The site-specific metocean survey undertaken (2023) recorded maximum significant wave heights up to 8.96 m and high energy periods of up to 20 s within the site boundary during November 2022, with dominant wave directions from the north and north-north-east. The dominant wind direction within the physical processes study area is from the south-west, with wind speeds likely up to 31.5 m/s. Currents typically flow in a south-south-westerly direction near the seabed and a southerly direction near the surface, with a mean spring tidal range of 2.34 m to 2.41 m. Mean current speeds up to 0.27 m/s were recorded during the site-specific metocean survey, with a maximum speed of 0.91 m/s. The average Suspended Particulate Matter (SPM) levels have to be between 0 mg/l and 1 mg/l across the physical processes study area (Cefas, 2016), with evidence of low sediment transport rates due to low current speeds in the physical processes study area.
  4. Salinity differences between the surface and seabed within the physical processes study area was found to be limited to 0.1 PSU throughout the year, with the greatest differences evident in the month of August (Berx and Hughes, 2009). The physical processes study area was considered to be subject to weak stratification (formation of distinct and stable water layers by density), with evidence of relatively thorough mixing, even in the summer months. Maximum nearbed temperatures recorded in the month of October, when the water column layers are considered to be fully mixed, following a warming period of the surface waters over the summer months (volume 3, appendix 7.1, annex A). Further data extracted from the Scottish Shelf Waters Reanalysis Service (Barton et al., 2022) confirmed that the effects of wind on seasonal stratification are very low within the physical processes study area.
  5. Three potential impacts of physical processes on receptors due to the construction, operation and maintenance, and decommissioning phases of the Array, were identified. These were noted as:
  • increased suspended sediment concentrations (SSCs) and associated deposition and sediment transport due to operation and maintenance activities;
  • impacts to the wind field due to the presence of infrastructure; and
  • impacts to seasonal stratification due to the presence of infrastructure.
  1. With the proposed designed in measures in place, each of these impacts result in effects of negligible or minor adverse significance, which is not significant in EIA terms.
  2. Cumulative impacts arising from the Array together with other projects and plans were assessed and predicted to result in effects of negligible or minor adverse significance (not significant in EIA terms) on the physical processes receptors. No physical processes mitigation is considered necessary because the predicted impacts in the absence of mitigation is not significant in EIA terms.
  3. No likely significant transboundary effects with regard to physical processes from the Array on the interests of EEA states were predicted.