Appendix 5 Physical Processes – Baseline Environment

Appendix 5
Physical Processes – Baseline Environment

5.1.        Desktop Study

  1. A number of desktop datasets have been identified in order to characterise the baseline environment for physical processes and to support this Scoping Report ( Apx Table 5.1   Open ▸ ).


Apx Table 5.1:
 Summary of Key Desktop Datasets and Reports

Apx Table 5.1:  Summary of Key Desktop Datasets and Reports


5.2.        Site-specific Survey Data

  1. A brief overview of existing and planned site-specific data sources with relevance to physical processes is presented below.
  2. Between March and July 2022, site-specific geophysical surveys were conducted across the site boundary. The results of these surveys provide geophysical and bathymetric data, which will be used to characterise the baseline environment within the site boundary for the relevant physical processes section of the Array EIA Report. A summary of the data collected during these surveys and how they will be used is as follows:
  • bathymetric data was collected using multibeam echo sounder (MBES) in order to determine topography, gradients;
  • high resolution side scan sonar (SSS) data was collected to determine seabed features, such as sediment composition and the presence of boulders and debris;
  • high resolution sub-bottom profiler (SBP) data was collected to determine the subsurface sediment conditions and composition, such as boulders and shallow geology features, that may influence foundation design;
  • multichannel two-dimensional (2D) ultra-high resolution seismic (UHRS) data was collected to determine the deeper sub-surface soil conditions that may influence foundation depth; and
  • dual magnetometer data across the site to support unexploded ordnance (UXO) interpretation.
  1. In addition, Metocean and Light Detection and Ranging (LiDAR) buoys were deployed within the site boundary in August 2022. They will remain there for 12 months, allowing one year of physical processes data to be collected. There were two floating LiDAR buoys, three metocean buoys and three mounted seabed Acoustic Doppler Current Profilers (ADCPs) deployed at three sites across the site boundary (KIS-ORCA, 2022).


5.3.        Baseline Characterisation

  1. An overview of the baseline environment for physical process is presented below. It has been informed by a review of relevant desktop data and the results of the site-specific geophysical surveys (Appendix 7).

5.3.1      Bathymetry

  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 lowest astronomical tide (LAT). The seafloor consists of gentle slopes and generally deepens towards the east Apx Figure 5.1   Open ▸ ). These gentle seafloor gradients range from 0o to 5o, 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.

5.3.2      Wind and Waves

  1. Strong winds can occur throughout the North Sea, with a large variation in wave height due to fetch limitations and water depth effects. In the absence of site-specific data at the time of writing, metocean data collected in the vicinity of the site boundary has been used to characterise the baseline for the wave regime and for tidal currents and elevation (section below). Eight metocean buoys were deployed across the Round 3 Firth of Forth Zone for 18 months, with the nearest buoys approximately 51 km and 57 km away from the site boundary, with others further inshore towards the Firth of Forth. The with the highest wave height of 6.7 m recorded in January 2012 (Fugro, 2012; Royal Haskoning DHV, 2012). Similarly, as the Round 3 Firth of Forth Zone is within the North Sea, it can experience long wave periods which are associated with storms in the Norwegian North Sea. Peak spectral wave periods of up to 20 s were recorded (Fugro, 2012; Royal Haskoning DHV, 2012).
  2. Scotland’s Marine Atlas and the Marine Scotland NMPi maps provided an overview of the wave regime within the site boundary:
  • annual mean wave power ranges from 14 kW/m to 19 kW/m; and
  • annual mean significant wave height (i.e. the average height of the highest third of waves) ranges from 1.81 m to 2.10 m (Marine Scotland, 2011 and 2022).

5.3.3      Tidal Currents and Elevation

  1. Tidal currents provide insight into the patterns and rates of naturally occurring sediment transport and are mainly driven by the tides themselves. 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 3.0 m (Marine Scotland, 2022).
  2. Similarly, 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 Seagreen Alpha and Bravo) Offshore Wind Farms (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).

Apx Figure 5.1:
Bathymetry Across the Site Boundary

Apx Figure 5.1   Open ▸ : Bathymetry Across the Site Boundary


5.3.4      Geology

  1. Baseline information on the geology within the site boundary provides an understanding of the origin and stability of the seabed and the geology which will be encountered when installing the offshore structures, such as platform foundations.
  2. 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 UHRS and SBP data integrated with bathymetric, backscatter and SSS data, which was collected during the 2022 site-specific surveys. A total of five geological units were separated, with a total of five interpreted horizons, aided interpretation through the delineation of localised geological features.
  3. The Quaternary sequence within the site boundary represents the repeated phases of erosions and deposition that are associate with the Pleistocene glaciations (Gatliff et al., 1994). The glacial and glaciomarine sediments from this period are highly eroded and then re-deposited, as seen in the large channel systems which are open, infilled and buried. These channels can reach depths of 200 m and widths of up to 3 km with very steep slopes up to 27o (Bone et al., 1991). The sedimentary infills of these channels can differ in density from the surrounding sediments, with organic rich layers with very high and variably coefficients of compressibility present in some channels. Boulders, and other coarse material, can occur at the bases of channels. Within the central North Sea, pockmarks in areas of soft clay indicate gas escape (Bone et al., 1991). Furthermore, trapped gas is widespread throughout the central North Sea, and is frequently trapped where lithological changes occur within the Quaternary or Tertiary, such as levels of glacial erosion (Bone et al., 1991).
  4. 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 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).
  5. The geological morphology within the site boundary is very varied and includes the following features:
  • megaripples;
  • sand waves;
  • boulders (primarily in the north-west of the site boundary);
  • recent marine soft sediment deposits; and
  • deep channel structures (down to 60 m) with sedimentary infill (south-eastern corner).

5.3.5      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. 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%.
  2. The recent geophysical surveys have identified that the sediment within the site boundary consists primarily of sand, with some areas of gravel and occasional diamicton (poorly mixed coarse sediments). The gravel areas are more frequent in the north-west, with occasional diamicton also observed in this area ( Apx Figure 5.2   Open ▸ ). The seabed 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.
  3. 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. The boulder fields within the areas of diamicton are denser than in other seabed substrates. Manmade seafloor features are present across the site boundary, with linear debris (such as wire and rope) observed the most frequently, and occasional linear scars or trawl marks (due to human activity, such as trawling) are also present ( Apx Figure 5.3   Open ▸ ).
  4. The substrate within the site boundary is not classified as a sensitive receptor, due to the presence of large homogenous areas of sand, and the absence of rapid variation between substrates and pockets of sand. 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. Further information on designated sites is presented in paragraph 185.

5.3.6      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 (formerly known as Seagreen Alpha and Bravo) Offshore Wind Farm in March and June 2011, albeit 50.72 km away from the site boundary, and situated within shallower water (39.77 km to 64.82 m). Nonetheless, these samples suggested total suspended sediments (TSS) to be low (<5 mg/l) with a maximum value of 10 mg/l recorded in March 2011 (Fugro, 2012). Although all values were low, a slight increase in TSS was observed in March.
  3. Tidal currents are the principal mechanisms which influence suspended sediment concentrations (SSC) (Moskalski and Torres, 2012), 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. It should be noted that SSCs can also be elevated temporarily by wave-driven currents during storms, in which SSC levels can rise significantly. Following storms, SSC levels will gradually decrease to baseline conditions, regulated by the ambient regional tidal regimes. Thus, SSC levels demonstrate a broadly seasonal pattern due to the seasonal nature and frequency of storms. 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, the tide within the site boundary 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 (section 5.3.5), 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.

Apx Figure 5.2:
Sediment Classification Across the Site Boundary

Apx Figure 5.2: Sediment Classification Across the Site Boundary

Apx Figure 5.3:
Superficial Geology and Seabed Features Across the Site Boundary

Apx Figure 5.3: Superficial Geology and Seabed Features Across the Site Boundary


5.3.7      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 25.06 km from the site boundary, towards the Firth of Forth. 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 also designated for the following physical processes receptors:
  • offshore subtidal sands and gravels;
  • shelf banks and mounds; and
  • moraines representative of the Wee Bankie Key Geodiversity Area (JNCC, 2021c).
  1. There are other sites designated for physical processes receptors elsewhere in the North Sea and the Firth of Forth, the next closest being the Swallow Sands Marine Conservation Zone (MCZ). This MCZ is designated for subtidal coarse sediment, subtidal sand, and Swallow Hole (a North Sea glacial tunnel valley) (JNCC, 2021d). However, it is located a minimum of 58.96 km from site boundary. Therefore, there will be no impacts to this MCZ, the Firth of Forth Banks Complex MPA, or any other site designated for physical processes receptors due to the Array.