4.5. Sedimentology

4.5.1.    Geology

40. Information on the geology of the Proposed Development allows for an understanding of the origin and stability of the seabed, and the geology which will be encountered during the installation of wind turbines, offshore platform foundations, inter-array cables and offshore export cables.
41. The Proposed Development array area is part of a dynamic landscape where quaternary and pre-quaternary formations have been shaped as erosional surfaces by different geomorphic factors and continue to be shaped and modelled by the present day offshore marine conditions (Fugro, 2020a). The morphology features present are due to advances and rapid retreats consistent with an oscillating and dynamic ice margin during British Ice Sheet (BIS) deglaciation (Graham et al., 2009).
42. Subsequent sea level rise without new sediments led to the deepening and eroding of the sea mounds and banks present in the area. Seabed bottom currents have been actively mobilising and redistributing surficial sediments, developing bedforms and filling up both depressions and channels.
43. The seafloor morphology within the Proposed Development array area is very varied and can be classified into four types of morphological features:

• large scale banks (the Marr Bank and the Berwick Bank);
• arcuate ridges;
• incised valleys, relic glacial lakes and channels; and
• bedforms.
  44. The seabed within the Proposed Development export cable corridor is variable with morphological features which are framed by relic pre-Holocenic landscape, and secondary morphological features characterised by bedforms and boulder fields formed by reworked and redeposition of available material in present-day shallow marine conditions.
  45. Geophysical surveys observed that the bedforms in the Proposed Development export cable corridor are comprised of principally flow-transverse structures (subaqueous dunes: ripples, megaripples); locally the bedforms can be linear, braided and lobe-shaped (bars and ribbons). The seabed within the Proposed Development export cable corridor can be classified into several types of morphological features, which include:
• primary morphological features:

–           outcrops and erosional surfaces and platforms;

–           ridges; and

–           high topographic mounds and incised valleys and channels.

• secondary morphological features:

–           subaqueous dunes;

–           irregularity of the seafloor;

–           features related to anthropogenic activity; and

–           boulder fields.

4.5.2.    Seabed Substrate

46. An overview of surficial sediment geology and the seabed features is presented in this section, based on interpretation undertaken of the side-scan sonar (SSS) data collected during the recent geophysical surveys. An understanding of seabed substrate types is required to assess the potential impacts which may arise due to the installation of wind turbines, offshore platform foundations, inter-array cables and offshore export cables. Figure 4.42   Open ▸ illustrates the seabed substrates present across the Proposed Development. The recent geophysical survey of the Proposed Development array area and Proposed Development export cable corridor identified that it is comprised of several distinctive features:

• boulders and boulder fields;
• areas of ripples;
• areas of megaripples and sand waves; and
• areas of trawl marks.
  47. The majority of the Proposed Development array area seabed is ‘featureless’ however the southern and north-western extent of the Proposed Development array area are dominated by megaripples, sand waves, ribbons and bars. Boulders are also prevalent across the area and are either represented as isolated boulders or as clusters. Seabed sediments present in the Proposed Development array area can be classified into several groups:
• coarse gravel, shelly gravelly sand with boulders;
• mixed sediment;
• mixed sediments with patchy coarse material or boulders; and
• muddy sand.
  48. The seabed within the Proposed Development export cable corridor was recorded as smooth with very few observed primary morphological features (such as high reliefs or ridges), while secondary morphological features such as ripples and megaripples, sand bars and ribbons characterise the seabed morphology. Seabed sediments present in the Proposed Development export cable corridor can be classified into several groups:
• hard substrate: coarse sediment with cobbles, boulders and rock outcropping or sub outcropping characterised by high reflectivity signature in the side-scan data;
• gravelly sand and coarse sediments with medium reflectivity; and
• sandy sediments including fine sand and muddy sand with low reflectivity.
  49. The Skateraw landfall is sited on the East Lothian coast and is characterised by a foreshore which is mostly made up of rock with areas of sand deposits, where the top of the beach is lined with a mixture of sand, pebbles, and small boulders. The seabed morphology comprises a rocky undulating Carboniferous platform with patches of megarippled sands where sediment has accumulated within larger channels. Extending offshore, the undulating rocky seabed becomes flatter, with areas of sediment with boulders interpreted as sediment overlaying rock, (XOCEAN, 2021).
  50. To inform the modelling study seabed sediment information was required beyond the extent of the survey data and the EMODnet Geology database was utilised. The seabed classification shown in Figure 4.43   Open ▸ also corresponds with the interpretation of the most recent study, particularly with respect to the location of the nearshore rocky outcrops, sandy regions of the Proposed Development export cable corridor and the mixed sediments within the Proposed Development array area.

Figure 4.42: Seabed Classification Fugro 2019 Survey and XOCEAN 2021

Figure 4.43: EMODnet Geology Seabed Substrate

4.6. Sediment Transport

51. The MIKE21 Sediment Transport module enables assessment of bed sediment transport rates and initial rates of bed level change for non-cohesive sediment resulting from currents or combined wave current flows. It was used to determine the sediment transport pattern within the model domain. The model combines inputs from both the hydrodynamic model and, if required, the wave propagation model. It used sediment characteristics provided by the recent survey and EMODnet data as presented in the previous section to determine the sediment transport characteristics. It is noted that for a detailed sediment transport study greater detail of sediment characteristics across the model domain and along the coastline would be required. In the context of a comparative study to identify the impact of the Proposed Development infrastructure on sediment transport patterns the sediment characteristics identified within the survey and sampling were interpolated to those areas in the EMODnet data with similar sediment classifications.  
52. The model domain was set up with a layer of mobile bed material as described by the survey data. In areas where sediment is present an initial layer depth was set to 3 m and tapered to zero in the areas of rocky outcrops to ensure that sediment was not exhausted during the simulated events. Two sediment transport scenarios were examined, one relating to calm conditions and a second relating to the one in one year return period event from 000°. In each case the evaluations were undertaken over the course of two spring tides. These simulations included a period for the hydrodynamics and wave fields to stabilise and develop across the domain prior to sediment transport being enabled (i.e. a “warm-up” period).
53. For each scenario three aspects were examined:

• residual current, which is the net flow over the course of the tidal cycle. This is effectively the driving force of the sediment transport;
• potential sediment transport over this period; and
• rate of change of the bed during flood and ebb tides. This provides information for a ‘snap-shot’ in time to enable the process to be illustrated.
  54. For the tidal current alone, the residual current is presented in Figure 4.44   Open ▸ . It is characterised by low residual current speeds within the Proposed Development area which are borne out by the low transport rates shown in Figure 4.45   Open ▸ . The transport plot includes a log scale palette as the values within the offshore banks are several orders of magnitude smaller than those along the coastline. The mechanism is more clearly illustrated in Figure 4.46   Open ▸ and Figure 4.47   Open ▸ , where the bed levels are reduced at certain locations during the flood tide and increase to the same degree on the return tide. This corresponds with the sand ripples that are evident in the area. A log scale was also applied in these figures to encompass the range of values at the coast and across the offshore banks and at peak currents these changes are of the order of a fraction of a millimetre per day. This indicates that although the bed is mobile the area is stable.
  55. When a storm approaches from the north, the flood tide currents are enhanced by the wave climate. This is reflected in an increase in the residual currents and thus an increase in the sediment transport capacity on the flood tide as illustrated in Figure 4.48   Open ▸ and Figure 4.49   Open ▸ . Similarly, the figures showing the changes in bed level peak tide indicate rates of change for both flood and ebb tide, Figure 4.50   Open ▸ and Figure 4.51   Open ▸ . It is however noticeable that the increases and decreases in bed levels remain largely in opposition during the storm event indicating that tidal factors remain dominant under these circumstances.

Figure 4.44: Residual Current Spring Tide

Figure 4.45: Potential Sediment Transport over the Course of One Day (Two Tide Cycles)

Figure 4.46: Rate of Bed Level Change – Peak Flood Tide

Figure 4.47: Rate of Bed Level Change – Peak Ebb Tide

Figure 4.48: Residual Current Spring Tide with 1:1 Year Storm from 000⁰

Figure 4.49: Potential Sediment Transport over the Course of One Day with 1:1 Year Storm from 000⁰

Figure 4.50: Rate of Bed Level Change – Peak Flood Tide with 1:1 Year Storm from 000⁰

Figure 4.51: Rate of Bed Level Change – Peak Ebb Tide with 1:1 Year Storm from 000⁰

4.7. Suspended Sediments

56. The principal mechanisms governing SSC in the water column are tidal currents, with fluctuations observed across the spring-neap cycle and across the different tidal stages (high water, peak ebb, low water, peak flood) observed throughout both datasets. It is key to note that SSCs can also be temporarily elevated by wave-driven currents during storm events. During high-energy storm events, levels of SSC can rise considerably, both near bed and extending into the water column. Following storm events, SSC levels will gradually decrease to baseline conditions, regulated by the ambient regional tidal regimes. The seasonal nature and frequency of storm events supports a broadly seasonal pattern for SSC levels.
57. Sampling was conducted at an offshore station for Seagreen 1 in March and June 2011, suggesting Total Suspended Solids (TSS) to be low. The samples collected illustrated a TSS of < 5 mg/l with a maximum reading of 10 mg/l during March 2011 (Fugro, 2012). Although all values are low, a slight increase in TSS was observed in March. Further monitoring at a nearshore location south of the proposed landfall (shown in Figure 4.6   Open ▸ ) captured how SSC can increase substantially due to meteorological conditions. Figure 4.52   Open ▸ shows the SSCs are generally < 5 mg/l (blue trace) however during storm conditions these may be increased to over 100 mg/l when wave heights are increased (black trace). Counter to this, in deeper water such as Fugro site A ( Figure 4.6   Open ▸ ), turbidity and hence SSC (which are of the same order of magnitude) are not influenced by the wave climate to the same degree. Figure 4.53   Open ▸ shows the turbidity levels (red trace) and corresponding significant wave height (black trace). Turbidity levels remain low even during periods of increased wave activity.
58. For more generalised conditions the Cefas Climatology Report 2016 (Cefas, 2016) and associated dataset provides the spatial distribution of average non-algal Suspended Particulate Matter (SPM) for the majority of the United Kingdom Continental Shelf (UKCS). Between 1998 and 2005, the greatest plumes are associated with large rivers such as those that discharge into the Thames Estuary, The Wash and Liverpool Bay, which show mean values of SPM above 30 mg/l. Based on the data provided within this study, the SPM associated with the Proposed Development has been estimated as approximately 0 mg/l to 1 mg/l over the 1998 to 2005 period, Figure 4.54   Open ▸ . Higher levels of SPM are experienced more commonly in the winter months; however, due to the tidal influence, even during summer months the levels remain elevated.

Figure 4.52: Measured Suspended Sediment Concentrations at Fugro Site F

Figure 4.53: Measured Turbidity at Fugro Site A

Figure 4.54: Distribution of Average Non-algal Suspended Particulate Matter - CEFAS

5. Potential Environmental Changes

5.1. Overview

59. The potential changes to the baseline hydrographic conditions as a result of the installation and presence of the Proposed Development are quantified in the following sections. These changes relate to the presence of the infrastructure within the water column and seabed and are therefore associated with wind turbine legs along with cable and scour protection. The potential changes to sea state and sediment transport regimes were established by repeating the modelling undertaken in the previous section with the inclusion of the Proposed Development. The modelling was undertaken using an indicative layout which included the following changes in line with the maximum design scenario for physical processes:

• leg structures 5 m in diameter relating to 179 wind turbine structures each comprising four legs;
• scour protection 80 m diameter and 2 m in height associated with 20 m caisson for each wind turbine leg;
• leg structures 4 m in diameter relating to eight HVAC OSPs/Offshore convertor station platforms each comprising six legs;
• leg structures 5 m in diameter relating to two HVDC OSPs/Offshore convertor station platforms each comprising eight legs;
• scour protection 60 m diameter and 2 m in height associated with 15 m caissons for each OSP/Offshore convertor station platforms leg; and
• cable protection to a height of 3 m for 15% of inter-array/interconnector cables and areas along the Proposed Development export cable corridor with limited burial depth and at cable crossings.
  60. The areas within the Proposed Development export cable corridor requiring cable protection were identified from the seabed classification data, Figure 4.42   Open ▸ . The rocky outcrops where full burial depth may not be achieved are located within the eastern end of the corridor. Trenchless techniques would be used in the nearshore/inter-tidal areas. It should be noted that the scale of the model mesh meant that the general flow and sediment patterns around the structures could be observed on the wider scale. However, the localised nature of the scour meant that a detailed assessment of the effectiveness of the scour protection proposed at each foundation location was not undertaken as this was not the purpose of the computational modelling. The scour protection does not have implications on the global scale and is restricted to reducing sediment erosion in the vicinity of the foundations; there would be larger implications if scour protection were not provided (Whitehouse et al., 2006).
  61. The methodology implemented for the modelling used parameters selected from the project description outlined in volume 1, chapter 3 of the Offshore EIA Report, to ascertain the most influential and likely scenario for each coastal process aspect under examination. The indicative layout used within the modelling study is presented in Figure 5.1   Open ▸ . For each stage of the following study, the maximum design scenario applied is outlined.

Figure 5.1: Modelled Array Indicative Layout

5.2. Post-Construction Hydrography

5.2.1.    Tidal Flow

62. The spring tide simulation was repeated with the addition of 780 structures (i.e. 716 wind turbine legs and 64 OSP/Offshore convertor station platforms legs). The wind turbine and HVDC OSP/Offshore convertor station platforms legs being of 5 m diameter and the remaining 48 HVAC OSP/Offshore convertor station platforms legs being 4 m in diameter; all with a circular plan form. This represented the largest obstruction to tidal flow as, although the 14 MW option has more elements in the water column, they are more slender in design. The bathymetry was also amended to take account of scour and cable protection. The following figures show the same mid flood and mid ebb steps from the simulation as were presented in Figure 4.22   Open ▸ and Figure 4.23   Open ▸ respectively, but with the Proposed Development foundation and structures in place. Where appropriate, the Proposed Development export cable corridor has been indicated on the figures along with the Proposed Development array area, to indicate the locality of the works without obscuring the model results. Due to the limited magnitude of the changes, difference plots have also been provided. These are the proposed minus the baseline condition, therefore increases in current speed will be positive. The same procedure for calculating differences and plotting figures has been implemented throughout this report.
63. Figure 5.2   Open ▸ shows the post-construction flood tide flow patterns with Figure 5.3   Open ▸ showing the changes and as the changes are limited to the vicinity of the development a more focused plot is provided in Figure 5.4   Open ▸ . In the difference figures a log scale has been introduced to accentuate the values for clarity. Similarly Figure 5.5   Open ▸ , Figure 5.6   Open ▸ and Figure 5.7   Open ▸ show the same information for the ebb tide. During peak current speed the flow is redirected in the immediate vicinity of the structures and cable protection at the south of the site. The variation is a maximum of 1 cm/s which constitutes less than 2% of the peak flows at 200 m from the structure and reduces considerably with increased distance from each structure.

Figure 5.2: Post-Construction Tidal Flow Pattern – Peak Flood (HW-1 Hour)

Figure 5.3: Change in Tidal Flow (Post-Construction Minus Baseline) – Peak Flood (HW-1 Hour)

Figure 5.4: Change in Tidal Flow (Post-Construction Minus Baseline) Proposed Development Array Area – Peak Flood (HW-1 Hour)

Figure 5.5: Post-Construction Tidal Flow Pattern – Peak Ebb (LW-1 Hour)

Figure 5.6: Change in Tidal Flow (Post-Construction Minus Baseline) – Peak Ebb (LW-1 Hour)

Figure 5.7: Change in Tidal Flow (Post-Construction Minus Baseline) Proposed Development Array Area – Peak Ebb (LW-1 Hour)

5.2.2.    Wave Climate

64. Using the same principle as for the tidal modelling, the wave climate modelling was repeated with the inclusion of the Proposed Development structures, foundations and cable protection. Again, changes were found to be indiscernible from the baseline scenario by visual inspection therefore difference plots have been provided and a log scale was employed to highlight the variation.
65. The presence of the Proposed Development was seen to have the greatest influence when storms approached from the northerly sectors. The one in one year storm for the 000° direction for the mean sea level condition is presented in Figure 5.8   Open ▸ and corresponds to the baseline plot in Figure 4.27   Open ▸ . The change in wave climate is presented in Figure 5.9   Open ▸ . The changes are seen as reductions in the lee of the site and increases where the waves are deflected by the structures. These changes are in the order of 2 cm which represents less than 1% of the baseline significant wave height. A similar pattern was observed between the one in one and 1 in 20 year return period storms from each direction therefore, for brevity, only the 1 in 20 year results are presented for the additional storm directions.
66. The more severe 1 in 20 year storm event results are presented in Figure 5.10   Open ▸ to Figure 5.17   Open ▸ for the four principal directions which effect the physical processes study area and coastal zone (000°, 045°, 090⁰ and 135°). In each case, the post-construction wave climate is followed by the difference plot. It is apparent where the wave field interacts with either a wind turbine structure or a protection feature which reduces the water depth, the origin of the changes are focussed on specific locations. In the case of the 1 in 20 year storms, the changes are seen to follow the same pattern, with decreases in the lee of the Proposed Development and increases either side. However, it is noted that the changes are not considerably increased from the more frequent return period scenario and in the order of 2 cm to 4 cm whereas the baseline wave heights are increased for the greater return period events giving rise to a less marked overall impact on wave climate.

Figure 5.8: Post-Construction Wave Climate 1:1 Year Storm 000° Mid-Tide

Figure 5.9: Change in Wave Climate 1:1 Year Storm 000° Mid-Tide (Post-Construction Minus Baseline)

Figure 5.10: Post-Construction Wave Climate 1:20 Year Storm 000° Mid-Tide

Figure 5.11: Change in Wave Climate 1:20 Year Storm 000° Mid-Tide (Post-Construction Minus Baseline)

Figure 5.12: Post-Construction Wave Climate 1:20 Year Storm 045° Mid-Tide

Figure 5.13: Change in Wave Climate 1:20 Year Storm 045° Mid-Tide (Post-Construction Minus Baseline)

Figure 5.14: Post-Construction Wave Climate 1:20 Year Storm 090° Mid-Tide

Figure 5.15: Change in Wave Climate 1:20 Year Storm 090° Mid-Tide (Post-Construction Minus Baseline)

Figure 5.16: Post-Construction Wave Climate 1:20 Year Storm 135° Mid-Tide

Figure 5.17: Change in Wave Climate 1:20 Year Storm 135° Mid-Tide (Post-Construction Minus Baseline)

5.2.3.    Littoral Currents

67. The previous sections established the magnitude of the changes in tidal currents and wave conditions individually, however sediment transport regimes are driven by a combination of these factors. Although the modelling has demonstrated that the principal contribution comes from tidal currents, for the sake of completeness, the influence on littoral currents was examined.
68. The modelling was extended to include the post-construction scenario (i.e. with 780 structures and associated scour protection relating to caisson foundations) for the one in one year storm from 000°. The baseline littoral currents for mid flood and mid ebb were presented in Figure 4.40   Open ▸ and Figure 4.41   Open ▸ respectively. The post-construction littoral currents are shown in Figure 5.18   Open ▸ and Figure 5.21   Open ▸ for the flood and ebb tides respectively. The corresponding changes for the flood tide are presented in Figure 5.19   Open ▸ with a more detailed plot in Figure 5.20   Open ▸ whilst for the ebb tide Figure 5.22   Open ▸ and Figure 5.23   Open ▸ show the corresponding information.
69. During the flood tide the influence of the wave climate is in concert with the tidal current and the tidal flow in the lee of each of the structures is further reduced. During the ebb tide, the tidal flow is in opposition to the wave climate and the resultant littoral current is reduced in magnitude. The presence of the structures has a limited influence on the wave induced reduction in flow and there is little difference between changes in littoral current magnitude and the tidal flows alone due to the installation. The magnitude of these changes remains limited to ±5% of the baseline currents at 200 m and reduces considerably with increased distance from each structure.

Figure 5.18: Post-Construction Littoral Current 1:1 Year Storm from 000° - Flood Tide

Figure 5.19: Change in Littoral Current 1:1 Year Storm from 000° - Flood Tide (Post-Construction Minus Baseline)

Figure 5.20: Change in Littoral Current 1:1 Year Storm from 000° - Flood Tide (Post-Construction Minus Baseline) Proposed Development Array Area

Figure 5.21: Post-Construction Littoral Current 1:1 Year Storm from 000° - Ebb Tide

Figure 5.22: Change in Littoral Current 1:1 Year Storm from 000° - Ebb Tide (Post-Construction Minus Baseline)

Figure 5.23: Change in Littoral Current 1:1 Year Storm from 000° - Ebb Tide (Post-Construction Minus Baseline) Proposed Development Array Area

5.3. Post-Construction Sedimentology

5.3.1.    Sediment Transport

70. The numerical modelling methodology for sediment transport was described in section 4.6, which indicated how the baseline information was discretised to form the basis of the modelled scenarios. For the post-construction scenario, in addition to the Proposed Development structures being included in the tide and wave models, the bed material map was edited to include the proposed additional scour and cable protection. In each case an area of fixed bed was applied overlain with a thin layer of sand to initialise the model and avoid instabilities. The scour protection was defined as the envelope of four times the 20 m caisson diameter for the wind turbine and four times the 15 m caisson diameter for the OSP/Offshore convertor station platforms legs. The models were then re-run for a spring tide under calm conditions and also for a one in one year storm from 000°.
71. There are a number of approaches for quantifying potential sediment transport, given that transport rates vary both across the area and due to tidal state and climate conditions. For this analysis, the residual current was calculated over the course of two tidal cycles (one day) with the structures in place and compared with that for the baseline ( Figure 4.44   Open ▸ ) for the calm condition. The post-construction residual current and changes are shown in Figure 5.24   Open ▸ and Figure 5.25   Open ▸ respectively. As with previous results a more detailed plot is presented in Figure 5.26   Open ▸ .
72. The corresponding sediment transport was simulated over the course of one day where the equivalent baseline daily sediment transport rate was shown in Figure 4.45   Open ▸ . The post-construction daily sediment transport rate and differences are shown in Figure 5.27   Open ▸ and Figure 5.28   Open ▸ respectively. It should be noted that both the sediment transport and difference plots use a log palette as there is a large range in sediment transport potential across the domain and even so the changes in sediment transport rates are very small in the order of 0.01 m3/m over the course of a day.
73. This process was repeated for the 1 in 1-year storm. The baseline residual current ( Figure 4.48   Open ▸ ) and daily potential sediment transport ( Figure 4.49   Open ▸ ) were compared with the equivalent post-construction residual current pattern as shown in Figure 5.29   Open ▸ ; with the difference in Figure 5.30   Open ▸ and in more detail in Figure 5.31   Open ▸ whilst a similar comparison of the potential sediment transport is shown in Figure 5.32   Open ▸ and Figure 5.33   Open ▸ .
74. This analysis shows that although there are changes as a result of the installation of the Proposed Development structures and associated scour and cable protection, the magnitude is limited in nature and log scales were required to illustrate values. As anticipated, in areas of reduced residual current in the lee of structures the sediment transport rate is also reduced and vice versa. Within the context of this comparative study there is a maximum change in residual current and sediment transport of circa ±15% which is largely sited within close proximity to the wind turbine foundation structures (less than 300 m elongated in the direction of principle tidal currents). It is noted that areas of reduced residual current and sediment transport are often accompanied by a similar increase in close proximity. This indicates that the residual current and resulting sediment transport paths are adjusted to accommodate the structures rather than transport pathways being cut off.

Figure 5.24: Post-Construction Residual Current Spring Tide

Figure 5.25: Change in Residual Current Spring Tide (Post-Construction Minus Baseline)

Figure 5.26: Change in Residual Current Spring Tide (Post-Construction Minus Baseline) Proposed Development Array Area

Figure 5.27: Post-Construction Potential Sediment Over the Course of One Day (Two Tide Cycles)

Figure 5.28: Difference in Potential Sediment Transport Over the Course of One day (Post-Construction Minus Baseline)

Figure 5.29: Post-Construction Residual Current 1:1 Year Storm from 000° Spring Tide

Figure 5.30: Change in Residual Current 1:1 Year Storm from 000° Spring Tide (Post-Construction Minus Baseline)

Figure 5.31: Change in Residual Current 1:1 Year Storm from 000° Spring Tide (Post-Construction Minus Baseline) Proposed Development Array Area

Figure 5.32: Post-Construction Potential Sediment Transport over the Course of One day 1:1 Year Storm from 000°

Figure 5.33: Difference in Potential Sediment Transport over the course of One day (Post-Construction Minus Baseline) 1:1 Year Storm from 000°