6.3.2. Offshore Export Cables
- The proposed Skateraw offshore cable route was examined using numerical modelling. The simulation assumed the same trenching rate as with the inter-array cables (i.e. 500 m/h), and that installation began from offshore and continued to the nearshore region of the proposed trenchless landfall. Each trench was 2 m at the surface extending to a depth of 3 m (i.e. the greatest burial depth proposed). The operation took approximately 5.5 days to complete encompassing a range of tidal conditions and mobilised 400,000 m3 of material. The composition was determined from the sampling data and was of generally more finely graded material than the inter-array material:
- medium sand: 22%;
- fine sand: 46%;
- very fine sand: 18%; and
- coarse silt: 14%.
- The landfall trenching route modelled is illustrated by the yellow trace in Figure 6.49 Open ▸ and the average suspended sediment plume during the course of the operation is shown in Figure 6.50 Open ▸ . The figure shows how the plume travels north and south on the tide as the release progresses along the route perpendicular to the tidal flow. This gives rise to average SSC <50 mg/l.
- The instantaneous SSC for peak flood and ebb tides are presented for day two, day four and the final day in Figure 6.51 Open ▸ to Figure 6.56 Open ▸ respectively. They show increases where sediment is released at the cable location but also at the extent of each tidal cycle as material is re-suspended. The plume travels north and south on the tide as the release progresses along the route perpendicular to the tidal flow and sediment concentrations reduce to background levels on slack tides. Average SSC along the route range between 50 mg/l and 500 mg/l.
- Finally, Figure 6.57 Open ▸ shows the average sedimentation whilst Figure 6.58 Open ▸ illustrates sedimentation levels one day following cessation of the sediment release. Tidal patterns indicate that although the released material migrates both north and south by settling and being re-suspended on successive tides, the sedimentation level is small. Although there is some migration of material, sediment remains within the cell and would be drawn into the baseline transport regime with small increases in bed sediment levels, typically <3 mm at the coastline.
Figure 6.49: Modelled Offshore Cable Route
Figure 6.50: Average Suspended Sediment Concentration During Offshore Export Cable Trenching
Figure 6.51: Suspended Sediment Concentration Day Two Peak Flood – Offshore Export Cables Installation
Figure 6.52: Suspended Sediment Concentration Day Two Peak Ebb – Offshore Export Cables Installation
Figure 6.56: Suspended Sediment Concentration Final Day Peak Ebb – Offshore Export Cables Installation
Figure 6.57: Average Sedimentation During Offshore Export Cables Installation
Figure 6.58: Sedimentation One Day Following Cessation of Offshore Export Cables Installation
7. Summary
- The Proposed Development is located within an area which encompasses the Firth of Forth Banks Complex MPA and a range of designations including bathing waters therefore the impact on physical processes is important in the assessment of the potential environmental impact. This report has outlined the baseline characteristics of the region in terms of coastal processes. This includes tidal current, wave climate and sediment transport under both calm and storm conditions. Numerical modelling has been used to quantify the changes in physical processes due to the installation of the Proposed Development. The presence of the wind turbine foundations redirects both waves and tidal flow and although some changes in sediment transport were revealed, these were limited in magnitude and represented an adjustment in the transport path alignment.
- The installation of the Proposed Development was seen to marginally reduce wave heights in the lee of the structures whilst a marginal increase was noted at the periphery, however during larger storm events these effects were less marked. Any considerable changes in tidal currents and wave climate would not extend to the coastline and there would be no change in coastal processes in this area.
- Finally, suspended sediment plumes for construction activities were quantified. In all cases, the material released was native to the bed sediments and, although there are short periods of increased turbidity, the material was retained in the sediment cell and would be subsequently assimilated into the existing sediment transport regime.
8. References
BERR (2008). Review of Cabling Techniques and Environmental Effects applicable to the Offshore Windfarm Industry. Technical Report, Department for Business Enterprise and Regulatory Reform (BERR), in association with Defra, 164pp.
Berx, B, Hughes, S. (2009). Climatology of Surface and Near-bed Temperature and Salinity on the North-West European Continental Shelf for 1971–2000. Elsevier.
Centre for Environment, Fisheries and Aquaculture Science (Cefas) (2016). Suspended Sediment Climatologies around the UK, CEFAS.
Fugro (2012). Seagreen Wind Energy Limited: Firth of Forth Zone Development – Metocean Study.
Fugro (2020a). Seagreen 2 and 3 Windfarm Zones Geophysical Survey – Final Survey Results Report – Export Cable Route. Unpublished report for SSE Seagreen Wind Energy Limited, Fugro Document No: P906089-RESULTS-008 (01).
Fugro (2020b). Seagreen 2 and 3 and ECR Windfarm Zone Geophysical Survey – Final Survey Results Report – Seagreen 2 and Seagreen 3. Unpublished report for SSE Seagreen Wind Energy Limited, Fugro Document No: P906089-RESULTS-012 (01).
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HR Wallingford (2012). Appendix E3 – Geomorphological Assessment. Seagreen Wind Energy. Available at: http://marine.gov.scot/datafiles/lot/SG_FoF_alpha-bravo/SG_Phase1_Offshore_Project_Consent_Application_Document%20(September%202012)/006%20ES/Volume%20III_Technical%20Appendices/Part%201_Technical%20Appendices/Appendix%20E3.pdf. Accessed September 2021.
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SSER (2021a) Berwick Bank Offshore Scoping Report. Available at: BERWICK BANK WIND FARM Offshore Scoping Report - Introduction (berwickbank-eia.com)
XOCEAN Ltd (2021) 00338 SSE Berwick Bank Lot 1 & 2 Operations and Results Report. Unpublished report for SSER