Appendix 6 Physical Processes – Baseline Environment
6.1 Desktop Study
- An initial desk-based review of literature and data sources to support this Offshore EIA Scoping Report has identified a number of baseline datasets. This includes a number of surveys that were undertaken to characterise the former Firth of Forth Zone, as summarised in Apx. Table 6. 1 Open ▸ .
Apx. Table 6. 1: Summary of Key Desktop Reports and Datasets
- This section provides an overview of both the planned and existing project specific data sources of relevance to physical processes.
- A recent geophysical survey campaign was completed across the Proposed Development Array Area and proposed ECC (Thortonloch landfall only) in July to August 2019. This survey provides both geophysical and bathymetric data which will support the development of the physical processes EIA for the Proposed Development. The aims of the data collection, and a summary of the data collected during these surveys includes:
- bathymetric data in order to determine site topography, gradients and a baseline for a seabed mobility study that may influence foundation design and cable installation using multibeam echo sounder (MBES);
- high-resolution sidescan sonar (SSS) data to determine seabed features and the presence of boulders, seabed sediments and debris;
- high-resolution sub-bottom profiler (SBP) data to determine the shallow sub-surface soil conditions that may influence foundation design and cable installation such as boulders and shallow geology features;
- multichannel 2D ultra-high resolution seismic (UHRS) data to foundation depth to determine the deeper sub-surface soil conditions; and
magnetometer data across the site (along the planned survey lines) to support unexploded ordnance (UXO) risk reduction.
- To support the acquisition of physical processes data, there are several surveys planned for summer 2020:
- geotechnical borehole campaign (approximately five weeks duration) to measure physical properties of soils;
- geotechnical survey cone penetrometer test (CPT) campaign (approximately two weeks duration) to test the geotechnical engineering properties of soils and soil stratigraphy;
- deployment of wave buoys and lidar to gather data relating to the metocean parameters within the Proposed Development Array Area and proposed ECC;
- subtidal benthic ecology surveys providing an overview of the seabed sediment composition to support the characterisation of the subtidal aspects of the Proposed Development; and
landfall walkover site survey to provide an overview of the nature of the foreshore area, including a review of sediments; evidence of erosion/deposition or littoral sediment transport and any defence assets present. Photographs will be gathered to support the characterisation of the landfall area.
- This section provides a detailed baseline for physical processes established through a review of desktop data and site-specific survey results.
- Proposed Development Array Area
- The bathymetry of the Proposed Development Array Area is influenced by the presence of large-scale morphological bank features, including the Marr Bank and the northern extent of the Berwick Bank. These two bank features are defined as Shelf Banks and Mounds and are part of the Firth of Forth Banks Complex.
- Geophysical data collected in 2019 suggests the water depth within the Proposed Development Array Area varies between 32.8 m and 68.5 m relative to LAT, and average depths of generally 51 m below LAT. Minimum water depths of approximately 38 m below LAT are found on top of the western central part of the Proposed Development Array Area and maximum depth around 68 m below LAT in the east of the banks. Apx. Figure 6. 1 Open ▸ illustrates the bathymetry recorded across the Proposed Development Array Area during the 2019 geophysical survey.
- Baseline Characterisation
188.8.131.52 Proposed ECC
- The bathymetry of the proposed ECC is relatively variable, varying between 53.1 m and 69.8 m below LAT at the time of geophysical investigation. This variance in depth is influenced by the seafloor typography which slopes gently, reaching 60 m depth below LAT approximately 20 km from landfall (Kilometre Point (KP) 20), before decreasing to 44 m below LAT at KP 32 and varying between 40 and 30 m below LAT in the area of the cable corridor over the over the southern part of Marr Bank. The depth of the water in the far east extent of the route extends down to 64 m below LAT.
- Apx. Figure 6. 2 Open ▸ illustrates the bathymetry recorded within the proposed ECC during the 2019 geophysical survey.
Wind and Waves
- Throughout the North Sea, strong winds can occur with wave heights varying greatly due to fetch limitations and water depth effects. Waves in the northern North Sea can be generated either by local winds or from remote wind systems (swell waves). East of the mouth of the River Tay, the dominant wave conditions approach from between 200N and 600N. However, extreme wave conditions (> 4 m) can be experienced from the entire eastern sector (00 to 1800) (HR Wallingford, 2012).
- Metocean surveys conducted across the former Firth of Forth Zone to characterise the zone provide an overview of the wave regime within the physical processes study area. During the stormiest event over the 18-month wave buoy deployment, a significant wave height of 6.7 m was recorded in January 2012, which correlated with a 1 in 1-year sea wave climate return period event (Fugro, 2012).
- As offshore waves transfer from the deep offshore water to shallower coastal areas (e.g. proposed ECC to landfall), a number of important modifications may result due to interactions of offshore deep-water waves with the seabed, with the resultant modifications producing shallow water waves. These physical ‘wave transformation’ interactions include:
- shoaling and refraction (due to both depth and current interactions with the wave);
- energy loss due to breaking;
- energy loss due to bottom friction; and
momentum and mass transport effect.
- Within the Offshore EIAR physical processes baseline assessment, a detailed baseline will be presented which provides an overview of the wind and wave regime within the region and specific to the Proposed Development, utilising data collected from deployed wave buoys.
Tidal Currents and Elevation
- An understanding of the tidal currents provides an insight into the patterns and rates of naturally occurring sediment transport. Currents are primarily driven by tides with a residual component generally dominated by storm driven currents (Ramsay and Brampton, 2000). Tidal elevations across the outer Firth of Forth are governed by a southerly directed flood tide which moves along the eastern coastline of Scotland into the Firth of Forth and around Fife Ness (HR Wallingford, 2009). Across the mouth of the Firth, the flood tidal stream has a general east-southeast pattern, whilst the ebb tidal stream runs in a west-north-west direction. The main peak flood tide occurs approximately two hours before high water (HW), with the main peak ebb tide occurring approximately four hours after HW (HR Wallingford, 2009). Tidal processes are often characterised by the natural tidal elevation of an area. The Firth of Forth Zone is characterised by a tidal regime which is semi-diurnal with variable mean spring tidal ranges, based on the metocean data collated within the 2011 survey campaign (HR Wallingford, 2012).
Apx. Figure 6. 1: Proposed Development Array Area Bathymetry Data
Apx. Figure 6. 2: Proposed ECC Bathymetry Data
Apx. Figure 6. 3 Former Firth of Forth Zone Metocean Survey Deployments
- Metocean surveys conducted across the former Firth of Forth Zone to support the development of the characterisation of the Zone provided an overview of the tidal current flows. The locations of the mooring positions used for the collection of data during these surveys are illustrated in Apx. Figure 6. 3 Open ▸ . The strongest current flows during the survey period were recorded at the two most northerly sites (sites A and B) which correlates to the location of Seagreen Alpha/Bravo . At these sites (A and B), a maximum current of 0.91 metres per second (m/s) in April 2011 during a period of spring tides that correlated with the maximum water level at most sites. Current speeds decreased slightly at the other sites with maxima ranging from 0.68 m/s to 0.77 m/s (Fugro, 2012). Further detail is presented in Apx. Table 6. 2 Open ▸ .
- Further, while sites C, D and G were characterised by a north to south tidal axis, site E and site H displayed axes parallel to their respective nearby coastlines, which were northeast to southwest at Site E and northwest to southeast at Site H (Fugro, 2012).
Apx. Table 6. 2: Summary of Tidal Current Statistics from the 2011 Metocean Survey Deployments
- Information of 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, array cables and offshore export cables. Apx. Figure 6. 4 Open ▸ to Apx. Figure 6. 6 Open ▸ illustrate the seabed features, seabed sediments and boulders present across the Proposed Development Array Area and proposed ECC.
- Proposed Development Array Area
- 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 due to advances and rapid retreats consistent with an oscillating and dynamic ice margin during British Ice Sheet (BIS) deglaciation (Graham et al., 2009).
- 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.
- 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
- Proposed ECC
- The seabed within the proposed ECC 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.
- The geophysical surveys observed that the bedforms in the proposed ECC 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 ECC 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.
6.3.5 Seabed Substrate
- An overview of surficial sediment geology and the seabed features is presented in this section, based on interpretation undertaken of the 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, array cables and offshore export cables.
- Apx. Figure 6. 4 Open ▸ to Apx. Figure 6. 6 Open ▸ illustrate the seabed substrates present across the Proposed Development.
- Proposed Development Array Area
- The recent geophysical survey of the Proposed Development Array Area 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.
- 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, sandwaves, 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
- Proposed ECC
- The recent geophysical survey of the proposed ECC identified that it is comprised of several distinctive features:
- boulders and boulder fields;
- area of ripples;
- area of megaripples and sand waves; and
area of trawl marks.
- The seabed within the proposed ECC 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 ECC 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 sidescan data;
- gravelly sand and coarse sediments with medium reflectivity; and
sandy sediments including fine sand and muddy sand with low reflectivity.
- SSER is currently assessing the feasibility of two landfall locations on the East Lothian coast, Thorntonloch Landfall and Skateraw Landfall. SSER intends to refine this to only one landfall option by the submission of the application. The geophysical surveys provided an overview of the Thorntonloch landfall area, identifying a band of approximately 2 km along the shore to be defined as the coastal area for the surveys. This coastal area is comprised of a sandy beach to the north, a rocky platform in the middle and a pebble and rocky beach in the south. The nearshore area of the proposed ECC consists of a submerged beach and the rocky platform from the lowest tide until around 30 metres depth, approximately 2 miles from the shore.
- Sampling was conducted at an offshore station within Seagreen Alpha/Bravo 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.
- 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 (HW, 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 significantly, both nearbed 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.
- The Cefas Climatology Report 2016 (Cefas, 2016) provides the spatial distribution of average non-algal Suspended Particulate Matter (SPM) for the majority of the UK continental shelf (UKCS). Between 1998 and 2005, the greatest plumes are associated with large rivers such as 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. 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.
Apx. Figure 6. 4: Proposed Development Array Area and Proposed ECC Seabed Features Data
Apx. Figure 6. 5: Sediment Interpretation from SSS Data for the Proposed Development Array Area and Proposed ECC
Apx. Figure 6. 6: Proposed Development Array Area and Proposed ECC Boulder Fields Data