6. Potential Changes During Construction

75. In addition to the changes in coastal process resulting from the presence of the Proposed Development, the construction phase influences were quantified. The principal construction elements relate to the transport and fate of sediment brought into suspension due to seabed preparation, the installation of the foundation structures and the laying of inter-array/interconnector cables between the wind turbines/OSPs/Offshore convertor station platforms and the offshore export cables to shore. An overview of the modelling techniques implemented is provide in Table 3.1   Open ▸ .
76. As with the post-construction aspects, the approach was to examine the construction technique which represents the maximum design scenario in terms of coastal processes. In practice, these changes are therefore likely to be of lesser magnitude. In each scenario the modelling examined excess SCC arising from the proposed activities (i.e. ambient SSC were not included). Baseline studies outlined in section 4.6 and section 4.7 indicate that turbidity levels are generally low, particularly in deep water areas, however sediment transport mechanisms are active with the associated bedload. Sedimented material arising from the construction phase activities would therefore be amalgamated into the sediment transport regime. The numerical modelling SSC provide depth average values and do not therefore differentiate between bed load and water column suspended sediment.
77. During each phase of the assessment the transport of suspended sediment was modelled by undertaking simulations which released sediment at a rate and location appropriate to each type of construction. The sediment released was defined according to the characteristics of grab samples collected at each specific location. Where a number of locations were encountered, such as a dredging path, then a representative grading was used. The sediment sample locations are presented in Figure 6.1   Open ▸ .

Figure 6.1: Benthic Subtidal Survey Autumn 2020 - Grab Sample Locations

6.1. Seabed Preparation

78. Due to the nature of the seabed in the Proposed Development array area and Proposed Development export cable corridor, the cable installation will require seabed preparation in the form of seabed features clearance. The Project Design Envelope (PDE) presented by the project description outlined in volume 1, chapter 3 of the Offshore EIA Report indicates that sand waves may be cleared for the offshore, inter-array and interconnector cabling along up to a 25 m wide corridor. Clearance activities may extend along circa 20% of the offshore cable route with an average clearance depth of up to 5 m and 30% of the inter-array cable route with an average clearance depth up to 1.3 m.
79. The modelling undertaken to quantify the potential increases in SSC and sedimentation simulated the use of a suction hopper dredger to remove material from the crest of sand waves and deposit material in the adjacent trough. In practice plough dredging may be undertaken however this type of operation would have less impact in terms of both SSC and sedimentation footprint.
80. Two representative clearance operations were assessed one relating to the offshore cabling and a second for the inter-array cables. The geophysical survey data was used to identify areas of sand waves and megaripples where the operations are most likely to be required. Figure 6.2   Open ▸ indicates these areas by grey shading and the clearance routes modelled are specified in yellow for the inter-array and green for the offshore cable. In each case the clearance was undertaken in a westerly direction with a dredging rate of 10,000 m3/h with a spill of 3%.

Figure 6.2: Sand Wave Clearance Paths Modelled

6.1.1.    Offshore Cable Sand Wave Clearance

81. The offshore cable route was cleared at 80 m/h along the 25 m wide route for a period of two hours, in line with dredging rate and plant required to carry out the operation. The material was then deposited over a half hour period from the hopper with the modelled route taking 3.5 days to prepare. The redistributed material was classified using the properties identified from the sampling undertaken along the route simulated.

• coarse sand: 20%;
• medium sand: 33%;
• fine sand: 39%; and
• very fine sand: 8%.
  82. The SSC vary greatly during the course of the dredge and disposal campaign. During the dredging phase when only 3% of the material is released the plume is very small with concentrations <100 mg/l as shown in Figure 6.3   Open ▸ . During the disposal phase the plume is slightly larger ( Figure 6.4   Open ▸ ) with concentrations reaching 2,500 mg/l at the release site. However, the most extensive increases are seen as the deposited material is redistributed on the successive tides, where sedimentation occurs on the slack tide reducing the SSC completely and resuspension and transport occurs when the tidal currents increase. Under these circumstance concentrations of 100 mg/l to 250 mg/l are seen as illustrated in Figure 6.5   Open ▸ which shows SSC arising at peak current speed. The average SSC during the course of the dredge and disposal campaign is presented in Figure 6.6   Open ▸ with values <100 mg/l with a plume width of 10 km which corresponds with the tidal excursion.
  83. The average sedimentation depth is shown in Figure 6.7   Open ▸ and illustrates how the deposited material is focussed within 100 m of the site of release with a maximum depth 0.5 m to 0.75 m whilst the finer sediment fractions are distributed in the vicinity at much smaller depths circa 5 m to 10 mm. The dispersion of the released material would continue on successive tides and be incorporated into the baseline sediment transport regime. The sedimentation one day following the cessation of the clearance operation is presented in Figure 6.8   Open ▸ and is consistent with this mechanism. 

Figure 6.3: Suspended Sediment Concentration During Dredging Phase – Offshore Cable Path

Figure 6.4: Suspended Sediment Concentration During Disposal Phase – Offshore Cable Path

Figure 6.5: Suspended Sediment Concentration with Sediment Re-Mobilisation – Offshore Cable Path

Figure 6.6: Average Suspended Sediment Concentration During Dredge and Disposal Campaign – Offshore Cable Path

Figure 6.7: Average Sedimentation During Dredge and Disposal Campaign – Offshore Cable Path

Figure 6.8: Sedimentation One Day Following Cessation of Dredge and Disposal Campaign – Offshore Cable Path

6.1.2.    Inter-array Cable Sand Wave Clearance

84. The inter-array cable route was cleared at 312 m/h along the 25 m wide route for a period of one hour, in line with the dredging rate and reduced removal depth. The material was then deposited over a 15 minute period from the hopper with the modelled route taking 2.1 days to prepare. As previously, the redistributed material was classified using the properties identified from the sampling undertaken along the route simulated.

• coarse sand: 28%;
• medium sand: 59%;
• fine sand: 10%; and
• very fine sand: 3%.
  85. The resulting SSC showed similar characteristics to the offshore cable clearance. The dredging phase plumes were even smaller, as 3% spill of the material is released with faster progress along the route and again concentrations are <100 mg/l as shown in Figure 6.9   Open ▸ . Similarly, the release phase plume is slightly larger than the dredging plume with concentrations reaching 2500 mg/l at the disposal site, Figure 6.10   Open ▸ . At this site the greatest area of increased suspended sediment concentration, extending a tidal excursion circa 10 km from the site, is also associated with re-mobilisation of the deposited material on subsequent tides with concentrations of 100 mg/l to 250 mg/l whilst average levels <100 mg/l as illustrated in Figure 6.11   Open ▸ and Figure 6.12   Open ▸ respectively.
  86. Due to the smaller volume the average sedimentation depth, shown in Figure 6.13   Open ▸ , is typically half that of the offshore cable works. The sedimentation one day following the cessation of the clearance operation is presented in Figure 6.14   Open ▸ and shows deposited material at the site of release with depth 0.2 m to 0.4 m whilst in the locality lower depths, typically <5 mm, are present at 50 m distance from the release.

Figure 6.9: Suspended Sediment Concentration During Dredging Phase – Inter-Array Cable Path

Figure 6.10: Suspended Sediment Concentration During Disposal Phase– Inter-Array Cable Path

Figure 6.11: Suspended Sediment Concentration with Sediment Re-Mobilisation – Inter-Array Cable Path

Figure 6.12: Average Suspended Sediment Concentration During Dredge and Disposal Campaign – Inter-Array Cable Path

Figure 6.13: Average Sedimentation During Dredge and Disposal Campaign – Inter-Array Cable Path

Figure 6.14: Sedimentation One Day Following Cessation of Dredge and Disposal Campaign – Inter-Array Cable Path

6.2. Foundation Installation

87. The PDE presented in the project description outlined in volume 1, chapter 3 of the Offshore EIA Report includes two potential foundation types, piled and suction caissons foundations. The caissons were applied in the hydrographic assessments as they created the largest potential obstruction to tidal flow and sediment transport however the installation produces much less seabed disturbance than installation of piled foundations. Therefore, the piled structures were assessed in terms of potential increases in suspended sediment.
88. The largest potential release would be from augured (drilled) piles, where the material would be jetted and released to the water column as a plume. It is anticipated that 10% of piles across the site may require drilling of up to 20% of the pile depth. The modelling assumed that at each site the material which is released has a similar composition to the sampled sediment. In reality, to require drilling (rather than driving) the sediments are generally less granular and augured material would be less easily brought into suspension therefore the modelled scenario provides a conservative assessment in terms of suspended sediment concentration.
89. A sample of three representative pile installations were simulated to cover the range of conditions across the area both in terms of water depth, tidal currents and sediment grading; these locations are shown on an indicative layout in Figure 6.15   Open ▸ . The sites were also selected in their proximity to the Firth of Forth Banks Complex MPA. The modelling was undertaken using the MIKE MT module which allows the modelling of erosion, transport and deposition of cohesive and cohesive/granular sediments. This model is suited to sediment releases in the water column and allows sediment sources which may vary spatially and temporally. In this case, the cohesive functions were not utilised as the material released comprised sand. The sediment grading was defined for each location and assumed two concurrent drilling operations located at separate legs of the same wind turbine location to provide the largest augmented sediment plume concentration.
90. At each location it was assumed that the auguring was required to the 16 m pile depth for an assumed 5.5 m diameter pile as a maximum design scenario (i.e. 380 m3 per pile). The drilling rate was taken as 0.5 m/h which was both prescribed in the PDE and also allowed the release to cover the full range of tidal conditions (i.e. the auguring was undertaken continuously over a 32 hour period with material released throughout the water column).
91. For each location a set of results are presented. Firstly, the average suspended sediment plume during the course of the installation is shown. Due to the variation in suspended sediment levels, instantaneous plots of the sediment plumes are also presented during peak flood and ebb tides on each of the two days. It should be noted that all the plots require the use of a log scale to cover this range and provide clarity and during slack water SSC decrease to background levels.
92. The final set of plots relates to sedimentation. Due to the fine sandy nature of the material, it is clear that the sediment will be dispersed. It will be transported mid-tide, settle on slack water and be re-suspended and further dispersed on the resumption of tidal flow. For all three locations, sediment levels after the cessation of construction would not be discernible from the background sediments due to the limited magnitude of deposition and the similar nature of the material.

Figure 6.15: Location of Modelled Piled Installations

6.2.1.    Modelling Location WIND Turbine A

93. Wind turbine labelled A is located in the north-west of the Proposed Development array area within the Marr Banks with the following sediment composition derived from the seabed sample data.

• coarse sand: 3%;
• medium sand: 17%;
• fine sand: 66%; and
• very fine sand: 14%.
  94. This location is sited between the Marr Banks at a relatively shallow point where current speeds are at their highest across the Proposed Development array area and, as a result, there is good dispersion potential. This is demonstrated in the average suspended sediment plot shown in Figure 6.16   Open ▸ , where concentrations are <1 mg/l away from the immediate vicinity of the two discharge locations. The following figures illustrate this further; Figure 6.17   Open ▸ and Figure 6.18   Open ▸ show the instantaneous concentrations on the first auguring for peak flood and ebb tides respectively whilst, Figure 6.19   Open ▸ and Figure 6.20   Open ▸ show the instantaneous concentrations on the second day. In each case the plume related directly to the sediment releases is <5 mg/l and this drops to single figures within a very short distance, typically less than 500 m. Again, a log scale was required to illustrate low level of SSC. Some small areas of increased suspended sediment can be seen where material has been deposited on slack tides and subsequently re-suspended.
  95. The sedimentation plots, shown in Figure 6.21   Open ▸ and Figure 6.22   Open ▸ , present the average sedimentation and sedimentation one day following cessation of the works respectively. These demonstrate the dispersive nature of the site, dispersing material the full extent of the tidal excursion, and even using a very small contour interval this settlement would be imperceptible from the background sediment transport activity with plotted sediment depths less than typical grain diameters.

Figure 6.16: Average Suspended Sediment Concentration – Pile Installation Wind Turbine A

Figure 6.17: Suspended Sediment Concentration Day One Peak Flood - Pile Installation Wind Turbine A

Figure 6.18: Suspended Sediment Concentration Day One Peak Ebb - Pile Installation Wind Turbine A

Figure 6.19: Suspended Sediment Concentration Day Two Peak Flood - Pile Installation Wind Turbine A

Figure 6.20: Suspended Sediment Concentration Day Two Peak Ebb - Pile Installation Wind Turbine A

Figure 6.21: Average Sedimentation During Pile Installation - Wind Turbine A

Figure 6.22: Sedimentation One Day Following Cessation of Pile Installation - Wind Turbine A

6.2.2.    Modelling Location Wind Turbine B

96. Wind turbine labelled B is located in the south-east of the Proposed Development array area on Berwick Bank with the following composition.

• very coarse sand: 6%;
• coarse sand: 10%;
• medium sand: 45%; and
• fine sand: 39%.
  97. This location exhibits slightly coarser graded material and current speeds are slightly lower when compared to the site of wind turbine A previously examined. As anticipated the average suspended sediment plot shown in Figure 6.23   Open ▸ , indicates a slightly smaller plume envelope however concentrations remain <1 mg/l a short distance, circa 400 m, from the two discharge locations. Figure 6.24   Open ▸ to Figure 6.27   Open ▸ illustrate the instantaneous concentrations on the peak flood and ebb tides on each day of the drilling. As with wind turbine A, areas of increased suspended sediment are evident where material has been deposited on slack tide and subsequently re-suspended. Typically, the plume concentration is <5 mg/l, and reduces with the distance from the site as the sediment is dispersed.
  98. Figure 6.28   Open ▸ and Figure 6.29   Open ▸ show the average sedimentation and sedimentation one day following cessation of the drilling operation. In line with the slightly coarser material and lower current speeds the footprint is marginally smaller than site A however sedimentation levels remain very small due to the limited volume of material released.

Figure 6.23: Average Suspended Sediment Concentration – Pile Installation Wind Turbine B

Figure 6.24: Suspended Sediment Concentration Day One Peak Flood - Pile Installation Wind Turbine B

Figure 6.25: Suspended Sediment Concentration Day One Peak Ebb - Pile Installation Wind Turbine B

Figure 6.26: Suspended Sediment Concentration Day Two Peak Flood - Pile Installation Wind Turbine B

Figure 6.27: Suspended Sediment Concentration Day Two Peak Ebb - Pile Installation Wind Turbine B

Figure 6.28: Average Sedimentation During Pile Installation - Wind Turbine B

Figure 6.29: Sedimentation One Day Following Cessation of Pile Installation - Wind Turbine B

6.2.3.    Modelling Location Wind Turbine C

99. Wind turbine labelled C is located in the north-east of the Proposed Development array area south of Montrose Bank with the following composition.

• very coarse sand: 17%;
• coarse sand: 10%;
• medium sand: 28%; and
• fine sand: 45%.
  100. This site was selected due to the proximity to Montrose Bank in the MPA close to the extent of the Proposed Development. The sediment composition and tidal flows are similar to site A and therefore the plume extent and subsequent footprint are of a similar scale and form. This is demonstrated in the average suspended sediment plot shown in Figure 6.30   Open ▸ and instantaneous figures Figure 6.31   Open ▸ to Figure 6.34   Open ▸ , where peak concentrations are <5 mg/l and average values are typically less than one fifth of this. At this location the transport cycle is also evident with material settling out on slack tides and re-suspended with increasing current speeds.
  101. The average sedimentation shown in Figure 6.35   Open ▸ also indicates this transport cycle with the material being dispersed further following the end of the operation as illustrated in Figure 6.36   Open ▸ . The resulting sedimentation depths are <0.001 mm from the two drilling operations and demonstrates that this settlement would be imperceptible from the background sediment transport activity.

Figure 6.30: Average Suspended Sediment Concentration – Pile Installation Wind Turbine C

Figure 6.31: Suspended Sediment Concentration Day One Peak Flood - Pile Installation Wind Turbine C

Figure 6.32: Suspended Sediment Concentration Day 2 Peak Ebb - Pile Installation Wind Turbine C

Figure 6.33: Suspended Sediment Concentration Day Two Peak Flood - Pile Installation Wind Turbine C

Figure 6.34: Suspended Sediment Concentration Day Two Peak Ebb - Pile Installation Wind Turbine C

Figure 6.35: Average Sedimentation During Pile Installation - Wind Turbine C

Figure 6.36: Sedimentation One Day Following Cessation of Pile Installation - Wind Turbine C

6.3. Cable Installation

102. The third aspect of the construction phase is cable installation, including the inter-array cables, interconnector cables and offshore export cables to shore. For the maximum design scenario in terms of release of sediment into the water column, cables were assumed to be trenched. A number of trenching techniques may be suited to the ground conditions; however, it was assumed within the modelling that a trench of material of the maximum depth presented in the project description outlined in volume 1, chapter 3 of the Offshore EIA Report was mobilised into the lower water column as a result of the burial process, in line with the Business Enterprise and Regulatory Reform (BERR) guidelines (BERR, 2008). In reality the preferred scenario would be jet trenching and the maximum depth may not always be achieved with a corresponding reduction in the amount of material mobilised.
103. Similar to the pile installation, the model simulations used the sediment grading determined from sediment sampling. However, the modelling was undertaken using the MIKE PT module. This module was implemented as it had the advantage that it could be used to describe the transport of material released in a specific part of the water column. In this way, the dispersion would not be over-estimated or the corresponding sedimentation under-estimated by the application of a current profile through the water column.
104. Trenching rates can vary widely depending on the bed material and equipment used; typically, rates are between 25 m/h and 780 m/h. For the simulation, a relatively high rate of 500 m/h was used over an extensive sample route ensuring that material was released at all tidal states over a number of tides and ensuring initial concentrations were not underestimated.

6.3.1.    Inter-array/Interconnector Cables

105. Inter-array and interconnector cable installation will be undertaken along a number of paths which connect groups of wind turbines to a local hub (i.e. the OSP/Offshore convertor station platforms) or which connect two OSPs/Offshore convertor station platforms to each other. Each route would be undertaken as a separate operation and thus a single example has been selected to quantify the potential suspended sediment levels during the installation. Figure 6.37   Open ▸ shows an indicative wind turbine layout with the modelled inter-array cable route shown in yellow. This route was run from the south-east corner of the site, perpendicular to the tidal flow to the west, then in line with tidal flows in a northerly direction before re-crossing the site. This ensured that the full extent of the site and tidal conditions were incorporated into the simulation.
106. The inter-array cabling was undertaken along the indicated route with a trench 2 m wide and 3 m in depth thus circa 469,000 m3 of material was mobilised during the seven day simulation. The sediment grading characteristics were derived from sediment sampling along the route and defined by the following sand fractions.

• very coarse sand: 22%;
• coarse sand: 12%;
• medium sand: 36%; and
• fine sand: 30%.
  107. The model results presented follow the same format as those for the wind turbine installation described in paragraphs 91 and 92. Figure 6.38   Open ▸ shows the average suspended sediment concentration over the course of the trenching phase. It is clear that the sediment is re-suspended and dispersed on subsequent tides as the plume envelope is most extensive towards the start of the route to the south-east of the site with peak values of 100 mg/l.
  108. Figure 6.39   Open ▸ to Figure 6.44   Open ▸ shows the suspended sediment patterns over the course of this operation, day two, four and five peak flood and ebb tides respectively. Whilst Figure 6.45   Open ▸ and Figure 6.46   Open ▸ show the flood and ebb tides at the end of the trenching. The volume of material mobilised is relatively large, and elevated tidal currents disperse the material giving rise to concentrations of up to 500 mg/l. As was evident in the previous operations, the material settles during slack water and then is re-suspended to form an amalgamated plume. This is further illustrated in Figure 6.47   Open ▸ and Figure 6.48   Open ▸ which show the average sedimentation and the sedimentation one day following cessation at slack water. The sedimentation is greatest at the location of the trenching and may be up to 30 mm in depth however within close proximity, circa 100 m, the depths reduce considerably which is indicated by the use of a log scale in all figures. Although the material is dispersed, it remains within the sediment cell and is therefore retained within the transport system.

Figure 6.37: Modelled Inter-Array Cable Route

Figure 6.38: Average Suspended Sediment Concentration During Inter – Inter-Array Cable Trenching

Figure 6.39: Suspended Sediment Concentration Day Two Peak Flood – Inter-Array Cable Installation

Figure 6.40: Suspended Sediment Concentration Day Two Peak Ebb – Inter-Array Cable Installation

Figure 6.41: Suspended Sediment Concentration Day Four Peak Flood – Inter-Array Cable Installation

Figure 6.42: Suspended Sediment Concentration Day Four Peak Ebb – Inter-Array Cable Installation

Figure 6.43: Suspended Sediment Concentration Day Five Peak Flood – Inter-Array Cable Installation

Figure 6.44: Suspended Sediment Concentration Day Five Peak Ebb – Inter-Array Cable Installation

Figure 6.45: Suspended Sediment Concentration Final Day Peak Flood – Inter-Array Cable Installation

Figure 6.46: Suspended Sediment Concentration Final Day Peak Ebb – Inter-Array Cable Installation

Figure 6.47: Average Sedimentation during Inter-Array Cable Installation

Figure 6.48: Sedimentation One Day Following Cessation of Inter-Array Cable Installation