7.12. Cumulative Effects Assessment

7.12.1. Methodology

  1. The CEA assesses the LSE1 associated with the Array together with other relevant plans, projects and activities. Cumulative effects are defined as the combined effect of the Array in combination with the effects from a number of different projects, on the same receptor or resource. Further details on CEA methodology are provided in volume 1, chapter 6.
  2. The projects and plans selected as relevant to the CEA presented within this chapter are based upon the results of a screening exercise (see volume 3, appendix 6.4 of the Array EIA Report). Volume 3, appendix 6.4 further provides information regarding how information pertaining to other plans and projects is gained and applied to the assessment. Each project or plan has been considered on a case-by-case basis for screening in or out of this chapter's assessment based upon data confidence, impact-receptor pathways and the spatial/temporal scales involved.
  3. In undertaking the CEA for the Array, it should be noted that other projects and plans under consideration will have differing potential for proceeding to an operational stage and hence a differing potential to ultimately contribute to a cumulative impact alongside the Array. Therefore, a tiered approach has been adopted which provides a framework for placing relative weight upon the potential for each project/plan to be included in the CEA to ultimately be realised, based upon the project/plan’s current stage of maturity and certainty in the projects’ parameters. The tiered approach which will be utilised within the Array CEA employs the following Tiers:
  • tier 1 – Array and Proposed offshore export corridor(s) and Proposed onshore transmission infrastructure and all plans/projects which became operational since baseline characterisation, those under construction, and those with consent and submitted but not yet determined;
  • tier 2 – All plans/projects assessed under Tier 1, plus those projects with a Scoping Report; and
  • tier 3 – All plans/projects assessed under Tier 2, which are reasonably foreseeable, plus those projects likely to come forward where an Agreement for Lease (AfL) has been granted.
  1. The specific projects scoped into the CEA for physical processes, are outlined in Table 7.13   Open ▸ . The projects scoped in fall within the physical processes CEA study area, which is defined by two tidal excursions (16 km ZoI from the Array). One mean spring tidal excursion of 8 km was defined for the north/south orientation, reducing to 4 km for currents moving to the east and west, as discussed within section 7.3 for the physical processes study area. Therefore, the physical processes CEA study area is defined by a distance of 16 km to the north and south of the site boundary and 8 km to the east and west. This will allow the interaction of sediment plumes from the Array and surrounding projects to be considered.
  2. Some of the potential impacts considered within the Array alone assessment are specific to a particular phase of development (e.g. construction, operation and maintenance or decommissioning). Where the potential for cumulative effects with other plans or projects only have potential to occur where there is spatial or temporal overlap with the Array during certain phases of development, impacts associated with a certain phase may be omitted from further consideration where no plans or projects have been identified that have the potential for cumulative effects during this period.


Table 7.13:
List of Other Projects and Plans Considered within the CEA for Physical Processes

Table 7.13: List of Other Projects and Plans Considered within the CEA for Physical Processes

 

Figure 7.2:
Other Projects/Plans Screened into the Cumulative Effects Assessment for Physical Processes

Figure 7.2: Other Projects/Plans Screened into the Cumulative Effects Assessment for Physical Processes

7.12.2. Maximum Design Scenario

  1. The maximum design scenarios identified in Table 7.7   Open ▸ have been selected as those having the potential to result in the greatest effect on an identified receptor or receptor group. The cumulative effects presented and assessed in this section have been selected from the details provided in volume 1, chapter 3 of the Array EIA Report as well as the information available on other projects and plans (see volume 3, appendix 6.4), to inform a ‘maximum design scenario’. Effects of greater adverse significance are not predicted to arise should any other development scenario, based on details within the Project Description (volume 1, chapter 3) (e.g. different wind turbine layout), to that assessed here, be taken forward in the final design scheme.

 

Table 7.14:
Maximum Design Scenario Considered for Each Impact as part of the Assessment of Likely Significant Cumulative Effects on Physical Processes

Table 7.14: Maximum Design Scenario Considered for Each Impact as part of the Assessment of Likely Significant Cumulative Effects on Physical Processes

7.12.3. Cumulative Effects Assessment

  1. An assessment of the likely significance of the cumulative effects of the Array upon physical processes receptors arising from each identified impact is given below.

Increase in Suspended Sediment Concentrations and associated Deposition and Sediment Transport due to operation and maintenance activities

  1. Increased SSCs and associated deposition on physical features may arise during the operation and maintenance of the Array, which may impact on the sediment transport regime within the physical processes study area. The potential of an increase in SSCs may arise as a result of mooring lines or cables making contact with and moving on the seabed, disturbing seabed materials and causing scouring and increased SSCs within the water column. Should the other projects cited take place concurrently with the Array operation and maintenance, there is potential for cumulative increased turbidity levels and increased impact on sediment transport.

                        Tier 1

                        Operation and maintenance phase
Magnitude of impact
  1. The magnitude of the increase in SSCs and associated deposition arising during the operation and maintenance of the Array, including the potential to impact on the sediment transport regime has been assessed as low for the Array alone, as described in section 7.11. The assessment was undertaken for mooring lines and cabling disturbing seabed sediments, with mooring lines noted as the primary potential impact pathway.
  2. The operation and maintenance phase of the Array coincides with the operation and maintenance phase of the Proposed offshore export cable corridor(s). Transmission maintenance activities such as cable repair and reburial may result in increased SSCs, however these activities would be of limited spatial extent and frequency. It is unlikely that cable repair and reburial activities will be undertaken during storm conditions, when the SSCs arising from the Array mooring lines will be greatest, as assessed in the MDS for the Array. As discussed under section 7.11, disturbed materials are more likely to move along the seabed, rather than becoming fully suspended in the water column and over only a short duration of the tidal cycle will be transported a short distance before being re-deposited on the seabed. Therefore, the changes to the overall sediment transport regime in the physical processes study area is unlikely.
  3. The cumulative impact is predicted to be of local spatial extent, short term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
Sensitivity of receptor
  1. As discussed under section 7.11, the sensitivity of the seabed morphology within the physical processes study area is of low value, comprised mainly of mobile sand materials, with low sediment transport rates. Materials will be redeposited close by after a short period of suspension, thus not impacting significantly on seabed morphology. There will be no interaction of sediment plumes between the Array and the Proposed offshore export cable corridor(s), as they will be advected on the same tidal current. Even if this was the case, any material will settle locally and the seabed morphology in the physical processes study area should be able to accommodate any additional impacts. Impacts are likely to be fully recoverable after some time.
  2. The seabed morphology is deemed to be of low vulnerability, medium recoverability and low value. The sensitivity of the receptor is therefore, considered to be low.
Significance of effect
  1. Overall, the magnitude of the cumulative effect is deemed to be low and the sensitivity of the receptor is considered to be low. The cumulative effect will therefore be of negligible adverse significance, which is not significant in EIA terms.
Further mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 7.10) is not significant in EIA terms.

                        Tier 2

                        Operation and maintenance phase
Magnitude of impact
  1. One Tier 2 project has been identified with potential cumulative effects associated with this impact: Morven Offshore Wind Farm ( Table 7.14   Open ▸ ). This project has been assessed in terms of operation and maintenance during the Array operation and maintenance period. Maintenance activities applicable to this impact for the Morven Offshore Wind Farm are cable repair and reburial (Morven Offshore Wind Limited, 2023). Cable repair and reburial may result in local and short term increases in suspended sediments, which are unlikely to occur simultaneously with the MDS for the Array for this impact. There may be potential impacts to sediment transport due to the presence of the Morven Offshore Wind Farm infrastructure within the water column, which are likely to be greater than the impacts during the operation of the Array, however due to the low sediment transport rates in the area, this cumulative impact is not expected to be significant.
  2. The cumulative impact is predicted to be of local spatial extent, short term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
Sensitivity of receptor
  1. As discussed under section 7.11, the sensitivity of the seabed morphology within the physical processes study area is of low value, comprised mainly of mobile sand materials, with low sediment transport rates. Materials will be redeposited close by after a short period of suspension, thus not impacting significantly on seabed morphology. There will be no interaction of sediment plumes between the Array and the Morven Offshore Wind Farm, as they will be advected on the same tidal current. Even if this was the case, any material will settle locally and the seabed morphology in the physical processes study area should be able to accommodate these additional impacts. Impacts are likely to be fully recoverable after some time.
  2. The seabed morphology is deemed to be of low vulnerability, medium recoverability and low value. The sensitivity of the receptor is therefore, considered to be low.
Significance of effect
  1. Overall, the magnitude of the cumulative effect is deemed to be low and the sensitivity of the receptor is considered to be low. The cumulative effect will therefore be of negligible adverse significance, which is not significant in EIA terms.
Further mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 7.10) is not significant in EIA terms.

                        Tier 3

                        Operation and maintenance phase
Magnitude of impact
  1. Two Tier 3 projects have been identified with potential cumulative effects associated with this impact: Bellrock Offshore Wind Farm and Morven offshore export cable corridor(s) ( Table 7.14   Open ▸ ). These projects have been assessed in terms of operation and maintenance during the Array operation and maintenance period. Maintenance activities applicable to this impact for the Bellrock Offshore Wind Farm and Morven offshore export cable corridor(s) are likely to be cable repair and reburial, which may result in local and short term increases in suspended sediments and are unlikely to occur simultaneously with the MDS for the Array for this impact. Due to the floating infrastructure anticipated for Bellrock Offshore Wind Farm, there may be temporary increases to SSCs due to mooring lines and dynamic cabling during the operational phase, however the Bellrock Offshore Wind Farm project is anticipated to include fewer wind turbines (less than 80) than the Array and as the impact for the Array has been assessed as being local to each wind turbine, rather than over the site boundary (section 7.11) there is no anticipated cumulative impact.
  2. The Bellrock Offshore Wind Farm has also been assessed in terms of potential decommissioning during the Array operation and maintenance period. Should decommissioning activities, such as foundation and cable removal, be undertaken, there may be a temporary increase in SSCs. As per the cable repair and reburial during the operational phase, it is unlikely that decommissioning works will be undertaken during storm conditions, and therefore will not occur simultaneously with the MDS for the Array for this impact.
  3. As discussed under section 7.11, disturbed materials are more likely to move along the seabed, rather than becoming fully suspended in the water column and over only a short duration of the tidal cycle will be transported a short distance before being re-deposited on the seabed. Therefore, the changes to the overall sediment transport regime in the physical processes study area is unlikely due to the cumulative effects of these projects.
  4. The cumulative impact is predicted to be of local spatial extent, short term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore, considered to be low.
Sensitivity of receptor
  1. As discussed under section 7.11, the sensitivity of the seabed morphology within the physical processes study area is of low value, comprised mainly of mobile sand materials, with low sediment transport rates. Materials will be redeposited close by after a short period of suspension, thus not impacting significantly on seabed morphology. There will be no interaction of sediment plumes between the Array and the Bellrock Offshore Wind Farm or Morven offshore export cable corridor(s), as they will be advected on the same tidal current. As material will settle locally, the seabed morphology in the physical processes study area should be able to accommodate these additional impacts. Impacts are likely to be fully recoverable after some time.
  2. The seabed morphology is deemed to be of low vulnerability, medium recoverability and low value. The sensitivity of the receptor is therefore, considered to be low.
Significance of effect
  1. Overall, the magnitude of the cumulative effect is deemed to be low and the sensitivity of the receptor is considered to be low. The cumulative effect will, therefore, be of negligible adverse significance, which is not significant in EIA terms.
Further mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 7.10) is not significant in EIA terms.

Impacts to the Wind Field due to the Presence of Infrastructure

  1. The presence of offshore infrastructure may impact on the wind field during the operation and maintenance of the Array. Should the other projects cited take place concurrently with the Array operation and maintenance, there is potential for a cumulative decrease in the wind field within the physical processes study area.

                        Tier 2

                        Operation and maintenance phase
Magnitude of impact
  1. The magnitude of changes to the wind field arising due to the presence of infrastructure within the Array has been assessed as medium for the Array alone, as described in section 7.11.
  2. The operation and maintenance phase of the Array coincides with the operation and maintenance phase of the Morven Offshore Wind Farm, therefore there may be potential cumulative impacts due to the presence of the wind turbine infrastructure from both projects on the wind field. However, as noted in paragraph 83 and 85, the velocity deficit is likely to be very minor (circa 2% of the baseline) over a distance of 5 km to 20 km to the lee of the wind turbines and will be most prominent in the centre of the Array. Wake interactions will decrease towards the extremities of the physical processes study area, where wake interactions with Morven Offshore Wind Farm may occur.
  3. The cumulative impact is predicted to be of local spatial extent, long term duration, continuous and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore, considered to be medium.
Sensitivity of receptor
  1. As discussed under section 7.11, the sensitivity of the wind field is deemed to be negligible, due to the offshore, exposed location. Thus the wind field within the physical processes study area should not be significantly impacted by additional impacts from neighbouring offshore wind farms. Any changes to the wind field would be fully recoverable by the removal of the infrastructure.
  2. The wind field is deemed to be of low vulnerability and highly recoverable. The sensitivity of the receptor is therefore considered to be negligible.
Significance of effect
  1. Overall, the magnitude of the cumulative effect is deemed to be medium and the sensitivity of the receptor is considered to be negligible. The cumulative effect will therefore be of minor adverse significance, which is not significant in EIA terms.
Further mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 7.10) is not significant in EIA terms.

                        Tier 3

                        Operation and maintenance phase
Magnitude of impact
  1. One Tier 3 project has been identified with potential cumulative effects associated with this impact: Bellrock Offshore Wind Farm ( Table 7.14   Open ▸ ). This project has been assessed for this impact in terms of operation and maintenance during the Array operation and maintenance period. As for the Tier 2 project, it is unlikely that there will be any significant cumulative impact on the wind field due to the Array and Bellrock Offshore Wind Farm. This Tier 3 project will be operational over a smaller spatial area than the Array and with a smaller number of wind turbines than both the Array and Morven Offshore Wind Farm. The orientation of the Bellrock site is also such that the majority of the wind turbines will be located further from the Array wind turbines than the Morven wind turbines to the Array wind turbines. Therefore, this cumulative impact is anticipated to be less than for the cumulative impact between the Array and Morven Offshore Wind Farm.
  2. The cumulative impact is predicted to be of local spatial extent, long term duration, continuous and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be medium.
Sensitivity of receptor
  1. As discussed under section 7.11, the sensitivity of the wind field is deemed to be negligible, due to the offshore, exposed location. Thus the wind field within the physical processes study area should not be significantly impacted by additional impacts from neighbouring wind farms. Any changes to the wind field would be fully recoverable by the removal of the infrastructure.
  2. The wind field is deemed to be of low vulnerability and highly recoverable. The sensitivity of the receptor is therefore, considered to be negligible.
Significance of effect
  1. Overall, the magnitude of the cumulative effect is deemed to be medium and the sensitivity of the receptor is considered to be negligible. The cumulative effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Further mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of further mitigation beyond the designed in measures outlined in section 7.10 is not significant in EIA terms.

Impacts to Seasonal Stratification due to the Presence of Infrastructure

  1. The presence of offshore infrastructure, both above and below the water line, may impact on seasonal stratification by altering the wind field, wave climate and tidal regime during the operation and maintenance of the Array. This may include the presence of OSP foundations, wind turbines and their foundations and associated infrastructure (including cable protection, scour protection and anchor mooring lines). Should the other projects cited take place concurrently with the Array operation and maintenance, there is potential for a cumulative impact on seasonal stratification within the physical processes study area.
  2. In response to the Array EIA Scoping Report, MD-SEDD representation noted that cumulative impacts on stratification due to large scale offshore wind development could occur and should be included in the assessment. MD-SEDD also noted that the impact would be difficult to quantify, as this is an area of ongoing research. An outline qualitative assessment has therefore been included for this cumulative impact.

                        Tier 2

                        Operation and maintenance phase
Magnitude of impact
  1. The magnitude of changes to the seasonal stratification arising due to the presence of infrastructure within the Array has been assessed as medium for the Array alone, as described in section 7.11.
  2. The operation and maintenance phase of the Array coincides with the operation and maintenance phase of the Morven Offshore Wind Farm, therefore there may be potential cumulative impacts due to the presence of infrastructure from both projects on seasonal stratification.
  3. A recent paper by Christiansen et al. (2023) discussed the uncertainties surrounding the impact of fixed monopile wind turbine foundations on stratification on a regional scale, in addition to local effects. There is some early evidence that alterations to stratification fronts may be far reaching, although limited in magnitude, and more pronounced for larger wind farms with less spacing between wind turbines Christiansen et al. (2023). However, in terms of the Array MDS, the floating foundations are estimated to cause a surface obstruction of less than 0.44% of the total Array area and be situated fully within the stratified layer. There is greater potential for the fixed foundations from Morven Offshore Wind Farm to impact on seasonal stratification due to the greater obstruction to flow within the water column. However, potential impacts to seasonal stratification are considered to be localised, with only minimal impacts in the far field. A study by Carpenter et al. (2016) concluded that there is expected to be very little impact on large-scale stratification at the current offshore wind farm capacity in the North Sea. This study provided a comparison of the estimated the timescales of mixing and advection for water bodies with offshore wind farms against baseline stratification (Carpenter et al., 2016).
  4. The cumulative impact is predicted to be of local spatial extent, long term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore, considered to be medium.
Sensitivity of receptor
  1. As discussed under section 7.11, the sensitivity of the tidal front is deemed to be negligible due to the weak seasonal stratification within the physical processes study area. Any changes to seasonal stratification are considered to be highly localised and additional impacts will not affect the tidal front significantly. Any changes to seasonal stratification would be fully recoverable by the removal of the infrastructure.
  2. The tidal front is deemed to be of low vulnerability and highly recoverable. The sensitivity of the receptor is therefore, considered to be negligible.
Significance of effect
  1. Overall, the magnitude of the cumulative effect is deemed to be medium and the sensitivity of the receptor is considered to be negligible. The cumulative effect will therefore be of minor adverse significance, which is not significant in EIA terms.
Further mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 7.10) is not significant in EIA terms.

                        Tier 3

                        Operation and maintenance phase
Magnitude of impact
  1. One Tier 3 project has been identified with potential cumulative effects associated with this impact: Bellrock Offshore Wind Farm ( Table 7.14   Open ▸ ). This project has been assessed for this impact in terms of operation and maintenance during the Array operation and maintenance period.
  2. As the Bellrock Offshore Wind Farm will utilise floating wind turbine infrastructure, the impact on seasonal stratification is anticipated to occur in a similar manner to the Array, however due to the smaller spatial extent of this wind farm and the smaller number of wind turbines, the impact is anticipated to be less.
  3. The cumulative impact is predicted to be of local spatial extent, long term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore, considered to be medium.
Sensitivity of receptor
  1. As discussed under section 7.11, the sensitivity of the tidal front is deemed to be negligible due to the weak seasonal stratification within the physical processes study area. Any changes to seasonal stratification are considered to be highly localised and additional impacts will not affect the tidal front significantly. Any changes to seasonal stratification would be fully recoverable by the removal of the infrastructure.
  2. The tidal front is deemed to be of low vulnerability and highly recoverable. The sensitivity of the receptor is therefore, considered to be negligible.
Significance of effect
  1. Overall, the magnitude of the cumulative effect is deemed to be medium and the sensitivity of the receptor is considered to be negligible. The cumulative effect will therefore be of minor adverse significance, which is not significant in EIA terms.
Further mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 7.10) is not significant in EIA terms.