11.3 Construction and Decommissioning
11.3.1 Increased Suspended Sediment Concentrations and Associated Sediment Deposition
99 Temporary increases in SSC and associated sediment deposition are predicted to occur during the construction and decommissioning phases of the Proposed Development as a result of the installation/removal of foundations, seabed preparation (i.e. sand wave and boulder clearance)and as a result of the installation of array, OSP/Offshore convertor station platform interconnector, and offshore export cables. Seabed preparation activities (e.g. sand wave and boulder clearance) will occur in advance of installation of offshore cables and dredged material will be disposed of within the Proposed Development array area and Proposed Development export cable corridor (see section 4 in Part One of the RIAA for further details regarding seabed preparation activities).
100 These activities may result in temporary changes to water clarity, smothering and siltation rates (see paragraph 115 et seq. for further detail).
101 The assessment of LSE during the HRA Stage 1 Screening process identified that during construction and decommissioning phases, LSE could not be ruled out for the potential impact of increased SSC and associated sediment deposition. This relates to the following designated site and relevant Annex I habitat features:
- Berwickshire and North Northumberland Coast SAC:
– mudflats and sandflats not covered by seawater at low tide;
– large shallow inlets and bays;
– reefs; and
– submerged or partially submerged sea caves.
102 The HRA Stage 1 Screening determined that this applies to the Proposed Development export cable corridor only which is, at its closest point, located 4.1 km from the SAC. The Proposed Development array area is at its closest point 34.69 km from this SAC which is beyond the ZoI predicted for increased SSC and associated sediment deposition (predicted precautionarily to be 20 km during LSE screening and refined to 10 km following modelling undertaken to inform this assessment, see paragraph 110 for further detail regarding deposition).
103 The impact is predicted to be of local spatial extent, short term duration, intermittent and medium reversibility.
104 The following sections explain how this potential impact on Annex I habitat features of the Berwickshire and North Northumberland Coast SAC has been quantified and assessed.
Maximum design scenario relevant to the assessment of adverse effects on integrity
105 The maximum design scenario considered for the assessment of potential impacts on Annex I habitat features from increased SSC and associated sediment deposition during construction and decommissioning is presented in Table 11.2 Open ▸ .
Table 11.2: Maximum Design Scenario Considered for the Assessment of Potential Impacts on Annex I Habitat Features from Increased SSC and Associated Sediment Deposition during Construction and Decommissioning
Designed-in measures relevant to the assessment of adverse effects on integrity
106 Designed-in measures (and the associated commitments) which are of relevance to the assessment of potential impacts on Annex I habitat features from increased SSC and associated sediment deposition during construction and decommissioning are presented in Table 11.3 Open ▸ .
Table 11.3: Designed-in Measures Relevant to the Assessment of Adverse Effects on Integrity on European Sites Designated for Annex I Habitat Features from Increased SSC and Associated Sediment Deposition
Information to inform Appropriate Assessment
107 The Appropriate Assessments for European sites for Annex I habitats are presented in section 11.5. Information common to inform the Appropriate Assessments in presented in this section.
108 The installation of the Proposed Development offshore export cables, and any seabed preparation works required prior to the installation of the offshore export cables, will disturb seabed sediments. This is predicted to lead to elevated levels of suspended sediments, above background levels, and sediment deposition as the material resettles back to the seabed, which may smother benthic habitats and species. Site specific benthic grab surveys showed that the sediments within the offshore section of the Proposed Development export cable corridor are characterised by muddy sand sediments which grade into slightly gravelly muddy sand, and rocky habitats with increasing proximity to the landfall. The sample stations with the highest percentage composition of mud are generally found along the inshore section of the Proposed Development export cable corridor.
109 Full details of the physical processes modelling undertaken to determine the fate of disturbed sediments along the Proposed Development export cable corridor, which has been used to inform this assessment, are presented in volume 3, appendix 7.1 of the Offshore EIA Report. This includes the individual scenarios considered and assumptions within these, and full modelling outputs for suspended sediments and associated sediment deposition. For the purposes of this assessment, the following activities have been considered:
- seabed feature clearance prior to offshore export cables installation; and
- installation of offshore export cables.
110 Sand wave clearance for the installation of offshore export cables will involve disturbance of seabed material within a corridor of up to 25 m in width for 20% of offshore export cables where it may be necessary. Modelling of suspended sediments associated with site preparation activities showed that during the dredging phase the suspended sediment plume is very small with concentrations <100 mg/l. Suspended sediment concentration is predicted to reach its peak in the disposal phase with concentrations reaching 2,500 mg/l at the release site, but the plume is predicted to be at its most extensive when the deposited material is redistributed on successive tides. Under these circumstances, concentrations of 100 mg/l to 250 mg/l are predicted with average values <100 mg/l extending up to 10 km, corresponding with a tidal excursion. Sedimentation of 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 mm to 10 mm over a maximum distance of 10 km from the site of activity. Sedimentation one day following cessation of operation is similar to during sand wave clearance operations with a small extension to the area over which sedimentation has occurred but with no increase in maximum sedimentation depth. The dispersal of the deposited material would continue on successive tides and be incorporated into the baseline sediment transport regime.
114 The Marine Evidence-based Sensitivity Assessment (MarESA) and the Feature Activity Sensitivity Tool (FeAST) have been drawn upon to support the assessment of sensitivity of the Annex I habitats within the Berwickshire and North Northumberland Coast SAC as detailed in the following sections.
Mudflats and sandflats not covered by seawater at low tide
115 On the basis of the mapped distribution of designated habitat features of the Berwickshire and North Northumberland Coast SAC, the qualifying Annex I habitat mudflats and sandflats not covered by seawater at low tide feature are concentrated in the English part of the SAC, and small patches of the feature in the Scottish part of the SAC are approximately 12 km from the Proposed Development export cable corridor. This feature is therefore located beyond the limit of the predicted maximum extent (10 km) of increased SSC and deposition arising from the installation of offshore export cables and associated sand wave clearance activities. Considering the project specific physical processes modelling outputs, there is no pathway to effect on the Annex I habitat mudflats and sandflats not covered by seawater at low tide feature as a result of increased SSC and sediment deposition.
Large shallow inlets and bays
116 As discussed in section 11.2.5, large shallow inlets and bays are habitat complexes which comprise an interdependent mosaic of subtidal and intertidal habitats.
117 Large shallow inlets and bays do not have any specific biotopes associated with it, although the feature consists of the following sub-features: intertidal sand and muddy sand; subtidal coarse sediment; subtidal sand; subtidal mud, subtidal mixed sediment, and saltmarsh habitat. On the basis of the mapped distribution of designated habitat features of the Berwickshire and North Northumberland Coast SAC, the qualifying Annex I large shallow inlets and bays feature are concentrated in the English part of the SAC, approximately 38 km from the Proposed Development export cable corridor. This feature is therefore located beyond the limit of the predicted maximum extent (10 km) of increased SSC and deposition arising from the installation of offshore export cables and associated sand wave clearance activities. Considering the project specific physical processes modelling outputs, there is no pathway to effect on the Annex I large shallow inlets and bays feature as a result of increased SSC and sediment deposition.
Reefs
118 According to the mapped distribution of designated habitat features of the Berwickshire and North Northumberland Coast SAC, the qualifying Annex I reef feature are distributed extensively throughout the SAC, including in the Scottish section of the SAC, and are within the ZoI of increased SSC from the Proposed Development offshore export cable works. As discussed in section 11.2.5, subtidal reefs within the Berwickshire and North Northumberland Coast SAC include the sub-features kelp forest communities and subtidal faunal turf communities. These sub-features of the reef habitat are moderately sensitive to smothering. Siltation can clog or block the feeding/respiratory organs of animals, which can lead to the death of individual organisms and potentially damage the community. Increased water turbidity influences the ability of kelp and other algal species to photosynthesise. Activities which result in a reduction in water clarity may have a detrimental effect on their physiological functioning which in turn may affect the growth and survival of kelp forest. Deposition of silt can also cover available substrate which interferes with the process of spore attachment. Young sporelings, or holdfasts which contain a diverse range of microniches for colonisation, are also sensitive to being covered by silt, which can inhibit their growth and development. Extensive physical damage may therefore lead to loss of habitat (English Nature and SNH, 2000). The MarESA also identifies that the biotopes which represent the subtidal reef feature habitats are of medium sensitivity to smothering and siltation rate change and water quality changes due to the inability of organisms to feed until the sediment is dispersed.
119 Most intertidal communities tend to be more resilient to physical change than subtidal communities because of their adaptation to the natural physical processes to which they are subjected. These harsh conditions generally result in a rapid recolonisation and re-growth of communities.
Submerged or partially submerged sea caves
122 According to the mapped distribution of designated habitat features of the Berwickshire and North Northumberland Coast SAC, the qualifying Annex I submerged or partially submerged sea caves feature are distributed throughout the SAC, including in the Scottish section of the SAC, and are within the ZoI of increased SSC from the Proposed Development offshore export cable works. The sensitivity of the communities associated with submerged or partially submerged sea caves ranges from not sensitive to medium according to the MarESA. Whilst the upper, vertical walls of caves are unlikely to be subject to any smothering or affected by increased SSC, the inner reaches of caves with shallow slopes or horizontal ledges have the potential to be. The effect could result in reductions in suspension feeding by characteristic species whilst some species, such as encrusting sponges, prefer these conditions and will not experience adverse effects in these conditions over short periods. The wave exposed conditions experienced by biotopes typical of sea caves are also likely to result in any settled sediment being removed quickly.
11.4 Operation and Maintenance
11.4.1 Increased Suspended Sediment Concentrations and Associated Sediment Deposition
124 Maintenance activities associated with the offshore export cables (e.g. cable repair and/or reburial) may lead to increases in SSCs and associated sediment deposition over the operational lifetime of the Proposed Development. These changes may result in temporary changes to water clarity, smothering and siltation rate changes.
125 The assessment of LSE during the HRA Stage 1 Screening process identified that during operation and maintenance, LSE could not be ruled out for the potential impact of increased SSC and associated sediment deposition. This relates to the following designated site and relevant Annex I habitat features:
- Berwickshire and North Northumberland Coast SAC:
– mudflats and sandflats not covered by seawater at low tide;
– large shallow inlets and bays;
– reefs; and
– submerged or partially submerged sea caves.
126 The HRA Stage 1 Screening determined that this applies to the Proposed Development export cable corridor only, which is at its closest point 4.1 km from the SAC. The Proposed Development array is at its closest point 34.69 km from this SAC which is beyond the ZoI predicted for increased SSC and associated sediment deposition (predicted precautionarily to be 20 km during LSE screening and refined to 10 km following modelling undertaken to inform this assessment; see paragraph 110).
127 The impact is predicted to be of local spatial extent, short term duration, intermittent and have medium reversibility.
128 Paragraphs 129 to 139 explain how the impacts of the Proposed Development on Annex I habitat features during operation and maintenance have been quantified and assessed.
Maximum Design Scenario Relevant to the Assessment of Adverse Effects on Integrity
129 The maximum design scenario considered for the assessment of potential impacts on Annex I habitat features from increases in SSC and sediment deposition during operation and maintenance are set out in Table 11.4 Open ▸ .
Table 11.4: Maximum Design Scenario Considered for the Assessment of Potential Impacts on Annex I Habitat Features from Increases in SSC and Sediment Deposition during Operation and Maintenance
Designed-in measures relevant to the assessment of adverse effects on integrity
130 There are no designed-in measures which are of relevance to the assessment of potential impacts on Annex I habitat features from increases in SSC and sediment deposition during the operation and maintenance phase.
Information to inform Appropriate Assessment
131 The Appropriate Assessments for European sites for Annex I habitats are presented in section 11.5. Information common to inform the Appropriate Assessments in presented in this section.
132 Maintenance activities within the Proposed Development benthic subtidal and intertidal Proposed Development export cable corridor may lead to increases in SSCs and associated sediment deposition over the operational lifetime of the Proposed Development. The maximum design scenario for offshore export cables is for cable repair of up to 4,000 m and reburial of up to 4,000 m of offshore export cables over the Proposed Developments lifetime (35 years), using similar methods as those for cable installation activities.
133 The length of cable affected during each repair or reburial event would be approximately 1 km, which is approximately 0.4% of the length of cable assessed for the construction phase. Repair/reburial events would be undertaken intermittently over the Proposed Developments lifetime. The sediment plumes and sedimentation footprints would be dependent on which section of the cable is being repaired however the impacts of the operation and maintenance activities (i.e. cable repair and reburial) are predicted to be no greater than those detailed for construction (which considered the entire length of offshore export cable) and, in reality, substantially smaller due to the reduced scale.
134 Impacts of increased SSC and associated sediment deposition during the operation and maintenance phase, are predicted to be negligible within the Berwickshire and North Northumberland Coast SAC given most sediment deposition will be localised, focussed within the Proposed Development export cable corridor and any fines reaching the coast within the SAC are predicted to be < 3 mm in thickness. The maximum distance that the modelling outlined in paragraph 110 predicted fines could travel from the Proposed Development export cable corridor, was 10 km.
Mudflats and sandflats not covered by seawater at low tide
136 As outlined in paragraph 115, the qualifying Annex I habitat mudflats and sandflats not covered by seawater at low tide feature of the Berwickshire and North Northumberland Coast SAC are concentrated in the English part of the SAC, approximately 12 km from the Proposed Development export cable corridor. This feature is therefore located out with the predicted ZoI of increased SSC and sediment deposition arising from cable repair activities. There is, therefore, considered to be no pathway to effect on the Annex I habitat mudflats and sandflats not covered by seawater at low tide feature as a result of increased SSC and sediment deposition.
Large shallow inlets and bays
137 As outlined in paragraph 117, the qualifying Annex I habitat large shallow inlets and bays feature of the Berwickshire and North Northumberland Coast SAC are concentrated in the English part of the SAC, approximately 38 km from the Proposed Development export cable corridor. This feature is therefore located out with the predicted ZoI of increased SSC and sediment deposition arising from cable repair activities. There is, therefore, considered to be no pathway to effect on the Annex I habitat mudflats and sandflats not covered by seawater at low tide feature as a result of increased SSC and sediment deposition.
Reefs
138 The sensitivity of reefs to increased SSC and sediment deposition is as described previously for the construction phase assessment in section 11.3.1 which assessed them to be of medium vulnerability, medium recoverability and therefore overall, medium sensitivity.
Submerged or partially submerged sea caves
139 The sensitivity of submerged or partially submerged sea caves to increased SSC and sediment deposition is as described previously for the construction phase assessment in section 11.3.1 which assessed them to be of medium vulnerability, medium recoverability and therefore overall, medium sensitivity.
11.4.2 Alteration of Seabed Habitats arising from effects of Physical Processes
140 Alteration of seabed habitats may arise from the effects of changes to physical processes, including scour effects and changes in the sediment transport and wave regimes resulting in potential effects on benthic receptors and Annex I habitats.
141 The presence of foundation structures, associated scour protection and cable protection may introduce localised changes to the tidal flow and wave climate, resulting in potential changes to the sediment transport pathways and long-term effects on benthic ecology for the lifetime of the Proposed Development. Volume 3, appendix 7.1 of the Offshore EIA Report provides a full description of the modelling used to inform this assessment.
142 The assessment of LSE during the HRA Stage 1 Screening process identified that during the operation and maintenance phase, LSE could not be ruled out for the potential impact of alteration of seabed habitats arising from effects of physical processes. This relates to the following designated site and relevant Annex I habitat features:
- Berwickshire and North Northumberland Coast SAC:
– mudflats and sandflats not covered by seawater at low tide;
– large shallow inlets and bays;
– reefs; and
– submerged or partially submerged sea caves.
143 The impact is predicted to be of long-term duration and continuous, but of local spatial extent, with high reversibility.
144 The following sections explain how the impacts of the Proposed Development on Annex I habitat features have been quantified and assessed.
Maximum design scenario relevant to the assessment of adverse effects on integrity
145 The maximum design scenario considered for the assessment of potential impacts on Annex I habitat features from changes in physical processes are set out in Table 11.5 Open ▸ .
Table 11.5: Maximum Design Scenario Considered for the Assessment of Potential Impacts on Annex I Habitat Features from Changes in Physical Processes
Designed-in measures relevant to the assessment of adverse effects on integrity
146 There are no designed-in measures which are of relevance to the assessment of potential impacts on Annex I habitat features from changes in physical processes during the operation and maintenance phase.
Information to inform Appropriate Assessments
147 The Appropriate Assessments for European sites for Annex I habitats are presented in section 11.5. Information common to inform the Appropriate Assessments in presented in this section.
148 The presence of the Proposed Development infrastructure will obstruct tidal flow and alter the wave climate within the benthic subtidal and intertidal ecology study area. Tidal flow is predicted to accelerate in the immediate vicinity of each structure as it is redirected around the foundation and there may be a zone of reduced speed in the lee of the structure. 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 within 200 m of the structure and reduces significantly with increased distance from each structure. These changes would also be largely limited to the immediate vicinity of the foundations and only extend a small distance beyond the southern and western boundaries of the Proposed Development array area. The limited nature of these changes would not influence the hydrodynamic regime. The modelling presented in volume 3, appendix 7.1 of the Offshore EIA Report demonstrates that tidal flows will not be affected in the nearshore.
149 Modelling of the predicted changes to wave climate for a one in one-year storm found the changes will be 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. For the more severe 1 in 20-year storm event, the changes are predicted to follow the same pattern with decreases in the lee of benthic subtidal and intertidal ecology study area and increases either side. However, the changes are not significantly 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.
150 Sediment transport is driven by a combination of tidal currents and wave conditions, the magnitude of these has been individually quantified as described in paragraphs 148 and 149. For a one in one year storm from 000° during the flood tide the wave climate is in concert with tidal flow reducing the tidal flow on the lee side of the structure further. However, during the ebb flow, the wave climate and tidal flow are in opposition, reducing the magnitude of the littoral current. With the presence of infrastructure, wave climate causes a small reduction in the magnitude of flow whilst there is little difference between the magnitude of littoral current flow and the tidal flows. Changes in magnitude compared to baseline current flow are ±5% (volume 3, appendix 7.1 of the Offshore EIA Report) which would not be sufficient to disrupt beach and offshore bank morphological processes or destabilise coastal features.
151 Residual currents are effectively the driver of sediment transport and therefore any changes to residual currents would have a direct impact on sediment transport which would persist for the lifecycle of the Proposed Development. However, if the presence of the foundation structures does not have a significant influence on either tide or wave conditions (see paragraphs 148 to 150 above for changes in tidal currents and changes to wave climate and littoral current) they cannot therefore have a significant influence on the sediment transport regime. For completeness, the residual current and sediment transport was simulated with the foundations in place. The maximum change in residual current and sediment transport is predicted to be approximately ±15% within close proximity to the structures (less than 300 m elongated in the direction of principal tidal currents). Changes in the residual current and sediment transport reduce with increasing distance from the wind turbines towards baseline levels.
153 The modelling presented in volume 3, appendix 7.1 of the Offshore EIA Report demonstrates that tidal flows will not be affected in the nearshore. For some wave climates (predominately storms approaching from the northerly sectors), there is predicted to be a very small change at the coast, but these are for specific storm directions and would be imperceptible from natural variation. The combination of the two (littoral currents) and thus the impact on sediment transport is also not predicted to give rise to any discernible change in physical processes at the coast and, therefore, within the Berwickshire and North Northumberland Coast SAC.
Mudflats and sandflats not covered by seawater at low tide
155 The FeAST assesses the sensitivity of mudflats and sandflats not covered by seawater at low tide to changes in tidal currents and wave exposure to be low. The MarESA finds that the sensitivity ranges from not sensitive to medium sensitivity. Increases in changes in tidal currents and wave exposure can lead to physical damage and reduction in suspension feeding, as well as potential shifts in sediment and community characteristics over extended periods. Whereas a reduction in flow from tides can result in the clogging of suspension and deposit feeders feeding apparatus. The MarESA finds that for both pressures, biotopes with organisms such as Z. nolteii and M. edulis are the most sensitive as changes to currents and wave exposure can impact feeding as well as their distribution due to species differing levels of tolerance.
156 Mudflats and sandflats not covered by seawater at low tide are deemed to be of medium vulnerability, medium recoverability and therefore overall, medium sensitivity to this pressure.
Large shallow inlets and bays
157 As discussed in section 11.2.5, large shallow inlets and bays are habitat complexes which comprise an interdependent mosaic of subtidal and intertidal habitats. The habitat - large shallow inlets and bays - does not have any specific biotopes associated with it, although the feature consists of the following sub-features: intertidal sand and muddy sand; subtidal coarse sediment; subtidal sand; subtidal mud, subtidal mixed sediment and saltmarsh habitat. The sensitivity of the component habitats is therefore likely to be similar to the equivalent biotopes. The FeAST determines equivalent biotopes to be not sensitive to tidal current and wave exposure changes.
158 Large shallow inlets and bays are deemed to be of low vulnerability, high recoverability and therefore overall, negligible sensitivity to this pressure.
Reefs
159 As discussed in section 11.2.5, subtidal reefs within the Berwickshire and North Northumberland Coast SAC include the sub-features kelp forest communities and subtidal faunal turf communities.
160 Kelp forest communities fringe the open coast within the SAC and are characterised by kelp species such as Laminaria hyperborean which support an understorey of red algal turf and short epifaunal turf. Beyond this fringing area, reefs in over 10 m water depth, are characterised by urchin grazed kelp habitats, small crabs, squat lobsters and anemones such as Urticina felina. Subtidal faunal turf communities are present beyond the kelp communities where, due to lack of light, this living turf comprises diverse assemblages of attached animals growing on subtidal substrate including encrusting sea mats and sponges, tall erect soft corals and sea fans. The MarESA identifies that the biotopes which represent these subtidal reef habitats are not sensitive to changes in local water flow from tidal currents or local wave exposure changes.
161 Intertidal reefs within the Berwickshire and North Northumberland Coast SAC includes the sub-feature rocky shore communities. The majority of rocky shores within the SAC are gently sloping bedrock or boulder shores, and within the stretch of coastline from Fast Castle Head to Eyemouth in south-east Scotland, tend to be animal dominated. The MarESA identifies that the biotopes which represent the intertidal reef range from not sensitive (rock with fucoids and barnacles) to being of medium sensitivity (rock with mussels and barnacles) to changes in local water flow from tidal currents but are not sensitive to local wave exposure changes.
Submerged or partially submerged sea caves
164 Intertidal sea cave communities which occur above the low water mark are subject to varying degrees of tidal inundation and splash from waves. The communities within these caves tend to be dominated by species which are tolerant of high energy water movement and exposure to the air and therefore not sensitive to tidal current and wave exposure changes.
165 Subtidal sea cave communities which occur in the subtidal are continually submerged in water and are not exposed to the air at low tide. Conditions in these caves, are not as harsh or extreme as in intertidal sea caves and are subject to less water movement from the surrounding sea. Subtidal sea caves are not sensitive to tidal current change and wave exposure change which may be attributed to the sheltered nature of subtidal sea caves.
166 Submerged or partially submerged sea caves are deemed to be of negligible sensitivity.