Review of Cabling Techniques and Environmental Effects Applicable

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Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind Farm Industry – Technical Report zone is very likely to be undertaken during periods of low tide, and as such the potential for resuspension of material is reduced. Some of the disturbed material will, however enter into suspension during the flood tide but the extent of this will depend on the sediment type and cohesiveness. Resuspension of sediment is not likely to be of concern where cabling occurs within cohesive or coarse sediments, but can be significant when cabling is undertaken in non-cohesive fine sediments or chalks. Potential effects of suspended sediment are detailed in Section 5.2.2. Within fine sediments there are also issues to consider in relation to potential contaminant release. Contaminants, such as oils and heavy metals, generally attach to fine sediments but certain chemicals can persist in coarser sediments. Disturbance of sediment can release associated contaminants into the water column. If contaminants reach a certain level they can cause effects on certain species or can bioaccumulate through the food chain. However, the effects of contaminant release on the environment tend to be localised and would only be of concern near industrialised areas. Investigation of potentially contaminated sites that may lie within the cable route would be considered during the screening and scoping phase. If nearby sites are identified where there is evidence of historic contamination, sediment sampling will be necessary in order to determine the level of concentration within the sediment. Predictions can then be made as to whether the release of contaminants could have an adverse effect. Settlement of material Settlement of suspended material has the potential for smothering to occur that could cause the burial of important or sensitive species and habitats (see Section 5.2.2). Given that the installation generally occurs during low tide, it is only likely to be the fine sediments that have been disturbed, which may become suspended in the water column during the flood tide. 5.3.3 MITIGATION MEASURES Access to a site requires careful planning to avoid any sensitive features, which can be marked to ensure effective avoidance by construction plant and staff. Such features include rare or notable plants for example on shingle banks or saltmarsh, the presence of biogenic reef areas and the presence of important sessile (non-mobile) communities or species. Vegetated shingle areas should be avoided, or a process of translocation agreed. Horizontal directional drilling (within Section 3.8.6) is an appropriate form of mitigation to avoid damage, particularly in the intertidal and landfall areas where habitats may be more sensitive (e.g. chalk cliffs, saltmarsh, etc.). This methodology has been proposed for the Thanet offshore wind farm due to the inter alia geological features of the area and the presence of saltmarsh habitat (Thanet Offshore Wind, 2005). 112
Potential impacts and mitigration measures In order to promote recovery within the intertidal zone, material displaced as a result of cable burial activities should be back filled. This reduces the potential for remobilisation of sediments and enables recovery of benthic organisms to occur within a much quicker timescale. Where sensitive habitats (e.g. vegetated shingle, saltmarsh, etc.) are present along a cable route it may be necessary to remove vegetation prior to installation and replant/enhance following installation. Stabilisation techniques may also be necessary in certain conditions. Guidance is available relating to translocation and enhancement for saltmarsh habitat in the Environment Agency/Defra publication ‘The Saltmarsh Management Manual’ (Environment Agency, 2005) and the Chartered Institute of Water and Environmental Management (CIWEM)/Royal Society for the Protection of Birds (RSPB) document ‘The saltmarsh creation handbook: a project managers guide to the creation of saltmarsh and intertidal mudflat’ (RSPB, 2005a). 5.4 Natural Fish Resource The installation and burial of export and inter-array cables has the potential to impact upon the natural fish resource (finfish and shellfish) in a number of ways including: ● ● ● ● Habitat disturbance; Noise and vibration; Smothering and contamination; and Electro magnetic field generation. 5.4.1 POTENTIALLY SIGNIFICANT EFFECTS Habitat disturbance The following fisheries habitat types have the potential to be affected by the cable installation process: ● ● ● ● ● ● Spawning grounds; Nursery grounds; Feeding grounds; Over-wintering areas for crustaceans; Migration routes; and Shellfish beds. Disturbance caused by the presence of installation vessels and equipment (and associated noise) will displace fish within the water column from the vicinity of 113
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REVIEW OF CABLING TECHNIQUES AND EN
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Review of Cabling Techniques and En
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Abbreviations AIS Automatic Identif
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TTS Temporary Threshold Shift TWA T
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3 Cable types and 18 installation t
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Page 82 and 83: 4 Physical change 4.1 Introduction
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Page 144 and 145: References Introduction Sinden, G.
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Printed in UK on recycled paper. De
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