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 Clay – A permanent scar will be left in stiff clay habitats following cabling activity. In soft clay, infilling is expected to occur rapidly. In harder or stiffer clays, a cutting wheel disc is often used which allows a wedge of soil to be cut by the action of the plough. This wedge is lifted by a ramp on the plough share, the cable is placed at the bottom of the trench and the wedge of soil then allowed to naturally backfill onto the cable. This process leaves minimum disturbance to the seabed with no spoil mounds. Spoil mounds are only found with ‘V’ shape plough shares more commonly associated with pipeline burial. Clay supports a species poor community due to the cohesive nature of the substrate. Cabling through soft clay is likely to put more sediment into suspension than in stiff clay where the habitat is more cohesive. Sand – Sand will infill rapidly following disturbance by ploughing or trenching. Burrowing species may be affected but are generally adapted to change through natural disturbance due to the mobility of the substrate. Gravel – Certain types of gravel habitat will infill immediately following cable laying activity, others may leave a shallow trough following initial infill. Generally, species inhabiting mobile gravel are adapted to harsh living conditions and would be expected to recover quickly. Suspended sediment Impacts resulting from cabling include the release of sediment into suspension (see Section 4.3). This can have a number of effects on the benthic species inhabiting areas adjacent to the cabling activity. The significance of the impact will be dependent on the type of sediment, the hydrodynamic conditions and the sensitivity of the species affected in addition to the type of installation method. The potential change in significance as a result of the type of sediment and hydrodynamic conditions is outlined in Section 4.3.5 and summarised in Tables 4.2 and 4.3. Increases in suspended sediment can affect filtering mechanisms of certain species, such as specific types of worm and brittle stars, through the clogging of gills or damage to feeding structures. Suspended sediment can also attach to fish eggs causing abnormalities or death. The sensitivity of the receptor is an important consideration when determining the significance of this effect. This includes its tolerance to a given effect, but also to its potential for recovery from such an effect (discussed in sub-section above). Its tolerance will be similar in all areas of the UK but will also depend to some extent on its adaptability to its ambient conditions. If a species has adapted to survive in a wide variety of conditions (such as in estuaries where suspended sediment concentrations (SSC) may be measured in grammes per litre), it is more likely to survive a small increase in SSC than a species which is exposed to a lower variation in SSC throughout the year. Information on the sensitivity (tolerance) of species should be obtained from published literature and consultation with relevant 106
Potential impacts and mitigration measures experts. Information can be found on the Marine Life Information Network (MarLIN) website (http://www.marlin.ac.uk) and the UK Marine SAC Programme website (http://www.ukmarinesac.org.uk/marine-communities.htm) that provide detailed information on the sensitivity/intolerance of many key features of the marine environment. However, it should be noted that the knowledge base for determining sensitivity of species is limited and research is needed to increase the confidence in such predictions. Many of the species in the marine environment are likely to have some degree of tolerance to increases in suspended sediment in order for them to adapt to natural perturbations and are therefore likely to survive localised short term effects. A prolonged increase in suspended sediment concentrations can affect the penetration of light through the water column affecting the photosynthetic activity of macroalgae, phytoplankton and eel grass. This is unlikely, however, during the installation of cables, as suspended sediment is only a very localised short term effect. As the material settles out of suspension, it can cause smothering to sensitive species and can change certain habitat characteristics. Studies outlined in Section 4.5.2 show the potential for settlement in site specific situations and generally conclude that the extent of the settlement will depend on the factors outlined above but, in general, the effect is expected to be short term and localised with one example showing settlement ranging from 20m up to a maximum of 200m either side of the cable, dependent on the state of the tide (Norfolk Offshore Wind, 2002). 5.2.3 OTHER EFFECTS Potential contamination due to sediment disturbance Consideration must also be given to the potential for contaminant remobilisation during cable installation activities due to disturbance of contaminated sediments. It is, however, less likely that high levels of contamination would be encountered away from the coast, unless the cable passes close to a historic or active disposal site. The screening/scoping process will identify if the cable route is likely to encounter potentially contaminated sites; in which case it would normally be a requirement to undertake sediment analysis prior to decision making relating to this aspect. Electro magnetic field generation Submarine power cables can generate electro magnetic fields (EMF) in the surrounding seabed and water. The potential impact of EMF on fisheries is discussed in Section 5.4. 107
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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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