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 Seacon concluded that the higher rates of sediment release from the pretrenching and backfilling was a result of the larger volume of sea bed strata disturbed during these operations and the fact that the material disturbed during trenching was lifted to the surface for inspection. This meant that the sediment was carried through the full water column before being placed alongside the trench. Although the trenching was undertaken using non-specialist equipment, a comparison of the monitoring results for the trenching and jetting operations at Nysted support the disturbance levels set in Table 4.3 above. Kentish Flats Offshore Wind Farm For Kentish Flats, EMU Ltd undertook turbidity monitoring during the cable installation in fulfilment of the FEPA licence conditions (EMU Ltd., 2005). Background data was collected from October 2004 to the beginning of February 2005. During the cable burial operations site measurements were taken 500m down-tide of the three export cables which were laid using ploughs. The results of the monitoring showed: ● ● 98 Marginal, short-term increases in background levels (approximately a 9% increase to the modal concentrations); and Peak concentrations occasionally reaching 140mg/l (equivalent to peaks in the natural concentrations driven by the tidal cycle). These site observations support the broad conclusions from the modelling undertaken for the Cromer, Sheringham and London Array wind farms and suggest that the environmental effects of cabling methods are likely to be shortterm and localised. Cable Burial Operations at Lewis Bay, Nantucket Sound In 2003 Applied Science Associates Inc undertook modelling simulations to estimate water column sediment concentration and sediment deposition resulting from the proposed embedment of submarine electricity cables in Lewis Bay, Nantucket Sound (Galagan et al., 2003). SSFATE model simulations were completed to quantify these impacts for cables buried to a depth of 6ft in sand-sized marine sediments. It was assumed that a jetting device would be used to create a trapezoidal trench measuring 6ft across at the top, 2ft across at the bottom and 8ft deep. It was also assumed that 30% of the total sediment fluidised within the trench would be evenly distributed vertically throughout the overlying water column with the remaining 70% remaining within the limits of the trench. The maximum flood and ebb tide currents of 0.6ft/s with a water depth of 3.5ft was adopted within the modelling. The modelling results indicate that sediment was re-deposited on the seabed within 200ft of the trench with a maximum depositional depth of around 25mm immediately adjacent to the cable route. The modelling also indicated that suspended sediment concentrations would reach a maximum value of 120 mg/l
Physical change along the cable route and reduce to less than 20 mg/l within 1000ft of the cable route. It is interesting to note that the results of the above modelling exercise completed in the USA are broadly in agreement with the modelling and site measurements discussed above for a number of wind farms in the UK. 4.5.3 CONCLUSIONS AND FURTHER RESEARCH The findings from the studies undertaken for a number of UK offshore wind farms indicate that the disturbance to seabed sediments during cable burial operations are likely to be short term and relatively localised, particularly if ploughing techniques are employed. It is unlikely that cumulative or in-combination impacts will present significant problems because of the short term nature of the burial operations. As noted earlier there is limited research and advice on the quantification of material arising from cable burial operations, particularly if jetting, dredging or cutting/excavating methods are adopted. This lack of base information on the release of sediment into suspension limits the benefits that could arise from the use of more sophisticated modelling techniques to establish the fate of released sediment. It is, therefore, suggested that this is an area where further research would be of value. It is appreciated that monitoring requirements included in FEPA licences for UK Round 1 and 2 offshore wind farms will provide valuable data for the quantification of the impacts of cable burial operations. 4.6 Mitigation Measures The previous sections have discussed the influence of cable burial operations on sediment disturbance. In practice the main areas where mitigation measures can be adopted to reduce this disturbance are during the selection of the cable route and cable burial method. Both the route and burial methods adopted will be a balance between the engineering issues associated with the route corridor (see Section 3.10 on the Burial Assessment Surveys) and the key environmental issues associated with the site. The range of environmental issues likely to be encountered is discussed in Section 5. An appropriate level of site investigation (see Section 4.2.1) is an essential to ensure that the optimum route and burial methods are selected for the cable. The level of data gathered should be sufficient to provide confidence in the selected route and burial method and allow the assessment of the potential environmental impacts. It is at this stage that mitigation (e.g. re-routing) may be considered should the environmental impacts be significant. The completion of an appropriate level of site investigation in the early stages of the project will reduce the risks of delays during the consenting process and the risk of major changes in the later stages of a project. It will also allow a more strategic 99
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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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Appendix A: Standards and codes of
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Printed in UK on recycled paper. De
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