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 Figure 4.2: Maximum Extent of Sediment Re-Deposition (Norfolk Offshore Wind, 2002) 94
Sheringham Shoal Offshore Wind Farm Physical change A broadly similar approach was adopted by HR Wallingford, for the assessment of ploughing operations for the Sheringham Shoal Offshore Wind Farm. However, within the study a more sophisticated approach was adopted for the dispersion of fine sediment using the dispersion model SEDPLUME-RW. The model used the hydrodynamic output from a TELEMAC-2D flow model and the assumption of a logarithmic velocity profile through the water column to track the 3-dimensional movement of fine sediment particles (mobile surface layer comprising sandy gravel with less than 4% fines). The results indicated that dispersion of sediment was rapid with concentrations dropping to less than 1mg/l above background within a single flood or ebb excursion. It was noted that dispersion occurred at a slower rate on a neap tide than a spring tide because of the lower rate of turbulent diffusion with footprints 4km and 9km either side of the cable route. The scenarios included ploughing through chalk, ploughing through a silt/clay/sand mix and trenching through chalk. For ploughing chalk during a neap tide, the dispersion footprint extends for around 9km in each direction, with concentrations dropping to levels of less than 1mg/l (above background) within a single flood or ebb excursion. For the spring tide simulation the higher turbulence causes the chalk concentrations to drop below 1mg/l (above background) within 4km of the cable route. The results for fines arising from other bed types with high percentages of fines, the neap tide footprint extends less than 2km, while the spring tide footprint is very small. As before the neap tide footprint is larger due to the lower rate of turbulent diffusion. The extent of the footprint on both tides is less than that of chalk due to the lower amount of material available per metre length of cable and the settling of silt during periods of slacker flows (chalk is assumed to have zero settling). This result is applicable to much of the inter-turbine cabling within the wind farm site where there are no exposures of chalk. The volume of material released by trenching through chalk is much higher, and therefore the extent and persistence of concentrations above 1mg/l is much greater. The predicted plume extends more than 10km in either direction at a level of up to 20mg/l (above background) on a neap tide. The model predicts a gradual drift of the plume towards the shore over the six tides, but that the plume has dispersed to less than 1mg/l concentration before the end of the model run. The footprint of silt deposition was found to extend over a wide area, but at an undetectable rate. Even under slack water conditions, the maximum rate of deposition over the six tide simulation was less than 0.5mm in the areas of greatest deposition, and in most of the footprint area the rate was far less. This result is anticipated as the deposited fines will be re-suspended on each tide, with no measurable material left in place. 95
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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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