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 Table 4.4: Examples of physical disturbance of the seabed by seabed uses Deep water 90 Activity Spatial Extent / Unit Area UK Offshore pipelines 5084km length of pipeline installed; giving total length of 5847km (assuming a 15% contingency for interlink pipelines) UK Oil and gas installations Shallow water UK Aggregate extraction (for 2005) UK Dredging disposal (for 2004) 240 offshore platforms 180,000 m 2 (assuming average plan area of 30 m x 25 m x 240 offshore platforms) 134 km 2 (area actually dredged) 1257km 2 (licensed) 21.2 m tonnes 310 km 2 (licensed) 30.1 m tonnes (wet tonnage disposed) 4.5 Dispersal and Re-Deposition of Sediment 4.5.1 METHODS Reference/Source Ongoing research by Bomel Ltd. on behalf of HSE Ongoing research by Bomel Ltd on behalf of HSE Crown Estate (2005) Cefas (pers. comm.) Once the sediment has been lifted into suspension it will disperse under the action of the tidal flows. The strength of these currents and the dispersion and settling characteristics of the suspended sediment will determine the footprint over which the sediment will be deposited. Sands and gravels disturbed during the cable burial operations will settle back to the seabed very rapidly and the footprint is unlikely to extend any great distance from the cable route. Silts, clay and chalk particles will remain in suspension for a greater period of time and will be dispersed over a much greater distance, depending upon the strength of the tidal currents. However, the depth of deposition over such a large area is likely to be small. Relatively simple dispersion and deposition calculations can be undertaken to assess the magnitude of the problem based on tidal flow data presented in the form of “tidal diamonds” on published Admiralty charts or measurements from the site. Alternatively more sophisticated methods may be adopted using sediment dispersion models to track the movement of sediment particles. Such models require tidal flow inputs from hydrodynamic models. When calculating the predicted physical changes it is important that the changes are put into context with natural and other anthropogenic changes that occur. The effect of the physical change on receptors within the footprint of change will be dependent on a number of factors including the ambient levels of suspended sediment and the degree of variation throughout the year, for example during storm events. If the natural levels of suspended sediment and the seasonal
Physical change variation are high then the degree of impact is likely to be less. There is considerable variation in the natural and anthropogenic induced levels of suspended sediment around the UK coast. Suspended sediment concentration varies around the UK from 1-327 mg/l around the English coast and 1-227 mg/l around the Welsh Coast but annual mean values are typically 1-110 mg/l (Parr et al., 1998; Cole et al., 1999). Other areas, particularly near estuaries will have higher concentrations. In the study area of the proposed Sheringham Shoal Offshore Wind Farm off the north Norfolk coast (Scira, 2006), concentrations varied from 10mg/l to 30mg/l and between 30mg/l (in summer) to 60mg/l (in winter) for the study area for Thanet offshore wind farm, off the east Kent coast (Royal Haskoning, 2005). The variability in the ambient levels of suspended sediment and the seasonal variation that can be experienced mean that it is not possible to state what the potential effect of cabling could be on certain receptors. In addition an area of seabed subject to disturbance already will be less affected by subsequent changes. In order to put the potential effects into context it should be considered against a number of other activities that occur within the marine environment, where studies have been completed to investigate potential and actual effects, e.g. aggregate extraction and fishing activity. Aggregate activity is a well controlled and monitored activity where levels of suspended sediment have been measured. There are a number of potential sources of increased suspended sediment concentrations including the release of material at the cutter or drag-head, overspill during hopper loading and sieving (which may be necessary in order to obtain an optimum sediment load). Of these potential sources the material suspended at the drag-head is the most likely to be similar to the material suspended during ploughing for cabling activities. Measurement of the plume generated by the movement of the drag-head alone have shown that the volume of sediment introduced into the water column is barely detectable and is in the order of 1% of the material introduced by screening and overflowing (Hitchcock et al., 1998, John et al., 2000). The scale of effect will obviously vary dependent on the sediment size and the relative amounts of overspilling/sieving undertaken but does provide a reasonable indication of the relative scale of effect when compared to aggregate extraction. It must also be borne in mind the temporary nature of the effect which is limited to one event (per cable) over a short time scale along with the immediate start of recovery of the seabed following disturbance. The low levels of sediment that are mobilised during cable laying mean that there will be only low levels of deposition around the cable route. The finer material will generally remain in suspension for longer but will settle and remobilise on each tide with no measurable material left in place. Coarser sediments are expected to settle within a few metres of the cable route and following disturbance are likely to recover rapidly, given similar communities in the vicinity. Case studies are provided below which illustrate some of the predicted physical changes resulting from cable laying activities. The resulting direct and indirect 91
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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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7 Gaps in understanding Research fo
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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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Review of Cabling Techniques and Environmental Effects Applicable

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