activities to cause sediment disturbance effects, potentially resulting in changes in suspended sediment concentrations and / or sea bed or, in the case of nearshore cable installation, shoreline levels due to deposition or erosion.
7.6.1.1 Impact 1: Changes in suspended sediment concentrations due to foundation installation
200. The installation of wind turbine foundations has the potential to disturb sediments from: (i) the sea bed (surface or shallow near-surface sediments); and (ii) from several tens of metres below the sea bed (sub-surface sediments), depending on foundation type and installation method. The worst case assumes that the disposal of any sediment that would be disturbed or removed during foundation installation would occur within the East Anglia THREE site.
7.6.1.1.1 Sea bed and shallow near-bed sediments 201. Sea bed sediments and shallow near-bed sediments within the East Anglia THREE site would become disturbed during any levelling or dredging activities that may be needed at each foundation location to create a suitable base prior installation. The worst case scenario for disturbance of these sediments assumes that sediment would be removed by means of dredging and returned to the water column at its surface layer as overflow froma dredger vessel.
202. This process would cause localised and short term increases in suspended sediment concentrations both at the point of dredging at the sea bed and, more importantly, at the point of its discharge back into the water column which, in the worst case scenario, would be at the water surface.
203. Mobilised sediment from these activities may be transported by wave and tidal action in suspension in the water column, ultimately resulting in its deposition elsewhere on the sea bed.
204. The disturbance effects at each wind turbine location are likely to last for no more than a few days of construction activity, with the overall construction programme lasting up to 2.5 years.
205. Baseline suspended sediment concentrations within the East Anglia THREE site are typically between 5 and 10mg/l in winter and below 5mg/l in summer, with a clear pattern of wave-stirring of sediment from the sea bed during storm conditions and relatively benign concentrations during ‘typical’ conditions (Appendix 7.5).
206. The mean grain size of sea bed sediments from samples taken across the East Anglia THREE site is in the range 0.21 to 0.36mm (medium sand). Most grab samples
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 42
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