species or habitats (Chapter 7 Marine Geology, Oceanography and Physical Processes section 7.6.1.7).
179. Increased suspended sediment load has the potential to affect the benthos through blockage to the sensitive filter feeding apparatus of certain species and / or smothering of sessile species upon deposition of the sediment.
10.6.1.2.1 East Anglia THREE site 180. Chapter 7Marine Geology, Oceanography and Physical Processes identifies that installation of wind turbine foundations has the potential to disturb sediments from the sea bed and from several tens of metres below the sea bed, depending on the foundation type and installation method.
181. To install foundations, the sea bed would also potentially require levelling which involves the removal of sediment by a dredge. The worst case scenario would involve the release of this sediment into the water column at its surface layer by the dredger vessel. This process would cause localised and short term increases in suspended sediment concentrations both at the point of dredging at the sea bed and, to a greater extent, at the point of its discharge back into the water column at the water surface.
182. The worst case scenario involves an excavation of the sea bed to level an area of sandwaves up to 5m in height, with a maximum volume of excavation per foundation of 26,000m3. This is associated with a 60m diameter gravity base structure. However the worst case scenario described in Table 10.5 involves the sea bed preparation for 174 smaller (40m diameter) gravity base foundations to support wind turbines and meteorological masts and up to six larger (60m diameter) gravity base foundations for collector and converter stations and an accommodation platform. The sea bed preparation for these foundations could result in the maximum of 3,201,000m3 of sediment being released into the water column. However, foundations would be installed over a 2.5 year period with a maximum of two sea bed preparation events for foundations within a 24 hour period.
183. The mean grain size of sea bed sediments across the East Anglia THREE site is in the range 0.21 – 0.36mm (medium sand). Very small percentages of gravels and muds were present. It is predicted that sediment disturbed from the sea bed during installation would remain close to the bed and rapidly settle. The material released at the water surface from the dredger vessel would rapidly (seconds to minutes) fall to the sea bed as a highly turbid dynamic plume immediately upon its discharge.
184. Some of the finer sand fraction from this release and the very small proportion of muds are likely to stay in suspension for longer and form a passive plume which
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 1 Introduction Page 63
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