of residual tidal flow. Sediment concentrations returned to background levels rapidly after cessation of the release into the water column.
217. Given this finding from the consecutive installation of fifteen wind turbine foundations, it is expected that effects from installation across the whole East Anglia THREE site would be similar, although with the point of release moving across the site with progression of the construction sequence. Indeed, the wave conditions across the East Anglia THREE site are more severe, suggesting that the passive plume would be similar or lower in concentration than that previously considered for East Anglia ONE.
7.6.1.1.2 Sub-surface sediments 218. Deeper sub-surface sediments within the East Anglia THREE site would become disturbed during any drilling activities that may be needed at the location of each monopile or 3 or 4 legged jacket (with pin piles) in order to install piles into the sea bed. The worst case scenario for a release from an individual wind turbine assumes that a 12m diameter monopile foundation would be drilled from the sea bed surface to a depth of 40m below the sea bed surface, releasing 4,524m3 of sediment into the water column per monopile. (This compares with a volume of 3,142m3 for each individual 7MWwind turbine with a maximum diameter of 10m drilled to the same 40m maximum depth).
219. For the total volume released during the construction phase, the worst case is associated with the maximum number (172) of 7MW monopiles of the maximum diameter (10m) for that wind turbine type. This yields a total volume of 540,353m3 for the wind turbine foundations (compared with 452,389m3 for the 12MW turbines because there be would be fewer of these wind turbines (100 in total) across the windfarm site). If it is assumed that similar foundations are used for the worst case of two meteorological masts, five offshore platforms and one accommodation platform, the total volume increases to 565,487m3.
220. To ensure a conservative approach to the assessment of effects, it has been assumed that the conservative average dredging volumes apply everywhere across the windfarm site, but in reality the foundation diameters would vary according to water depth and therefore this assessment is a deliberate over-estimate of potential effects.
221. This process would cause localised and short term increases in suspended sediment concentrations at the point of discharge of the drill arisings. Released sediment may then be transported by wave and tidal action in suspension in the water column, ultimately resulting in its deposition elsewhere on the sea bed.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 45
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