contained only very small percentages of gravels and muds, with most being sands (see Appendix 7.2 for further details of the grab samples).
207. The worst case scenario involves the maximum quantity of sediment released through sea bed preparation activities for the different wind turbine sizes being considered.
208. For a release from an individual wind turbine, the worst case is associated with the conservative average dredging volume for each individual 12MW wind turbine (with a maximum diameter of 60m). This yields a conservative average dredging volume of 26,000m3 per wind turbine (compared with a conservative average dredging volume of 17,500m3 for each individual 7MW wind turbine with a maximum diameter of 40m).
209. For the total volume released during the construction phase, the worst case is associated with the maximum number (172) of 7MW gravity base structures of the maximum foundation diameter (40m) for that wind turbine type. This yields a conservative average total dredging volume of 3,010,000m3 for the wind turbine foundations (compared with 2,600,000m3 for the 12MWwind 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 and that the larger (60m diameter foundations) are used for the up to five offshore platforms and one accommodation platform, the total volume increases to 3,201,000m3.
210. 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.
211. Expert-based assessment suggests that, due to the sediment grain sizes present across the proposed windfarm area, the sediment disturbed from the sea bed by the drag head of the dredger would remain close to the bed and rapidly settle, whilst the majority of material released at the water surface from the dredger vessel would rapidly (order of minutes or tens of minutes) fall to the sea bed as a highly turbid dynamic plume immediately upon its discharge.
212. Some of the finer sand fraction from this release and the very small proportion of muds that are present are likely to stay in suspension for longer and form a passive plume which would become advected by tidal currents. Due to the sediment sizes
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 43
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