to that on the existing sea bed and therefore there would be no significant change in sediment type.
238. In addition to the localised mounds, some of the sediment from this release (mainly the fine sand fraction and the very small proportion of muds) is likely to form a passive plume and become more widely dispersed before settling on the sea bed. Expert-based assessment suggests that due to the dispersion by tidal currents, the thickness of deposits across the wider sea bed area would be very small (within the order of millimetres).
239. This assessment is supported by an evidence-base obtained from research into the physical impacts of marine aggregate dredging on sediment plumes and sea bed deposits (Whiteside et al. 1995; John et al. 2000; Hiscock and Bell 2004; Newell et al. 2004; Tillin et al. 2011; Cooper and Brew 2013).
240. The earlier Delft3D plume modelling studies (ABPmer 2012b) considered the bed level changes resulting from deposition of sediments from the passive plume due to sea bed preparation for 15 foundations. This involved a worst case sediment release of 22,500m3 per foundation (i.e. around 85% of the value of the average conservative volume considered as the worst case for an individual wind turbine in East Anglia THREE). For the most part, the deposited sediment layer across the wider sea bed was found to be less than 0.2mm thick and did not exceed 2mm anywhere. The previous assessment also concluded that this deposited sediment also has the potential to become re-mobilised and therefore would rapidly become incorporated into the mobile sea bed sediment layer, thus further reducing any potential effect.
7.6.1.2.2 Sub-surface sediments 241. Expert-based assessment suggests that due to the finer-grained nature of any sub- surface sediment released into the water column from drilling, there would be greater dispersion across a wider area, in keeping with the pattern of the tidal ellipses.
242. The earlier Delft3D plume modelling studies (ABPmer 2012b) considered the bed level changes resulting from deposition of sediments from drilling for 8 piled (jacket) foundations. The coarser sediment become deposited near to the point of release to thicknesses of up to a few centimetres, but over a sea bed area local to each foundation (within a few hundred metres). For the most part, the deposited sediment layer across the wider sea bed area was found to be less than 0.025mm thick.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 49
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