222. 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.
223. Although the sub-surface sediment release quantities involved under a worst case scenario are considerably lower than those involved in the worst case scenario for the surface and near-bed sediments, the sediment types would differ, with a larger proportion of finer materials.
224. The sediment types likely to be encountered have been estimated from the two available borehole logs that are located directly within the East Anglia THREE site.
225. Expert-based assessment suggests that the coarser sediment fractions (medium and coarse sands and gravels) and aggregated ‘clasts’ of finer sediment would settle out of suspension in relatively close proximity to the foundation location, whilst disaggregated finer sediments (fine sands and muds) would be more prone to dispersion across a wider area. Due to the small quantities of sediment release involved, however, these disaggregated finer sediments are likely to be widely and rapidly dispersed, resulting in only low elevations in suspended sediment concentration and low changes in bed level when they ultimately come to rest on the sea bed.
226. Modelling simulations undertaken for the proposed East Anglia ONE project using the Delft3D plume model (ABPmer 2012b) confirm the above expert-based assessments of suspended sediment concentrations arising from disturbance of deeper sub-surface sediments. As agreed with Natural England and Cefas through the Evidence Plan, the earlier modelling studies make a suitable analogue for the present assessments, with any key differences between the two sites being explicitly noted.
227. In the earlier modelling studies, 982m3 of variably graded fine sediment (sand, clay and silt) was released into the water column once every two days to simulate the construction of 8 consecutively drilled (jacket) foundations over a 15 day simulation period. This value is similar to the worst case volume that would be released from the jacket foundations that are being considered for the East Anglia THREE site (831m3), with a similar distribution of envisaged sediment types at depth, but is acknowledged to be less than the worst case scenario for the monopile foundations being considered for East Anglia THREE (4,524m3).
228. Nonetheless, the previous modelling results support the general principles of the expert-based assessments in that, away from the immediate release locations,
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 46
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