cables because parts of the offshore cable corridor are in considerably shallower water and in closer proximity to the identified morphological receptor groups.
269. The types and magnitudes of effects that could be caused have previously been assessed within an industry best practice document on cabling techniques (BERR 2008). This document has been used alongside expert-based judgement and analysis of site conditions to inform the assessments presented below.
7.6.1.5.1 Assessment of effect magnitude and/or impact significance 270. It is anticipated using expert-based assessment that the changes in suspended sediment concentration due to offshore cable installation would be less than those arising from the disturbance of sea bed and near-bed sediments during foundation installation activities, although the location of effect would differ as it would be focused along the offshore cable corridor.
271. This assessment is based on the overall sediment release volumes being low and confined to near the sea bed (rather than higher in the water column) along the alignment of the offshore cable route, and the rate at which the sediment is released into the water column from the jetting process would be relatively slow.
272. It is likely that the concentrations would be enhanced by the greatest amount in the shallowest sections of the offshore cable corridor, but in these locations the background concentrations are also greater than in deeper waters, typically up to 170mg/l (ABPmer 2012a).
273. Modelling simulations undertaken for the proposed East Anglia ONE project using the Delft3D plume model (ABPmer 2012b) confirm the above expert-based assessments and provided the following quantification of magnitude of change:
Sand-sized material (which represents most of the disturbed sediment) would settle out of suspension within less than 1km from the point if installation within the offshore cable corridor and persist in the water column for less than a few tens of minutes.
Mud-sized material (which represents only a very small proportion of the disturbed sediment) would be advected a greater distance and persist in the water column for hours to days.
In water depths greater than 20m LAT, peak suspended sediment concentrations would be typically less than 100mg/l, except in the immediate vicinity (a few tens of metres) of the release location.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 54
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