7.6.1.6 Impact 6: Changes in sea bed levels due to offshore export cable installation 278. The increases in suspended sediment concentrations associated with Impact 5 have the potential to result in changes in sea bed levels as the suspended sediment deposits on the sea bed.
7.6.1.6.1 Assessment of effect magnitude and/or impact significance 279. Given that the changes in suspended sediment concentration due to offshore export cable installation would be lower than those arising from the disturbance of sea bed and near-bed sediments during foundation installation activities, so the magnitude of bed level changes would also be lower, although the location of effect would differ as it would be focused along the offshore cable corridor.
280. Modelling simulations undertaken for the proposed East Anglia ONE project using the Delft3D plume model (ABPmer 2012b) confirm that bed level changes of up to 2mm would be observed within a few hundred metres of the inshore sections of the offshore cable corridor and further afield the bed level changes are not expected to be measureable.
281. Using this as a basis, the worst case changes in bed levels due to offshore cable installation are likely to have the following magnitudes of effect (Table 7.22):
Table 7.22Magnitude of effect on bed level changes due to offshore cable installation under worst case scenario
Location
Near-field* Far-field
Scale Low Negligible Duration
Negligible Negligible
Frequency
Negligible Negligible
Reversibility Magnitude of Effect
Negligible Negligible
Low Negligible
* The near-field effects are confined to a small area of sea bed (likely to be of the order of several hundred metres up to a kilometre from the offshore cable corridor), and would not cover the whole offshore cable corridor.
282. These effects on sea bed level are considered highly unlikely to have the potential to impact directly upon the identified receptor groups for marine geology, oceanography and physical processes due to separation distances, except for parts of the Suffolk Natural 2000 site across which part of the offshore cable corridor crosses or comes within close proximity.
283. For most receptor groups the magnitude of effect is lower than that associated with foundation installation and there is a large separation distance (well beyond one tidal ellipse) which does not support the existence of a pathway between the source and receptor.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 56
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