elevations in suspended sediment concentration above background levels were low (<10mg/l) and within the range of natural variability. Indeed, concentrations were generally no greater than 5mg/l at a distance of 5km from the release location, indicating wide dispersion in low concentrations. Net movement of fine grained material retained within a plume was to the north, in accordance with the direction of residual tidal flow, although gross movement to both the north and south was possible depending on timing of release. Sediment concentrations arising from one foundation installation were deemed unlikely to persist for sufficiently long that they significantly interact with subsequent operations and therefore no cumulative effect was anticipated.
229. The larger release volumes associated with the worst case for the East Anglia THREE site and the slightly lower tidal currents compared with the East Anglia ONE site may combine to result in larger concentrations above background levels than previously modelled (but likely to still be modest; of the order of tens of mg/l) but the principle of wide dispersion in relatively low concentrations remains valid. Furthermore, a conservative assumption was made in the modelling that all drilled sediment would fully disaggregate into component particle sizes but in reality some would arrive at the surface as larger aggregated clasts which would settle more rapidly.
230. The changes in suspended sediment concentrations (magnitudes, geographical extents and durations of effect) that are anticipated above would move across the site with progression of the construction sequence as the point of sediment release (and hence geographic location of the zone of effect) changes with the installation of foundations at different wind turbine locations.
7.6.1.1.3 Assessment of effect magnitude and/or impact significance 231. Given that the expert-based assessments of the dynamic and passive plume effects on suspended sediment concentrations for the proposed East Anglia THREE project are consistent with the findings of the earlier modelling studies for the proposed East Anglia ONE project, there is high confidence in the assessment of effects.
232. The worst case changes in suspended sediment concentrations due to foundation installation are likely to have the following magnitudes of effect (Table 7.17):
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 47
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