In shallow water depths nearer to shore (less than 5m LAT) the potential for dispersion is more limited and therefore the concentrations are likely to be greater, approaching 400mg/l at their peak. However, these plumes would be localised to within less than 1km of the location of installation and would persist for no longer than a few hours.
After 180 hours following cessation of installation activities any plume would have been fully dispersed.
274. There are similarities in water depth, sediment types and metocean conditions between the offshore cable corridor for the proposed East Anglia ONE project and for the proposed East Anglia THREE project (indeed the inshore section is common to both proposed projects), making the earlier modelling studies a suitable analogue for the present assessments.
275. Using this as a basis, the worst case changes in suspended sediment concentrations due to offshore cable installation are likely to have the following magnitudes of effect (Table 7.21):
Table 7.21Magnitude of effect on suspended sediment concentrations due to offshore cable installation under worst case scenario
Location
Near-field* (nearshore)
Near-field* (offshore)
Far-field Scale High Low Negligible Duration Negligible Negligible Negligible Frequency Negligible Negligible Negligible
Reversibility Magnitude of Effect
Negligible Negligible Negligible Medium Low Negligible
* The near-field effects are confined to a small area (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.
276. These effects on suspended sediment concentrations due to offshore cable installation do not directly impact upon the identified receptors groups for marine geology, oceanography and physical processes, so there is no impact associated with the proposed project. The impacts arising from subsequent deposition of the suspended sediments on the sea bed are discussed under Impact 6 (section 7.6.1.6).
277. The effects do, however, have the potential to impact upon other receptors and therefore the assessment of impact significance is addressed within relevant chapters of the PEIR, taking into consideration the tidal ellipses presented in Figure 7.4, which represent the potential pathways between the source and receptor.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 55
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