8.6 Potential Impacts 8.6.1 Potential Impacts during Construction
8.6.1.1 Impact 1: Change in Water Quality due to Re-suspension of Sediments during construction of the East Anglia site (Foundation Installation and Inter-array Cabling).
78. A number of activities associated with the construction of an offshore windfarm have the potential to re-suspend sediments, thus impacting on the turbidity of the water. The specific activities that could contribute to the deterioration in water quality due to the re-suspension of sediments include: sediment release from sea bed preparation or any drilling required for the installation of foundations (i.e. wind turbines and the offshore collector and converter stations); vessel movements (i.e. jack-up feet or anchors); and inter-array and offshore export cable installation.
79.
Site specific modelling of the potential increase, dispersion and deposition of sea bed sediments associated with the proposed East Anglia THREE project has not been undertaken. However, due to similarities in water depth, sediment types and metocean conditions and proximity of the East Anglia ONE and East Anglia THREE sites, it was agreed with Cefas and Natural England that the use of the modelling and studies undertaken for the proposed East Anglia ONE project were valid proxies for use within this assessment. For full details of the assessment, please refer to Chapter 7 Marine Geology, Oceanography and Physical Processes.
80.
In relation to the disturbance of surface sediments, the predominantly medium sand grain size present at the site indicates that any sediment disturbed during sea bed preparation for gravity base structure foundations (up to 26,000m3 per foundation), would remain close to the site and settle rapidly. There is a possibility that some of the finer sand andmud fractions could stay in suspension for longer, however, Chapter 7Marine Geology, Oceanography and Physical Processes concludes that these would fall to the sea bed in relatively close proximity (<1km) to its release within a short period of time.
81.
Further evidence for this prediction was provided by modelling simulations undertaken for the proposed East Anglia ONE project. This work concluded that, with the exception of the immediate area around the foundations, concentrations of suspended sediment in the plume associated with foundation installation remain within the limits of natural variation and returned rapidly to background levels following cessation of the activity.
82.
In relation to the release of deeper sub-surface sediments associated with the installation of monopiles, based on estimates from the two available borehole logs
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 8 Marine Water and Sediment Quality Page 27
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