Impact
Impact 2: Change in Water Quality due to Re-suspension of sediments during construction in the offshore cable corridor (installation of cabling).
Key design parameters forming the realistic worst case scenario Rationale
reality, trenches would be V shaped and therefore less sediment would be disturbed.
The maximum cable installations within the offshore cable corridor includes the installation of up to 620km of HVDC export cable, which would result in the disturbance of up to 31,000,000m3 of sediment (some of which would be released into the water column) by cable installation.
The maximum realistic speed of cable installation is likely to be approximately 450/hr.
Up to four export cables would be installed.
The worst case cable installation techniques would be jetting / vertical injector techniques, which relies on either fluidisation or liquefactions of the sea bed.
The sediment disturbance during cable installation takes a very conservative approach assuming that all cable trenches would measure 5m depth by 10m width as those are the maximum possible dimensions (Chapter 5 Description of the Development). In reality, trenches would be V shaped and therefore less sediment would be disturbed.
Impact 3: Change in water quality due to re-suspension of contaminants within sediment.
As above for Impacts 1 and 2.
The worst case scenario relates to activities that involve the most re- suspension of near surface sediment.
Sub surface sediments have not been exposed to the water column and contaminant sources; therefore, they are likely to contain low levels of contamination. Whereas, near-surface sediments are likely to contain higher concentrations of contaminants due to their exposure to the water column. Therefore, the worst case scenario would involve the installation of gravity base structures.
The rationale relating to sea bed preparation and installation of gravity base structures as well as the cable installation are therefore relevant here.
Impact 4: Change in Water and Sediment Quality due to Accidental Releases or Spills of Construction Materials or Chemicals
It is anticipated that a maximum of 45 vessels would be on site at any time during the construction of the proposed East Anglia THREE project with up to 39 vessels involved in the construction and a further six commissioning components of the windfarm where construction had been completed.
Preliminary Environmental Information May 2014
The greater the number of vessels on site the greater the chance of accidental spillage.
East Anglia THREE Offshore Windfarm
Chapter 8 Marine Water and Sediment Quality Page 6
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