Impact
Worst case parameter for assessment
Rationale
being placed on that location. By assessing the two impacts (Construction impact 1 and Operation impact 1) separately all possible impacts will be quantified although there would be a degree of “double counting”. This is a precautionary approach but ensures a comprehensive assessment.
Impact 2: Increased Suspended Sediment Concentrations (SSCs) and Sediment Re-deposition
The worst case scenario would involve the maximum amount of sediment disturbance including:
1. Sea bed preparation of 40m diameter gravity base foundations calculated as 17,500m3 per foundation (see Chapter 7 Marine Geology, Oceanography and physical processes Table 7.4). Therefore for 174 foundations (see Rationale column) the maximum expected amount sediment released into the water column is 3,045,000m3.
2. Sea bed preparation for up to six 60m diameter gravity base foundations (see rationale) would result in a maximum sediment release into the water column of 156,000m3
3.
Installation of up to 550km inter-array cables and 195km HVAC export cables between collector and converter station. Therefore the maximum expected amount of disturbed sediment (with worst case trench of up to 10m wide and 5m depth) is 37,250,000m3. However, only a small amount of this would be become suspended.
The total sediment released into the water column during construction within the East Anglia THREE site by foundation preparation would be up to 3,201,000m3 with a maximum of 37,250,000m3 of sediment disturbed (some of which would be released into the water column) by cable installation.
The installation of cables and foundations would be spread across 2.5
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 11 Fish and Shellfish Page 13
Only the smallest wind turbines (7MW) would be installed on 40m diameter gravity base foundations and therefore the worst case would be 172 wind turbine foundations, It has been assumed that the two meteorological masts would be installed on foundations which, if gravity base, would require similar sea bed preparation no greater than that of the 40m diameter foundations.
It has been assumed that the worst case for up to: five foundations for converter and collector stations, and one accommodation platform would result in a similar amount of sea bed preparation as that for the largest wind turbine foundation considered for East Anglia THREE which is a 60m diameter foundation (see Chapter 7 Marine Geology, Oceanography and physical processes Table 7.4)
Should the installation of jackets or tripods using pin piles be required, drilling may also be undertaken which would release subsurface materials into the water column.
Should the installation of jackets or tripods using pin piles be required, drilling may also be undertaken which would release subsurface materials into the water column.
Sub-surface sediments have a different physical composition to near-surface sediments and may therefore
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