Impact
Impact 5: Change in Water Quality due to works at the offshore export cable Landfall
Key design parameters forming the realistic worst case scenario Rationale
Installation of cable by the short Horizontal Directional Drilling (HDD)method described in Chapter 5 Description of the Development.
At landfall, the offshore export cable would be installed to depths of 3 to 10m below the sea bed under the existing coastal line using HDD methods.
A pre-cut trench would connect the onshore export cable to the offshore export cable. The option requiring the largest pre-cut trench would be the worst case scenario as this option would release the greatest amount of sediment into the water column.
There are two variants of the HDD method, a ‘long’ or ‘short’ HDD. The ‘long’ method would use HDD to install the cable below the sea bed up to 1,100m from the base of the cliff. The ‘short’ method would have a shorter offshore set-back distance and an offshore trench would be excavated towards deeper water.
Operation
Impact 1: Deterioration in water quality due to re- suspension of sediments associated with scouring.
The maximum amount of sediment that would be placed into suspension due to changes in tidal regime around infrastructure with no scour protection has been calculated based on scour produced by 60m diameter gravity base foundations which has been calculated as 4,580m3 per foundation (see Chapter 7 Marine Geology, Oceanography and physical processes Table 7.6). Therefore for 108 foundations (see Rationale column) the maximum expected amount scour released into the water column is 494,640m3.
All of the above are based on a 1 in 50 year return period.
Impact 2: Change in sediment and water quality as a result of the release of hazardous
Sources of hazardous materials would be from the installed structures and fromvessels visiting the site. The worst case scenario involves:
Preliminary Environmental Information May 2014
The need for scour protection would not be determined until the wind turbine location and associated foundation types are known. Therefore, the worst case scenario involves the use of no scour protection which may potentially cause the formation of scour holes around the foundations.
The largest wind turbines (12MW) would be installed on 60m diameter gravity base foundations and therefore the worst case would be 100 wind turbine foundations. It has been assumed that the worst case for up to: five foundations for converter and collector stations, two meteorological masts and one accommodation platform would result in a similar amount of scour as that for a wind turbine foundation.
Installed infrastructure would include; 172 wind turbines, three collector and two converter stations, two meteorological masts and one accommodation platform.
East Anglia THREE Offshore Windfarm
Chapter 8 Marine Water and Sediment Quality Page 7
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