Table 17.3 Worst Case Assumptions Impact Construction
Direct Impacts: Direct disturbance to archaeological receptors and / or their physical setting
Sea bed Preparation (including the deposition of dredged material).
Installation of foundations.
Installation of scour protection if required.
Cable burial and installation (including inter-array, export and interconnector).
Installation of offshore ancillary structures. Sea bed contact from the anchors of support vessels.
Sea bed contact from the legs of jack-up vessels.
The worst case scenario is defined by the development options which result in the maximum possible disturbance to the sea bed. This includes consideration of the largest sea bed footprint area, the greatest volume of spoil and the highest number of locations which may be subject to physical impacts.
The sea bed preparation and installation of Conical Gravity Base foundations typically comprise the worst case scenario for archaeological and cultural heritage receptors at and under the sea bed (dependent on the depth of sea bed preparation). This is due to requiring the largest foundation and scour protection footprint and the substantial volume of sea bed sediment dredged from the sea bed in preparation for installation, leading to significant vertical and horizontal removal of sediment. This is also the worst case scenario for the deposition of dredged material, whereby 90% of all sediment released at the water surface from the dredger is deposited from the highly turbid dynamic plume which would rapidly fall to the sea bed following its release, potentially impacting upon archaeological and cultural heritage receptors located within the deposition footprint.
The sea bed footprint from jack-up vessels comprises the worst case situation for attendant vessels during construction due to the largest footprint and the compression of the sea bed which may damage potential archaeological and cultural heritage receptors of all types.
For potentially prehistoric archaeology receptors buried at depth within sub-sea bed sediments the worst case situation comprises the installation of 12m diameter monopoles (Chapter 7 Marine Geology, Oceanography and Physical Processes).
At the landfall site, the worst case situation comprises ‘short’ HDD due to the larger footprint of physical disturbance to sea bed sediments (Chapter 7 Marine Geology, Oceanography and Physical Processes).
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 17 Offshore Archaeology Page 16
Key Design Parameters forming the Worst Case Scenario
Rationale
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