7.6.2.5 Impact 5: Changes to the sea bed morphology due to the presence of foundation structures
346. The sea bed morphology would directly be impacted by the footprint of each foundation structure on the sea bed within the East Anglia THREE site. This would constitute a ‘loss’ in natural sea bed area during the operational life of the proposed project.
347. This direct footprint could be further enhanced due to the presence of foundation structures in one of two ways.
348. Under a scenario of no scour protection being provided, a scour hole would be likely to develop around each foundation. This would have two implications for sea bed morphology, in addition to the direct foundation footprint. The scour hole would directly affect an area of the sea bed, lowering sea bed levels locally around each foundation and causing mobile sediments to be suspended into the water column. These sediments would ultimately settle back to the sea bed potentially causing bed level changes due to deposition.
349. Under an alternative scenario of scour protection being provided, the sea bed would be further occupied by material that is ‘alien’ to the baseline environment, such as concrete mattresses, fronded concrete mattresses, rock dumping, bridging or positioning of gravel bags.
350. The worst case direct sea bed footprint for a foundation structure is associated with the upper diameter of the gravity base structures in the size in the greatest water depth. These maximum foundation sizes would be associated with the highest-rated wind turbines (12MW) and therefore there would be up to 100 foundations, each with a footprint of 2827.8m2 (Table 7.3). There would also be up to two meteorological masts, each with a footprint of 315m2, and up to six offshore platforms (three collector stations two converter station and one accommodation platform) presumed to have the same size footprint as the largest wind turbine foundation size. Thus taking the total worst case for foundation numbers to 108. This arrangement would result in a total worst case direct foundation footprint area across the project of 300, 377m2. This represents 0.1% of the total sea bed area within the East Anglia THREE site (304.8km2).
351. When the area of predicted scour hole development for the worst case largest diameter gravity base structures is combined with the direct foundation footprints at each of the 108 foundation locations, the combined area of sea bed affected increases to a maximum of 904,694m2. This represents 0.30% of the total sea bed area within the East Anglia THREE site. The changes in bed levels due to settling of
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 69
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