contribute to changes in the sea bed morphology due to alteration of sediment transport patterns (see operation impact 3, section 7.6.2.3) or due to initiation of sea bed scour (see Impact 4, section 7.6.2.4).
313. Expert-based assessment suggests that each foundation would present an obstacle to the passage of currents locally, causing a small modification to the height and/or phase of the water levels and a wake in the current flow. This latter process involves a deceleration of flow immediately upstream and downstream of each foundation and an acceleration of flow around the sides of each foundation. Current speeds return to baseline conditions with progression downstream of each foundation and generally do not interact with wakes from adjacent foundations due to the separation distances.
314. There is a strong pre-existing scientific evidence base which demonstrates that the changes in the tidal regime due to the presence of foundation structures are both small in magnitude and localised in spatial extent. This is confirmed by existing guidance documents (ETSU 2000; ETSU 2002; COWRIE 2009) and numerous Environmental Statements for offshore windfarms.
315. This is further supported by previous Delft3D numerical modelling of changes in hydrodynamics associated with the proposed East Anglia ONE project (ABPmer 2012b). This modelling was based on a worst case of 240 gravity base structures (50m base diameter) and showed changes in water level of less than ±0.007m across a small geographical area.
316. In respect of changes in tidal currents, the previous modelling predicted maximum reductions in peak flow speeds of 0.05 to 0.1m/s and maximum increases in peak flow speeds of 0.05m/s, from peak baseline values for the proposed East Anglia ONE project of around 1m/s. The geographical extent of these maximum changes was largely confined to the near-field environment (a wake zone local to each wind turbine foundation).
7.6.2.1.1 Assessment of effect magnitude and/or impact significance 317. Given that the expert-based assessments of the changes in the tidal regime associated with the presence of foundation structures for the proposed East Anglia THREE project are consistent with the findings of the earlier modelling studies for the proposed East Anglia ONE project, there is high confidence in the assessment of effects.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 62
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