across a greater sea bed area, but the changes in wave heights across this wider area would be notably lower (a few %) than the changes local to each wind turbine foundation (tens of %). Since it is expected that the changes in tidal flow and wave heights during the operation phase would have no significant far-field effects, then so the changes in sediment transport would be similar, with the likely following magnitudes of effects (Table 7.30):
Table 7.30Magnitude of effects on the sediment transport regime due to the presence of foundations under worst case scenario
Location
Near-field Far-field
Scale Low Negligible Duration
High High
Frequency
Medium Medium
Reversibility Magnitude of Effect
Negligible Negligible
Low Negligible
337. The impacts on the sediment transport regime would not extend beyond the zones of influence previously illustrated for the changes to the tidal and wave regimes and therefore, there is no impact associated with the proposed project on the marine geology, oceanography and physical processes receptor groups.
338. The effects of the changes in tidal regime and wave heights on the local sediment transport regime are manifest in terms of scour hole generation (see Impact 4 below).
7.6.2.4 Impact 4: Changes in suspended sediment concentrations due to scour around foundation structures
339. The localised changes in the tidal and wave regimes around each foundation structure are likely to result in localised scour of the sea bed, under a worst case that involves no scour protection being provided.
340. Scour assessments using empirical methods presented by Bos (2002a; 2002b), Harris et al.(2010), Khalfin (1983; 2007a, b), Sumer and Fredsoe (2002) and Whitehouse et al.(2011a, b) have been performed to determine scour depths, plan areas and associated sediment volumes for the worst case foundation type of gravity base structures (See Table 7.6) . These methods have been further informed by the theories of Soulsby and Clarke (2005) in relation to combined waves and currents and Annandale (1995; 2006) and Annandale and Smith (2001) in relation to the strength properties of the sea bed sediments. Findings from the approaches have been verified against field measurements and laboratory scale physical model tests (Bolle et al. 2009; 2010; Khalfin 2007b; Larsen and Frigaard 2005; Margheritini 2012; Raaijmakers and Rudolph 2008; Stahlmann and Schlurmann 2010; Whitehouse et al.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 67
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