present, this is likely to exist as a measureable but modest concentration (tens of mg/l) plume for around half a tidal cycle and sediment would fall to the sea bed in relatively close proximity to its release (within a few hundred metres up to around a kilometre, along the axis of the tidal flow) within a short period of time (order of hours).
213. This assessment is supported by an evidence-base obtained from research into the physical impacts of marine aggregate dredging on sediment plumes and sea bed deposits (Whiteside et al. 1995; John et al. 2000; Hiscock and Bell 2004; Newell et al. 2004; Tillin et al. 2011; Cooper and Brew 2013).
214. Modelling simulations undertaken for the East Anglia ONE site using the Delft3D plume model (ABPmer 2012b) confirm the above expert-based assessments of suspended sediment concentrations arising from disturbance of surface and shallow near-surface sediments. There are good similarities in sediment types and distributions between the East Anglia ONE (5% gravel, 93% sand and 2% mud) and East Anglia THREE (1.5% gravel, 94.5% sand and 4% mud) sites. The water depths for each site are also within a similar range. Whilst the East Anglia THREE site is located closer to the amphidromic point than the East Anglia ONE site, and therefore the tidal currents are slightly lower, it is located further offshore and therefore the wave conditions are higher. Overall, therefore, the earlier modelling studies for the East Anglia ONE site represent a sufficiently suitable analogue for broadly verifying the conclusions of the more qualitative expert-based assessment described above.
215. In the earlier modelling studies, consecutive daily releases of 22,500m3 of sediment (mostly medium sand, but also with small proportions of gravel, other sand fractions and muds) were made at the water surface at fifteen wind turbine locations across the East Anglia ONE site. This sediment release is greater than the conservative average release volume from each of the 7MWwind turbines, but slightly lower (85% of the value) than the conservative average release volume from each of the 12MWwind turbines. Nonetheless it does represent a suitable analogue for the type and broad magnitude of effect.
216. The previous modelling showed that away from the immediate release locations, where suspended sediment concentrations were very high (orders of magnitude in excess of natural background levels) for a very short duration (seconds to minutes) as the dynamic plume falls to the bed, elevations in suspended sediment concentration above background levels within the passive plume were low (<10mg/l) and within the range of natural variability. Net movement of fine grained material retained within the passive plume was to the north, in accordance with the direction
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 44
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