Table 7.17Magnitude of effect on suspended sediment concentrations due to foundation installation under worst case scenario
Location
Near-field* Far-field
Scale
High Low
Duration
Negligible Negligible
Frequency
Negligible Negligible
Reversibility Magnitude of Effect
Negligible Negligible
Medium Low
* The near-field effects are confined to a small area (likely to be of the order of several hundred metres up to a kilometre from each foundation location), and would not cover the East Anglia THREE site.
233. The effects on suspended sediment concentrations due to foundation installation for the proposed project do not directly impact upon the identified receptor groups for marine geology, oceanography and physical processes, so there is no impact associated with the proposed project.
234. The effects do, however, have the potential to impact upon other receptors and therefore the assessment of impact significance is addressed within relevant chapters of the PEIR, taking into consideration the tidal ellipses presented in Figure 7.4, which represent the potential pathways between the source and receptor.
7.6.1.2 Impact 2: Changes in sea bed levels due to foundation installation 235. The increases in suspended sediment concentrations associated with Impact 1 (section 7.6.1.1) have the potential to result in changes in sea bed levels as the suspended sediment deposits on the surrounding sea bed potentially raising the seabed level slightly. There would be different settling rates for the sediment types associated with the sea bed and shallow near-bed sediment disturbance and the deeper sub-surface sediment disturbance, so each is discussed in turn.
7.6.1.2.1 Sea bed and shallow near-bed sediments 236. Expert-based assessment suggests that the coarser sediment would rapidly (within the order of minutes or tens of minutes) fall to the bed as a highly turbid dynamic plume immediately upon its discharge, forming a deposit (‘mound’) local to the point of release. Due to the sediment grain sizes observed across the site (predominantly medium sand or coarser, with very little fine sand or muds), a large proportion of the disturbed sediment would behave in this manner.
237. The resulting mound would be a measureable protrusion from the sea bed (likely order of tens of centimetres to a few metres in height) but would remain highly localised to the release point. The precise configuration of height and spreading distance of each mound would vary across the windfarm site, depending on the prevailing conditions, but in all cases the material within the mound would be similar
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 48
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