301. The landfall location lies at the shoreline within an identical offshore cable corridor to the proposed East Anglia ONE project, which also is intended to be installed by means of HDD.
302. The EIA for the proposed East Anglia ONE project has therefore previously assessed the effects of cable installation at the landfall using HDD techniques.
303. The baseline characteristics within the cable corridor at the landfall and across relevant adjacent lengths of shore are provided in Appendix 7.4. Consideration of final burial depths and shoreline set-back distances would be made during detailed design, based upon observations of past coastal change and projections of future coastal change, taking into consideration climate change effects (especially sea-level rise) during the operational lifetime of 25 years.
304. There are presently two installation options, a ‘long’ method where HDD extends seaward a distance of 1,100m from the base of the cliff and a ‘short’ method with a shorter HDD distance and an offshore trench is then excavated to deeper water. The ‘short’ method may be required in areas where there is only a thin veneer of surface sediment overlying the London Clay geology. This enables the cable to remain buried even if the surface sediment veneer is reduced or removed by natural sediment transport processes.
305. The ‘long’ HDD option would cause minimal direct disturbance to the existing coastline during construction.
306. The ‘short’ HDD option would cause minimal direct disturbance over the most landward sections at the cable landfall but some disturbance would occur where trenching and back-filling is needed in the London Clay further seaward in the inter- tidal or nearshore zone. However, these effects would be highly localised and temporary in duration. The trenching into London Clay would likely result in clumps ofmud to be displaced and back-filled, rather than the material breaking down into its constituent silt and clay particles. It is therefore unlikely that significant changes in suspended sediment concentration would be noted during these works. The back- filling of the trench would result in no noticeable change in coastal morphology after completion of the offshore cable installation at the landfall.
7.6.1.8.1 Assessment of effect magnitude and/or impact significance 307. The worst case changes to suspended sediment concentrations and coastal morphology at the cable landfall are likely to have the following magnitudes of effect (Table 7.25):
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 60
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