60.
The background electrical field in the area would depend upon the tidal flow moving through the local geomagnetic field. Using a conservative estimate of maximum seabed flows of 1.2m/s, background electric fields could therefore be expected to reach a maximum of approximately 60µV/m.
61.
Appendix 9.2 shows that B fields generated by the AC cable designs proposed are expected to be well below the magnitude of the background geomagnetic field at the seabed (assuming 1m burial). iE fields associated with 75kV AC are expected to be below background electrical fields. In the case of the higher voltage AC cables, iE fields would be above the background E field induced by tidal flow. However, they would attenuate quickly within 0.5 to 1m for 132kV and 5 to 10m for the likely worst case rating cable of 220kV.
62.
In the case of DC cables, both B fields and iE fields are predicted to be similar to or below background levels where cables are buried more than 1m from the seabed surface (both depend on the same tidal flow flowing through the B fields).
63.
For the likely worst case, 320kV cables buried to 0.5m, both fields would be reduced to below background within 0.5m if cables are bundled, and 4.5m if cables are laid separately. For 500kV cables buried to 0.5m, fields would be reduced to background within 2.5m (bundled) and 9.5m (separated).
64. How, or whether, the fields generated by the East Anglia THREE project would interact with background fields is not certain. Current understanding is that the B field that is more intense is the one likely to be more easily detected and is therefore of greater relevance to an organism. Once B fields attenuate to below the geomagnetic field, they may be of less relevance to an organism, however owing to differences in field’s geometries and characteristics, the two fields may be decipherable.
65.
Similarly, once iE fields generated by AC cables attenuate to below the background (tidally induced) iE field, they may be less relevant to organisms, although the two fields may still be decipherable.
9.10.2 Electromagnetic Field Detection 66. A relatively large number of organisms in the marine environment are either known to be sensitive to electromagnetic fields or have the potential to detect them (Gill and Taylor 2001; Gill et al. 2005).
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 9 Underwater Noise Page 16
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