Table 12.16 Summary of likely avoidance (fleeing response /TTS onset) areas (around mid-water column) and estimated number of individuals affected for pile driving during construction at East Anglia THREE Scenario
Hammer energy
Spatial worst case (two vessels monopiles, no overlap)
Temporal worst case (single vessel jacket)
3,500kJ
Area (km2)
281.6
Density (individuals per km2)
0.187 (harbour porpoise)
0.284 (harbour porpoise and unidentified dolphin and porpoise)
2,000kJ 72.1
0.187 (harbour porpoise)
0.284 (harbour porpoise and unidentified dolphin and porpoise)
No. individuals
53 80 13 20
Percent of reference population (227,298) 0.023%
0.035% 0.006% 0.009%
12.6.1.1.2.4.1.2 Temporal worst case 291. In the temporal worst case scenario pile driving will be completed by a single vessel. Using the jacket approach and a 1,800kJ hammer (modelled hammer energy of 2,000kJ used as a proxy) pile driving could occur over two years for a total of 1,272 hours (Table 12.2). Therefore, in this scenario pile driving would occur for a maximum of 6.9% of the time over a two year period. Using a single vessel to install 12MWmonopiles would lead to a maximum of 444 hours and36 minutes of pile driving over the two years (or piling for 2.5% of the time).
292. If it is assumed that individuals will not return to the area between piling events (average waiting time is 8 hours 46 minutes between piling events for jacket foundations or 37 hours 59 minutes for 12MWmonopiles) likely avoidance would occur for approximately 41% or 26% of the two year construction period in the jacket scenario and 12MW monopile scenarios respectively. The jacket scenario can therefore be considered the worst case for temporal displacement of marine mammals.
293. The number of grey seal that are likely to avoid the area for up to 41% of the construction period is less than one seal (0.274 individuals) based on a maximum mean at sea density of 0.014 individuals per km2. In the case of harbour seal, the effect equates to 0.006 individuals (based on a maximum mean at sea density of 0.0003 individuals per km2).
294. The number of harbour porpoise which are likely to avoid the area for up to 41% of the two year construction period is summarised in Table 12.16. Based on the higher
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 12 Marine Mammal Ecology Page 77
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