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12.6.1.1.2.4.1 Likely avoidance/TTS 281. In harbour porpoise and pinnipeds the range at which likely avoidance occurs is defined as the fleeing response or TTS onset range (Table 12.15).


Table 12.15 Summary of fleeing response /TTS onset distances (around mid-water column) estimated for pile driving during construction at East Anglia THREE for different hammer energies. Species or species group


Impact criteria SEL (dB re 1 μPa2·s)


Harbour porpoise Pinnipeds (in water)


164 179


Maximum hammer energy kJ 2,000


3,500


~4-6km <1.5km


~5-8km <2.5km


12.6.1.1.2.4.1.1 Spatial worst case 282. The spatial worst case assesses the maximum area over which displacement could occur at any one time based on two concurrent monopile foundations being installed.


283. The predicted fleeing response range for pinnipeds during construction at East Anglia THREE is less than 2.5km from the pile for the maximum hammer energy. At such short ranges, the area of this disturbance can be assumed to approximate to a circle ( with an approximate area of 19.6km2).


284. In order to calculate the number of individual seals that could be exposed to this impact the maximum mean at-sea seal density within the East Anglia THREE site ( section 12.5.2) has been multiplied by the area of effect. In the case of grey seal, this equates to 0.274 individuals (based on a maximum mean at sea density of 0.014 individuals per km2). In the case of harbour seal, this equates to 0.006 individuals (based on a maximum mean at sea density of 0.0003 individuals per km2).


285. During the construction of East Anglia THREE there could be two pile driving vessels operating concurrently across the site. As such the maximumnumber of individuals that could be affected would be doubled at most (as the range at which likely avoidance could occur would not overlap). However, less than 1% of the reference population for each species (and less than 1% of the South east England MU in the case of harbour seal) would be affected. The location of the displacement area will not be constant throughout the construction period, but will move around the site as construction progresses.


286. This worst case spatial impact of disturbance (concurrently installing two 12MW monopiles using the 3,500kJ hammer) would be limited to the duration of any concurrent driving of monopiles. Concurrent pile driving could occur over a maximum of 222 hours and 18 minutes over the two year construction period.


Preliminary Environmental Information May 2014


East Anglia THREE Offshore Windfarm


Chapter 12 Marine Mammal Ecology Page 75


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