12.6.1.1.2.4.1.3 Likely avoidance/TTS summary 299. The duration of pile driving noise and potential displacement will be greater in the jacket scenario than the monopile scenario. The former is considered the worst case temporal impact and the latter the worst case spatial impact.. The assessment makes the precautionary assumption that s that animals will be excluded from the area from the onset of the first piling event, until the end of the last piling event. During this worst case temporal scenario only a single piling vessel would be operating at once to provide the maximum duration for the construction period. The footprint approach has been used to consider the worst case area over which displacement can occur during pile driving, and given the greater range of noise propagation from the installation of monopiles using a 3,500kJ hammer; this is considered the worst case.
300. There is no evidence to suggest that the East Anglia THREE site is of particular importance for grey or harbour seal (section 12.5.2). Possible avoidance is not assessed in pinnipeds. For both seal species, given the very low at sea densities, the number of individuals likely to avoid the area is effectively no change. However, based on the worst case temporal impact (with avoidance occurring for up to 41% of a two year construction period) the magnitude of the effect of TTS is assessed as negligible.
301. For harbour porpoise the magnitude of effect for exposure of individuals to noise levels that could lead to instantaneous TTS over the construction of the entire windfarm is negligible (less than 1% of the population).
302. The assessment of behavioural response to pile driving noise in harbour porpoise also considers the range of possible avoidance. Therefore the magnitude of the combined behavioural effect in harbour porpoise is assessed for both types of behavioural response in the following section.
12.6.1.1.2.4.2 Possible avoidance 303. Ranges of possible avoidance for harbour porpoise are between approximately 37km and 70km for full hammer strike energy at 3,500kJ (e.g. Plate 9.5 Appendix 9.1). The spread in these ranges is due to variations in bathymetry and therefore propagation efficiency. For lower hammer energies, harbour porpoise are expected to exhibit disturbance over shorter ranges. Using the 2,000kJ hammer in the jacket scenrio the ranges are 29km to 51km.
304. In the simplified assessment that is taken forward, the worst case area of behavioural effect is based on the area over which possible avoidance could occur during the construction of the entire windfarm using the ‘footprint’ approach.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 12 Marine Mammal Ecology Page 79
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