45.
The model used was the AcTUP V2.2L version of RAM, a PE model (described in Annex A), with the actual implementation based on RAMGeo. This has previously been benchmarked, with good agreement, against the slower RAMSGeo implementation which allows for shear wave propagation in the substrate.
46.
The results of the RAM modelling are shown in Plate 9.2 for the two transects diverging from the pile using the same environmental properties as outlined in Section 9.4.1. The modelling shows two important points:
•
The noise level, as a result of complex interaction of the sound wave with the seabed, can be several dB lower near the seabed compared to around mid- water column at ranges exceeding the first few km. The effect is potentially weakest where the seabed is rapidly up-sloping. The implication of this is that animals, which dwell on or near the seabed, such as some fish for example (e.g. demersal fish), would be exposed to sound pressures that are potentially lower than those predicted in the energy flux model described in Section 9.4.1 (this would also be true of the particle velocity component of the acoustic field). However, these are broadband levels and this generalisation would not be true at all frequencies. This model does also not account for the vibration travelling along the seabed, which may generate a surface wave in the sediment with a velocity or displacement component to which flat fish, for example, may be sensitive (Hawkins 2009). Whilst sound originating from the section of the pile below the sea bed would generally attenuate more rapidly that in the water, the surface wave travelling along the surface of the sea bed may generate particle velocity components to which flat fish would likely be sensitive (Hawkins 2009; Hazelwood and Macey 2012); and
•
The noise level close to the sea surface is tens of dB lower compared to the mid-water noise levels at relatively short distances from the pile. This would result in a reduced exposure of any animal travelling at the surface which would likely reduce the area of avoidance from the pile. It would also result in a substantially reduced SEL dose (further described in Section 9.4.6) for any animal which might swim away, from the sound source at shallow depth (i.e. near the surface).
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 13
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