Annex C of this report). The noise footprint can be considered to contain the possible impact ranges for a given threshold which might occur, irrespective of the timing, specific location or number of piling vessels operating within the project boundary. Examples of the construction noise footprints are illustrated in Plate 9.6 and Plate 9.12 for harbour porpoise Phocoena phocoena and fish, respectively.
9.4.4 Modelling the Effect of Multiple Piling Vessels 41.
The use of multiple piling vessels, considered for the foundation installation at East Anglia THREE, could potentially increase the area of the sea where the noise from piling is present at levels which might result in an impact. The area effected during concurrent piling would depend on the separation distance between piling vessels (i.e. whether the impact zones from each vessel overlap or not). An illustrative scenario has been modelled with two piling vessels with different separation distances. As it is highly unlikely that the sound pulses would interfere constructively, the sound levels would not be expected to increase as a result of summation. The results of this multiple piling vessel modelling are presented and discussed further in Section 9.6.1.
9.4.5 Modelling Sound Pressure as a Function of Position in the Water Column 42.
For the propagation conditions around East Anglia THREE, the broadband noise levels at larger ranges from the source (greater than around 1km from the source) resulting from impact piling are expected to be lower near the sea bed than they are around mid-water depth. Due to the pressure release effect of the surface, the noise levels towards the water surface would also be much lower than deeper down around the mid-water column.
43.
The energy flux model described in Section 9.4.1 only considers the sound energy propagating through the water column and so does not provide vertical profile data. As described above, this would not be the case in reality, where it would be expected that the broadband sound pressure would be reduced near the surface and the seabed. This is important when considering the potential impact of underwater noise on seabed dwelling species and species near the surface.
44.
To investigate this effect a more comprehensive propagation model (computationally more intensive) was used in addition to the energy flux model. Underwater sound propagation was modelled along two transects over a distance of around 50km, radiating northward and southward from an example pile location in East Anglia THREE. This example location and respective transects were chosen as they allow demonstration of sound propagation along both, up- and down-sloping environments.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 12
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