published in Robinson et al. (2007) for a UK windfarm. The SEL source level or acoustic pulse energy source level has been assumed to scale up linearly with hammer energy as demonstrated by measurements during a full piling sequence, including the piling soft-start, by Robinson et al. (2007 and 2009b). This scaling was applied using a theoretical maximum scaling of SEL, directly with the hammer energy i.e., a doubling of hammer energy results in a 3dB increase in acoustic energy expressed in dB units (SEL). The dimensions of the pile are not expected to have a significant effect on the radiated noise energy if the hammer energy remains the same (Nehls et al. 2007). The source level is therefore scaled by hammer energy regardless of the pile size.
30.
The received levels for each TOB are obtained from the difference between the source level and the propagation loss in each TOB (see Section 9.2 and Annex A), which can be summed up to obtain the broadband or pulse SEL for the given bandwidth.
31.
The peak pressure level of the sound pulse generated by the impact piling can decay at a slightly higher rate compared to the energy in the pulse (the SEL is proportional to pulse energy) due to temporal dilation of the pulse that results from multiple reflections from the seabed and the sea surface as the sound pulse propagates. To allow the peak pressure level to be propagated as a function of range, an extra loss term was applied to the energy flux model to account for this more rapid peak pressure level decay. This loss term was established using the OASES wavenumber integration transmission loss model by estimating the difference in transmission loss between the pulse energy and the peak pulse pressure for a single flat bathymetry transect for various water depths and was validated against previous underwater noise measurement data from impact piling of the peak pressure level and pulse SEL metrics. For the propagation of peak pressure level, a scaling factor was applied to convert the pulse energy source level to a peak pressure source which was based on measurement data where sound exposure level of the pulse and peak pressure data were measured at ranges of 750m and less to the pile, and where the hammers were operating in the upper range of their rated hammer energy (i.e., where the hammer has the shortest impact time on the anvil or pile).
9.4.2 Modelled Sound Propagation to Estimate Potential Impact Ranges 32. Underwater sound propagation was modelled at twenty locations within East Anglia THREE with the aim of estimating the potential impact ranges. The locations were selected to encompass a range of sound propagation conditions resulting from variation in bathymetry, including locations near up-sloping and down-sloping
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 9
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