9.4 Underwater Noise Propagation Modelling
9.4.1 Noise Propagation Model 27.
The underwater sound propagation modelling employed for this study has been undertaken by NPL. Potential impact ranges from pile driving have been estimated using an energy flux solution by Weston (1976), which is capable of propagation over large distances whilst accounting for range-dependent bathymetry. The energy flux underwater sound numerical propagation model has been implemented;
• •
• with the frequency-dependent absorption formula from Thorpe (1967);
including the effect of surface scattering (Coates 1988; Medwin and Clay 1998; Ainslie et al. 1994);
using sediment acoustics properties across the modelled area, which are representative of those expected to result in greater propagation distances (Hamilton 1980; Lurton 2003); and
•
using GEBCO Digital Atlas bathymetry data over an area extending approximately 100km around the windfarm boundary, augmented with higher resolution bathymetry data, where available (both lowest and highest astronomical tides are considered to ensure the longest propagation ranges are captured).
28.
For shorter range modelling, for the purpose of establishing injury ranges, a higher data resolution was used. The Weston energy-flux model assumes a homogenous sound speed profile which is often the case in coastal waters due to tidal mixing. The Weston energy-flux model has been benchmarked, with good agreement, against other transmission loss models published in the literature including the Range-dependent Acoustic Model (RAM) implementation of the parabolic equation (PE) solution (Collins 1993) based on AcTUP V2.2L (Maggi and Duncan 2010), an image source model (Urick 1983), a wavenumber integration transmission loss model (OASES), and a normal mode model (Kraken) and against measurement data. Validation of the Weston energy-flux model against measurement data is discussed in Annex B.
29.
The energy flux propagation model has been used to propagate an SEL source level to establish the SEL received level as a function of range. To derive a source level for use in the model, a monopole SEL source level was specified in TOB using a spectral source level shape taken from Ainslie et al. (2012) based on a broadband SEL source level calculated by De Jong and Ainslie (2008) from piling measurement data
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 8
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