4. Summary

  1. This document provides an overview of the magnitude of injury and disturbance to marine mammals from underwater noise resulting from piling activities. Modelled noise contours from three selected conversion factors were applied: 10% reducing to 1% conversion factor, 4% reducing to 0.5% conversion factor and 1% constant conversion factor (as presented in the subsea noise sensitivity assessment; volume 3, appendix 10.1, annex B). As highlighted in the technical note on conversion factors provided in volume 3, appendix 10.1, annex A, the application of 10% reducing to 1% conversion factor in modelling of injury and noise disturbance contours is considered to result in overestimated impact ranges and subsequently these results have not been taken forward to the impact assessment of marine mammals. Instead, results generated using either a 4% reducing to 0.5% conversion factor (recommended in the technical note on conversion factors; volume 3, appendix 10.1, annex A) or the 1% constant conversion factor (commonly applied to previous offshore wind farm subsea noise assessments) have been taken forward to the assessment of significance in volume 2, chapter 10.
  2. Supplementary information on a 4% and 10% constant conversion factor has also been presented for the assessment of instantaneous injury at the request of stakeholders. The ranges of effect (SPLpk) predicted using a constant conversion factor of either 4% or 10% for the SPLpk metric are less than the range predicted for cumulative exposure for minke whale (2,319 m) based on SELcum and using the 4% reducing to 0.5% conversion factor. Therefore, as a precautionary approach, the potential to mitigate for injury was considered with respect to the largest potential injury zone for all species (2,319 m).
  3. The reason for considering two different conversion factors was to adopt the more precautionary approach since the larger predicted ranges switched between the 4% reducing to 0.5% and 1% constant conversion factor across the marine mammal hearing groups and depending on the acoustic metric applied. Thus, maximum injury ranges were predicted for different species using either the 4% reducing to 0.5% conversion factor or the 1% constant conversion factor depending on which of the dual acoustic metrics (SPLpk or SELcum) resulted in the largest predicted ranges ( Table 4.1   Open ▸ ). For behavioural effect ranges, where the unweighted SELss metric was applied, the 1% constant conversion factor resulted in the larger impact ranges compared to the 4% reducing to 0.5% and therefore this conversion factor was used for the marine mammal behavioural assessment for all species.

 

Table 4.1:
Summary of Injury Ranges and Corresponding Acoustic Metric (SPLpk or SELcum) and Conversion Factor (1% Constant or 4% Reducing to 0.5%) Taken Forward for the Marine Mammal Impact Assessment

Table 4.1: Summary of Injury Ranges and Corresponding Acoustic Metric (SPLpk or SELcum) and Conversion Factor (1% Constant or 4% Reducing to 0.5%) Taken Forward for the Marine Mammal Impact Assessment

 

5. References

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