Piling work on foundations for wind turbines during the construction of wind farms in the North Sea generates high levels of underwater sound that can disturb harbour porpoises and seals. Given the expected developments in the Netherlands and other countries, it cannot be excluded that the accumulated effects of this impulsive sound as a result of multiple initiatives may impact entire populations.
At the request of the Ministry of Economic Affairs, Rijkswaterstaat agreed to develop an ‘Ecology and accumulation of effects assessment framework’ for Round 3 offshore wind energy. As part of that process, the Underwater Sound Working Group developed a 'line of reasoning' (in other words, a staged procedure) for determining the cumulative effects of impulsive underwater sound on relevant populations of marine mammals in the North Sea. In doing so, it used the Interim PCoD model, which was developed recently in the United Kingdom by researchers from the University of Saint Andrews and SMRU Marine.
As a guideline for environmental impact assessments and appropriate assessments for future Dutch Offshore Wind Energy projects, the following phased approach has been recommended:
- Calculation of sound propagation per piling strike
- Calculation of disturbance area for harbour porpoises and seals
- Calculation of number of harbour porpoises and seals possibly suffering disturbance
- Calculation of number of animal disturbance days (in other words, the number of disturbed animals per day multiplied by the number of impulse days)
- Estimation of the possible effect on the population on the basis of the number of animal disturbance days using the Interim PCoD model or the estimation formula described in this report
- Calculation of the cumulative exposure of harbour porpoises and seals in the vicinity of the pile and the determination of the distance within which there is a PTS risk for animals; investigation and description of how this risk will be mitigated.
The staged procedure (the 'line of reasoning') was applied to obtain an initial estimate of the possible extent of the cumulative effects of impulsive underwater sound on the harbour porpoise population of a number of scenarios for both the construction of wind farms on the DCS and in the rest of the North Sea and for seismic surveying in the period 2016-2022.
In addition, this report also discusses a possible approach for seals and provides an overview of the current knowledge gaps and areas of uncertainty in the proposed approach.