Abstract
The Dutch government has set ambitious targets for the development of offshore wind energy, with 3.5GW being developed in the period up to 2023. The development of these various offshore wind farms has an impact on the marine ecosystem, which is being closely monitored by the Dutch government.
In 2016, the Ministry of Economic Affairs and Climate (EZK) commissioned Rijkswaterstaat to setup an integrated research program to reduce the knowledge gaps regarding the effects of offshore wind farms on the North Sea ecosystem. This Wind op Zee Ecologisch Programma (WoZEP) runs from 2016 to 2023 and the results of the studies carried out are used in the Ecology and Cumulation Framework (KEC) in which cumulative effects of current and planned wind farms on protected species are determined.
The WoZEP project aims to:
▪ Reduce uncertainties of assumptions and knowledge gaps in the KEC, environmental impact reports (EIA) and appropriate assessments (AA);
▪ Reduce uncertainties of assumptions and knowledge gaps regarding long-term effects due to scaling up of wind energy at sea;
▪ Gain insight in the effectiveness of mitigation measures to reduce adverse effects.
Pile driving impulses are audible over ranges of tens of km’s and propagate more or less evenly in all directions in areas without differences in depth. Bottom topography in the Borssele area is more differentiated, with roughly deeper water west of the future wind farms and shallows east of them. Geelhoed et al. (2018) concluded that the avoidance distance of harbour porpoise during construction of the Gemini wind farms were between 10 and 20 km. It should be noted that no mitigation measures were taken during the construction of these wind farms. Studies on Danish and German wind farms, however, show avoidance distances between 17-21 km with mitigation measures to reduce sound emission (Brandt et al. 2011, 2016; Tougaard et al. 2009).
One of the current knowledge gaps in determining the effects of underwater noise on harbour porpoises is whether frequency weighting needs to be applied to noise thresholds for avoidance of the pile driving location. Factors that influence the frequency spectrum of the underwater sound at a distance from the piling location are (among other things) the water depth and the sound mitigation measures used.
In 2019/2020, the Borssele wind farm was constructed. The Borssele wind farm consists of lots I + II and III + IV and is located offshore of the southwestern coast of the Netherlands (Figure 1.1). In both lots monopiles are used as foundations for the windmills. Monopiles are installed by means of pile driving, a method that results in high levels of emitted underwater noise. To reduce the impact on marine life, the Dutch government set a season-dependent noise threshold level that is not to be exceeded. To remain below the noise thresholds, noise mitigation measures are required to reduce emitted sound levels. Blauwwind built the wind mills in lots III + IV and installed between October 2019 and April 2020. Blauwwind used as mitigation measures an AdBM Noise Abatement System and/or a (single or double) Big Bubble Curtain. Ørsted constructed the wind farms of lots I + II between January 2020 and May 2020. The mitigating measures that Ørsted used are an Hydro Sound Dampener (HSD) and/or a (single or double) Big Bubble Screen.
The different lots of the Borssele wind park have a different depth and different noise mitigating measures were used during the installation, this made the construction of this wind park an ideal moment to collect data to address the before mentioned knowledge gaps. With the knowledge this gains, the models that calculate the effects of sound on the harbour porpoise population in the North Sea can be improved.
This report describes the acoustic measurements that were collected from October 2019 up to September 2020 during the construction of the Borssele OWF and addresses questions related to the data availability and quality that are relevant for the analysis of the collected data