Offshore wind farms are a sustainable energy source, but as any industrial development they also have an impact on the surrounding environment. The Objectives for a Substantiable Development (ODS in Spanish) highlight the importance of reducing the CO2 fingerprint by using offshore natural resources in a sustainable way. Therefore, the environmental impact of any offshore windfarm development shall be evaluated with the same level of detail as it is currently done for mainland windfarms.
As the number of offshore windfarms are rapidly increasing the impact on the nature is getting more relevant due to the accumulative impact. As a result of this, underwater noise assessments on these developments are getting more popular. Every other week we can see on the news about cetaceans being found away from their migration routes, this should be used as a waring signal about the noise pollution, the impact in the fauna and the importance of a thorough noise impact assessment before any offshore development is carried out.
The “Hoja de ruta para el Desarrollo de la Eólica marina y de las energías del mar en España” was published on the 10th December 2021. The document describes the strategy to boost Spain to the top of offshore windfarms developments. The document written by the “Ministerio para la Transición Ecológica y el Reto Demográfico”, gives 20 guidelines aiming towards reaching the objective of producing 1 to 3GW of energy using windfarms by 2030.
The Asociación Empresarial Eólica highlights in the Hoja de Ruta del Sector Eólico 2021-2024 two key points:
- Develop a strategy for offshore windfarms prioritizing the update of the current legislation framework.
- Reaffirm the commitment the windfarm industry has with the environment which was established in the Plan Nacional Integrado de Energía y Clima (PNIEC) in 2019.
Underwater Noise Impact Assessment
When carrying out any Noise Impact Assessment the are two fundamental fields to work on: measurements and predictions. For both of them there are multiple methods to carry out the work, although not all of them are suitable. Hence choosing the wrong measurement or prediction methodology can lead to wrong results, which obviously will lead to the wrong conclusions and mitigations measurements in the Noise Impact Assessment. The correctness of the methodology depends on the physical characteristics of the environment such as: deepness, type of maritime soil and frequencies which are going to be assessed (this usually depends on the noise source and the affected species)
Three main theories are used when modelling underwater noise impact assessments:
- Ray Trace Modelling.
- Normal-mode Theory.
- Multipath Expansion models.
Software use one or a combination of those theories, each of them have some advantage and disadvantages depending on the environment where the assessment is carried out, hence the importance of knowing them in dept so the results can be accurate.
An accurate noise impact assessment not only allow us to use oceans and seas in a sustainable way, it also has a positive impact on the economy due to tourism by keeping maritime species where they belong.
As underwater noise assessments are fairly new it does not exist a unified legislation framework.
For 13 years ago, the European Union has been used the Marine Strategy Framework Directive (2008/56/CE) which is being used as a European framework for the maritime environment.
In Spain, the Ley de Protección del Medio Marino (41/2010) establish the obligation to develop strategies to assess the underwater noise. A later directive, Ley de Evaluación Ambiental (21/2013), establish that any development in the environment including the use of natural resources of the maritime waters shall include an Environmental Impact Assessment.
None of those documents give noise limits. The Documento Técnico sobre Impactos y Mitigación de la Contaminación Acústica Marina gives some noise limits but these can only be used as a recommendations as the document is not a law.
The only standard or law which gives noise limit is the Ley de Ruido (37/2003), which gives noise limits for humans. It is possible to convert those limits for the maritime environment, but the A-weighting curve used in those limits made their usage completely wrong as the audibility range of humans is completely different than maritime species. For example, in mainland noise impact assessment frequencies under 20Hz are not considered in the assessment. When carrying out an Underwater Noise Impact assessment a specific methodology shall be used for frequencies below 20Hz as they are highly relevant, some experiments has measured noise from a source located 19.810 kilometres away.