Ralf van Hal

Seafloor mapping trial in Princess Amalia Wind Farm : use of ME70 on board of RV Tridens

The first wind farms in the Dutch coastal area are in operation for about 10 years now. The question is how the area of the wind farm developed in terms of marine environment in those years, as fishing activities were excluded from the area. One of the plans for evaluating this development is a benthic survey, similar to the survey that was conducted five years after the construction of the wind farms. However, a benthic survey can only sample a very small part of the wind farm area and the idea was that a prior survey using a multi-beam echosounder could provide information of seafloor structures on a larger scale. The results might be used to inform the benthic survey in sampling specific areas in order to detect and identify potential seafloor features of interest. A ship of opportunity, the RV Tridens II during the International Bottom Trawl Survey Q1 2017, was used to collect multi-beam echosounder data. Due to technical, practical and weather issues only a single afternoon was available for this work. During this afternoon, multiple lines were steamed through the Princess Amalia Wind Farm, recording data with the ME70 system in bathymetric mode. This way a small part of the wind farm area was covered. The results for the acoustic backscatter showed no specific seafloor features in this area, providing no new guidance for the benthic survey. The closest multi-beam recording was made at a distance of approximately 75 m from a monopile. As a result, the obvious hard structures (the scour beds around the monopiles) could not be sampled. This limitation is caused by the safety rules in place that vessels are not allowed closer than 100 m from the monopiles.