Acoustic analysis of seabed at the Luchterduinen wind farm zone
In this project the seabed of the Luchterduinen wind farm (North Sea, Netherlands Continental Shelf) has been investigated towards indicators for presence of biogenic reef structures based on acoustic data. The wind farm went operational in 2015 with 42 wind turbines spread over an area of about 5 km by 7 km. Multibeam echosounder (MBES) bathymetry and side-scan sonar (SSS) backscatter data were acquired in April and May 2020 as well as in August 2020 in the wind farm.
The MBES bathymetry data revealed a seabed morphology with two dominant bedform types covering the entire wind farm. The water depths range between 13 and 27 m (LAT). Mega ripples with a wavelength between 3 and 8 m and wave heights around 15 cm are superimposed on the larger sand waves with a wavelength between 300 and 850 m and heights up to 3 m. Paulsen et al. (2016) estimated the average migration speed of sand waves for the Hollandse Kust Zuid, an area next to Luchterduinen, between 0.7 and 3.0 m/year. This would lead to a minimum sand wave cycle (crest to crest) of 100 years for the Luchterduinen area.
The mega ripples are only interrupted by the monopiles and their scouring protection. The side-scan data shows that the scouring protection reaches an extent with a radius of about 20 m around the monopile. The inner circle of the scour protection consist of larger rocks (d50 = 40 cm, causing high backscatter and acoustic shadows) and the outer circle is covered with a bit finer material (only high backscatter). Between the monopiles the backscatter is lower and appears to be very consistent across the entire wind farm. Acoustic shadows are mostly caused by mega ripples. A few single boulders, debris or man-made structures were visible as well. Often it is difficult to distinguish between these features based on the acoustics and only ground truthing would yield to a clear conclusion. Video data around the turbine shows that a variety of different biota is located around the monopile. However, the side-scan data does not indicate that the biota and hard structures continue towards the areas in between the monopiles. It rather shows a homogenous distribution of mega ripples consisting of sand between the monopiles across the entire wind farm.
The assumption of the presence of sand is based on the combination of the acoustics and video data from two wind turbines showing sand as the main sediment type on the seabed. In addition, an acoustic study conducted in Hollands Kust Zuid (HKZ) (Gaida, 2020), indicates sand as the dominant sediment type as well. Since specific acoustic patterns, which are expected
for biogenic reefs, are not visible in the SSS images, the acoustic results rather pled for the non-existence of biogenic reefs in the area between the monopiles at the Luchterduinen wind farm. In case nature enhancement structures are planned to be placed in the area between the monopiles, the seabed dynamics of moving mega ripples and in particular sand waves should be considered. Therefore, they should not be located in the trough very close to the crest, otherwise the sand waves are likely to migrate over the structures (see migration rates above). Based on the acoustic results, there is no reason to expect biogenic reefs in this area. In case a ground truthing campaign will still be carried out, the lee side of the monopile is probably the most promising location, since the shear stress is lower and calmer condition can be found, which might be the preferred location for reef-building species in the Luchterduinen area. In addition, the SSS backscatter images indicate for these areas an increased backscatter. However, it is more likely that the material of the filter layer is moved with the current and deposited in this area causing the increased backscatter.
The full report can be read here.