Seafloor Classification in a Sand Wave Environment on the Dutch Continental Shelf Using Multibeam Echosounder Backscatter Data

Abstract

High resolution maps of sandy seafloors are valuable to understand seafloor dynamics, plan engineering projects, and create detailed benthic habitat maps. This paper presents multibeam echosounder backscatter classification results of the Brown Bank area of the North Sea. We apply the Bayesian classification method in a megaripple and sand wave area with significant slopes. Prior to the classification, corrections are implemented to account for the slopes. This includes corrections on the backscatter value and its corresponding incident angle. A trade-off in classification resolutions is found. A higher geo-acoustic resolution is obtained at the price of losing spatial resolution, however, the Bayesian classification method remains robust with respect to these trade-off decisions. The classification results are compared to grab sample particle size analysis and classified video footage. In non-distinctive sedimentary environments, the acoustic classes are not attributed to only the mean grain size of the grab samples but to the full spectrum of the grain sizes. Finally, we show the Bayesian classification results can be used to characterize the sedimentary composition of megaripples. Coarser sediments were found in the troughs and on the crests, finer sediments on the stoss slopes and a mixture of sediments on the lee slopes.

Sediment Characteristics over Asymmetrical Tidal Sand Waves in the Dutch North Sea

“…The few field studies available on sand waves have mostly observed coarser sediments on crests compared to troughs, but did not consistently study the composition of slopes. Also, the focus has been on grain size rather than on related components such as biogeochemical compounds [3,5,21,34,[44][45][46][47][48][49]. [23,26,[30][31][32][33] and sand wave emergence and growth.…”

Conservation Implications of Sabellaria spinulosa Reef Patches in a Dynamic Sandy-Bottom Environment

“…Smaller scaled megaripples are found superimposed on the sand waves. These megaripples have wavelengths of ∼10 m, amplitudes of ∼0.5 m, and an east-west orientation (Koop et al, 2019). Both sand waves and megaripples are known to migrate.…”

“…In 2017, S. spinulosa reefs were discovered for the first time on the Dutch Continental Shelf, in the Brown Bank area (van der . This area is characterized by large-scale (5-10 km) tidal ridges, superimposed with dynamic, migrating bedforms at smaller scales: sand waves with wavelengths of ∼200 m, and megaripples with wavelengths of ∼10 m (Knaapen, 2009;van Dijk et al, 2012;Koop et al, 2019). In addition, the area is fished at least once a year by demersal fisheries (van der .…”

The feedbacks among tidal sand waves, benthic organisms and sediment sorting processes

“…Due to the horizontal displacement of the sand wave, the ner grains originating from earlier deposits become mixed in the top layer, leading to a ning of the stoss slope. In contrast to the lack of eld evidence for sand waves, a study by Koop et al (2019) into migrating mega ripples did reveal a similar sorting pa ern where both the lee and stoss side are ner than the crest and trough. However, all these eld data were obtained by box-and multicorers which only describe the upper part (∼ 20 cm) of the bed, so that the internal structure cannot be assessed.…”

“…e bed of the North Sea contains a great variety of sandy sediments, which show large spatial variations (Damen et al, 2018a). However, within areas with marine bed forms sediments tend to be remarkably well sorted, both on a small scale (mega ripples (Koop et al, 2019)) and on a large scale (sand banks (Verfaillie et al, 2006;Van Dijk et al, 2012)).…”