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

Abstract

The behavior of asymmetrical bedforms, which include many tidal sand waves, is challenging to understand. They are of particular interest since they are mostly located within areas prone to offshore engineering activities. Most experimental investigations regarding asymmetrical bedforms consider the riverine environment, are limited to a single sand wave or a few scattered ones, and focus only on differences between crest and trough. Hardly any information is available on sediment compositional changes along asymmetrical tidal sand waves, despite their abundance offshore. An asymmetrical sand wave field located off the coast of Texel Island in the North Sea was studied in June and October 2017. A total of 102 sediment samples were collected over two seasons along a single transect that covered five complete sand waves to measure the grain size composition, organic carbon concentration, chlorophyll-a (chl-a) concentration, and sediment permeability. We found significant variations in these sediment parameters between the sand wave trough, crest, and gentle and steep slopes, including a difference in permeability of more than 2-fold, as well as a difference in median grain size exceeding 65 µm. Based on these characteristics, a sand wave can be divided into two discrete halves: gentle slope + crest and steep slope + trough. Our results indicate a distinct sediment-sorting process along the Texel sand waves, with a significant difference between the two halves of each sand wave. These data could serve as input for process-based modeling of the link between sediment-sorting processes and seabed morphodynamics, necessary to design offshore engineering projects.

Horizontal and Vertical Sediment Sorting in Tidal Sand Waves: Modeling the Finite‐Amplitude Stage

“…Usually, sand wave crests are reported to be coarser and better sorted compared to the troughs (Passchier & Kleinhans, 2005; Stolk, 2000; Svenson et al, 2009; Terwindt, 1971; Van Lancker & Jacobs, 2000), although at some field sites the opposite is observed as well (Anthony & Leth, 2002; Roos, Hulscher, et al, 2007). Some monitoring campaigns have also recorded the sediment characteristics on the slopes, and they revealed that the lee slopes are generally composed of finer sediments with respect to the stoss slopes (Cheng et al, 2020). Moreover, field observations have shown that sand wave morphology is strongly related to the characteristics of the sediment, as sand wave heights increase for increasing grain size (Damen et al, 2018a; Ernstsen et al, 2005; Flemming, 2000).…”

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

“…Other than crest/trough sorting, observations on sorting pa erns over the slopes are more scarce. Cheng et al (2020b) present a detailed eld campaign where they found that the stoss slopes of sand waves are coarser than the lee side slopes. In case of a super-…”

“…Since the study by Cheng et al (2020b) did not report on hydrodynamic conditions, it is unknown what caused this contradiction. However, an explanation for the model result might be found in the uvial environment.…”

“…Also in sand wave elds distinct sorting pa ens are observed. In general, troughs are reported to consist of ner sands and mud, whereas the crests are characterised by coarse, well-sorted, sediments (Terwindt, 1971;Van Lancker et al, 2000;Passchier et al, 2005;Roos et al, 2007a;Cheng et al, 2020b).…”

“…Usually, sand wave crests are reported to be coarser and be er sorted compared to the troughs (Terwindt, 1971;Van Lancker et al, 2000;Stolk, 2000;Passchier et al, 2005;Svenson et al, 2009), although at some eld sites the opposite is observed as well (Anthony et al, 2002;Roos et al, 2007a). Some monitoring campaigns have also recorded the sediment characteristics on the slopes, and they revealed that the lee slopes are generally comprised of ner sediments with respect to the stoss slopes (Cheng et al, 2020b). Moreover, eld observations have shown that sand wave morphology is strongly related to the characteristics of the sediment, as sand wave heights increase for increasing grain size (Flemming, 2000;Ernstsen et al, 2005;Damen et al, 2018a).…”