Abstract
AbstractTidal sand waves are dynamic bedforms found in coastal shelf seas. Moreover, these areas are inhabited by numerous benthic species, of which the spatial distribution is linked to the morphological structure of sand waves. In particular, the tube‐building worm Lanice conchilegais of interest as this organism forms small mounds on the seabed, which provide shelter to other organisms. We investigate how the interactions between small‐scale mounds (height ∼dm) and large‐scale sand waves (height ∼m) shape the bed of the marine environment. To this end, we present a two‐way coupled process‐based model of sand waves and tube‐building worm patches in Delft3D. The population density evolves according to a general law of logistic growth, with the bed shear stress controlling the carrying capacity. Worm patches are randomly seeded and the tubes are mimicked by small cylinders that influence flow and turbulence, thereby altering sediment dynamics. Model results relate the patches with the highest worm densities to the sand wave troughs, which qualitatively agrees with field observations. Furthermore, the L. conchilegatubes trigger the formation of sandy mounds on the seabed. Because of the population density distribution, the mounds in the troughs can be several centimetres higher than on the crests. Regarding sand wave morphology, the combination of patches and mounds are found to shorten the time‐to‐equilibrium. Also, if the initial bed comprised small sinusoidal sand waves, the equilibrium wave height decreased with a few decimetres compared to the situation without worm patches. As the timescale of mound formation (years) is shorter than that of sand wave evolution (decades), the mounds induce (and accelerate) sand wave growth on a similar spatial scale to the mounds. Initially, this leads to shorter sand waves than they would be in an abiotic environment. However, near equilibrium the wavelengths tend towards their abiotic counterparts again. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd
Highlights
The underwater landscape of sandy coastal shelf seas, such as the North Sea, is characterised by fascinatingly rhythmic bed features of various shapes and sizes (McCave, 1971; Stride, 1982)
Tidal sand waves are highly relevant to study from an engineering perspective, as their occurrence, size and dynamic behaviour interfere with offshore engineering applications (Nemeth et al, 2003; Roetert et al, 2017)
We showed that the wavelengths of the emerging bedforms were significantly smaller than that of the supposed fastest growing mode (FGM), which was true for the random initial bed topography
Summary
The underwater landscape of sandy coastal shelf seas, such as the North Sea, is characterised by fascinatingly rhythmic bed features of various shapes and sizes (McCave, 1971; Stride, 1982). Sand waves have heights up to 10 m, wavelengths on the order of hundreds of metres and migrate several metres per year (Van Dijk and Kleinhans, 2005; Damen et al, 2018) After dredging they regenerate in several years time (Knaapen and Hulscher, 2002). Since the tube-building worms are sensitive to low water temperatures, high mortality rates within the communities are observed during severe winters (Strasser and Pieloth, 2001; Alves et al, 2017) These long-lasting sandy mounds attract other species through which they alter the local benthic communities, making them very valuable ecosystem services (Zuhlke, 2001; Rabaut et al, 2010)
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