Modelling the effect of suspended load transport and tidal asymmetry on the equilibrium tidal sand wave height

Abstract

Tidal sand waves are rhythmic bed forms found in shallow sandy coastal seas, reaching heights up to ten meters and migration rates of several meters per year. Because of their dynamic behaviour, unravelling the physical processes behind the growth of these bed forms is of particular interest to science and offshore industries. Various modelling efforts have given a good description of the initial stages of sand wave formation by adopting a linear stability analysis on the coupled system of water movement and the sandy seabed. However, the physical processes causing sand waves to grow towards equilibrium are far from understood. We adopt a numerical shallow water model (Delft3D) to study the growth of sand waves towards a stable equilibrium. It is shown that both suspended load transport and tidal asymmetry reduce the equilibrium sand wave height. A residual current results in asymmetrical bed forms that migrate in the direction of the residual current. The combination of suspended load transport and tidal asymmetry results in predicted equilibrium wave heights comparable to wave heights found in the field.

Video Transects Reveal That Tidal Sand Waves Affect the Spatial Distribution of Benthic Organisms and Sand Ripples

“…This study reveals for the first time that both epibenthos and endobenthos abundance is significantly higher in sand wave troughs compared to the crests. Our results are similar to patterns in larger-scale offshore bed forms, such as sand banks (van Dijk et al, 2012) and with observed differences in endobenthic community clusters over sand waves (Baptist et al, 2006). In addition, our results demonstrate that in sand wave troughs, sand ripples are generally less abundant than on sand wave crests, and if present, sand ripples have a less regularly shaped pattern.…”

“…Hulscher () was the first to use a process‐based model to study the initial growth phase, after which many additions were made (e.g., Campmans et al, , and references therein). Also, more complex models are available which describe equilibrium amplitudes (Campmans et al, ; van Gerwen et al, ). Until now, these morphological models lack interaction with spatially varying small‐scale biological processes.…”

“…Contrarily, coastal bed forms are reported to drive the spatial distribution of benthic communities. The crests of large-scale tidal sandbanks in the North Sea were shown to harbor a less dense and less diverse community, compared with the troughs (van Dijk et al, 2012). Baptist et al (2006) studied sand wave effects on benthos spatial distribution.…”

Modeling storm effects on sand wave dynamics

“…Within process-based modeling two types exist: A complex modeling approach (e.g., Borsje et al, 2013;Van Gerwen et al, 2018) and an idealized modeling approach. Both modeling approaches have their own advantages and disadvantages.…”

“…All of these studies were able to describe equilibrium sand wave profiles on a domain with a length in the order of hundreds of meters and horizontally periodic boundary conditions. Van Gerwen et al (2018) used the model Delft3D (Lesser et al, 2004) to model sand waves of finite amplitude. Shortcomings of these type of models are generally that the sand wave height is overpredicted by far and that the wavelength of the bed forms keeps increasing for larger model domains, especially at long time scales.…”