Simulating the role of tides and sediment characteristics on tidal flat sorting and bedding dynamics

Abstract

Understanding sediment sorting and bedding dynamics has high value to unravelling the mechanisms underlying geomorphological, geological, ecological and environmental imprints of tidal wetlands and hence to predicting their future changes. Using the Nanhui tidal flat on the Changjiang (Yangtze) Delta, China, as a reference site, this study establishes a schematized morphodynamic model coupling flow, sediment dynamics and bed level change to explore the processes that govern sediment sorting and bedding phenomena. Model results indicate an overall agreement with field data in terms of tidal current velocities, suspended sediment concentrations (SSCs), deposition thicknesses and sedimentary structures. Depending on the variation of tidal current strength, sand-dominated layers (SDLs) and mud-dominated layers (MDLs) tend to form during spring and neap tides, respectively. Thinner tidal couplets are developed during daily scale flood-ebb variations. A larger tidal level variation during a spring-neap tidal cycle, associated with a stronger tidal current variation, favours the formation of SDLs and tidal couplets. A larger boundary sediment supply generally promotes the formation of tidal bedding, though the bedding detail is partially dependent on the SSC composition of different sediment types.Sediment properties, including for example grain size and settling velocity, are also found to influence sediment sorting and bedding characteristics. In particular, finer and coarser sediment respond differently to spring and neap tides. During neap tides, relatively small flow velocities favour the deposition of finer sediment, with limited coarser sediment being transported to the upper tidal flat because of the larger settling velocity. During spring tides, larger flow velocities transport more coarser sediment to the upper tidal flat, accounting for distinct lamination formation. Model results are qualitatively consistent with field observations, but the role of waves, biological processes and alongshore currents needs to be included in further studies to establish a more complete understanding.

Morphodynamic Modeling of Tidal Basins: The Role of Sand‐Mud Interaction

“…In order to reduce complexity and restrict the degrees of freedom (and thus number of simulations) we strongly schematized tidal basin geometry and boundary conditions. We have quantified the effects of sand-mud interaction by defining a number of morphologic metrics, but we also observe that some of these metrics vary even more in reality due to for instance the large-scale variability in tidal prism, storminess or suspended mud concentrations (Zhou et al, 2021(Zhou et al, , 2022 and the basin geometry on a smaller scale (e.g., comparing adjacent Wadden Sea basins; Cleveringa & Oost, 1999;Ridderinkhof et al, 2014). And even though our model simulations fit within the range of morphological metrics that describe the basins in the Wadden Sea, they are most useful to analyze in comparison with each other.…”

Modelling spatial and temporal patterns in bioturbator effects on sediment resuspension: A biophysical metabolic approach

“…Following the degrading slope towards the channel, the submersion time becomes longer and the hydrodynamic stress stronger. In these conditions, only the coarser and heavier particles (sand < 2 mm) settle (Fagherazzi & Wiberg, 2009;Friedrichs, 2011;Zhou, et al, 2021). Fine sediment particles (silt, clay) exert a net attractive force between them, called cohesion (Grabowski, et al, 2011).…”