Modelled and Observed Estuarine Tidal Bar Sedimentology with Sand and Mud

Mud deposits may form on accreting tidal bars in estuaries. With this research we aim to understand when and where mud deposits form and how they are preserved. To better understand the sedimentology of tidal bars, we compare field data with numerical modelling results of a tidal bar in the Western Scheldt in the Netherlands. The field data are a combination of historic bathymetry, biomorphological maps, soil samples and cores collected in October 2017. We developed a depth averaged (2-DH) morphodynamic model in Delft3D which was calibrated on water levels. The model includes sand and mud processes, builds stratigraphy and represents 25 years of morphological development. The results show that mud deposits mainly form on high intertidal areas during high water slack. From model results and field data we observe that the bar increases in elevation when mud is able to deposit on the top of the bar, resulting in a higher equilibrium elevation compared to models without mud. This elevation change can create a window of opportunity for vegetation settling. The enhanced elevation with cohesive material decreases overflow and chute channel formation as also observed in earlier modelling and physical experiments, which leads to more channel stability and a decrease in tidal prism. While the spatial extent of mudflats is large at the surface, only limited mud is preserved in the stratigraphy. We distinguish two types of mud layers in our analysis: thin drapes ranging from 2-20 mm in thickness discovered in cores and thicker layers of +10 cm at the surface. Diatom analyses shows that the thin drapes contain high abundances of marine planktonic species, most likely related to storm events, because our studied bar is in the brackish zone of the estuary. These thin drapes are not captured in the model. The thicker layers at the surface are mudflats which are observed in the model and the field data. Field data also shows that they can significantly increase in thickness in the presence of vegetation, potentially up to several decimetres. These results will contribute to better estuarine management, since most species favour a muddy substrate and mud is therefore an important criterion for ecological valuable intertidal area in estuaries.