Morphodynamics and Sedimentology of Estuaries with Sand and Mud

Abstract

A better understanding is required about the relation between morphodynamics and mud in estuaries. Currently, only limited studies account for mud in morphodynamic models, which is due to model limitations and a lack of spatially and temporally dense data of mud in the bed. Therefore, the effect of mud on the long-term evolution of estuaries is uncertain. The aim of my PhD project is to; (1) get a better understanding about the spatial patterns and processes of mud deposition; (2) determine the influence of mud on estuary morphology, and (3) understand the relation between mud deposition and preservation in the geological record. I will address these aims on the spatial scale of whole estuarine systems to individual bars and temporal scales of decades to millennia. As a secondary objective, I want to understand how these processes can be created in a model environment, both physical and numerical. These objectives related to mud will generate a better understanding of natural dynamics in estuaries in general. To achieve the objective I complementary used numerical modelling, flume experiments and field data. I applied the depth-averaged numerical model Delft3D on the scale of a whole idealised estuary and on the scale of one intertidal bar in the Western Scheldt; Walsoorden. The numerical model includes sand and mud processes, morphodynamics and builds stratigraphy. As a second methodology, I used flume experiments in which we created complete and dynamic estuaries with sand and a mud simulant. The experiments were conducted in a novel periodically tilting flume, the Metronome, which prevents many scaling problems that occurred in previous estuary experiments. Field data and the small scale numerical model of Walsoorden indicated that thick mud layers dominantly occur at the surface and no layers of significant thickness were discovered in deeper stratigraphy. Deposition of mud mainly occurs at high intertidal elevations and therefore increases bar height, decreases flow over shoals and focusses flow. The large-scale numerical model showed that mud is predominantly stored in mudflats on the sides of estuaries, while the experiments showed dominantly deposits on bars. Both models showed relatively more deposits in the landward direction and higher mud supply concentrations lead to narrower and shorter estuaries due to filling and increased cohesion. Notably, in contrast to rivers, channels do not increase in depth when they get narrower. It was hypothesised that mudflats in estuaries have a similar effect as river floodplain formation: creating narrower and deeper channels. However, due to the changes in bar elevation, filling and reduction of flow over the shoals by mudflat formation, the tidal prism decreases in the estuary. Due to the decrease in the water volume entering the estuary during ebb, the channels do not increase in depth. The depth does not adapt to the flow, but the tidal prism adapts to the morphology. Increased mud deposition also decreases channel-bar migration in estuaries, due to increased cohesion, a lower chance for cross-cutting channels on higher shoals, but also lower flow velocities due to an overall smaller tidal prism.