Simulating and classifying large-scale spatial sand-mud segregation using a process-based model for a tidal inlet system

Abstract

Tidal inlet systems, as found in the Dutch Wadden Sea, often feature both sand and mud. Due to differences in sediment properties, sand and mud particles respond different to identical forcing conditions, like short waves and tidal currents. Because of this, sand and mud can get transported to different locations. This process is referred to by sand-mud segregation. Sand-mud segregation can have considerable influences on bathymetry, potential pollution and flora & fauna. It is because of these aspects that predictions on sand-mud segregation are needed. To predict sand-mud segregation, commonly for a practical scenario, a morphodynamic model can be used. Though, modelling a practical scenario often comprises a complex bathymetry and various non-linear processes, that contribute to sand-mud segregation, occur. Because of this, practical sand-mud segregation models are often hard to understand, give little insight in the overall processes and discrepancies with reality are often found within the results. These problems can be overcome by considering a schematized scenario, where only the overall forcing conditions (tidal currents and short waves) are considered, along with a schematized bathymetry and geometry. By considering a schematized scenario of the Amelander tidal inlet system, the large-scale sand-mud segregation patterns can be reproduced. It is found, e.g. in observations from the field and theoretical descriptions, that mud is commonly found in less hydrodynamic active areas (as long as mud is available). Within the Amelander tidal inlet system, mud is therefore found in the shallow intertidal areas, far from the deeper hydrodynamically active areas, like the tidal inlet and tidal channels. The schematized process-based model (a newly developed sand-mud version of Delft3D, which accounts for (non) cohesive regimes, a layered stratigraphy and consolidation lag) also reproduces these large-scale spatial sand-mud segregation patterns, with the schematized process-based approach. By combining the schematized model approach with variations in relative forcing domination (by tidal currents or short waves), various scenarios can be considered. From observations in the field and associated sand-mud segregation theory, one always expects mud deposition in less hydrodynamically active areas. Given the properties of tidal currents and short waves, it is hypothesized that mud is commonly found in deeper/shallow areas, respectively for a relative dominance of tidal currents/short waves. Results from the schematized process-based model support this hypothesis. Mud is transported to deeper areas when short wave domination is imposed, while mud is commonly found in shallow areas for a tidally dominated system. A schematized process-based sand-mud segregation model is able to reproduce large-scale spatial sand-mud segregation patterns for a practical case (the Amelander tidal inlet system). Furthermore, a classification can be set up, relating these large-scale spatial sand-mud segregation patterns to relative forcing dominance (from tidal currents or short waves), which is based on observations, theory and the schematized process-based model.