Reproduction of estuarine bathymetry by means of a process-based model: Western Scheldt case study, the Netherlands

Abstract

The current study aims to investigate the processes governing the evolution of major morphological features in alluvial estuaries as well as their morphodynamic interaction with the geometry by means of a 3D morphodynamic, process-based numerical model (Delft3D). The main hypothesis is that channel-shoal patterns evolve through the interaction between tidal movement, the available sediments and the tidal basins' geometry. Once such a system is not disturbed too much by extreme events, stable morphological patterns will emerge that compare well with observed features.Starting from a flat bed and imposing the Western Scheldt geometry, model results show two distinct time-scales, i.e. channel-shoal pattern formation within decades and a slow longitudinal profile development continuing for centuries. The developing channel-shoal patterns reflect major morphological features observed in the Western Scheldt estuary. Using the 1998 bathymetry as a reference, we applied different indicators and techniques to evaluate the model performance over time, namely: visual comparison, hypsometries, Brier Skill Score (BSS), mean basin depth and longitudinal profiles. Sensitivity analysis shows that trends in development are similar for different model parameter settings, like introducing salt-fresh water density differences or variations in sediment grain size and sediment transport formulation. Varying river discharge and including non-erodible layers, dredging and dumping activities and 3D hydrodynamics led to occasional improvements. The geometry is an important governing factor for pattern development. Applying an erodible geometry reveals a timescale for geometric adaptation that is much longer than the pattern development timescale within the basin. Model runs including highly schematized extreme conditions suggest that extreme events play a limited role.The main conclusion of the study is that the interaction between tidal forcing and basin geometry plays a major role in the development of morphological features in an alluvial tidal basin such as the Western Scheldt. A process-based model taking into account these major forcing mechanisms can predict such features with significant skill. The bathymetries generated in this study may form the basis for further studies considering, for example, the impact of sea level rise on estuarine morphodynamic development, or a rapid assessment of estuarine bathymetry in the absence of measured data.