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.