Impact of small-scale transverse bed slope effects on large-scale morphology: Experimental and modelling studies

Morphodynamic models are essential tools to predict the development of fluvial and tidal systems in scientific and engineering studies, and are increasingly used for decision making regarding climate change mitigation, flood control, navigation and engineering works. However, many existing morphodynamic models predict unrealistically high channel incision, which is masked by increasing downward sediment transport on the side-slopes of channels up to two orders of magnitude higher than the default value. The formulation for this slope effect is overly simplistic and based on a few experiments with a small range of flow conditions and fine sediments. In this thesis, firstly the slope effect was quantified for a large range in flow conditions and sediment sizes by executing over 250 experiments. Secondly, model simulations were conducted for various scales and environments to identify possible explanations for the severe incision and to study the effect on large-scale morphology. Experimental results showed a new relation, but are still in the same order as the default value for the slope parameter in morphodynamic models. Model results show that the severe incision in morphodynamic models depends on grid size and the amount of suspended sediment. An arbitrary bed slope calibration to obtain realistic morphology may cause an order of magnitude error in rate of morphological change, channel depth and bar dimensions. Furthermore, it is impossible to calibrate a model on both sediment transport magnitude and morphology, which has major implications for calibrated models that are used for decision making, and suggests a critical knowledge gap.