Sediment transport processes in dune morphology and the transition to upper-stage plane bed

Delta areas around the world are densely populated, and feature an incredible amount of economic activity. Especially with regard to changing climate, it is vital to protect these areas from flooding by rivers. For flood modelling of lowland rivers specifically it is important to understand the interaction between river flow and bedforms. Bedforms arise when sediment (e.g. sand) that is transported downstream, organizes itself in (rhythmic) patterns on the river bed. Dunes are common bedforms in the sandy rivers found in low-lying land, and affect the water depth in the river strongly, especially under flood conditions. This is because under increasing discharge and flow strength, they grow rapidly and become more asymmetric and steeper. Due to this, water is slowed down more and more by the dunes (the dunes impose hydraulic roughness) during increasing discharge. However, if the discharge increases enough, dunes eventually degrade due to processes that dampen the bedform. Eventually a\ud transition to the upper-stage plane bed regime can occur; the flow is so powerful that the dunes are completely washed away and the hydraulic roughness and water depth decrease sharply. It is valuable to be able to describe the aforementioned processes with models that are accurate but also computationally cheap.\ud \ud This thesis aims to i) better understand the sediment transport processes along dunes and which processes control the transition from a dune regime to an upper-stage plane bed regime, and, ii) based on this understanding, to investigate the possibilities of an idealized dune evolution model to represent a wide range of dune shapes including upper-stage plane bed