Modelling of the Bovenrijn nourishment : comparison of two different transport layer approaches

After centuries of river training in the Dutch Rhine, the river has developed to a waterway which conveys water downstream very efficiently. Moreover, the corridor Waal-Bovenrijn-Niederrhein has developed into a main shipping route between the port of Rotterdam to the German hinterland. Decades of monosectoral projects (e.g., focussing on flood safety only), have rendered the river the way it is today. Dealing with climate change and other current needs have shifted the paradigm towards multisectoral projects, such as Integral River Management (IRM), which look at the wishes of different stakeholders. One of the measures, which is adaptive and is considered within the context of IRM, is performing large scale nourishments. This is expected to reduce the bed level degradation seen in the downstream Bovenrijn and Upper Waal. Reducing this bed degradation is expected to raise water levels during the low water periods, which has a benefit for nature and shipping. A pilot nourishment was performed in 2016, with a follow-up nourishment in 2019. The bed level development is the key indicator in such intervention and an important parameter controlling bed level changes is grain size (of the parent material and the nourished sediment). This research was set to compare the bed level development using two different approaches for modelling changes in bed composition, namely the state-of-the-art Hirano concept and the novel HANNEKE concept. The novel concept was successfully applied to the nourishment in the Bovenrijn. This shows that the modelling concept can be applied at a field scale. Analysis of the results showed comparable behaviour using both approaches. Unfortunately, a one-to-one comparison proved to be troublesome, as errors were found in both results. Nevertheless, the results are encouraging as the analysis have guided us to issues which need to be improved in the software.