Location in the Rhine at the German-Dutch border where sediment is added to the river bed to counteract the effects of the lowering-bed. Image sources are, Left: Rijkswaterstaat / Joop van Houdt. Right: Nieuws de Vries & van de Wiel.
Fast algorithm to approximate changes in the river bed level due to opposite stream flow or backwater effects.
In the last decades, sand-bed rivers like the Dutch Rhine are lowering their bed level at a rate of a few centimeters per year. Especially for the navigability of the river, this degradation has negative effects in the long-term since an uneven lowering lead to humps in the river bed. Therefore, numerical models are typically used to understand these long-term changes. To make computational times feasible, traditional models often neglect the effects of temporal variations in the flow rate and the addition-extraction of sediment to the river bed (see Figure). However, these natural or induced variations can create an opposite flow in the river also known as backwater effect. This backwater effect may change the bed level and contribute to the decreasing sediment size and therefore should be taken into account in morphodynamic modelling.
Key goals: Fundamental understanding
The schematization of the top and side view of the river zooming into the river bed dynamics at the low and high flow levels. Source: Adapted from Arkesteijn et al. (2017) and Blom et al. (2017).
Existing numerical models do not incorporate significant fluctuations of bed elevation and texture when calculating the mean river bed level and are therefore only applicable in a river segement so-called normal flow zone. Our numerical approach describes the river’s behaviour also outside of the normal flow zone, in the so-called backwater segment. We further reduced the computational times of traditional approaches that requires hours to days by using the following approach:
- We model the backwater segment by considering a river channel that is forced by varying flow rates and a constant base level (see Figure).
- To estimate the change in the river elevevation and possible sediment fining, we numerically solve the equations that describe the system in upstream direction.
- We start from a local river bed level that can be derived from the imposed downstream water level(s).
This approach has been so far validated by comparing its results to traditional modelling approaches.
Form whom and where?
Policy advisors modelling river bed dynamics and addition-extraction of sediment in sand-gravel rivers such as the Dutch Rhine.
Data-collection methods: Numerical modeling
Temporal scale: 1-10years
Application and findings
We applied our approach for an idealized case that was inspired yet not fully applicable to the Dutch Rhine. For the above schematization (see Figure), we considered that the difference between the incoming and the outgoing sediment load is zero. Therefore, we used an extra set of equations to calculate the morphodynamic equilibrium of the river bed at a given distance from the downstream boundary. Sufficiently far upstream of the backwater zone, the solution of our model reduces to the traditional numerical models that work under the assumption of normal flow. For a wide range of parameter settings, our model is able to capture the behavior in the ‘normal’ flow zone and backwater segment very well. Furthermore, the reduction in computation time is significant. While our model requires only a few minutes, the traditional approaches require a few days, dependent on the initial conditions.
Status for day-to-day practice
The model is validated by comparing its results with those obtained by traditional approaches. Although the model performs well, especially for rivers with a mild slope, the accuracy of calculation reduces when other sources than the backwater effect become dominant in the river bed dynamics. So far, we mainly considered situations where the sediment was assumed to be uniform and the channel geometry is simplified.
Location along the river Rhine that inspired our simplified model schematization.
Key locations: Abroad Netherlands (NL)
Spatial scale: River section
An important matter in the design of interventions to add or extract sediment to the river bed is the range of grain sizes of the mixture. Therefore, we will extend our analysis to include mixed-size sediments and include a more realistic channel geometry by for example allowing for width variations.
Last updated: 28/09/2019
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Delft University of Technology
As soon as available, explore the storyline to get to know more about the main methods or prototype tools that were developed within this project.
Explore the output details for available publications to get a glance of the innovative components and implications to practice as well as the links to supporting datasets.
Research outputs are currently under review. Meanwhile, the following are the main conference proceedings for additional details about this project:
- L. Arkesteijn, R. Labeur and A. Blom. (2018) From time series to probability density functions at the boundaries in morphodynamic modelling. In: Huismans, Y., Berends, K.D., Niesten, I., Mosselman, E (Eds.). The future river: NCR DAYS 2018 Proceedings. Netherland Centre for River Studies publication 42-2018, 8-9 February 2018, Deltares, Delft, pp. 116.
- Arkesteijn, L., Blom, A. & Labeur, R. J. (2017). A space-marching model to assess the morphodynamic equilibrium behaviour in a river’s backwater dominated reaches. RCEM 2017 – Back to Italy: The 10th Symposium on River, Coastal and Estuarine Morphodynamics, Trento-Padova, 15-22 September 2017, Book of Abstracts. Lanzoni, S., Redolfi, M. & Zolezzi, G. (eds.). p. 151.
- Arkesteijn, L., Labeur, R. J. & Blom, A. (2017). The morphodynamic equilibrium state of a river in backwater dominated reaches. NCR days 2017: Book of abstracts. Hoitink, A. J. F., de Ruijsscher, T. V., Geertsema, T. J., Makaske, B., Wallinga, J., Candel, J. H. J. & Poelman, J. (eds.). Vol. NCR publication 41-2017, p. 2-4
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