Rhine river in the Pannerdensch Kanaal nearby the location of the nourishment field experiment. Source (Rijkswaterstaat / Bart van Eyck).
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Model simplifications that give similar morphodynamic predictions using less computational power.
Rijkswaterstaat has started a large scale nourishment field experiment in the German-Dutch border area, a part of the Rhine River specially relevant for its mixed-sediment properties. The mixed character of the sediment is a property necessary to explain physical phenomena as downstream fining of rivers, the gravel sand transition zone, the formation of bedload sheets, the evolution of bars, or bed surface armoring. When a morphodynamic model accounts for mixed-size sediment it can lose its predictive capabilities. We conduct laboratory experiments aimed at gaining insight into the origin of the problem and in search of a possible solution.
Key goals: Fundamental understanding
Top and side view of the river schematisation including the possibility of width variations and changes in the bed level.
For instance for modelling the nourishments in the Rhine, Delft3D has been used to forecast the effects and assumptions are made associated with neglecting the dynamics of flood waves to make computation times feasible. In other models the river is schematized to a 1D channel, excluding two and three dimensional processes such as the flow in river bends or bifurcations. Our aim is:
- To assess the relevance of flood waves and multiple dimensions for long term morphodynamic changes, and address their implications associated with the applicability of numerical models.
- To give guidelines on the possible simplifications in the modelling of flood waves, and the necessity to use higher-dimensional models (2D-3D) to accurately model the longitudinal trend of the river profile.
Form whom and where?
Policy advisors modelling sediment management measures in sand-gravel rivers such as in the Dutch Rhine.
Data-collection methods: Numerical modeling Process-based modeling
Temporal scale: 10-50 years more than 50 years
Application and findings
Over time rivers tend to an equilibrium state in which the morphodynamic state is dynamic (varies around a mean state). The prediction of a long-term trend using a numerical model can only be accurate if the system tends to the correct equilibrium state. Therefore, we are developing a fast algorithm to numerically approximate the dynamic equilibrium state by comparing the performance of several simplified flow models. Models are compared with respect to: (i) the model response in time to perturbations to the system, such as the flood waves traveling downstream; (ii) a correct response to base level change; and eventually (iii) the time the river needs to find a new equilibrium. The test cases for the numerical experiments are idealized, though the river characteristics and hydrograph are inspired by the Rhine characteristics (see image on the right).
Status for day-to-day practice
Our analysis of the morphodynamic equilibrium state is still ongoing and so far we have mainly considered situations where the sediment was assumed to be uniform. An important matter in the design of nourishment measures, however, is the range of grain sizes of the mixture. Next we therefore aim to extend the analyses and the approximation algorithm to include mixed-size sediments. Furthermore, we will extend the approximation to include a more realistic channel geometry by allowing for width variations. With respect to the applicability of existing numerical models and techniques, we found that mainly discharge variability and an accurate local hydrograph are of importance for accurate predictions. Including the full wave dynamics, i.e. accurate velocities and their spatial gradients is less relevant. However, we will extend the study of the accuracy of simplified models in time to obtain more general conclusions.
Key locations: Abroad Netherlands (NL)
Spatial scale: River section
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Last modified: 25/01/2019
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