D2) Numerical morphodynamic modelling

Liselot Arkesteijn

"We search for simplifications of the river morphodynamic models to assess the evolution of the river bed profile over several decades using less computational power"

Practical motivation

River morphodynamic numerical models require quite some computational effort to assess the evolution of the river bed profile over several decades . Such computational effort may be reduced when flood waves are modelled in a simplified manner. In this project we search for simplifications that give similar morphodynamic predictions using less computational power.

Rhine river in the Pannerdensch Kanaal nearby the location of the nourishment field experiment. Source (Rijkswaterstaat / Bart van Eyck)

In the last decades the bed of the Rhine river has been degrading at a rate of a few centimeters per year. This has several negative side-effects on the long term, especially for the navigability of the river since an uneven degradation may lead to humps in the river bed. Recently Rijkswaterstaat started a large scale nourishment field experiment near the German-Dutch border to counteract this. Sediments will be supplied and tracked during the coming years to obtain more experience for the future design of (potentially) regular measures. Since morphodynamic changes are typically slow and take place on a timescale of hundreds or thousands of years. Numerical models can be used to assess this long-term impact of sediment management measures.

Management phase: Problem identification |

Management goals: Morphological understanding |

Research aim

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.

Temporal scale: Long term (more than 50 years) |

Methods

  • Study the equilibrium profile. 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.
  • Comparison of the performance of several simplified flow models. Models are compare with respect to the model response in time to perturbations to the system, such as the flood waves traveling downstream, a correct response to base level change, and eventually 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.

Data-collection methods: Numerical modeling | Process-based modeling |

Top and side view of the river schematisation including the possibility of width variations and changes in the bed level. (Source: Liselot Arkesteijn).

Main progress and next steps

  • 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.

Last modified: 17/06/2018

Main researcher

Liselot Arkesteijn

Supervisory team

dr. ir. Astrid Blom

astrid.blom@tudelft.nl

Dr.ir. Robert Jan Labeur

R.J.Labeur@tudelft.nl

dr. Ralph Schielen

r.m.j.schielen@utwente.nl

Prof. dr. ir. Wim Uijttewaal

w.s.j.uijttewaal@tudelft.nl

Contributing partners

Rijkswaterstaat Ministry of Infrastructure and Water Management

LievenseCSO Infrastructure & Environment

Witteveen+Bos Engineering and Consulting

HKV Consultants

University of Twente

Delft University of Technology

NWO Applied and Engineering Sciences

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Events

15/09/2017

Symposium on River, Coastal and Estuarine Morphodynamics 2017 in Padova (Italy)

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Newsletters

DeltaLinks Magazine | oktober 2016: Rivieren

01/10/2016 by dr. ir. Astrid Blom

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Details

Status
Active
Start
12/2014
End
12/2018
Project group
Project D

Contact details

Liselot Arkesteijn

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