Insights into how model uncertainties affect water levels and flood risks in a bifurcating river system.
Rhine bifurcation at Pannerden kannal where the discharge of the Rhine river is distributed approximately into 2/3 to the Waal river and 1/3 towards the Nederrijn and IJssel rivers (source: Rijkswaterstaat).
Motivation and practical challenge
In the past decade, the Dutch government carried out the Room for the River program. Under this program, several large-scale interventions such as dike relocation or floodplain excavation were implemented to reduce the water levels along the main Dutch rivers. As a hydraulic engineer, I expect that when carried out in the vicinity of a bifurcation, these interventions may also influence the discharge distribution within the river branches. The discharge distribution at the main bifurcations of the Dutch Rhine (see photo) has a dominant influence on the downstream water levels. These water levels are the driving hydraulic load, determining the required height and strength of the 1430 km river dikes in The Netherlands. I also expect that the effect of local river interventions, regulation structures at the bifurcations, and the roughness of the river bed have a dominant influence on the water levels and the discharge distributions.
To support more accurate and robust dike design, I aim to quantify and possibly reduce uncertainties in river water levels related to an uncertain discharge distribution.
Three main components of my research related to the interaction between water levels and discharge distribution. Component 3) is ongoing research.
The distribution of discharge over the river branches has a dominant influence on the design water levels, which respond very differently to changing conditions in a bifurcating river in comparison to a single branch river. Focusing on the bifurcating Dutch river Rhine system and its three main branches (on the top-right figure), new components of my research quantify:
- Water levels in the three branches as a result of uncertainties in the hydraulic roughness of the main channel. Using a 1D model, I estimated the sensitivity of water levels to various combinations of low and high roughness values for the river branches (see top-left figure).
- Changes in the water levels due to a dike relocation intervention on the upper reach of the Waal River. I used a simplified 1D schematization to model these changes under both a free discharge distribution and a hypothetical fixed discharge distribution at the bifurcation (see bottom-left figure).
- Changes to design water levels as a result of the interactions of water levels with the discharge distribution. See the left-bottom figure, which is ongoing research.
Relevant for whom and where?
Contributors to the project, researchers and others who are involved with overarching design and planning of river interventions.
Rhine river and its branches pointing out the main bifurcations at Pannerden kannal and IJsselkop.
Progress and practical application
The interaction between the river water levels and the discharge distribution at the bifurcations strongly reduce the uncertainties in river water levels throughout the entire bifurcating system. For example, these interactions cause a high water level in a branch to be counteracted by a decrease in discharge towards this branch. These counteracting effects strongly depend on the size of the downstream branches. Conditions in the Waal branch, which carries the largest portion of discharge, dominate the uncertainties in water levels throughout the entire Rhine system. The other branches, particularly the IJssel branch, have little to no influence. Furthermore, our findings for a dike relocation intervention show that the counteracting effect between water levels and discharge distribution must be accounted for in future planning of human river interventions. When taking it into account, the lowering of the water levels by the dike relocation is smaller than for a single branch river. However, at the same time, the uncertainty related to the lowering of the water levels was smaller. For details about findings please click on related outputs below.
Status for day-to-day practice
It is very important that any river intervention, ranging from dredging to Room for the River like interventions are also assessed at the system level. Local changes can seriously affect water levels and distributing discharge along all river branches.
Future work will aim at improving the accuracy of the model results. This could for example be achieved by improving the roughness scenarios with more data from river bedforms, especially for the IJssel and Nederrijn branches. Furthermore, the effects and uncertainties of river interventions near the main bifurcation should be studied with a two-dimensional model.
Last modified: 27/04/2021
Matthijs R.A. Gensen
University of Twente
Feedback mechanism in bifurcating river systems
We quantify the water level variations for high and low roughness in the main channel when considering or not the feedback mechanism between the discharge at the bifurcation and the water levels.
27/04/2021 by Matthijs R.A. Gensen et al.
Bevat: Conference proceedings Publication open access journal
Compensating human interventions at a river bifurcation
Impact of river interventions under a range of discharges and roughness conditions to possibly counteract the feedback mechanism between water levels and discharge at a bifurcating river.
11/02/2021 by Matthijs R.A. Gensen et al.
Bevat: Conference proceedings