Our research has given insight into the functioning of a bifurcating river. We have shown that the largest downstream branch in a bifurcating river dominates the water levels throughout the entire system by steering the discharge distribution at the bifurcation. The bifurcation also strongly affects the impact of human interventions. Specific intervention design is necessary to avoid unwanted water level increases. We have also shown that the water balance of discharges at a bifurcation is not necessarily closed when estimating discharges from available water level observations. Discharge and water level estimation can be improved by explicitly accounting for water balance.
Figure 1. Rhine bifurcation at Pannerden 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 these interventions may also influence the discharge distribution within the river branches when carried out in the vicinity of a bifurcation. The discharge distribution at the main bifurcations of the Dutch Rhine (see Figure 1) 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 expect that the roughness of the river bed and the effect of river interventions have a dominant influence on the uncertainty in water levels and the discharge distribution.
To support a more accurate and robust dike design, I aim to quantify and possibly reduce uncertainties in river water levels related to an uncertain discharge distribution.
Figure 2. Three main components of my research related to the interaction between water levels and discharge distribution. Component 3) is ongoing research. 1) Water levels due to high, low roughness 2) Water level reduction due to dike relocation 3) Changes in design water levels.
The discharge distribution over the river branches has a dominant influence on the design water levels, which respond very differently to changing conditions in a bifurcating river compared to a single branch river. Focusing on the bifurcating Dutch river Rhine system and its three main branches (Figure 2 on the top-right), 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 (Figure 2 top-left).
- 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 (Figure 2 bottom-left).
- Changes to design water levels as a result of the interactions of water levels with the discharge distribution. See the Figure 2 bottom-right, 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 Pannerdensche Kop 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. A second intervention aimed at balancing the discharge distribution is ideally of the same type, e.g. two floodplain excavations. Otherwise, large deviations in the discharge distribution can occur for medium-high or extreme discharges. For details about findings, see the related outputs.
Recommendations for practice
- Explicitly consider the bifurcating river as one interconnected system in which water levels and discharges are interdependent.
- Consider water balance closure in the derivation of rating curves (Qh-relations).
- Measure discharges in multiple branches on the same day to consider the water balance and assess the accuracy of the measurements.
- Assess the effect of discharge distribution uncertainty on system-wide flood risk.
Last modified: 23/11/2021
Matthijs R.A. Gensen
University of Twente
Multidecadal Analysis of an Engineered River System Reveals Challenges for Model-Based Design of Human Interventions
The relation between water levels and discharges in the Waal river has been estimated from observations, not finding an effect of Room for the River interventions on water levels.
Bevat: Publication open access journal
Human interventions in a bifurcating river system: Numerical investigation and uncertainty assessment
For intervention design in a bifurcating river, it is important to consider the entire river system and explicitly account for a range of discharge conditions to avoid unwanted water level increases throughout the river system.
05/10/2021 by Matthijs R.A. Gensen et al.
Bevat: Publication open access journal
Using data analysis, expert elicitation and hydraulic models to quantify uncertainties in a bifurcating river
The uncertainties of these three methods must be taken into account in the assessment of flood risks.
01/07/2020 by Matthijs R.A. Gensen et al.
Bevat: Conference proceedings
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
Reflection: The biggest flood risk in the rivers - bifurcation points or piping?
Flood risk along the Rhine branches: Flood risks along the major rivers continuously demand our attention. During this All-Risk webinar, we called attention to new research on the role of two of the mechanisms that influence flood risk: the two main bifurcations of the Rhine branches and the role of the failure mechanism piping. Can it be plausibly argued that one of the two causes are dominant?