Cooperating with the river bifurcations
On the possibility of allowing more management freedom around the river bifurcations
A river bifurcation balances out the water level variations from downstream changes in the river. In the Dutch Rhine river, the conditions nearby the bifurcation along the Waal branch drive the water level variations throughout all the Rhine branches. The conditions in the Nederrijn and IJssel drive only minor water level variations, and therefore, we may allow for more management freedom in the Nederrijn and IJssel, for instance, in nature development.
The bifurcating Dutch Rhine river
Soon after the Rhine river (locally known as Bovenrijn) enters into the Netherlands it splits into three downstream branches: The Waal, the Nederrijn and the IJssel. At the bifurcation, discharge and the related flood risks are distributed over the three branches. Like many other rivers around the world, these branches of the Dutch Rhine are under constant change driven by both natural river dynamics and human interventions. Change should not necessarily be a problem if we try to adapt by creating new opportunities. Over the last 20 years, we have adapted our traditional management approach by giving more space to the river while also allowing recreation and nature development. Opportunities for adapting river management continue. For instance, with the Flood Protection Program, the government is reinforcing the primary dikes to the changing climate. Thereby, river managers at the national and regional water authorities are considering options to strengthening the dike sections while strategically developing the surrounding area.
Location of a future dike reinforcement project along the river Waal (Source: https://www.dijkversterkingwolferensprok.nl/)
A strict discharge distribution
Despite the opened-up opportunities, there is one element that we remain strict about: the discharge distribution over the three branches of the Dutch Rhine. In Dutch policy, the amount of discharge that every branch should receive is specified very precisely. This policy discharge distribution is set for a Bovenrijn discharge of 16,000 m³/s with a precision of 1 m³/s. Each year, river managers have to check if the current situation around the bifurcation would result in the exact policy discharge distribution.
Limited freedom for river managers
The policy also specifies that, throughout the year, any human intervention around the bifurcation points cannot lead to deviations in the discharge distribution of over 5 m³/s. If an intervention would exceed this limit, additional measures should be taken to avoid deviations. Such strict policy strongly limits the freedom of river managers. The riverine conditions around the bifurcation should remain as much as possible, otherwise, large deviations from the policy discharge distribution could occur. Therefore, for instance, vegetation in the surroundings of the river bifurcations is strictly managed, not allowing for the natural development of the area.
The feedback mechanism between discharge distribution and water levels
At a river bifurcation, the discharge is distributed over the downstream branches. At high discharges, the Rhine river is free-flowing, whereby the water levels in the downstream branches determine the discharge distribution at the bifurcation points. However, the amount of discharge in each branch also determines the water level along with several riverine conditions. The most important riverine conditions include: (i) the geometry of the river cross-section, (ii) the flow resistance or roughness of the river bed caused by the natural formation of river dunes on the river bed, and (iii) the roughness caused by the vegetation on the floodplains.
Quantifying the water level variations due to changing conditions
As a researcher at the University of Twente, one of the aspects that I have explored as part of my work in the All-Risk program is how the feedback mechanism at river bifurcations affects water level variations. I used a hydraulic model to estimate the variation in water levels that are caused by a change in the roughness of the river bed in one of the branches. I used two modelling approaches: a "single branch" and a “bifurcating river” approach. In the “single branch”, only one branch is modelled, the feedback mechanism is disabled and the amount of discharge that the branch receives is equal to the policy discharge distribution. In the “bifurcating river”, all branches are modelled, the feedback mechanism is enabled and the discharge distribution is let free.
Variations in the water levels
The following three slides show how water levels respond to changes in roughness in a "single branch" and in a "bifurcating river". Variations in water levels are reduced by the feedback mechanism due to a redistribution of discharge over the branches.
Lessons learned
The roughness of the Waal branch is a dominant driver of water level variations in all of the Rhine branches. Being the largest branch of the three branches, the Waal is better able to steer the discharge distribution than the other two branches. As such, water level variations driven by the Nederrijn or IJssel roughness are much smaller. If we consider the differences between these branches, it makes sense that we strictly manage the upper reach of the Waal. On the other hand, we may have more management freedom in the Nederrijn and IJssel, for instance, in nature development.
Two issues with the policy discharge distribution
Following my research, I think that the policy discharge distribution is too strict and that we should talk more about how more flexibility in the discharge distribution at the bifurcations can open up opportunities in both the short-term (e.g. fewer restrictions on nature development) and the long-term (e.g. a more cost-effective discharge distribution). I think there are two specific issues with the policy discharge distribution:
- Firstly, we should clearly acknowledge that we cannot control the discharge distribution to the precision of 1 m³/s that we wish. Riverine conditions are unknown for the design level of the flood protection system. Therefore, large deviations from the policy discharge distribution may occur, even if we don’t account for them.
- Secondly, I think that we should look at variations in the water levels instead of the deviations from the policy discharge distribution. My research showed that a deviation in the discharge distribution will result in different variations in water levels along the river branches. Currently, we do not acknowledge that the water level variations are different in each branch and we might miss opportunities for improving the river management as a result. As long as the variations in the water levels remain low, we may allow some deviations in the discharge distribution to occur.
The bifurcation can be our friend
I think that we should talk more about how we can "cooperate" with the bifurcations instead of restricting it the way it is done. We should acknowledge that we cannot exactly predict water levels and discharge distribution. However, it is the bifurcation that actually helps us with reducing major variations in water levels. A bifurcation does not have to be our enemy; it can be our friend!
Interested to read more?
This storyline is based on the results of the following open access publications:
Gensen, M.R.A., Warmink, J.J., Huthoff, F., Hulscher, S.J.M.H., 2020. Feedback Mechanism in Bifurcating River Systems: the Effect on Water-Level Sensitivity. Water 12, 1915. https://doi.org/10.3390/w12071915
All-Risk project page " Large scale uncertainty in river water levels "
