Waal river draws more water, signaling potential Rhine system shift

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Historical observations and model computations point to the Rhine system having crossed a tipping point. Since extreme peak flows in the 1990s, the Waal has gradually attracted more water at the Pannerdense Kop bifurcation. This is shown in research by TU Delft, Leiden University, Utrecht University, and Rijkswaterstaat published in Geophysical Research Letters.

Right downstream of where the Rhine crosses the border between the Netherlands and Germany, the river splits into the Waal and the Pannerdensch Kanaal. Water discharge via the Waal continues to increase, at the expense of the northern branch, the Pannerdensch Kanaal (which then splits into the Nederrijn-Lek and the IJssel).

Based on measured data and model computations, researchers Astrid Blom and Ralph Schielen conclude that the Rhine system may have reached a tipping point. However, there is no absolute certainty.

"After all, a river is not a flume in a laboratory in which we can examine the effect of a single modified condition. Nevertheless, we think it is important to draw attention to this now. These changes can have consequences for both flood safety and shipping," says Blom, associate professor at TU Delft.

A peak flow sequence triggered tipping

Due to the peak flows in 1993, 1995 and possibly 1998, a significant quantity of sand and gravel transported over the river bed reached the Pannerdense Kop bifurcation. This led to sudden changes in the Rhine system around the Pannerdense Kop bifurcation in the eastern part of the Netherlands.

Nearly half a meter of sediment was deposited in the Pannerdensch Kanaal, which subsequently has gradually increased the water discharge through the other branch, the Waal. "As a result, the Waal continues to erode," explains Blom.

So the peak flows of the 1990s caused a sudden change, which triggered a gradual adaptation of the river system. The system now seems to be moving towards a new stable situation, with an increasing share of river discharge flowing through the Waal. "It is not obvious if and how this change can be reversed, as back-tipping is not trivial," says Blom.

Flood safety

It is crucial that water managers keep a close eye on this redistribution of water discharge over the branches to keep the Netherlands safe, to stabilize the river bed, and to include this in future plans, such as the Program Room for the River 2.0 (formerly Integrated River Basin Management, IRM).

The discharge increase in the Waal implies a reduced water discharge via the Pannerdensch Kanaal and, as a result, the IJssel. When making future plans, we should not only consider peak flow conditions, Blom emphasizes. "During drought conditions, the observed changes will increase pressure on the freshwater supply to the IJsselmeer. We need to be prepared and prepare the system in a timely manner for these changes."

More information: Astrid Blom et al, Indications of Ongoing Noise‐Tipping of a Bifurcating River System, Geophysical Research Letters (2024). DOI: 10.1029/2024GL111846

Journal information: Geophysical Research Letters

Provided by Delft University of Technology