Four years ago, summer floods in Limburg - in the south of the Netherlands - drastically altered the riverbed of the Meuse, making accurate high-water forecasts even more difficult than usual. "This shows just how vulnerable and unpredictable the Meuse really is," says researcher Hermjan Barneveld.
In July 2021, heavy rainfall from storm depression 'Bernd' left parts of Germany, Belgium, and the Netherlands submerged. In South Limburg, the river Geul burst its banks, damaging 2,300 homes in Valkenburg alone. The Meuse also struggled to handle the enormous volume of water - chemical pipelines were suddenly exposed, and a ferry landing site collapsed.
For Hermjan Barneveld and Professor Ton Hoitink of the Hydrology and Environmental Hydraulics group, the high water levels prompted a new investigation into what was happening beneath the surface - on the riverbed. Their research led to unexpected insights that could ultimately improve flood risk assessments. The results were published on 10 July in the scientific journal Nature.
Bottlencks and currents
Limburg experienced major floods twice in the 1990s, leading to new strategies for flood protection along the Meuse. Whereas rivers had previously been confined and narrowed, the new approach focused on giving water more room. Floodplains were widened and allowed to overflow during high-water events. But many of these projects have yet to be completed - and that's proving to have consequences.
Barneveld explains: "The problem lies in the variation of flow speeds. In floodplains, water moves slowly and high water levels decrease, but in narrower sections of the river, the flow accelerates - sometimes reaching speeds of up to 20 kilometres per hour. You'd expect that in mountain rivers, not in the Netherlands. We need a far better understanding of what happens in those bottlenecks in order to predict both the timing and peak levels of high water."
Shifting riverbed
During the 2021 floods, the flow rate at Maastricht surged from 50 m³/s to 3,310 m³/s in just two days. Suddenly, for every litre of water, the equivalent of half a bathtub was surging through the Meuse. Some 150 kilometres downstream, near Lith in North Brabant, the river had calmed somewhat. Although it only overtopped its banks in the floodplains, residents in some in-between areas narrowly avoided flooding - sometimes by as little as three centimetres.
Even during the flooding, the Dutch water authority Rijkswaterstaat deployed survey vessels to measure the fast-flowing riverbed. Helicopters equipped with lasers also flew over the area, mapping the water surface in fine detail. As soon as the rain stopped, Barneveld and his colleagues took action as well. They noticed deep holes in the riverbed and large sand dunes that had shifted position. The researchers walked the floodplains to trace where the eroded sand and gravel had ended up.
Pits and sand dunes
So, what exactly happened on the Meuse riverbed during those days? At the so-called bottlenecks, vast quantities of gravel and fine sand were swept away - a
process known as erosion. Near the town of Stein in Limburg, this created 16
pits in the riverbed, some as deep as 15 metres.
Hoitink explains: "First the gravel layer was washed away. In that spot, the layer was very thin. Beneath it was fine sand, and once that started eroding, it happened extremely quickly." It turns out the Meuse moved as much sand and gravel in a few days as it would normally do in ten years. (See below the situation before and after. Graphic by Joris Beemster, animation by Bram Vernhout)
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Much of the sand only travelled a few kilometres downstream and ended up in the floodplains, where it formed large mounds - sometimes over three metres high.
Lessons for other rivers
In some places, the water level on the Meuse was half a metre higher than expected. "There were several factors," says Barneveld. "It was summer, so the floodplains were full of vegetation and crops, which played a role. But if the riverbed changes so quickly and dramatically, that also has an impact. We still need more research on that."
There's a reason the world's leading scientific journal chose to publish this study. Until now, sudden, severe erosion hadn't been factored into high-water forecasts. "And yet we know this fine sand lies close to the surface in many places, bottlenecks are widespread, and high water events will become more frequent due to climate change," says Barneveld. "This is the first time we've seen just how significant erosion can be - in the Netherlands and globally."
According to Barneveld, Rijkswaterstaat is now developing plans to better map the riverbed to improve flood predictions. "It's an important step - accurate forecasts are crucial when deciding whether to build emergency barriers with sandbags or evacuate people. That's how we can prevent future disasters."
