Common flood management tactics risk doing more harm than good without improved monitoring and understanding of rivers, according to a new study co-authored by Simon Fraser University researchers.
Published in the journal Nature, the study looked at the 2021 flood of Western Europe's Meuse River that caused dozens of fatalities and billions of dollars in infrastructure damage. The team discovered a regularly used flood mitigation strategy may have played a key role in exacerbating impacts of the flood.
The findings revealed that river widening, a common practice used to reduce flood risks accompanying severe rain events, actually had the opposite effect on the Meuse River.
Instead, when water volume rose with precipitation from a storm, it rapidly altered the river's morphology causing an unanticipated increase in erosion and sediment mobility, explains explains Environmental Science Professor Jeremy Venditti, who co-authored the study with Wageningen University.
Running from France through Belgium and the Netherlands, the Meuse was previously engineered and narrowed, controlling flow and allowing for infrastructure near the riverbanks. In more recent efforts to manage future flooding, sections of the river were re-naturalized, meaning the river was widened and infrastructure pulled back from the river bank.
The management technique is simple in theory; widening rivers increases the water carrying capacity and reduces the risk of overflows that threaten human life and infrastructure. But in practice, two problems emerged.
The first problem was that the river was widened unevenly, which caused bottlenecks. During the 2021 Meuse flooding event, the water discharging into the Netherlands was lower than levels experienced during previous flooding events in 1926, 1993 and 1995.
But even though the storm itself was shorter, the bottlenecks caused more damage, Venditti says, adding the velocity of the water increased 30 per cent compared to its 1995 equivalent prior to the river widening.
"There was nothing exceptional about this flood. This kind of flood flow had gone through this channel many times in the past, both before and after the river had been engineered," says Venditti. "This time it did something different, because of the uneven de-engineering of the river."
The second problem was that the structure of the riverbed itself, made up of fine sand protected by only a thin layer of gravel.
Venditti, a geomorphologist who has led front-running research on what happens when rivers naturally narrowed in bedrock canyons, notes that it's equivalent to creating a jet along the bed of the river that creates giant scours - or holes - on the bottom.
"You cause velocity inversions to happen. Water comes into a narrow constriction of the river and instead of the fast water being on top of the river and slow water being on the bottom, it inverts," he says. "When you put the fast-moving water on the bottom, you're really increasing the erosion potential of the river, and that's exactly what happened in the Meuse."
Once this unanticipated erosion process began and water penetrated the thin layer of gravel, there was nothing to stop it; the sand eroded over 15 meters deep in some sections, he explains. These holes caused water levels to rise and overflow above the riverbanks, devastating infrastructure.
According to the authors, many rivers around the world have these scours, the majority of which are linked to human interference, mining and similar sediment structures to the Meuse. These rivers, including the Saskatchewan, Rhine and Mississippi Rivers, could be susceptible to similar erosion futures.
