Tracking flood frequency key to protecting B.C. communities: UBC study
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Samadhee Kaluarachchi, UBC master's student
view moreCredit: Photo credit: Samadhee Kaluarachchi
A new study from the University of British Columbia shows that even modest increases in river flows can dramatically raise flood frequency, with major implications for infrastructure and community safety. The researchers call for a shift in flood management – from focusing solely on rare, large floods to tracking how often floods occur.
“We’re seeing floods that were once considered ‘once-in-100 year’ events now occurring once or twice every decade,” says Dr. Younes Alila, a professor at UBC’s faculty of forestry. “Our models and management strategies need to evolve to reflect how often floods are happening, not just how big they are.”
The study, led by master’s student Samadhee Kaluarachchi and Dr. Alila, reveals that traditional flood management approaches based on the "200-year flood" underestimate the growing risks posed by climate change and land-use pressures.
Flood risks start upstream
Previous research from Dr. Alila’s group showed how forestry practices like clear-cutting amplify both the size and frequency of floods. This new study goes further, highlighting that while many studies emphasize changing flood sizes, the greater threat lies in how frequently floods occur.
By examining factors such as forest cover, urbanization and climate change across international research, the study shows that many watersheds, especially in B.C., can be highly sensitive to disturbance. In some cases, forest harvesting has led to seven- to 10-fold increases in flood frequency.
“Floods in Vancouver often start hundreds of kilometres away in headwater forests. Protecting those areas is key to reducing risk downstream,” said Dr. Alila.
Working with nature
B.C.’s natural landscape—lakes, wetlands and forests—offers built-in flood protection by storing and gradually releasing stormwater, preventing sudden surges.
The research identifies ways to work with these natural systems. Logging upstream of lakes has less downstream impact than harvesting below them, while cutting at higher elevations can worsen snowmelt floods. Urban planners are also testing “sponge city” designs that let rain soak into the ground rather than rush into storm drains.
“Our study challenges skepticism around nature-based solutions like tree planting and wetland restoration,” said Kaluarachchi. “The literature shows they can significantly reduce flood frequency, even for extreme events. These approaches work with watersheds, not against them.”
Rethinking infrastructure
Many engineering structures fail not during record-breaking floods but from moderate events happening more often, eroding riverbanks, scouring foundations and clogging waterways.
Traditional defences were often designed under assumptions that no longer reflect today’s climate and land use. In B.C., a 2015 assessment found that 69 per cent of dikes in the Lower Mainland were rated poor to fair, with none meeting provincial standards — underscoring the need to rethink flood infrastructure.
With climate change making extreme weather more common, the authors urge governments, planners and engineers to factor flood frequency into risk management and infrastructure design. “Flood-related lawsuits are multiplying at an alarming rate. It’s time for strong science to guide regulations and policies, not courtroom battles,” Dr. Alila said.
The findings were published in Frontiers in Environmental Science, a peer-reviewed, open-access journal, making the framework available to flood managers and communities worldwide.
Interview languages: English (Kaluarachchi, Alila), French (Alila)
Please contact charlotte.fisher@ubc.ca or lou.bosshart@ubc.ca to arrange interviews with Dr. Alila or Samadhee Kaluarachchi.
Method of Research
Literature review
Subject of Research
Not applicable
Article Title
Managing extremes in the Anthropocene: A causal, stochastic physics approach
Article Publication Date
7-Oct-2025
Devastating floods made worse by river widening, finds SFU-Wageningen study
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.
Simon Fraser University
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.
As river management continues to play a key role in climate adaptation and flood mitigation, the team highlights the importance of even widening of rivers in naturalization efforts, and the need for more information on the structure of rivers world-wide.
This is especially true for rivers that have been altered by humans, as standard numerical models may no longer apply.
“Half-measures don’t work. If you want to re-naturalize a river, you have to re-naturalize the whole thing and give the whole river enough room to move,” says Venditti.
Journal
Nature
Article Title
Extreme river flood exposes latent erosion risk
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