A new study from UBC 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-a-century’ 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 focused on the “200-year flood” underestimates the growing risk as climate change and land-use pressures intensify.
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.
Media contact:
Charlotte Fisher
Marketing & Media Relations Strategist
Faculty of Forestry, UBC
charlotte.fisher@ubc.ca