Governments need to prepare for more frequent large floods

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Academic rigour, journalistic flair floodwaters around an inundated barn in a farm area

Flooded farmland in Abbotsford, B.C. In November 2021, floodwaters inundated parts of B.C.’s Lower Mainland, damaging properties and roads, and cutting off Vancouver from the rest of Canada. THE CANADIAN PRESS/Darryl Dyck Governments need to prepare for more frequent large floods Published: December 3, 2025 11.03pm GMT Samadhee Kaluarachchi, Younes Alila, University of British Columbia

Authors

Samadhee Kaluarachchi
PhD Student in Forest Hydrology, University of British Columbia
Younes Alila
Professor, Department of Forest Resources Management, University of British Columbia

Disclosure statement

Samadhee Kaluarachchi receives funding from the Natural Sciences and Engineering Research Council of Canada, the Faculty of Forestry at the University of British Columbia, the Gordon and Nora Bailey Fellowship in Sustainable Forestry, and the Mary and David Macaree Fellowship.

Younes Alila receives funding from Mitacs Canada and the Natural Sciences and Engineering Research Council of Canada.

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University of British Columbia provides funding as a founding partner of The Conversation CA.

University of British Columbia provides funding as a member of The Conversation CA-FR.

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DOI

https://doi.org/10.64628/AAM.ejq56d3ht

https://theconversation.com/governments-need-to-prepare-for-more-frequent-large-floods-269251 https://theconversation.com/governments-need-to-prepare-for-more-frequent-large-floods-269251 Link copied Share article

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Flood management is a priority for many governments around the world. Recent floods have led to hundreds of deaths and caused significant damage in Indonesia, Sri Lanka, Thailand, Vietnam, Albania, Kenya and elsewhere.

Canada too is no stranger to floods. Notably, in 2021, flooding in British Columbia cut off access to Metro Vancouver from the rest of the country and caused up to $14 billion in damages.

While many scientific and technical reports show that floods are becoming larger and more common, these reports may be underestimating how their frequency is changing. Flood sizes get the spotlight, however governments and experts need to also consider their frequency to address implications overlooked by traditional management methods.

Frequency and size together must tell the story, because even modest increases in size can lead to surprisingly big jumps in frequency. For example, timber harvesting in the B.C. Interior has led to a 19 to 26 per cent increase in flood size, and turned the former 100-year flood into a once-a-decade flood. Despite floods becoming more frequent, today’s practices still dominantly focus on flood size.

The consequences are severe. We can build infrastructure like dikes and dams bigger so they withstand larger once-in-a-century floods. But if we don’t capture how floods of all sizes (including the 100- and 200-year events) are becoming way more common, infrastructure can weaken and fail faster than we expect.

In our recently published study, we examined a range of scientific, technical and governmental documents to assess whether practices today help us reliably predict flood risks. We found that many of the factors contributing to the severity of a flood could respond much more strongly to climate and landscape changes than traditional methods imply, calling for change in our flood prediction practices.

We’re underestimating flood risk

tractors and diggers clear debris following a flood

Restoration work underway following November 2021 flooding damage at Tank Hill on Highway 1 in B.C. (B.C. Ministry of Transportation and Transit/flickr), CC BY-NC-ND

Nature’s flood “ingredients” include rainfall, snow, soil wetness and energy for snowmelt, which combine in many “recipes” to trigger floods. Human influences like climate change, land use and land cover changes can alter these recipes, making floods bigger and more common. Understanding how human activity causes these effects on floods means predicting flood frequency and size together.

However, short flood records make it difficult to estimate the frequency and size of large floods. Without overcoming this challenge, assessments can produce unreliable results.

Additionally, many studies lump distant flood records with more recent records, suggesting that floods today have similar odds as those decades ago. Yet, experts agree that changes in the climate and landscape alter floods more strongly today.

These practices together produced a widespread perception in risk assessments where flood sizes rise rapidly, or steeply, per change in frequency (called a “heavier tail”).

Our recently published study challenges that perception, which implies that human influence shouldn’t greatly alter floods. In many places, human activities are making large floods more common. By giving little attention to how our activities affect flood frequencies, our practices don’t seem to capture just how sensitive floods are and how much they’re changing.

Without adapting our practices, we risk the loss of lives and livelihoods, misallocating funds, economic losses and lawsuits against governments, municipalities and professionals. Reliable flood projection and management is vital.

The traditional perception where flood sizes rise steeply (left) suggests smaller frequency changes under human influence than the perception where flood sizes rise gently (right). (Samadhee Kaluarachchi/Younes Alila), Author provided (no reuse)

Considering flood frequency

To make reliable flood projections, we first need to identify a region’s natural flood frequencies and sizes, and which climate and landscape features drive them. With this solid baseline, we can determine how human activities shift flood frequencies and sizes, if floods are sensitive to human influence and what this means for society.

We can do this by predicting how different human activities affect floods through modelling or landscape experiments. We can work with flood records, using methods that recognize how current and future floods are far more affected by human activities than past floods.

We can use existing techniques to overcome challenges with short records and ensure that our estimates reflect a strong understanding of the natural and human drivers of flood frequencies and sizes.

By adopting stronger practices, our study predicts that many regions could see very different frequency-size relations: flood sizes could increase more slowly per change in frequency.

It signals a more “fragile,” or super sensitive, flood regime than what current methods imply. When we disturb the climate or landscape, large floods can react strongly; they become much more common, reflecting what we see in many places today.

This knowledge can help governments effectively manage the land while mitigating major jumps in flood frequency.

The way forward

Floodwaters wash away part of Highway 8 in B.C. in November 2021. (B.C. Ministry of Transportation and Transit/flickr), CC BY-NC-ND

Effective flood management must include strong policies, nature-based solutions, and infrastructure designed for size and strength to withstand both larger and more frequent floods.

Nature-based solutions such as green areas, permeable surfaces and water-retaining features are being adopted by governments worldwide. Studies suggest that measures like increased forest cover have little impact on large floods; however, this may reflect the focus on flood size. Natural landscapes like forests can greatly reduce flood frequencies, even for very large floods.

In B.C., landscape features like mountains, forests, lakes, wetlands and floodplains spread out floods, lowering their peaks and making large events rarer. However, these same features make floods react strongly to changes in the climate and landscape.

Flood risk management must work with nature, maintaining or increasing the landscapes’ ability to store floodwaters. Our policies must address flood risk at the source through effective land management, recognizing that key causes of urban floods could lie thousands of kilometres away in the distant uplands. With strong policies and interventions both upstream and downstream, we can proactively manage floods.

Climate change
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