Why are the B.C. floods so bad? Blame the wildfires, at least in part

A few short months after the end of a devastating wildfire season, many B.C. communities are cleaning up after disastrous floods that have swept away highways, submerged homes, triggered deadly landslides, stranded hundreds of people and forced thousands more to evacuate. 

While climate change and (bad) luck each had some role to play, previous wildfires are known to boost the risk of disastrous flooding following a heavy rain or snowmelt.

Here's why, and how to mitigate the risk.

One short-term problem is very hot fires form a "hydrophobic" — or water-repellent — crust on the soil, due to the dispersion of waxy compounds from rotting vegetation on the forest floor, writes John Clague, emeritus professor of Earth sciences at Simon Fraser University in Burnaby, B.C., in The Conversation.

He noted that many areas that burned during 2021's wildfires, such as the area around Merritt, were the same ones deluged by extreme rainfall on the weekend of Nov. 13.

According to the B.C. Ministry of Forests, the risk increases if an intense rainfall, such as 10 millimetres or more within 20 or 30 minutes, follows a dry period, which can increase the water repellency of fire-altered soils.

Bob Freitag is director of the Institute for Hazards Mitigation, Planning and Research at the University of Washington. The neighbouring U.S. state is typically hit with the same weather as parts of B.C. and has also experienced record flooding.

Freitag said following a fire, since the soil no longer absorbs water, "the water runs down very, very quickly [and] starts to scour out things. And so you have a situation… where you have lots of water come down and also lots of sediment coming down with it."

Landslides that include lots of sediment, rocks and trees are called debris flows, and they're common after fires.

Matthias Jakob, a geoscientist at BGC Engineering, a consulting firm that works on landslide preparedness, told CBC's What On Earth radio show that while it usually takes an extreme rainfall to trigger them, an ordinary storm may be enough to do so on a wildfire-affected slope.

"In other words, [debris flows] become almost certain when these wildfires reach these areas," he said. "And that's exactly what we have seen with these recent rainstorm events."

Fire-induced rock cracking can also increase rockfalls, as can the loss of stumps, logs and roots holding loose rock in place.

Not only does all that increase the risk of landslides, but when it reaches the bottom, that sediment and debris fills up rivers, leaving less room for water, potentially damming the river altogether, and boosting the flood risk. Meanwhile, rocks and debris carried by the river erode and damage the banks, along with nearby property, including homes or docks that might have been raised to protect from flooding, Freitag said.

According to the B.C. Ministry of Forests, the additional risk begins as soon as an area is severely burned and lasts for "another two or more years."