One-two punch: Why floods pummel wildfire-ravaged areas | CBC News

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

How do wildfires boost the risk of severe flooding and landslides?

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.

How long does this additional flood risk last?

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.”

At that point, vegetation starts to regrow, reducing the water repellency of the soil and holding more of it in place.

“However, increased floods and debris flow risks in some severely burned areas may last much longer,” the ministry said.

WATCH | Summer fires in interior B.C. may have made flooding worse:

Summer fires in B.C. Interior may have made flooding, mudslides worse

As some communities in the B.C. Interior remain cut off from the rest of the province, some are pointing to the summer’s fires as one reason the water came in so quickly. 3:17

FEMA, the U.S. government agency responsible for disaster preparedness and relief, says flood risk remains significantly higher in burned areas until vegetation is restored — up to five years after a wildfire.

Even as the vegetation starts to regrow, absorbing more water with its roots and stabilizing the slope, there are some new, longer-term risks. Freitag noted that initially, the roots of the dead trees do continue to hold soil.

“But after a while, they start to disintegrate, too.”

That can lead to more erosion.

Meanwhile, the wetter winters and warmer summers that the region is getting with climate change lead to the growth of more grasses and bushes during regeneration. During hot, dry summers that are also becoming more frequent, they burn readily.

“You never get to re-establish the forest, because you have now a lot of burning,” Freitag said.

He said the longer-term threat from all the fires and erosion is that the lower slopes of mountains can become completely denuded, with no soil at all for recovery or for the future absorption of rainfall or snow melt.

What can be done to mitigate the risk?

Replanting trees following wildfires or clearcuts can help. The B.C. Ministry of Forests told CBC News in an email that it “uses a science-based reforestation approach to reduce flooding.”

However, to reduce risk to people, awareness is key. 

Freitag said wildfire awareness workshops in at-risk communities now highlight the increased risk of floods after wildfires.

B.C.’s Ministry of Forests recommends that you: 

• Be alert when driving in an area that has had a recent fire, looking out for hazards such as washed-out bridges or culverts and landslides onto roads below steep banks.

• In areas where a wildfire has recently occurred, avoid camping on flood plains, near small steams, in alluvial fans (areas where flowing water has deposited sediment) or at the base of burned slopes.

• If you live in an at-risk location, sleep in an upper floor during severe weather, listen for unusual sounds such as trees cracking or boulders knocking together and watch for changes in local waterways.

• Find out if floods and landslides have occurred in your area in the past, as that can increase risk.

• Contact local government offices or Emergency Management BC after wildfires on Crown land to see if a risk analysis has been done.

FEMA recommends that property owners buy flood insurance following a wildfire if they don’t have it already.