Landslides an urgent threat after wildfires burn, UBC engineering professor says

A University of British Columbia (Okanagan campus) engineering professor is warning wildfire-threatened towns could be in the path of future, damaging and deadly landslides.

"We probably have, literally, hundreds if not thousands of homes and structures that could be potentially impacted by future mudflows and debris flows,"  said Dwayne Tannant.

"It's kind of a one-two punch. Maybe you've been lucky enough to survive the actual fires ... and now we come along with another hazard, these mudflows."

Tannant, a specialist in geohazard assessment and slope stability evaluation, says the extent of fire damaged hillsides this season makes increased mud and debris slides increasingly likely.

The B.C. Wildfire Service reports more than 8,620 square kilometres of land have burned so far this season.

Heat-hardened soil and steep terrain creates threat

Tannant says slides in the aftermath of a wildfire are triggered by intense rainstorms washing through fire and heat hardened, steep, terrain.

"The soil can't absorb the rainfall like it did prior to the fire, so when it does rain, that water quickly runs off ... and concentrates in small streams," he told Chris Walker, the host of CBC's Daybreak South. "By the time it gets down to where people live, you get a mudflow or debris flow."

He warns slides can hit suddenly, and residents should be aware of the risk during any sudden thunderstorms or prolonged precipitation.

"There's little warning time between an intense rainfall and ... these mud flows.   It's really quite difficult to know where and when these will occur."

"But they will occur."

Last week, the Trans-Canada highway was briefly closed by a landslide near the fire-ravaged town of Lytton.  

Tannant points to the 2017 Elephant Hill wildfire that spawned dozens of 2018 mudslides in the Cache Creek/Clinton/Lillooet area.

Flows cut off highways, forced 24 people from the Bonaparte Indian Band out of their homes, and heavily damaged at least five houses.

In response, the Transportation Ministry developed new slide protocols, mandating detailed weather forecasting for specific areas,  installing backcountry rain gauges and real-time weather reports.

Tannant says the province now conducts a post-fire review of all burn areas, mapping and assessing the fire severity, and comparing the data with topographic maps and downstream structures.

But he warns that assessment takes time, while the immediate threat remains active for years.

"We have to live through this fall, all through next spring and summer, and then Mother Nature and vegetation starts to mitigate the watersheds and reduce the risk."