As infernos blazing across swaths of B.C.’s forest grow in intensity and number, experts say the number of wildfire-caused landslides is expected to increase as well.
Thomas Pypker, Thompson Rivers University natural resource science professor, told Castanet Kamloops that wildfires can cause landslides by disrupting vegetation and soil.
He said soils stay wet for a longer period of time after vegetation is burnt away. This is because less precipitation is absorbed by the vegetation where it can eventually evaporate into the atmosphere.
Pypker said about 20 per cent of incoming precipitation is normally reduced through “interception loss” in forest canopies, where it then evaporates.
“So you will get generally 20 per cent more water coming in, plus you're losing less water through transpiration,” Pypker said.
“If you go deeper in the soil profile, the soil stays wetter, and that's a bit of a recipe to allow for landslides to happen. Because the soils, once they saturate, they become liquid and they'll run downhill.”
Roots, which will hold soil in place, will also be killed off by a fire which can further facilitate landslides.
“It's a sort of a double whammy. More water in the soil combined with less items in the soil, basically the root holding the soil in place,” Pypker said.
While over saturation of soils can cause issues when there is little vegetation to keep it in place, wildfires can also prevent water from being absorbed into the ground. This can lead to further problems.
Immediately following a fire, Pypker said soils can become hydrophobic — unable to absorb water — due to an “oily residue” that can coat top soils and keep water from penetrating the earth.
“When water runs across the surface, as opposed to penetrating the soil, you'll get more erosion,” he said.
Pypker said intense wildfires will completely burn away all vegetation, exposing soils to more precipitation which can cause “soil crusting.”
“The soils become individual grains and they'll crust, and this decreases the ability for the water to infiltrate into the soil, and again, promotes more overland flow,” he said.
Pypker said the top later of soil is the most valuable as a source of nutrients for vegetation. When it’s eroded away by precipitation following a fire, it can take longer for vegetation to regrow.
“Basically, you're resetting the clock on that soil back in time for it to redevelop into a more productive soil,” he said.
With climate change predictions going forward, Pypker said increased wildfires will bring higher landslide risks.
Fires 'not the underlying reason' for Chilcotin slide
Last week, a massive landslide dammed the Chilcotin River at Farwell Canyon, creating a lake behind the slide. Water started to flow over the debris on Monday morning, creating a surge that has moved through the Chilcotin and into the Fraser River.
In 2017, 19 wildfires merged and burned an area about 130 kilometres in size in the place where the landslide occurred.
Pypker said the soils where the landslide occurred are primarily silty and sandy, and he doubted the slide was caused by these fires.
“Those locations are actually very susceptible to landslides in general, so this may just be a landslide that just happened,” he said.
“It's quite a deep landslide with a lot of mass movement of material into the creek, so wildfires may be a contributing factor, but probably not the underlying reason why this particular landslide happened.”
