Worsening wildfires fuel alarming trend for more flooding: UBC prof
Wildfires cripple our forests’ natural systems that absorb rain and melting snow, which leads to more flooding as B.C.’s fire seasons get progressively worse.
Not only are the trees and absorbent organic material on the forest floor incinerated by intense forest fires, the burned material and ash cause water to bead on the surface, speeding its journey into swelling streams, said University of B.C. forestry Prof. Lori Daniels.
The trend lines are far from encouraging.
The summer of 2017 set a record for the most area burned at 12,000 square kilometres, driven by the Plateau Fire west of Quesnel, the biggest wildfire ever recorded in this province.
In fact, four of the worst fire seasons in B.C. history have come in the past eight years. That is bad news for people living near last year’s biggest fire zones and for regions such as the Okanagan and the Similkameen where snowpacks are more than 150 per cent of normal.
“We are already noticing that big fires are influencing flooding, they go together,” said Daniels. “We are seeing this pattern play out all over B.C.”
After the Boulder Creek fire in 2015, November rains led to flooding around Pemberton Meadows and the downstream areas on the Lillooet River. Cache Creek — where a state of emergency has been declared — is at the base of last summer’s 1,920-square-kilometre Elephant Hill fire zone.
A healthy, mature forest manages rain and snow in multiple ways. Snow caught on tree branches may evaporate straight back into the atmosphere and the green canopy will catch spring rain before it ever gets to the ground, said Daniels. Thick organic matter on the forest floor acts like a sponge, greatly reducing how much water flows down to creeks when the snow melts.
“Those organic materials can absorb a large amount of water,” she said. “Then it trickles down into the mineral soil and maybe reaches groundwater.”
A post-wildfire forest is a radically different place.
Not only are the trees destroyed, but depending on the intensity of the fire, a thick absorbent layer of organic matter may also have burned off. And if that isn’t bad enough, burned forest duff is full of fats and waxes that create a slick surface that allows water to bead, pool and run off.
“That waxy film makes the soils hydrophobic, so they don’t absorb the water any more, they shed it,” she said.
“After a fire where there is white ash you can see rivulets of water forming little channels instead of soaking in.”
As the water flows off, it quickly overwhelms streams and swells our rivers. The speed and volume of the water also lead to erosion, which washes out riverbanks and releases huge amounts of debris that can lead to logjams and block drainage systems. The risk of landslides and debris flows is also elevated in the burned-off area, especially after intense rainfall, according to the Ministry of Forests, Land and Natural Resource Operations.
In general, the severity of flooding is dependent mainly on the snowpack and a combination of warm temperatures and precipitation, said Brett Gilley of UBC’s department of earth, ocean and atmospheric sciences.
Recent summer-like temperatures have triggered flooding, but the peak of flood season is still likely weeks away. Rain on snow may accelerate the melt and increase the volume of run-off.