The Pacific Northwest Is in the Midst of a ‘Cascading Hazard’

16 November 2021

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CANADA – Things are a deadly, dangerous mess in the Pacific Northwest right now. An atmospheric river dumped rain across the region over the weekend, prompting widespread flooding and mudslides. Much of the damage occurred in places still trying to recover from a devastating wildfire season this summer, showing how climate change can compound disasters.

In British Columbia, around 275 people, including 50 children, had to be rescued off of flooded highways near Vancouver on Monday after they became trapped Sunday evening by a deluge of water, rocks, and mud. At one point on Tuesday, every major road out of Vancouver to the rest of Canada was closed.

About 7,000 people in the town of Merritt, British Columbia, were ordered to evacuate Monday due to flooding and after the wastewater treatment system failed. This is the second time in less than six months that the town of Merritt was under an evacuation order. In August, residents were placed under an evacuation order as two wildfires raged nearby. The wildfire flames have long since been put out, but they’ve left an imprint on the land that’s making the flooding worse.

Wildfires burn off an important layer of vegetation that, Brent Ward, a professor of earth sciences at Simon Fraser University and co-director of the Center for Natural Hazards Research, said “captures some of the rain and can slow it down before it gets into soil.”

Wildfires can also change how the soil interacts with rainfall. “When the organic layer in the upper part of the soil burns, it creates these organic compounds that travel into the upper part of the mineral soil and kind of precipitate,” Ward said. “It’s this waxy substance, and it makes that impervious to water—we call that hydrophobic. The water, instead of kind of centering down into the soil, it can’t go anywhere. It runs off, and runs down the slope, and hits a sort of steeper slope, and it erodes a lot. The sediment that’s eroding is often enough to generate a debris flow.”

Ward pointed out that while the term “mudslides” is more commonly known, the term “debris flow” is more accurate to what we’re seeing in the Pacific Northwest right now. After all, it’s not just mud that’s posing a danger, but all sorts of stuff, from rocks to trees to anything people leave lying around outside. This debris can be really big, too. A video posted to Twitter on Monday of flooding in Merritt shows an entire house being swept along by a current of muddy water. Merritt is only one example of a region in crisis from too much rain after a damaging wildfire season.

Around 9 inches (23 centimeters) fell in the town of Hope, British Columbia, over the weekend; the average rainfall for the entire month of November in Hope is 13.5 inches (34.4 centimeters). In Bellingham, Washington, the average daily record rainfall before this weekend was 0.88 inches (2.2 centimeters), which was handily beaten by Sunday’s 2.8 inches (7 centimeters) of rain. And evacuation orders are in place for communities in the valleys as floodwaters keep rising.

“This is changing so quickly that you might think you’re OK one minute and literally half an hour later you’ll see the change in the water levels,” Abbotsford Police Chief Mike Serr said at a press conference.

South of the border, the rains are also creating dangerous conditions. Washington Gov. Jay Inslee declared a state of emergency on Monday for 14 counties in the state, and evacuations were ordered in several places. One person was reported missing in the small town of Everson after they were swept away by floodwaters. More than 40,000 households were still without power in western Washington as of Tuesday and major highways remained closed.

There can be debris flows in places untouched by fire, of course, and fires can burn in places that won’t see flooding. But the correlation between the two is very strong—and an ominous sign of what the coming decades hold.

Climate change has driven an increase in the hot, dry weather that can worsen wildfires and the chances of heavy downpours. (The latter is due to the fact that the hotter atmosphere can hold more water.) Both these impacts have been on clear display in the Pacific Northwest as well as other parts of the world this year.

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“We call this a cascading hazard,” Ward said. “We have increased heat, increased drought, resulting in increased wildfires, resulting in increased landslides. One hazard triggers another, which triggers another.”

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