How wildfire is introducing a new weather hazard to the Pacific Northwest
Recent rain and wind across Western Oregon provide an early fall warning that winter weather is coming. And with it, could come a hazard we really haven’t had to deal with before: flash flooding.
Parts of the Pacific Northwest are being hit hard by rain and wind this weekend thanks to a bomb cyclone offshore. The National Weather Service called it the “most impressive” cyclone event in the “recent history off the Pacific Northwest coast.”
And the rain is expected to continue throughout the week as other systems move through the region.
“We’re having a lot of near misses. I’ll put it that way,” Andy Bryant, a hydrologist with theNational Weather Service in Portland, said of the cyclone’s forecasted path.
It’s unclear whether the current systems will be severe enough to trigger dangerous situations for communities. But they provide an early fall warning that winter weather is coming. And with so much of the Cascades and Siskiyou Mountains burned in last year’s Labor Day fires, forecasters are now paying heightened attention to the potential risk of flash floods and debris flows.
“Usually when we had heavy rain over the foothills of the Cascades, it’s like there’s nothing up there… We watch the river forecast for the rivers that drain the Cascades, but it’s just not an immediate concern,” Bryant said. “Now we have to really focus on what’s happening over those areas.”
Bryant says traditionally, flash floods are not something we have to worry about in the Pacific Northwest. This region experiences slower-developing floods, not the quickly-rising water that’s associated with major thunderstorms in places like the Midwestern and Northeastern United States.
But with so many acres burned, that could be changing.
“With that huge amount of area where all the vegetation is gone and the soil is in some places behaving almost like pavement, that just completely changes the amount of runoff that we would see out of the rain events that we get here,” he said.
After wildfire, forest soils can become coated in a hardened waxy substance left behind after tree and plant materials burn. This coating repels water,making the soil hydrophobic — water no longer soaks in. It just runs off.
With excess runoff, debris flows also become a concern. These arerapid flows of mud, boulders, water and logs that travel down slopes and stream corridors. And they can cause serious problems for infrastructure – like roads.
As a rule of thumb, the National Weather Service in Portland considers 0.8 inches of rainfall in 60 minutes over one area enough water to trigger flash floods and debris flows. Bryant says the actual rainfall needed could be higher or lower, depending on fire severity and other conditions on the ground.
The NWS is now using new tools to help forecast when conditions are ripe for this kind of flooding.
“[We’re using] a lot of sophisticated software that’s looking at information from the radar and from gauges on the ground and making very quick assessments — like in the order of a 10-minute turnaround — of how much water is going to be coming off the land,” he said.
This kind of information is needed to help communities prepare and protect infrastructure from damage.
For example, after the Holiday Farm Fire east of Eugene,an analysis from the Erosion Threats Assessment and Reduction Team found that debris flows, rock fall and landslides in the years after the fire posed a “high risk” to life and safety at a mobile home park, a school, to major roadways and thee different communities along the McKenzie River. In addition, post-fire flooding threatens park users, boat landings, bridges and the state highway that runs through the corridor.
And the potential for community impact isn’t just restricted to within the Holiday Farm burn scar.
“The Mackenzie is a sole source of drinking water for our community: Eugene, 200,000 people. So we’ve always invested in the Mackenzie as a safeguard of this critical resource,” said Karl Morgenstern, watershed restoration program manager for the Eugene Water and Electric Board.
Morgenstern is heading a task force focused on post-fire watershed recovery. EWEB is working on revegetation, installing erosion control measures and eventually floodplain restoration to help maintain water quality in the river. Floods and debris flows put huge amounts of sediment, vegetation and potentially toxic materials from burned properties into the water. Getting advanced notice helps the water treatment system prepare.
“We saw in the November/December storms (last year) a big uptick in organic carbon, metals, and nutrients. But then after that, it dropped back down to the pre-fire levels, and we have not seen that happen again. Of course, last winter was kind of a dud,” he said. “So we expect that to happen maybe this winter.”
Morgenstern expects the threat of debris flows will continue for several years. That means people living, working or traveling in burn scars would be wise to start preparing for the unexpected this winter.
“This is nothing out of the ordinary with what we’ve encountered before in the Pacific Northwest – you know, rain,” said Lane County Emergency Manager Patence Winningham.
But in the Pacific Northwest, where extreme wildfire events are becoming more and more common, rain isn’t just rain anymore. It now could be a harbinger of far more catastrophic consequences for communities touched by wildfire.
“We need to have a plan. We…learned that in the Holiday Farm Fire. All events start and end locally. So the more prepared we are as individuals and as communities, the stronger we’re going to be in our recovery.”
Copyright 2021 Oregon Public Broadcasting. To see more, visit Oregon Public Broadcasting.