DNR to study impact on debris flows in Bolt Creek Fire burn area after heavy rainfall

Despite the driving rain and high winds, Department of Natural Resources (DNR) crews set out to install rain gauges Friday in the area of the Bolt Creek Fire. The instruments will help scientists with some groundbreaking research into how much rain will trigger debris slides in burn areas in the Pacific Northwest. 

The Washington Department of Natural Resources says most of the research on debris slides has come from California, so new research in Western Washington will help scientists to understand how much rain it takes to cause a debris flow in this region.   

"West side fires are kind of a new emerging science, on how many years our structures are at risk downstream of the fire. That can be up to 5 years we think," said Kate Mickelson, the landslide hazards program manager with the DNR.  

She says in Oregon's 2017 Eagle Creek Fire, the larger debris flows weren't reported until 2020 and the DNR's study will provide important data west of the Cascades.  

"Debris flow modeling that the USGS uses is based out of southern California, which has a very different climate and vegetation than especially us on the west side of the state," said Mickelson.  "The rainfall thresholds that are used in those models may not be applicable for the western side of the Cascades."

She says the team will try to learn how much rain it takes to trigger flash flooding and debris flows by surveying local burn areas after atmospheric rivers or major storms pass through. 

"The idea for that would be to give that back to the USGS, and they can fine-tune their model, to be more Pacific Northwest Specific," said Mickelson.  "Also, that data can be used to give to Weather Service, so that they know a rainfall threshold that they need to be concerned about when they are issuing alerts."

"I don’t want this bridge to be taken out, that’s for sure, so I’m watching it," said Betsty Wright, a Grotto Resident. 

The creek behind her community is very full of water from recent rains. 

"I like to see it flowing like this. If I come here, and it’s raining like this, and the water is just stopped, I’m going to be taking a trek up there to see where it’s going," said Wright. "What’s falling now is rolling off the mountainside and coming down to this creek."   

She doesn't mind the driving rain or swift water, noting that it can be normal for this time of year.  However, with the Bolt Creek Fire leaving a large burn scar nearby, it does increase the potential for debris slides or falling trees. 

"If the trees fall and start blocking up and diverting the water from this creek. I will be worried," said Wright.

The National Weather Service says a general rule of thumb indicates that half an inch of rainfall in less than an hour is sufficient to cause flash flooding in a burn area.  However, that can vary.    

In the coming weeks, a study by the DNR will likely shed more light on how the rainfall will impact areas like Grotto. 

"We are going out to install rain gauges in the next couple of weeks, and then we will be going out to do surveys after atmospheric rivers or major storms that affect Bolt Creek, to kind of do an assessment. Did that rainstorm trigger anything? Then, How much rain did the burn scar receive?" Mickelson said. 

Communities in burn areas at a 'high risk' for slides, flooding with recent rainfall

The relief many have felt from seeing snow and rain is short-lived for those living in the shadow of the steep slopes and mountainsides burned by the late-season fires.