Jan Wesner Childs
After months of devastating wildfires and waiting desperately for the upcoming wet winter season to bring relief, some communities in California are now at high risk of potentially deadly mudslides.
The fires burned thousands of square miles of land and left scorched and barren hillsides vulnerable to an especially dangerous fast-moving type of landslide that scientists call "debris flow." Known less formally as mudslides, these flows are typically triggered by short, intense storms and can send a wall of soil, ash, vegetation, rocks and even cars and homes careening downhill, destroying everything in their path.
“A debris flow is kind of a flood on steroids," Jason Kean, a debris flow expert with the U.S. Geological Survey, told weather.com in an interview Wednesday. "It’s all bulked up with rocks, mud, even boulders and boulders can be the size of cars."
Areas of particular concern include parts of Santa Cruz and San Mateo counties, according to a state risk assessment published earlier this month.
"It’s imperative that we act swiftly to save lives by educating our community on the risks of debris flows and the only effective means of protection — early warning and evacuation," Santa Cruz County senior civil engineer Carolyn Burke said at a recent county meeting, according to the Santa Cruz Sentinel.
(MORE: What Winter Could Do to the Most Expansive Drought Since 2013)
Besides destroying vegetation that would normally hold soil and debris in place, wildfires exacerbate mudslide risk by changing characteristics of the soil itself. It becomes less likely to absorb water, which creates conditions ripe for flash flooding and debris flow, according to Kean.
“As (the water) runs off it picks up all these loose soil particles that no longer have vegetation to hold them in place and then it bulks up into that debris flow that can move really fast and start actually within minutes of intense rainfall," he said.
Debris can rush down a hillside at a rate up to 30 mph and then flatten out and spread even farther when it reaches the bottom of a slope, Burke said at the county meeting.
"[A] debris flow is something you really can’t plan for," Ian Larkin, Cal Fire's San Mateo-Santa Cruz unit chief, told the San Francisco Chronicle. "The only planning you do for it is to get people out of its way. ... It’s going to happen, and when it does happen, it’s instantaneous and dangerous."
The risk assessment for Santa Cruz and San Mateo counties included data from USGS maps of post-wildfire debris flow hazard areas that Kean helps create. The two counties were singled out for a closer look because of the proximity of residential areas and critical infrastructure to areas burned by the CZU Lightning Complex Fire, which charred more than 135 square miles in the region, destroyed nearly 1,500 homes and other structures and killed at least one person.
The report from the California Division of Forestry and Fire Prevention, known as Cal Fire, and the State Department of Conservation notes the blaze left behind vast areas with moderate to high "soil burn intensity," a measure of damage to the ground and an indicator of mudslide potential.
"When you have higher intensity fires … you have more of the landscape that can produce these post-fire debris flows," Ann Youberg, a senior research scientist with the Arizona Geological Survey, told Popular Science.
(MORE: California Wildfires, in Photos)
The elevated risk can last up to five years after a fire, and many of the areas impacted by wildfires have burned more than once in recent years.
They've also seen catastrophic post-wildfire debris flows. At least 20 people were killed when mudslides with walls of debris as tall as 30 feet struck the Santa Barbara County community of Montecito in 2018. On average, 25 to 50 people are killed by landslides every year in the United States, according to the USGS.
The threat for California will increase as the rainy season moves in, usually starting in November and then ramping up through February.
The state is no stranger to extreme weather.
"California has a distinct wet season that lasts from late fall into early spring, and a dry season with little to no rainfall from late spring to early fall," said weather.com meteorologist Chris Dolce. "That extreme variance is related to an area of high pressure that sets up off the West Coast in summer and prevents any significant moisture from reaching the state. Conversely, in winter, that high weakens and shifts farther south, which opens the door for Pacific storms to affect California with heavy rain and mountain snow."
Wildfire danger, meanwhile, peaks in the fall because that's when vegetation is completely dried out prior to the arrival of the first significant Pacific storms of the season, Dolce said.
Research has linked California's progressively more extreme fire seasons to climate change, and a recent study published by the University of California Institute of Transportation Studies found that the risk of post-wildfire debris flows is also likely to go up.
Communities in the potential path of mudslides across the state are preparing in any way they can. The community of Yucaipa in San Bernardino County plans to install new signs and traffic-control devices to warn of the danger, and officials are considering purchasing concrete barriers, Redlands Daily Facts reported.
Boulder Creek Community Church, which barely survived the CZU Complex Fire and is in one of the highest-risk areas in Santa Cruz County, recently installed a half-dozen 20-foot concrete barriers weighing 4 tons to help protect against a debris slide on the charred hillside behind church property, according to the San Francisco Chronicle.
Government officials, first responders and scientists are scrambling to come up with a rapid evacuation and response plan to hopefully save lives, while at the same time educate residents and avoid evacuation fatigue.
"I don’t expect to be able to relax until spring," Santa Cruz County geologist Jeff Nolan told the Chronicle.
This satellite image shows the burn scars left by the CZU and SCU lightning complex fires in Northern California. Areas burned by the fires are at higher risk of mudslides as the winter rainy season sets in.The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
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