Mountain residents brace themselves for winter rains
Fierce debris flows threaten homes, businesses located below denuded burn areas
The wildfires in the Santa Cruz Mountains have passed, but they have left behind foreboding scars. Black, charred slopes loom above homes and businesses, poised to become furious rivers of boulders and mud.
The extraordinary heat generated by summer’s CZU Lightning Complex fires ravaged the mountain range, scorching its forests and soils. And with winter rains now beginning to pound Northern California, debris flows, flash floods and rockfalls threaten to come crashing down on mountain communities.
But predicting these often tragic events — particularly debris flows — is a complicated business, largely because most of what we know about them comes from Southern California, where the landscape is markedly different.
“We just don’t know how these areas are going to respond,” said Drew Coe, a Cal Fire hy
drologist who led the agency’s Watershed Emergency Response Team during its assessment of areas burned by the CZU fires. That, Coe said, is because wildfires in the Santa Cruz Mountains are relatively uncommon and don’t typically burn as intensely as this year’s fires did.
The wildfires, sparked by lightning storms on Aug. 16, destroyed 1,490 homes and other structures and burned 86,509 acres during a monthlong siege. But now fire-weary residents face a new peril.
Because debris flows can strike quickly, Santa Cruz County officials have released an evacuation map and are urging residents to get to know their evacuation zones and risk levels, to prepare to leave ahead of time and to get an evacuation plan ready. Authorities are also encouraging community members to register for the county’s aler t system, CodeRed, keep an eye on weather forecasts and share any information they have with neighbors.
“T he threat of w inter evacuations has us pretty nervous,” said Jesse Keesaw, a 36-year- old Ben L omond resident who during the CZU fires was forced to evacuate with his wife and children for three weeks.
Keesaw, a maintenance technician at Ben Lomond’s Quaker Center, has lived in the San Lorenzo Valley his entire life. But he’s never had to contend with debris flow evacuations before.
“We’re trying to be optimistic,” he said. “But I know the mountains above us are really, really steep, and some of them are pretty devoid of vegetation.”
Flash floods and debris flows are more likely to occur after wildfires incinerate vegetation and damage soils, making it easier for short bursts of rain to flow
down and erode slopes.
While f lash floods are more common, debr is flows — sometimes called mudslides or mudflows — are more dangerous. The destructive slurries of mud and rock can appear with little warning.
In January 2018, debris f lows in posh Montecito near Santa Barbara killed 23 people and destroyed more than 100 homes after wildfires torched the area.
“A debris flow is a lot like wet cement,” said Noah Finnegan, a professor of geomorphology at UC Santa Cruz. “It’s capable of moving very large boulders, and it moves a lot faster than water.”
Wildfires prime slopes for debris flows in several different ways, Finnegan said. They incinerate vegetation, removing stabilizing root systems and the soil’s natural protections from rain.
Fires also make soil surfaces impervious to water. Scientists are not entirely certain why this oc
curs, but there are several theories. One possibility, Finnegan said, is that ash gets into the pore spaces of soils, plugging them up and preventing water from soaking in.
As water flows down a slope, Finnegan said, it picks up sediment. And as the sediment accumulates, the moving mass becomes more dense, powerful and treacherous.
When California teams surveyed the CZU burn area, they looked for geologic evidence of past debris flows and homes that might be at risk. They also studied the soil to measure what Coe calls “soil burn severity.”
While precisely predicting debris f lows is challenging, scientists know that the more severe the soil damage, the more likely a debris f low will occur. Although other factors such as slope steepness and geology are used to calculate the probability of debris flows, assessing just how badly soil was
burned tells the Cal Fire teams how a wildfire has increased the probability of a debris flow.
To measure the burn severity, the teams look at the soil’s color, integrity and water resistance. They also determine how much plant matter it contains.
Scientist s were surprised by how intensely the CZU fires burned. A coastal forest shouldn’t burn that ferociously because it’s so often shrouded by fog, Coe said, adding that “something was very different that day the fires took off.”
Many scientists suspect climate change played a role. When the fires ignited, they note, the region was experiencing an extremely high “vapor pressure deficit,” a measure of the degree to which the atmosphere can dry vegetation.
A 2019 study led by Park Williams, a Columbia University bioclimatologist, showed a strong link between California’s rising temperatures, increasing
vapor pressure deficits and larger wildfires.
Northern California scientists, however, are hampered by the fact that the computer models used for predicting debris f lows here are based on data from Southern California. Because the relatively parched Southland has more severe fires than the wetter, forested Santa Cruz Mountains, Coe said, “we don’t have a good idea” about how well the models will work here.
This uncertainty presents county officials with a dilemma, as they attempt to organize their evacuation protocols.
“The only way to really plan for debris flow is to get people out of the way because there’s very little you can do to prevent it,” said Ian Larkin, chief of Cal Fire’s Santa Cruz- San Mateo Unit.
At the same time, Coe said, issuing unnecessary alerts can cause “evacuation fatigue — and that does nobody any good.”
Seeking to refine their e v a c u a t ion pl a n n i n g , Santa Cruz County officials looked at Big Sur’s fire histor y, said Mark Strudley, manager of the county’s flood control program. They studied how the landscape responded after events like the 2016 Sobera nes f ire, which burned 132,127 acres and destroyed 57 homes.
County officials also ordered more field surveys to refine Cal Fire’s initial assessment. In addition, the California Geological Survey has scrutinized the burned terrain surrounding the residential areas of Boulder Creek. A nd Santa Cruz County geologists spent about a month carefully combing the entire burn scar, focusing on identifying all of the homes and other structures at risk.
All of these sources of information were considered when constructing a debris flow hazard area map, which the subsequent evacuation map was based on, said Carolyn Burke, senior civil engineer for the county.
To improve future predictions, the county, the National Weather Service, the U. S. Geological Survey and the California Department of Water Resources are working together to deploy a fleet of rain gauges in the burn area that will help them monitor the relationship between rainfall and debris flows.
Rainfall rates of threetenths of an inch in 15 minutes, half an inch in 30 minutes or seven- tenths of an inch in a hour will trigger evacuation warnings, Strudley said.
Such a deluge is not uncommon in the Santa Cruz Mountains. “It’s something we typically experience, on average, once or a few times every year,” he said.
And that frightens many mountain residents.
“I really don’t know how fast everything will work — whether we will be able to physically leave in time, whether the evacuation will get to us in time,” Keesaw said.