The wildfires are out. But a new winter menace looms in the fire-scarred hills of California.
Both Northern and Southern California got hit this week with the first round of post-fire debris flows, essentially flash floods that can appear from nowhere.
Heavy rains this week caused small ash and fire-debris laden flows in the Butte County hills as well as in Malibu and in the hills of eastern Orange County – all spots where massive wildfires denuded hundreds of thousands of acres of hillsides, some of them above housing tracts.
The debris-flow risk prompted some evacuations in Butte, Los Angeles and Orange counties on Thursday.
So, far officials report damage was minimal, with no reports of injuries or damage to homes. But the flooding has stoked fears that the state may be in for a winter of road closures and even worse, physical danger to homes and lives, in fire-burn areas.
Much of the fear stems from lesson learned in a shockingly devastating debris flow in January that killed 23 people in the upscale Santa Barbara town of Montecito, days after the Thomas wildfire scorched the coastal hillsides there.
That slide, triggered by a brief spate of intense rain, sent a boulder- and tree-carrying flood wall through town, destroying hundreds of homes, burying a quarter-mile of Highway 101 under 10 feet of sludge for a week, and depositing crumpled cars and trucks on the beach below town.
“The Montecito event was not a particularly strong storm, it just happened to be localized, one (burst of rain) in a bad spot,” Daniel Swain, a UCLA climate scientist, warned. “Those situations are hard to predict. Giant storms are not going to catch us by surprise, but these individual heavy cells can catch us by surprise.”
Weather officials and hydrologists say the Montecito flood was an extreme event, but they warn California could be in for more damaging debris flows because of the increase in hot summer and fall fires.
“What we are seeing is the risk (of damage and injury) is not limited to fire season,” Swain said.
With more rain on the way this weekend, emergency services officials are warning residents of fire areas to keep their heads on a swivel.
Thursday in Butte County, where the Camp Fire ripped through hillside towns of Paradise, Magalia and Concow, 2 inches of rain on Wednesday forced evacuations of several rural areas and forced closure of a key mountain road as a slurries of ash, water, mud and fire debris washed down slopes and over roads.
There were no reports of damage, although about 50 drivers were temporarily stranded on the mountain when water coursed over Honey Run Road.
Lower-lying areas of Butte County were hit with flooding as well Thursday, prompting Cal Fire water rescues of some residents and closure of Highway 99 for one hour. That flooding in the lower areas, however, was not related to the Camp Fire and was not considered a debris flow.
In Southern California, the Trabuco Canyon in the Santa Ana Mountains of eastern Orange County is considered the inhabited area most at risk this winter for debris flows. The Holy Fire ripped through those mountains in August, burning 23,000 acres, including in canyons.
Thursday rains caused Trabuco Creek to swell dramatically, and prompted some mandatory evacuations in that area. The state Office of Emergency Services, Cal Fire and others sent crews there to do prep work this fall to protect residents from flows.
In Montecito, rain caused some mud to flow over one road, but the incident was minor. Some residents, still alarmed by January’s catastrophe, reported hearing boulders moving in stream beds overnight. But National Weather Service official Mark Jackson said storm runoff in Montecito was under control and staying within normal creek beds.
In Redding, where the Carr Fire and two other major fires hit this summer, Caltrans officials have their eye on Interstate 5 and several rural highways they believe are susceptible.
Spokeswoman Lupita Franco reported Thursday the agency has been dealing with some rocks and mud on roads, but no significant flooding. Maintenance crews are on alert.
“No news is good news,” she said. “We are monitoring it closely.”
Climatologist and weather service officials said the winter’s first major rains, overall, were healthy for California, bringing snow to the mountains and needed water to the state’s dry watersheds.
“Over all, you could consider this a beneficial rain,” Jackson of the weather service said. “We are still 10,000 miles away from busting the drought. But these are the kinds of rains we need.
“They are good tests of the burn areas. The Thomas burn area (above Montecito) withstood the rain.”
The rain will cause new vegetation to grow on burned hillsides, but that vegetation likely will not be strong enough until next year to help stabilize those hills, Jackson said.