Downpour, slopes are deadly pairing
North Bay holds as slides kill 13 in Southern California
As crushing mudslides buried homes and left more than a dozen dead in fire-ravaged Southern California on Tuesday, state and local officials were closely watching burned swaths of Wine Country to make sure the state’s biggest storm of the season didn’t bring similar devastation.
Authorities reported minor rock slides along the Sonoma County coast, but inland areas stripped of vegetation by October’s firestorm and made vulnerable to slipping appeared to hold. Residents in hilly areas were told to prepare for evacuations, though none was initiated.
Loose rock and debris was reported to have contributed to a fatal car wreck on Highway 121 in
Napa County on Tuesday night, but it was not immediately clear what role the burn scar might have played.
Geologists and fire experts blame the losses in Santa Barbara County, where mud and rock poured in and around the community of Montecito, on steep mountainsides and heavy downpours, creating conditions far more hazardous than in the North Bay.
“Generally we don’t get mudflows or debris slows on gentler slopes,” said Gary Griggs, a professor of earth sciences at UC Santa Cruz. “I think it’s really what’s the topography around you. The steepness and the rainfall intensity may have been the factor in Santa Barbara.”
Rates of rainfall approached 1.6 inches per hour in the mountains above Montecito, three times what geologists consider the safe threshold for the area, according to the U.S. Geological Survey’s Landslide Hazards Program.
By contrast, the North Bay mountains generally saw 0.2 to 0.3 of an inch of rain per hour during this week’s storm, possibly slightly more in isolated areas, according to the National Weather Service.
The worst of the system was moving out Tuesday night, with only a small chance of showers forecast for Wednesday.
“We ended up with a good amount of rainfall over our area, but it didn’t come down that hard, not really comparable to what they’ve seen down south,” said Rick Canepa, meteorologist for the Bay Area’s National Weather Service office. “It’s definitely extraordinary what they’ve seen down there.”
In areas near Montecito, 13 people died, 25 were injured and multiple homes were leveled Tuesday after rains broke loose mudslides and flooding in mountain canyons recently charred by the Thomas Fire, according to county officials.
Some of the deadly slides occurred in areas that were not under mandatory evacuation orders, which were imposed only on areas in the Thomas Fire burn area, Santa Barbara County Sheriff ’s Office spokeswoman Kelly Hoover said. Tuesday’s fatal mudslides spread to areas that were away from the fire line and under only voluntary evacuation orders, she said.
The massive blaze that had smoldered up until this week’s storm burned more than 281,890 acres in Santa Barbara and Ventura counties, making it the largest wildfire in modern California history.
Elsewhere in Santa Barbara County, firefighters rescued 50 people stuck in floodwaters and debris, said Amber Anderson, a spokeswoman for Santa Barbara County.
In San Bernardino County, to the east, 911 dispatchers received multiple calls that people were stranded on small islands and in trees in river bottoms, according to the county Sheriff's Department. At least five people were retrieved by air rescue.
Throughout Southern California where a handful of smaller fires had burned alongside the Thomas Fire, mudslides and floodwaters kept crews hopping as they worked to clear downed power lines, fallen trees and blocked roadways, including Highway 101 in Santa Barbara.
The risk of damaging slides is heightened when fire has left hills and valleys free of the vegetation that helps stabilize slopes and runoff is apt to flow more quickly.
Dennis Staley, a geologist with the USGS Landslide Hazards Program who was on his way to Montecito, said the deadly slide likely originated miles above the community where masses of ash and debris from the fire were greased by rain.
“You have all the flow converging into a stream channel and it progressively gets larger and larger as it goes downstream,” he said. “When it exits the mountain, it can be a massive flow like we saw.”
In the North Bay, officials have been on guard all week for possible disasters in the charred landscape of October’s sweeping fires, where 44 people died and nearly 9,000 structures were leveled.
Local and state work crews have spent the past two months installing straw bales and planting seed and mulch in a process known as hydroseeding to prevent erosion. Although the burn scar is far too large to stop slides, authorities can limit their likelihood in specific neighborhoods and along roads.
USGS hazard maps indicate that the most vulnerable spots are Sonoma County’s Mark West Creek area, the ridge between Sonoma Valley and Napa Valley, particularly Mount Veeder, Hogback Mountain and Hood Mountain, and parts of Napa County around Atlas Peak and Mount George.
Many of the people living in these areas have been given instructions by county officials on how to prepare for slides.
Postcards went out to 3,000 residents alerting them that their neighborhood was designated as a high-risk area due to the wildfires and what might happen during moments of high rainfall, said Adriane Mertens, a spokeswoman for the city of Santa Rosa.
The postcard emphasizes that the National Weather Service’s general rule is that half an inch of rainfall in an hour can cause flash flooding in areas affected by fires.
By Tuesday evening, however, there were no reports of major issues in the fire zone, according to the weather service.
“We are prepared for the next time a rainstorm comes to town,” Mertens said.
In all, Santa Rosa received about 3 inches of rain between Monday morning and Tuesday.
Elsewhere in the Bay Area, the monster storm left San Francisco with its 16th wettest day on record Monday with 3.64 inches of rain, breaking the same-day record of 2.36 inches set in 1872, according to the weather service.
Roadways and public transportation systems throughout the Bay Area were chaotic as crews worked to deal with fallen debris, collisions and equipment issues throughout the morning commute.
Major delays were reported at various BART stations throughout San Francisco and the East Bay due to equipment problems, officials said.
One of the worst crashes of the commute happened when a wrong-way driver caused a four-car crash on Interstate 880 in Milpitas just before 5 a.m., according to the California Highway Patrol. Three lanes of the highway were shut down while the crash was being cleared, turning the commute there into a forehead-pounding ordeal.