The images on social media are dramatic, even for a region accustomed to winter storm mayhem. A tall tree gliding, roots and all, down a hillside and over a road in the Santa Cruz Mountains. A river of mud and logs coursing through the Hollywood Hills. Massive boulders tumbling onto a road east of Fresno.
While all this rain is great news for hopes of ending California’s punishing, multi-year drought and its accompanying wildfire risk, it comes with a hazard apart from the floods that have dominated the news this week: landslides. How much has that risk grown from day after day of rain? Geologists say it comes down to soil saturation.
“When you start to see hillslopes collapsing onto roads around you, it’s telling you the ground is saturated,” said Noah J. Finnegan, an associate professor of earth and planetary sciences at the University of California Santa Cruz. “I suspect we’re close to a sort of critical state.”
While most landslides end up being a matter of inconvenience to motorists when roads are blocked, they can also have tragic consequences in California. This week marks the fifth anniversary of one of the worst: a Jan. 9, 2018, mudflow that killed 23 people and damaged or destroyed 500 properties when heavy rains soaked the massive Thomas Fire burn scar in Santa Barbara County. This week’s rains prompted evacuations in the same area over landslide concerns after heavy rains overflowed creeks and flooded streets.
And last week marked the 41st anniversary of the Jan. 5, 1982, Love Creek landslide that killed 10 people and buried homes in the Santa Cruz Mountains community of Boulder Creek. The steep, tree-covered canyon hillsides gave way after persistent heavy rains that oversaturated the soil.
Brian Collins, a civil engineer with the U.S. Geological Survey’s landslide hazards program, monitors soil saturation and landslide risk around the Bay Area and advises the National Weather Service and other agencies. He said there have been about 1,000 landslides documented from the recent winter storms, mostly from the extended downpours that slowly rolled across the state on New Year’s Eve. Geologists still are assessing the impacts of the latest series of storms, he said.
Gauging risk is complicated — a place such as the Santa Cruz Mountains, accustomed to heavy winter rains, might not see as many landslides from four inches of rain as the hills in the East Bay, Collins said. But once the ground is saturated, anywhere that gets a lot of rain, either in a short burst or stretched out over many hours, is a place where the land can give way.
“At this point, with the soil saturated with back to back to back storms, wherever that next storm’s going to go is at risk,” Collins said. “There’s not any area that’s more at risk.”
Incredible footage! A @PGE4Me lineman captures the moment a pine tree in Santa Cruz Mountains gives way due to a mudslide along HWY 9 today. Lineman was not hurt, but issues like these are making it unsafe for crews to work in some locations. More info: https://t.co/zC3ax4QudN pic.twitter.com/B8w4URx0pV
— Mayra Tostado (@PGE_Mayra) January 10, 2023
How wet has it been compared to other years when devastating landslides struck? Bay Area meteorologist Jan Null of Golden Gate Weather Services said Dec. 26-Jan. 9 is the third-wettest 15-day period in San Francisco history with more than 12 inches, eclipsed only by a couple of 15-day periods in December and January of the 1860s that saw nearly 14 inches and 20 inches.
This is now San Francisco’s 3rd wettest 15-day period, going back to the Gold Rush in 1849. But a long way, thankfully, from the Winter of 1861-62 which saw 19.77″ in 15-days, a record 24.36″ for the month and a record 49.27″ for the season. pic.twitter.com/Tka0746r0d
— Jan Null (@ggweather) January 10, 2023
Continuous storms the winter of the Love Creek disaster also caused widespread flooding and mudslides throughout the Bay Area. According to the USGS, the Santa Cruz Mountains received more than 25 inches of rain over Jan. 3, 4 and 5, 1982.
Collins said that in many past wet winters, heavy rains have been scattered between dry spells, allowing the soil to dry out. If the soil isn’t saturated, there is less chance of another storm triggering landslides.
“This year is considerably different,” Collins said. “Things became saturated after the 27th and have remained saturated.”
A California Highway Patrol officer captured incredible footage of a huge landslide in Fresno County Monday afternoon. #CAwx #AtmosphericRiver https://t.co/Wqcngxk5Ea pic.twitter.com/h0bNiB8Qo3
— AccuWeather (@accuweather) January 10, 2023
He said that after a couple of winters of vegetation growth, burn scars from massive wildfires in recent years don’t necessarily lead to a higher risk of mudslides (Southern California, in contrast, is now contending with much more recent burn scars). And last year was a relatively mild wildfire season.
But just how much wildfires add to the odds of debris flows happening in different parts of the state remains under study, he said.
He noted that the Montecito disaster in 2018 involved heavy rains on an active wildfire zone and in a part of the state with different vegetation — such as chaparral — and risk characteristics.
“You can’t say there’s no risk, but from a post wildfire standpoint, there is a recovery period,” Collins said.
What does worry geologists are shallow landslides and debris flows in which top layers of soil are dislodged by heavy rain, either concentrated in a short period or persisting over a long time, Collins said. Those can move quickly and come with little warning.
If we continue to see a wet rainy season, that raises the risk of the slower, deeper land movement that people often first notice when doors and windows stop opening and closing smoothly and cracks appear in walls and foundations, he added.
“A winter with a lot of precipitation,” Collins said, “tends to mobilize those.”
Source: www.mercurynews.com