SAN DIEGO — Relentless storms from a series of atmospheric rivers have saturated steep mountains and bald slopes scarred by wildfires along much of the California coast, causing hundreds of mudslides this month.
So far, the debris has mostly blocked roads and highways and hasn’t damaged communities like it did in 2018, when mudslides roared through Montecito, killing 23 people and leveling 130 homes.
But more rain is forecast, raising the threat.
Experts say California has learned important lessons from the Montecito tragedy, and has more tools to identify hot spots and more basins and nets to catch falling debris before it reaches homes. Recent storms are testing those efforts as climate change brings more severe weather.
WHY IS CALIFORNIA PRONE TO MUDSLIDES?
California has relatively young mountains from a geological point of view, which means that much of its steep terrain is still moving and covered in loose rock and soil that can easily dislodge, especially when the ground is wet, according to the geologists.
Nearly the entire state has received 400% to 600% above average rainfall totals since Christmas, with some areas receiving up to 30 inches of precipitation, causing massive flooding. Severe weather has killed at least 19 people since late December.
Since New Year’s Eve, the California Department of Conservation’s landslide mapping team has documented more than 300 landslides.
The state’s prolonged drought has made matters worse.
Dan Shugar, an associate professor of geoscience at the University of Calgary, said the drought may have a counterintuitive effect when combined with the incredible rainfall California has seen in recent days.
“You’d think if the soil is dry it should be able to absorb a lot of water, but when the soil gets too dry, the permeability of the soil actually goes down,” he said. As the water drains from the hardened soil, moves downward, and picks up energy, it can begin to carry soil and debris with it, she said.
On top of that, wildfires have left some hillsides with little to no vegetation to hold the soil in place.
WHICH ARE THE MOST VULNERABLE AREAS?
The most vulnerable areas are hillsides that have burned in the last two to three years with communities below them, said Jeremy Lancaster, who leads the California Department of Conservation’s geological and landslide mapping team.
That includes recently burned areas in Napa, Mariposa and Monterey counties, he said.
In 2018, the deadly mudslides in Montecito occurred about a month after one of the largest fires in California history tore through the same area, charring 280,000 acres.
Montecito is located between the Santa Ynez Mountains and the Pacific coast. On the fifth anniversary of that tragedy, the entire community was ordered to evacuate on January 9 when rains lashed the area and debris blocked roads.
Lancaster warned that the threat of mudslides will persist long after the rains have subsided as water seeps 50 to 100 feet into the ground, dislodging things.
“They can happen weeks later, if not months,” he said.
WHAT CAN BE DONE TO PROTECT COMMUNITIES?
Lancaster said California has dramatically increased its efforts to identify hotspots since the Montecito mudslides. Her department continually updates its map so that local communities are aware and can make decisions, even whether to evacuate an entire community.
The state is also working on a system to better identify the amount of rain that could trigger a landslide.
Marten Geertsema, who studies natural hazards and terrain analysis at the University of Northern British Columbia, said agencies use a variety of tools to gauge the probability of landslides in a given area, including terrain maps and lidar: pulsed laser light to penetrate foliage to see the ground. They can then look for early warnings, such as changes over time in photos taken from the air, or from satellites, or in data from GPS monitoring stations, slope gauges, or other on-site instrumentation.
WHAT IS THE MOST EFFECTIVE DEFENSE AGAINST MUDSLIDES?
One of the best ways to manage landslides is with debris basins: pits dug into the landscape to catch material flowing downhill.
But the basins, which can require a lot of land, can also disturb the natural ecosystem and cause beaches to be replenished by collecting sediment that flows from the canyons, experts say.
And they’re expensive, said Douglas Jerolmack, a professor of environmental sciences and mechanical engineering at the University of Pennsylvania. And if old debris isn’t removed, it can be overwhelmed by new landslides or mudslides.
Some might not be big enough to cope with future slides made worse by climate change, Jerolmack said.
After the 2018 mudslides hit Montecito, the Los Angeles Times reported that the debris basins above the community were too small and had not been emptied sufficiently.
The tragedy galvanized the community, which raised millions to address the problem, said Patrick McElroy, a retired Santa Barbara fire chief who founded the nonprofit organization, The Project for Resilient Communities.
The organization hired an engineering firm to map the canyons and installed debris nets. He said recent storms have put them to the test: A net that was 25 feet high was almost completely filled.
McElroy said he’s still haunted by memories of 2018, but he feels better knowing the community might be safer now.
“I still haven’t gotten over it. But waking up, you know, the other day and seeing no injuries or deaths. I can’t tell you how impressed I am,” he said about the networks.
The best solution for the Montecito and Santa Barbara area is to have debris nets and basins, according to Larry Gurrola, the organization’s contract engineering geologist.
But nothing is cheap. Santa Barbara County spent $20 million on a new basin after 2018, while McElroy’s organization spent about $2 million installing the networks, which includes liability insurance and other fees. They have a five-year network permit, which will be withdrawn if not renewed.
Gurrola said the alternative is more expensive. With the recent storms, more than half of California’s 58 counties have been declared disaster areas, and the damage can cost more than a billion dollars to repair.
“The most important thing is that these things protect the community and save lives,” he said.
Glass reported from Minneapolis.
Copyright © 2023 by The Associated Press. All rights reserved.