California is tackling mudslides with boulder nets and better warnings

Relentless storms from a series of atmospheric rivers have filled steep mountains and bare hillsides scarred by wildfires along California’s long coast, triggering hundreds of landslides this month.

So far, the debris has mostly blocked roads and highways and not destroyed communities like in 2018, when a mudslide in Montecito killed 23 people and destroyed 130 homes.

But more rain is in the forecast, increasing the threat.

Experts say California has learned important lessons from the Montecito tragedy, and has more tools to pinpoint hot spots and more basins and nets to catch falling debris before it reaches homes.

The recent storms are testing as climate change causes more severe weather.

Why is California prone to mudslides?

California has relatively young mountains from a geological point of view, meaning that much of the steep terrain is still moving and covered with rocks and soil that can be easily removed, especially when the soil is wet, according to geologists.

Almost the entire country has received total rainfall of 400 to 600 percent above average since Christmas, with some areas receiving as much as 30 inches of rain, causing massive flooding.

Severe weather has killed at least 19 people since late December.

Dry soil can make things worse

Since New Year’s Eve, the California Department of Conservation’s landslide mapping team has documented more than 300 landslides.

The prolonged drought is getting worse.

Dan Shugar, associate professor of geoscience at the University of Calgary, said the drought could have a counterintuitive effect when combined with the extraordinary rainfall California has seen in recent days.

“You would think that dry soil should be able to absorb a lot of water. But when the soil becomes very dry, the permeability of the soil actually decreases,” he said.

When water flows over hard ground, it descends and picks up energy, so it can carry soil and debris, he said.

In addition, forest fires left some hillsides devoid of vegetation to maintain soil.

Where are the vulnerable areas?

The most vulnerable areas are hillsides that have burned in the past 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 areas recently burned in Napa, Mariposa and Monterey counties, he said.

In 2018, the deadly mudslide in Montecito occurred about a month after one of the largest wildfires in California history tore through the same area, burning 280,000 hectares (about 113,000 acres).

Montecito is between the Santa Ynez mountains and the Pacific coast. The community was ordered to evacuate on January 9, 2023, as heavy rain washed away the debris, which blocked the road.

Lancaster said that even though the rain has subsided, the threat to survival remains, as water seeps 15 to 30 meters into the ground, damaging things.

“They could happen weeks later, if not months,” he said.

Lancaster said California has dramatically increased efforts to identify hotspots and potential triggers since the Montecito mudslides.

Marten Geertsema, who studies natural hazards and terrain analysis at the University of Northern British Columbia, said the agency uses a variety of tools to measure risk, including terrain maps and lidar – pulsed light from a laser to penetrate the ground.

What is an effective avalanche defense?

One of the best ways to manage landslides is with debris basins – holes carved out of the landscape to catch material that flows down.

But the basin, which can require a lot of land, can also disrupt natural ecosystems and cause beaches that need to be replenished by collecting sediment that flows out of the canyon, according to experts.

They are expensive, said Douglas Jerolmack, a professor of environmental science and mechanical engineering at the University of Pennsylvania.

Old debris must be removed and the basin could be overwhelmed if the slide gets bigger, worse because of climate change, Jerolmack said.

After the 2018 mudslide hit Montecito, the Los Angeles Times reported that the debris basin above the community was low and had not been adequately emptied.

The tragedy took its toll on the community, which raised millions to address the problem, said Patrick McElroy, a retired Santa Barbara fire chief who founded the nonprofit Project for Resilient Communities.

The organization hired an engineering firm to map the canyon and install debris nets. He said the recent storm was trying: One 25-foot-tall net was filled almost to capacity.

McElroy said he is still haunted by memories from 2018, but feels more aware that the community may be safer now.

The best solution for the Montecito and Santa Barbara areas is to have nets and waste basins, according to Larry Gurrola, an engineering geologist hired by the organization.

Gurrola and McElroy said about $22 million US was spent on new basins and nets starting in 2018.

But they say it’s cheap, compared to the $1 billion estimated to repair roads and cities after the recent floods left 58 California counties as disaster areas.