California is tackling mudslides with boulder nets and better warnings

Relentless storms from a series of atmospheric rivers have saturated the steep mountains and bald hillsides scarred from wildfires along much of California's long coastline, causing hundreds of landslides this month.

So far, the debris has mostly just blocked roads and highways and has not harmed communities as in 2018, when mudslides roared through Montecito, killing 23 people and wiping out 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 the hot spots and more basins and nets are in place to capture the falling debris before it hits homes.

The recent storms are putting those efforts to the test as climate change produces more severe weather.

Why is California prone to mudslides?

California has relatively young mountains from a geological standpoint, meaning much of its steep terrain is still in motion and covered in loose rocks and soil that can be sloughed off easily, especially when the ground is wet, according to geologists.

Almost all of the state has received rainfall totals of 400 to 600 per cent above average since Christmas, with some areas receiving as much as 30 inches of precipitation, causing massive flooding.

The severe weather has killed at least 19 people since late December.

Dry ground 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 state's prolonged drought has made matters worse.

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

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With so much precipitation since Dec. 26, the soil in parts of California has been unable to absorb all the excess moisture, leading to mudslides and sinkholes.

"You'd think if the ground is dry it should be able to absorb a lot of water.But when ground becomes too dry, the permeability of the ground actually decreases," he said.

As water runs off the hardened soil, moving downward and picking up energy, it can begin carrying soil and debris away, he said.

Added to that, wildfires have left some hillsides with little to no vegetation to hold the soil in place.

Where are 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 that recently burned in Napa, Mariposaand Monterey counties, he said.

In 2018, the deadly mudslides in Montecito occurred about a month after one of the largest fires in California's history tore through the same area, charring 280,000 acres (about 113,000 hectares).

Montecito is sandwiched between the Santa Ynez mountains and the Pacific coast. The community was ordered to evacuate on Jan. 9, 2023, as rains dislodged debris, whichblocked roads.

Lancaster says even when rains subside, the threat of slides remains, as water seeps 15 to 30 metresinto the soil, dislodging things.

"They can occur weeks later, if not months," he said.

Lancaster said California has dramatically increased its 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 agencies use a variety of tools to gauge risk, includingterrain maps and lidar pulsed light from lasers to penetrate theground.

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Tens of millions in California are dealing with flooding and power outages after a series of massive storms hit the state, with more turbulent weather on the way.

What's an effective landslide defence?

One of the best ways to manage landslides is with debris basins pits carved out of the landscape to catch material flowing downhill.

But basins, which can require a lot of land, can also disrupt the natural ecosystem and lead to beaches needing to be replenished by collecting sediment that flows out of the canyons, according to experts.

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

Old debris must be removed and basinscan be overwhelmed if slides become bigger, worsened by climate change, Jerolmack said.

After the 2018 mudslides hit Montecito, the Los Angeles Times reported that debris basins above the community were undersized and hadn't been sufficiently emptied.

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 organizationthe Project for Resilient Communities.

The organization hired an engineering company to map the canyons and installed debris nets. He said the recent storms put them to the test: One net measuring 25 feet tall filled nearly to capacity.

McElroy said he's still haunted by memories from 2018, but feels betterknowing that the community might be safer now.

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

Gurrola and McElroy say about $22-million US was spent on a new basin and nets since 2018.

But they say that's cheap, compared to the $1-billion estimate to repair roads and townsafter recent floods left58 California counties as disaster areas.