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Dealing with the threat of mudslides in soggy California

SAN DIEGO — Relentless storms from a series of atmospheric rivers have saturated craggy mountains and wildfire-scarred bald hills along much of California’s long coastline, triggering hundreds of landslides this this month.

So far, the debris has mostly blocked roads and highways and hasn’t harmed communities like in 2018 when mudslides roared through Montecito, killing 23 people and wiping out 130 homes.

But more rain is forecast, increasing the threat.

Experts say California has learned important lessons from the Montecito tragedy and has more tools to identify hotspots and more ponds and nets in place to catch falling debris before it hits. reach the houses. Recent storms are putting these efforts to the test, as climate change produces more severe weather.

Mudslide and damage to homes in Montecito, California on January 10, 2018.Matt Udkow/Santa Barbara County Fire Department via AP File

Why is California prone to landslides?

California has relatively young mountains from a geological perspective, which means that much of its steep terrain is still shifting and covered in loose rocks and soils that can be easily pulled out, especially when the ground is wet, according to geologists.

Almost the entire state has received rainfall totals 400% to 600% above average since Christmas, with some areas receiving up to 30 inches of precipitation, causing massive flooding. Bad weather has killed at least 19 people since late December.

Since New Year’s Eve, the California Department of Conservation Landslide Mapping Team has documented more than 300 landslides.

The state’s prolonged drought has made matters worse.

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

Dealing with the threat of mudslides in soggy California

“You would think that 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 decreases,” he said. As the water drains from the hardened ground, moving downward and picking up energy, it can begin to wash away dirt and debris, he said.

Additionally, wildfires have left some hills with little or no vegetation to hold the ground in place.

What are the most vulnerable areas?

The most vulnerable areas are hillsides that have burned in the past two or three years with communities below, said Jeremy Lancaster, who leads the California Department of Conservation’s geological and landslide mapping team. .

That includes areas that recently burned in Napa, Mariposa and Monterey counties, he said.

In 2018, the deadly Montecito mudslides came about a month after one of the largest fires in California history tore through the same area, charring 280,000 acres.

Montecito is sandwiched between the Santa Ynez Mountains and the Pacific Coast. On the fifth anniversary of this tragedy, the entire community was ordered to evacuate on January 9 as rains battered the area and debris blocked roads.

Lancaster warned that the threat of landslides will linger long after the rains have ended, as water seeps 50 to 100 feet into the ground, dislodging things.

“They can occur weeks later, even months,” he said.

What can be done to protect communities?

Lancaster said California has significantly increased its efforts to identify hotspots since the Montecito mudslides. His department continually updates its map so local communities are informed and can make decisions, including whether to evacuate an entire community.

The state is also working on a system to better determine the amount of rain likely to 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 assess the likelihood of landslides in a given area, including terrain maps and lidar – the pulsed light of lasers to penetrate foliage until you see the ground. Then they can monitor for early warnings, such as changes over time in photos taken from the air or from satellites, or in data from GPS monitoring stations, inclinometers and/or other instruments on site.

What is the most effective defense against landslides?

One of the best ways to deal with landslides is to use debris ponds – pits dug into the landscape to collect materials that flow downstream.

But the pools, which can be land-intensive, can also disrupt the natural ecosystem and lead to beaches that need to be replenished by collecting sediment that drains from the canyons, experts say.

And they’re expensive, said Douglas Jerolmack, a professor of environmental science and mechanical engineering at the University of Pennsylvania. And if old debris is not removed, it can be overwhelmed by new landslides or mudslides.

Dealing with the threat of mudslides in soggy California

Some also might not be large enough to handle future landslides made worse by climate change, Jerolmack said.

After the 2018 landslides hit Montecito, the Los Angeles Times reported that debris ponds above the community were undersized and had not been emptied sufficiently.

The tragedy galvanized the community, which raised millions to address the issue, said Patrick McElroy, a retired Santa Barbara fire chief who founded the nonprofit The Project for Resilient Communities.

The organization hired an engineering company to map the canyons and install debris nets. He said the recent storms had put them to the test: a net measuring 25 feet high filled almost to capacity.

McElroy said he was still haunted by memories of 2018, but felt better knowing the community could be safer now.

“I haven’t recovered from it yet. But waking up, you know, the other day and seeing no injuries or death. I just can’t tell you how impressed I am,” he said of the Nets.

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

But nothing is cheap. Santa Barbara County spent $20 million on a new pond after 2018, while McElroy’s organization spent nearly $2 million installing the nets, which includes liability insurance and other costs. They have a five-year license for the nets, which will be revoked if not renewed.

Gurrola said the alternative is more expensive. With recent storms, more than half of California’s 58 counties have been declared disaster areas and repairing the damage could cost more than $1 billion.

“Most importantly, these things protect the community and save lives,” he said.

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