Thursday, February 2, 2023

Associated Press Extension: Dealing with the threat of landslides in California

Discontinuous storms fueled by a series of atmospheric rivers have saturated steep hills and bare slopes with wildfires along the California coast, triggering hundreds of landslides this month.

So far, the debris has blocked most roads and highways and hasn’t damaged communities, as it did in 2018, when a mudslide in Montecito killed 23 people and swept away 130 homes.

But more rain is forecast, increasing the risk.

Experts say California has learned important lessons from the Montecito tragedy, has more equipment to identify hotspots, and more basins and networks to contain falling debris before it reaches homes. Recent storms are testing those measures amid more severe weather due to climate change.

Why is California prone to landslides?

California has relatively young mountains from a geological point of view, meaning that according to geologists, much of it is still moving and covered in loose rock and dirt that can be easily removed, especially when the ground is wet.

Much of the state has received 400% to 600% above average total rainfall since Christmas, with some areas receiving up to 30 inches (762 mm) of rain, causing widespread flooding. The inclement weather has killed at least 19 people since late December.

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

The prolonged drought in the state has made matters worse.

Dan Sugar, an assistant professor of geology at the University of Calgary, said the drought could have the opposite effect when combined with the incredible amount of rain that California has received in recent days.

“You would think that if soil is dry, it should be able to absorb a lot of water, but when soil becomes very dry, its permeability actually decreases,” he explained. He said that as the water seeps down through the hardened earth, moves downstream and increases in speed, it can begin to carry away dirt and debris.

In addition, wildfires have left some slopes with little or no vegetation to hold the soil.

Which are the most sensitive areas?

Jeremy Lancaster, head of the California Department of Conservation’s Geological and Landslide Mapping Team, said the most vulnerable areas are hills that have burned over the past two to three years and communities have been established beneath them.

Areas that have burned recently include Napa, Mariposa and Monterey counties, he said.

In 2018, the deadly landslide in Montecito occurred about a month after one of the largest fires in California history, which had spread to 280,000 acres (113,312 ha).

Montecito is located between the Santa Ynez Mountains and the coast of the Pacific Ocean. On January 9, the fifth anniversary of that tragedy, the entire community was ordered to be evacuated as rain pounded the area and debris blocked roads.

Lancaster warned that the risk of landslides would remain long after the rain subsided, as water seeped into the ground 50 to 100 feet (15 to 30 m) and stirred things up.

“It could be 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 mudslide. His department continually updates its maps so that local communities can stay informed and make decisions, which may include evacuating entire communities.

The state is also working on a system to better identify how much rain can trigger landslides.

Maarten Geertsema, who studies natural hazards and terrain analysis at the University of Northern British Columbia, said agencies use a variety of tools to measure the likelihood of landslides in a given area, including: Terrain maps and lidar, which uses a pulsed laser beam. See the leaves and the ground. It can monitor for early warnings, such as changes over time in photographs taken from the air or from satellites, or changes in data from GPS monitoring stations, slope gauges and other instruments on site.

What is the most effective defense against landslides?

One of the best ways to manage runoff is with basins or debris deposits, which are pits dug into the environment to catch downstream flowing material.

Experts say that basins, which can require large amounts of land, can also disrupt ecosystems and that beaches need replenishment by collecting sediment flowing from basins.

And they’re expensive, said Douglas Gerolmack, professor of environmental science and mechanical engineering at the University of Pennsylvania. And if the debris they hold is not removed, they could become overwhelming in the event of new landslides.

In addition, some future landslides from climate change may not be large enough to deal with, Jerolmac said.

After a landslide in Montecito in 2018, the Los Angeles Times reported that the debris retention basins upstream of the city were too small and had not been emptied adequately.

The tragedy spurred the community to take preventive action, raising millions of dollars to address the problem, said Patrick McElroy, a retired Santa Barbara fire chief who founded the nonprofit The Project for Resilient Communities. ,

The organization hired an engineering firm to map the valleys and set up nets to collect debris. He said recent storms have put them to the test: One trap that was 25 feet (7.6 m) high was almost completely filled.

McElroy said he’s still haunted by the memories of 2018, but he feels good knowing the community can be safe now.

“I’m still not over it. But waking up the next day (from the landslide) and not seeing any injuries or deaths. I can’t tell you how impressed I am,” he said of the network.

According to Larry Gurola, the organization’s contract engineering geologist, the best solution for the Montecito and Santa Barbara area is both traps and basins to retain the debris.

But nothing is cheap. Santa Barbara County spent $20 million on a new basin after 2018, while McElroy’s organization spent about $2 million setting up the network, which covers liability insurance and other fees. They have a five-year permit to use the network, which will be withdrawn if not renewed.

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

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

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Glass reported from Minneapolis.

Nation World News Desk
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