Tackling threat of mudslides in soaked California

SAN DIEGO (AP) – Relentless storms from a series of atmospheric flows have saturated the steep mountains and barren slopes scarred by wildfires along much of California’s long coast, causing hundreds of landslides this month.

So far, the debris has mostly blocked roads and highways and done no harm to communities, like in 2018 when mudslides swept through Montecito, killing 23 people and obliterating 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 locate the hot spots and more basins and nets to catch the falling debris before it hits homes. Recent storms are testing those efforts as climate change leads to harsher weather conditions.

WHY IS CALIFORNIA APPLICABLE FOR MUD SURFACES?

California has relatively young mountains from a geological standpoint, meaning much of its steep terrain is still in motion and is covered with loose rock and soil that geologists say can be easily eroded, especially when the ground is wet.

Almost the entire state has received rainfall levels 400% to 600% above average since Christmas, with some areas receiving as much as 30 inches of rainfall, leading to massive flooding. The storm has claimed at least 19 lives since the end of December.

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

The ongoing drought in the state has made matters worse.

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

“You’d think that dry soil would hold a lot of water, but when the soil gets too dry, the soil’s permeability actually decreases,” he said. As water drains away from the hardened earth, moving down and absorbing energy, it can start carrying away soil and debris, he said.

The story goes on

Additionally, wildfires have left some slopes with little to no vegetation to hold the soil in place.

WHAT ARE THE MOST VULNERABLE AREAS?

The areas most at risk are hillsides that have burned in the past two to 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 happened about a month after one of the largest fires in California history tore through the same area, charring 280,000 acres.

Montecito nestles 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 rain fell on the area and debris blocked roads.

Lancaster warned that the risk of landslides will remain long after the rains have subsided as the water seeps 50 to 100 feet into the ground, displacing things.

“They can show up weeks, if not months, later,” he said.

WHAT CAN BE DONE TO PROTECT COMMUNITIES?

According to Lancaster, California has dramatically increased its efforts to identify hotspots since the Montecito mudslides. His department is constantly updating 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 how much rain 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 estimate the likelihood of landslides in a given area, including terrain maps and lidar — pulsed light from lasers to spot foliage see through the ground. Then they can look out for early warnings, such as: B. Changes over time in photographs taken from the air or from satellites, or in data from GPS monitoring stations, inclinometers and/or other on-site instruments.

WHAT IS THE MOST EFFECTIVE DEFENSE AGAINST MUD SURFACES?

One of the best ways to deal with landslides is with debris ponds — pits dug into the landscape to catch material flowing downhill.

But basins, which can take up a lot of land, can also disrupt the natural ecosystem and result in beaches having to be replenished by collecting sediment that flows out of the canyons, experts say.

And they’re costly, 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 also might not be large enough to deal with future landslides, which will be exacerbated by climate change, Jerolmack said.

After the 2018 mudslides hit Montecito, the Los Angeles Times reported that debris pools over the community were too small and under-emptied.

The tragedy excited the community, who 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 were testing them: a 25-foot-high net filled almost to capacity.

McElroy said he’s still haunted by memories of 2018 but feels better knowing the community could be safer now.

“I’m not over it yet. But waking up the other day and seeing no injuries and no deaths. I can’t tell you how impressed I am,” he said of the Nets.

According to Larry Gurrola, the engineering geologist employed by the organization, the best solution for the Montecito and Santa Barbara area is to have both meshes and debris ponds.

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

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

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

_____

Glass reported from Minneapolis.

Julie Watson and Doug Glass, The Associated Press