Southern California's next potential threat? Floods, landslides and toxic debris flows

As devastating wildfires continue to ravage the Los Angeles metropolitan region, local officials are warning of potential floods, landslides and toxic debris flows in the weeks and months ahead.

"The properties have been damaged beyond belief," Mark Pestrella, director of Los Angeles County Public Works, said at a joint LA County and city press conference Thursday.

"They are full of sediment, debris, silt and hazardous materials," added Pestrella, referring to burn scar sites in the vicinities of the Palisades and Eaton fires.

The Palisades Fire, which has destroyed the Pacific Palisades neighborhood, is now 23,713 acres and only 31 percent contained, according to Cal Fire. The Eaton Fire, burning north of Pasadena, is 14,117 acres and 65 percent contained.

"Both areas suffered watershed damage, burned to such a significance that we expect massive debris-laden flows when it rains," Pestrella said.

"In an event that we have major rain, we do expect that all of the street areas and all the communities will be impacted by debris flows that could be hazardous to human health," he warned.

To address this looming threat, Pestrella said that he and his teams are developing plans to both capture and hold back debris as much as possible during a heavy precipitation event.

"We do expect the window to open for rain in the later part of this month, and so we are already pre-deploying labor forces into the area," he added.

Asked at the press conference to address why a specific spot in Pacific Palisades had already undergone a hillside collapse, Pestrella explained that this occurrence was due to a geophysical process called "dry ravel."

The phrase “dry ravel” refers to the rolling or sliding of particles down a slope in steep, arid landscapes, according to scientists. During wildfires, such particles can move via the collapse of sediment chunks that have amassed behind vegetation.

"It is a landslide — debris into the streets and into properties," Pestrella said. "Some of this material is supporting some of the structures that have remained behind."

He also noted a certain amount of water inundation, due to the amount of the resource that was used during firefighting efforts, as well as some destruction of water and gas lines.

"A warning to all the residents, no matter where you live in LA County, if you have slopes behind your homes, or if you're located on top of a slope, these slopes have become fragile," he said.

"The soil that is supporting your home have all become fragile and damaged, if you will, due to the events that we've had, wind included," Pestrella added, stressing that debris flow hazards exist "even when it's not raining."

Regarding the toxic nature of the debris flow, Pestrella noted that the federal Environmental Protection Agency (EPA) is working with local partners to clear properties of hazardous substances.

Tara Fitzgerald, an EPA on-scene coordinator, explained that the agency has been tasked by the Federal Emergency Management Agency with conducting a "household hazardous materials removal in the burn footprint."

That removal, which she said began Thursday, involves eliminating "the most immediate risk to public health and workers."

Among the targeted materials for removal are pesticides, lithium-ion and other batteries, fuel and other items that would typically be sent to a household hazardous waste management landfill, according to Fitzgerald.

"We are working quickly to ramp up and look forward to being able to support this recovery effort," she added.

As for possible flooding in the upcoming weeks and months, Daniel Swain, a climate scientist at the University of California, Los Angeles, expressed some trepidation about what might be in store at a webinar earlier this week.

While forecasts do not indicate that such a deluge is imminently on the horizon, Swain noted that major rain is "still quite possible in February or March or even April."

And if such an event occurs, he warned that "there will be major concern regarding the potential for flash floods and debris flows in and downstream of all of these areas."

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