Southern California storm poses debris threat

from Rachel BeckerCalMatters

This story was originally published by CalMatters. Sign up for their newsletters.

An unusually strong storm system has reached Southern California, raising fears that the rain could unleash a threat that remains in the fiery scars of wildfires that have ravaged Los Angeles communities in recent years.

These are called debris flows fast moving slurries of flood water and sediment can flow down slopes, carrying cars, trees and even rocks with it.

They’re like a “flood on steroids,” said Jason Keen, a research hydrologist with the US Geological Survey’s Landslide Hazards Program. “It’s really hard to stop these things. The best thing to do is get out of the way.”

Forecasters expect the heaviest rain Friday into Saturday night; the forecast is for wet days through next week. Storms may extend from Santa Barbara County south into Los Angeles County and may spread inland into parts of Orange County and the Inland Empire.

Burn scars—smoothed by fire and stripped of vegetation—are particularly vulnerable. A storm following the 2018 Thomas Fire triggered debris flows in Montecito that killed 23 people. And in February, a debris flow in the burning area of ​​the Palisades Fire swept away a Los Angeles fire department member and his SUV in the Pacific Ocean.

The Los Angeles County Department of Public Works warns that there is a risk of moderate debris and mudflows capable of blocking roads and endangering some structures in the burn scars of nearly a dozen fires — including the Eaton, Hurst and Palisades fires in January.

The county has issued evacuation warnings, as well as some targeted evacuation orders for specific properties “at higher risk of being impacted by mud and debris flows.”

The Department of Public Works says most burned properties have been cleared of fire debris, and the rest have been propped up with gravel bags and other materials to hold the debris in place.

Governor Gavin Newsom also announced today that more than 400 personnel and resources, including fire trucks, helicopters and search and rescue teams, have been pre-deployed to Southern California counties.

“It’s a pretty serious situation,” said National Weather Service meteorologist David Gomberg.

By Friday morning, the storm had already unleashed up to 5 inches of rain in parts of Santa Barbara County, Gomberg said, and Southern California was bracing for more.

There is also a chance for thunderstorms, small tornadoes and a disturbing amount of rain to hit the burn scars of Eaton and the Palisades. Even as little as a half to 0.6 inches of rain could trigger a debris flow in those areas, said Gomberg, who added that his office is forecasting between half an inch and one inch per hour in those areas.

“And the further you go over the threshold, the likelihood of a more damaging debris flow increases,” he said.

We spoke with Keane of the US Geological Survey, an expert on debris flows after wildfires, about what to expect. This conversation has been edited and shortened.

As this storm really takes hold in Los Angeles and Southern California, I’m hearing a lot of concern about it hitting areas that burned last year, including the Eaton and Palisades fires. Why is this such a big concern? What can happen?

Last January, those fires removed a lot of vegetation on really steep slopes, and that made those slopes really vulnerable to erosion during heavy rains. This protective blanket of vegetation is gone and heavy rain can quickly cause flash flooding. And that flood can in some cases pick up material and become what we call a debris flow – which is like a flood on steroids.

These burn areas are still vulnerable, even though many months have passed since the fire and streams are now present. There is still much material that can be mobilized. So the threat is still there. And so we know they’re bad actors, and we worry that they might be bad actors again.

I hear a lot of different terms: mudslide, debris flows, landslide. What are the differences and what are the risk marks of burns?

Landslide is a general term that covers all types of mass movements, from rock falls to debris flows—those floods on steroids—to large, slow-moving ones. The type of flow we are most concerned about in a recent burn area is debris flow. It is also called a mudslide. But geologists don’t like to use the word mudslide that much because it sounds like there’s a little mud in your driveway—no big deal, not something that can kill you. And these things, if you’re in the wrong place at the wrong time, they can cause serious damage.

You called it a steroid flood. What happens in a debris flow?

Flash floods are bad and can cause a lot of problems too. They can become even worse if they collect enough sediment to become the consistency of wet concrete. But it’s worse than just concrete because it can contain rocks the size of cars. And very close to the mountain front, it can move very fast – faster than you can run. And when everything is filled with debris, stones, gravel, mud, trees, the flow can be much greater. It just becomes a different animal.

Debris flows now pack a bigger punch than floods, but luckily they don’t have as much range. So usually debris flows are confined very close to mountain fronts. That’s where they put those waste basins to catch them. But if there is no such protection, then they can travel downslope and affect neighborhoods, and then the floods can spread further down.

Is there something about Southern California that makes it more risky?

Southern California is sort of the event capital of the world. There’s this combination of very steep topography, like the San Gabriel Mountains that just go up, the Santa Monica Mountains, the Santa Ynez — very steep topography. Burns relatively often. Also, there are a lot of people who live very close to the mountain front, so that’s what increases the risk.

The thing about the burn zone is that it takes much less rainfall to cause a problem than it would in burn conditions. So now we’ve made the slopes really vulnerable. They are very steep. There are many people there. That is why the risk is so great.

We’ve seen debris flows in the forests of Northern California as well. It’s not just a Southern California problem, and it’s not just a California problem.

Is there anything that could be done to reduce this risk? Something that needs to be done now?

Shortly after the fires, particularly the Palisades fire, (there were) a number of fairly widespread debris flows that destroyed roads. There was also flooding and debris flows in the Eton fire. Fortunately, Los Angeles County has a dense network of debris basins designed to catch the material before it enters neighborhoods, and they largely saved the day.

The designers planned ahead and put in these waste basins – these big, giant holes in the ground – designed to catch the material. This is the best defense against them. They are not everywhere, but there is a good safety net. Also, it’s really hard to stop these things.

What should people who live near recent burn scars know? What should they do now that the rain has started?

The best thing to do if you are really close to a drain in one of these burn zones is to get out of the way. You will receive a warning from the National Weather Service, which closely monitors rain storms. They know how much rain it will take to cause a problem, and they will issue warnings and local authorities will reach out to get people off the road. So there are many views on the situation. So, at this point, the best thing to do is listen to the weather service, listen to your local authorities.

If they ask you to get out of the way, take their advice. These things can happen very quickly if there is intense rain, a flash flood where the debris flow can start within minutes.

So there’s no escaping debris once it starts?

It’s quite difficult. If you have a two-story home and find yourself there at the wrong time, get on that second floor for sure. Fight the hell out if you get trapped. But best to get out of the way.

This article was originally published on CalMatters and is republished under Creative Commons Attribution-NonCommercial-No Derivatives license.