The dangerous reality of mudslides

Mudslides can be a dangerous and deadly natural event and are often triggered after widespread wildfires. In California, emergency responders and the U.S. Geological Survey are researching better ways to be prepared in the event of a mudslide.


Mudslides can be a dangerous and deadly natural event, and are often triggered after widespread wildfires.

In California, emergency responders and the U.S. Geological Survey are researching better ways to be prepared in the event of a mudslide.

Here's a look.

December 2017, Southern California fell victim to the worst wildfire ever recorded in state history.

The Thomas Fire burned nearly 300,000 acres, scorching everything in its path, all the way from Ventura County to Santa Barbara County, which is about two hours north of downtown Los Angeles.

No sooner were Californians out of harm's way from the devastating blaze, Mother Nature dealt a different kind of wrath.

In January, heavy, intense rains caused unprecedented mudslides and debris flows in the Montecito neighborhood of Santa Barbara County because of all the scorched earth from the Thomas Fire.

21 people died in this disaster.

More than 100 homes were destroyed, over 300 damaged, and part of one of California's busiest highways, Highway 101, was shut down for nearly two weeks.

Kate Scharer is a geologist with the United States Geological Survey.

She was part of the response team which aided in the Montecito rescue effort, and also helped with damage surveillance and assessment.

Today, we met Scharer out here, at a debris basin the Sylmar community of Los Angeles County, located at the base of the San Gabriel Mountains.

In December, parts of Sylmar were also scorched from what was known as the Creek Fire.

The material that you can see on the landscape is what becomes a debris flow.

A common word that's used to describe that is actually a mudflow.

We like to use the term 'debris flow' because it includes not just mud from the soil, but also the big rocks, the trees, big chunks of the bedrock or the hillslope that can get entrained in these flows as they go downhill.

So the reason we like to say debris flow is it's a huge variety of material that comes down, not just mud.

Is that what causes the devastation?

Is it the velocity and the violence of the flow?

Or is it something else?

It's certainly the velocity of it.

Debris flows can move as fast as 30 to 40 miles an hour.

If you think of yourself driving in a car at that speed, you certainly wouldn't want to hit anybody, right?

When you envision that you could take all the material in the landscape and have it move that quickly in a slurry that has a concentration of, like, cement, for example, and it's moving that fast, it can take you out very quickly.

Scharer explains these charred hills here in Sylmar are exactly like the hillsides in Montecito after the Thomas Fire.

The inferno burned all the living and dead vegetation that had sheltered the soil.

The lands quickly eroded because there was no protective shield or barrier from the debris flow.

You can see where there used to be vegetation, before the fire.

And that vegetation, all of the roots that go in, the smaller plants that have burned up, all of those roots go in and basically lock the soil into the bedrock.

Once you have this burned, those roots that act like the staples that hold the soil into the ground are all gone, and so for two to five years after one of these fires, you can expect that, if the right kind of rainstorm comes through, material will get mobilized.

Just this thin veneer of soil and rubble will get mobilized, and can come down in a debris flow.

There is no way to quickly get away from this once the land starts crumbling down?

That's correct.

The descriptions you hear from people, that it sounds like a freight train coming at you.

So you can envision -- you have boulders out in the Montecito area.

We were measuring boulders that were two meters high.

That's taller than me.

And you're 6 feet tall.

Yes. [ Laughs ] And so those things were coming down with a velocity of 40 miles an hour.

That's, equivalently, a truck coming at you.

I have not seen a debris flow this bad within the last 20 years.

Janis Hernandez is with the California Geological Survey.

Hernandez was also part of the team out at Montecito, helping emergency responders and also documenting the path of the debris flow and mudslide.

Debris flows are characterized as sediment, rock, wooded debris, materials, air, and water that just get entrained and mobilized in a rapid deposit.

They come barreling down the canyons at a rapid pace, and as this material works its way down the slopes, it's picking up stuff.

We call it entraining.

But it's picking up stuff from the sides, from the front, and it's just making the mass even greater.

If, at the bottom of the canyon, we have a debris basin down there, it's supposed to catch this heavy material and allow the lighter material, the water, to flow out of it.

But in the case after a fire, there was just too much material to come down, and the basins filled up, and they over-topped, and all that material was spilling out into the neighborhoods uncontrollably.

If you happen to be home, you most likely will become part of that deposit.

Enough material came out of the canyons to fill, perhaps, 300 football fields.

Now the California Geological Survey is educating the public.

CGS has compiled a safety-and-preparedness checklist for all California residents, and any other U.S.

residents who live in parts of the country where they could be victims of wildfires that could lead to mudslides or debris flows in the future.

First, CGS says it is critical that residents heed all evacuation warnings from local law enforcement and Weather Service officials.

Secondly, CGS tells residents to expect debris flows for two to five years after a wildfire.

It takes intense rain, typically about 1/2 inch per hour to a recently burned slope, to trigger a debris flow or mudslide.

Thirdly, monitor all National Weather Service or local weather forecasts for flash-flood watches or flash-flood warnings.

And fourthly, if you must shelter in place, choose your location in advance and stay there, and find the highest point of shelter, such as a second story or a rooftop.

To help with these life-saving efforts, geologists are combining science and technology to map the post-fire danger zones for potential mudslides and debris flows.

It's a collaborative effort with USGS and CGS, and other technical specialists such as hydrologists, engineers, the U.S. Forest Service, and the Weather Service.

Together they are helping create specialized landslide maps.

Scientists track, and then map, things like soil texture, burn severity, and topographic information.

Geologists are also using aerial imagery and satellite data, and a technology called lidar.

Lidar is another mapping technique using laser beams flowing from an airplane to collect information.

Lidar can see through things like trees, brush, and other vegetation, and with digital processing get detailed imagery of everything on the ground, like homes, buildings, creeks, rivers, drainage systems, and it can identify any type of terrain, be it rough, hilly, or mountainous, helping geologists study landforms as if they're under a magnifying glass.

Is there any way that this can be prevented, or is Mother Nature just gonna do what Mother Nature does?

Mother Nature is gonna do what Mother Nature does.

When the rain comes down, no vegetation, loose soils, it's just a recipe for disaster.

You basically get buried by the debris.