Drought, deluge turned stable landslide into disaster

Drought, deluge turned stable landslide into disaster
The Mud Creek landslide in photographic and radar images. The radar velocity map shows the pre-collapse (solid line) and post-collapse (dashed line) extent of the sliding area, with faster sliding velocities before the collapse shown in darker shades of red. The highest velocities were about 16 inches (40 centimeters) per year. Credit: Google/SIO/NOAA/U.S. Navy/NGA/GEBCO/Landsat/Copernicus

"Stable landslide" sounds like a contradiction in terms, but there are indeed places on Earth where land has been creeping downhill slowly, stably and harmlessly for as long as a century. But stability doesn't necessarily last forever. For the first time, researchers at NASA's Jet Propulsion Laboratory in Pasadena, California, and collaborating institutions have documented the transition of a stable, slow-moving landslide into catastrophic collapse, showing how drought and extreme rains likely destabilized the slide.

The Mud Creek landslide near Big Sur, California, dumped about 6 million cubic yards (5 million cubic meters) of rock and debris across California Highway 1 on May 20, 2017. The damage took more than a year and $54 million to repair. No long-term motion had been documented at Mud Creek before this event, but workers in the state's transportation department had noticed small mudslides in the weeks before the collapse and closed the highway as a precaution.

The JPL-led team identified Mud Creek as a stable landslide using an eight-year data set from an airborne JPL instrument called the Uninhabited Airborne Vehicle Synthetic Aperature Radar, processed with a technique called interferometric synthetic aperture radar processing (InSAR). They calculated that Mud Creek had been sliding at an of about 7 inches (17 centimeters) per year since at least 2009. They used the European Space Agency's Sentinel-1A/B to document how the sliding area's behavior changed.

The airborne and satellite data measure changes only at the ground surface, however. "From that, we tried to infer what may have happened to the landslide's sliding surface, tens of meters underground, that allowed the Mud Creek slide to transition from stable to unstable," said the study's lead author, Alexander Handwerger, a NASA postdoctoral fellow doing research at JPL.

The collapse happened after several days of heavy rainfall during one of the wettest years in over a century for this area. Before 2017, a five-year drought had produced several of California's hottest and driest years ever. Using a of how water affects soil, the researchers studied what would happen as the intense rains saturated the parched ground. Water would replace air in the tiny spaces between soil particles, greatly increasing the pressure on the particles. This pressure change could have destabilized the sliding surfaces belowground and triggered the collapse.

Drought, deluge turned stable landslide into disaster
Velocity changes in the slide area, March 2015 to May 2017. In winter 2015-16, slide speeds increased with the winter rains and then slowed steadily until the following winter. In winter 2016-17, heavy rains caused the landslide to accelerate twice, first in December 2016 and then again in March 2017. The scientists think the double acceleration may have been a signal of the impending collapse in May. Credit: EO

California alone has more than 650 known stable landslides. If one began losing stability in the future, could InSAR data reveal the change? To answer that question, the team compared the Mud Creek images with images of two other stable landslides in similar types of soil and rock.

Paul's Slide, only 13 miles (21 kilometers) north of Mud Creek, went through the same weather conditions yet did not fail catastrophically. A landslide in Northern California received over 3 feet (1 meter) more rainfall than Mud Creek without catastrophic failure. "We thought if we compared these two cases that didn't fail to the one that did, we might find some characteristic velocity pattern that would be a warning that a slide was going to fail catastrophically," Handwerger said.

The idea paid off. Handwerger found that all three stable slides accelerated slightly after the winter started and then, as the season continued, slowed down again and stabilized. This is their usual annual pattern. But after the late-season rains, Mud Creek accelerated again, increasing in speed until its ultimate collapse. The other slides did not.

"We think that second speed-up may be a signal of interest, but we only have this one case," Handwerger said. "Since we now know that stable landslides in this region can fail catastrophically and we have good data coverage here, our plan is to monitor this whole stretch of the Pacific Coast Highway and look for these unusual velocity changes. If we get enough examples, we can start to actually figure out the mechanisms that are controlling this behavior."

The paper on the research, titled "A shift from drought to extreme rainfall drives a stable to catastrophic failure," was published today in Scientific Reports.

Explore further

Image: Landslide on the radar

More information: Alexander L. Handwerger et al. A shift from drought to extreme rainfall drives a stable landslide to catastrophic failure, Scientific Reports (2019). DOI: 10.1038/s41598-018-38300-0
Journal information: Scientific Reports

Provided by NASA
Citation: Drought, deluge turned stable landslide into disaster (2019, February 8) retrieved 21 September 2019 from https://phys.org/news/2019-02-drought-deluge-stable-landslide-disaster.html
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User comments

Feb 08, 2019
build in stupid places,
have stupid things happen

Feb 08, 2019
gosh, shootsownfoot.
Must be mughty nice living in a cocoon of first-worlder entitlement.
Where all the sacrifices to make your privileges possible are forgotten history.

Or at least far enough away to not disturb your pampered, self-absorbed, well-protected, heavily subsidized lifestyle of smuggery with harsh reality.

Feb 08, 2019
rrwills: you're exposing your lack of knowledge on the subject. Shootist is correct, there are stupid places to build. It's called a geographical survey that's done prior to putting a building or ?? somewhere that validates one's choice. Just forging ahead hoping for the best is dumb. It has nothing to do with all your adjectives like "pampered".

Feb 08, 2019
It would be bad luck if your town got smashed by a meteorite. It would be bad planning if your town had been built on a known flood plain. Unfortunately, most home buyers don't consider such things. The developers should be held responsible, along with the local authorities that gave them permission to build. They aren't, of course. The peasants pay for it via the insurance companies.

Feb 08, 2019
build in stupid places,

The ONLY road on a coast is not a stupid place.

Feb 09, 2019
California SR-1 was first built in 1934 and has been steadily moving inland as fragile stretches are repaired. The Big Sur stretch was built in 1919. Replacement in this area will be way inland to avoid Ventana / Big Sur. Cliff side sites will become isolated.

A family property in Capitola is gone, so am I.

Feb 09, 2019
kel & shot. I realize that you are wallowing safe & secure down in your mommies basement, where it is nice & cozy.
Your only personal contact is with other video-game players. Also hiding from the World in their mommies basements.

For the rest of us, who are out here in the real world? It ain't cozy or nice, Anthropospecific Induced Climate Change is a harsh reality that we need to deal with.

Or should we all become useless, whiny slugs like the two of you?

Conservative is not a political slogan.
Conservative is an action.
For efficient technology,
Organization of resources, including Human Beings.
Not just causing thoughtless waste & destruction vy treating People as disposable cattle.

Moral Character & Practicing Civil Virtues are the difference between Real Conservatives & you altright fairytails.

As denierbots it does not really matter whether you are a saudi stooge pr a russian quisling.
The agitprop you spew is the same rotten crap.

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