In early December 2017, the Thomas Fire burned nearly 300,000 acres in Southern California. The intense heat of the flames not only killed the trees and vegetation on the slopes above Montecito, but also vaporized their roots.
A month later, in the pre-dawn hours of January 9, a heavy storm hit the barren hillsides with more than half an inch of rain in five minutes. The rootless soil was transformed into a powerful slurry, churning up a canyon cut by a stream and picking up boulders in a hurry before spreading to the bottom and into the houses. Twenty-three people died in the disaster.
Could this tragedy have been avoided? What is the tipping point at which a solid slope begins to flow like a liquid? The new findings by a team led by Douglas Jerolmack of Penn’s School of Arts and Sciences and School of Engineering and Applied Sciences in collaboration with Paulo Arratia of Penn Engineering and researchers at the University of California, Santa Barbara (UCSB), apply cutting-edge physics. to answer these questions. Their study, published in the Proceedings of the National Academy of Sciences, conducted laboratory experiments that determined how the failure and flow behavior of Montecito mudslide samples related to soil material properties.
“We weren’t there to see it,” says Jerolmack, “but our idea was, ‘Could we learn something about the process of how a solid slope loses its stiffness by measuring how water-soil mixtures flow when they are?’ at different concentrations?’”
Merging the theoretical and the applied
During the winter of 2018, Jerolmack was on sabbatical and traveled to UCSB’s Kavli Institute for Theoretical Physics, but not to study mudslides. “It’s a place to come and solve problems that are frontier topics in physics,” he says. “I’m a geophysicist, but I wasn’t there to do geoscience. I was there to learn about this frontier physics, especially the physics of dense suspensions.”
Three days after Jerolmack’s arrival, however, the debris flows occurred. About a month later, when it was safe to do so, Thomas Dunne, a UCSB geologist and co-author of the paper, invited him to collect samples from Montecito.
It was a sad task. Some samples came from the devastated remains of dwellings, where the hillside mudflows were strong enough to push massive boulders down stream beds into and sometimes through houses. “When we got close to the mouth of the canyon, it was almost like a phalanx of boulders,” says Jerolmack. “The houses were buried up to their roof lines; the cars were pulverized and unrecognizable.”
Taking the samples to the lab, the researchers’ goal was to model how the mud’s composition and stresses are affected when it begins to flow, overcoming the forces that stiffen the substances, what scientists call a “state of stranding”. .”
It was not the first time that engineers and scientists attempted this type of modeling from field samples. Some studies had tried to simulate conditions in the field by placing shovels of dirt and mud in large rheometers, a device that spins samples rapidly to measure their viscosity, or how their flow responds to a defined force. Typical rheometers, however, only give accurate results if a substance is homogeneous and well-mixed, unlike the Montecito samples, which contained varying amounts of ash, clay, and rock.
More sensitive and high-tech rheometers, which measure the viscosity of small quantities, can overcome this drawback. But they come with another: Samples containing larger particles, such as rocks in mud, could clog their delicate operation.
“We realized that we could take measurements that we knew were reliable and accurate by using this exquisitely sensitive device,” Jerolmack says, “even if it came at the cost of having to sift through the coarsest material in our samples.”
A clear sign of “dirty” samples.
The research was based on the experience of each team member. UCSB postdoctoral researcher Hadis Matinpour prepared, recorded, and drew the first samples and analyzed the composition of the natural particles. Sarah Haber, then a research assistant at Penn, determined the chemical composition of the materials, including important quantities such as clay content.
“We had all this raw data and we were having trouble making sense of it,” Jerolmack says. “Robert Kostynick, then a master’s student at Penn, picked up the project for his thesis and put a lot of work and thought into organizing, interpreting and trying to collapse a lot of data.”
These contributions were supported by an understanding of cutting-edge physics related to the forces acting on dense suspensions. These include friction, as particles rub against each other; lubrication, if a thin film of water helps the particles slide past each other; or cohesion, if sticky particles like clay come together.
“We had the audacity, or maybe the naivety, to try to apply some very recent developments in physics to a really messy material,” says Jerolmack.
Penn postdoc Shravan Pradeep, who has a deep background in rheology, or the study of how complex materials flow, also joined the team. He pointed out precisely how the material properties of the soil (particle size and clay content) determined its failure and flow properties. Their analysis showed that an understanding of particle adhesion, measured as “yield stress” and how closely particles can be packed into a “stuck state,” could almost completely explain the results observed in the samples from Montecito.
Performance stress can be imagined by imagining toothpaste or hair gel, says Jerolmack. In a tube, these materials do not flow. Only when a force is applied to the tube, a firm pressure, do they begin to flow. The jamming state can be thought of as the point at which the particles are so crowded together that they cannot pass each other.
“What we realized was with debris flows, when you’re not pushing them hard, their behavior is completely governed by yield stress,” says Jerolmack. “But when you’re pushing really hard (the force of gravity carrying a debris flow down a mountainside), the viscous behavior comes to dominate and is determined by the distance of the stuck state particle density “.
In the lab, the researchers were unable to simulate failure, the point at which a solid soil, bounded by “sticking,” turned into a mobile mud. But they could approach it the other way around, evaluating muddy materials mixed with water at different concentrations to extrapolate the stranding state.
“The beauty is that when you get samples from nature, they can be all over the place in terms of their composition, how much ash they contain, where you collected them,” says Arratia. “Even so, in the end, all the data collapsed into a single master curve. That tells you that you now have a universal understanding that applies whether you’re in the lab or in the mountains of Montecito.
With climate change, the frequency and intensity of forest fires are increasing in many regions, as is the intensity of precipitation events. Thus, the risk of catastrophic landslides is not going away anytime soon.
The researchers say the new findings for predicting yield stress and stranding can help inform modeling done by federal and local governments to simulate debris flows. “Say, if it rains this hard and I have this kind of material, how fast is it going to flow and how far,” says Jerolmack.
And more generally, Jerolmack and his colleagues hope that the work, which combined theoretical and empirical science, will lead to more interdisciplinary approaches. “We can take newer discoveries in physics and relate them quite directly to a significant environmental or geophysical problem.”
Douglas Jerolmack is a professor in the Department of Earth and Environmental Sciences in the School of Arts and Sciences at the University of Pennsylvania and in the Department of Mechanical Engineering and Applied Mechanics in the School of Engineering and Applied Sciences.
Paulo Arratia is a professor in the Departments of Mechanical Engineering and Applied Mechanics and Chemical and Biomolecular Engineering in Penn’s School of Engineering and Applied Sciences.
Jerolmack and Arratia’s study was co-authored by Robert Kostynick, Sarah Haber and Shravan Pradeep of Penn and Hadis Matinpour, Alban Sauret, Eckart Meiburg and Thomas Dunne of the University of California Santa Barbara.
The study was supported by the Army Research Office (Grants W911NF2010113 and W911NF-18-1-0379), National Science Foundation (Grants 1720530 and 1734355), Petroleum Research Fund (Grant 61536-ND8 ) and the John MacFarlane Foundation.
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