In early December 2017, the Thomas Fire destroyed nearly 300,000 acres of Southern California. Not only did the intense heat of the flames kill the trees and plants on the hillsides above Montecito, they also evaporated their roots.
A month later, in the predawn hours of January 9, a powerful storm hit the barren slopes with more than half an inch of rain in five minutes. The rootless soil turned into a strong slurry, overturning a ravine carved by the creek and picking up boulders in the rush before spreading to the bottom and spurting into homes. 23 people were killed in the disaster.
Can this tragedy be avoided? What is the tipping point at which a solid slope begins to percolate like a liquid? New findings by a team led by Douglas Gerolmack of the Pennsylvania College of Arts and Sciences and College of Engineering and Applied Sciences in collaboration with Paolo Aratia of the University of Pennsylvania and researchers from the University of California, Santa Barbara (UCSB), apply sophisticated physics to answer these questions.
Their study was published in Proceedings of the National Academy of Sciencesconducted laboratory experiments that determined how the failure and flow behavior of samples from Montecito mudslides relate to soil material properties.
“We weren’t there to witness that happen, but our idea was, ‘Can we learn something about the process of how hard ridges lose their stiffness by measuring how a mixture of water and soil flows when they are in different concentrations?'” Jerulmak says. ”
Confusion between theory and practice
During the winter of 2018, Jerulmak was full-time and traveled to the Kavli Institute for Theoretical Physics at UCLA, but not to study mudslides. “It’s a place we come to and solve problems that are elementary issues in physics,” he says. “I’m a geophysicist, but I wasn’t around to work in Earth sciences. I was there to learn about boundary physics, especially the physics of dense suspensions.”
Three days after the arrival of Gerulmac, debris flows occurred. About a month later, when it was safe to do so, Thomas Dunn, a geologist at the University of California at San Francisco and one of the paper’s co-authors, invited him to collect samples from Montecito.
It was a bleak task. Some of the samples came from the remains of destroyed homes, where mudflows from the hillside were strong enough to push huge boulders down the creek bed all the way to – and sometimes through – the houses. “By the time we got close to the mouth of the valley, it was like a battalion of rocks,” Gerolmac says. “Homes were buried up to their railings, cars were destroyed and can no longer be recognized.”
By taking samples to the lab, the researchers’ goal was to model how mud forms and the stresses it experiences when it starts to flow, and to overcome the forces that harden materials, what scientists call a “blurring state.” ”
This wasn’t the first time engineers and scientists had attempted this type of modeling from field samples. Some studies have attempted to simulate conditions in the field by placing shovels of dirt and mud into large rheometers, a device that spins samples rapidly to measure their viscosity, or how their flow responds to a specific force. However, typical rheometers give accurate results only if the material is homogeneous and well mixed, unlike Montecito samples, which contain different amounts of ash, clay, and rock.
High-tech and sensitive hygrometers, which measure the viscosity of small quantities, can overcome this drawback. But it does come with another: Specimens with larger particles — like rocks in clay — can clog their delicate work.
“We realized that we could make measurements that we knew were reliable and accurate if we used this very sensitive device, even if it came at the cost of having to extract the coarser material from our samples,” says Gerolmak.
A clear indication of “dirty” samples
The investigation relied on the experience of each team member. Researcher Hadis Matinpour of the University of California, San Francisco, prepared, recorded and plotted the first samples and analyzed the composition of the natural particles. Sarah Haber, then a research assistant at the University of Pennsylvania, determined the chemical composition of the materials, including such important quantities as clay content.
“We had all this raw data and we had a hard time understanding it,” says Gerolmak. “Robert Kostenek, who was a master’s student at the University of Pennsylvania, chose the project for his thesis and put in a huge amount of legal process and thought about organizing and interpreting a lot of data and trying to deconstruct it.”
Those contributions were based on an understanding of the evolving physics related to forces working in dense suspensions. These include friction, where particles rub against each other; lubrication, if there is a thin film of water that helps the particles slide over each other; or cohesion, if sticky particles such as clay are bound together.
“We had the audacity, or perhaps the naivety, to try to apply some really recent advances in physics to really chaotic matter,” says Gerolmac.
Also joining the team was Dr. Ben postdoc, Shravan Pradeep, who has a deep background in rheology, or the study of how complex materials flow. And specify precisely how the soil’s physical properties–particle sizes and clay content–determine its failure and flow characteristics. His analysis showed that understanding particle adhesion, which is measured as the “yield stress,” and how well the particles stick together in the “jammed state,” can almost fully explain the results observed in the Montecito samples.
Jerulmak says yield stress can be visualized by photographing toothpaste or hair gel. In the tube, this material does not flow. Only when a force is applied to the tube – strong pressure – does it begin to flow. The state of interference can be thought of as the point at which the particles are so lumped together that they cannot pass each other.
“What we realized was that with debris flows, when you don’t stress them too hard, their behavior is completely governed by productivity stress,” says Gerolmac. “But when you push hard—the gravitational force that carries debris down a mountainside—the viscous behavior is dominant and is determined by how far the particle density is from the jammed state.”
In the lab, the researchers were unable to simulate failure, which is the point at which hard soil, restricted to “disturbing,” turns into a moving clay. But they can approximate the opposite, and evaluate muddy materials mixed with water at different concentrations to extrapolate the jamming state.
“The beauty of this is that when you get samples from nature, they can be scattered all over the place in terms of their composition, the amount of ash they contain, and the location from which they were collected,” Aratia says. “However, in the end, all the data just collapsed into one major curve. This tells you that now, you have a global understanding that proves whether you’re in the lab or you’re on the Montecito Mountains.”
With climate change, the frequency and intensity of wildfires is increasing in many regions, as is the intensity of precipitation events. Thus, the threat of catastrophic mudslides is not going away any time soon.
The researchers say the new findings for predicting crop stress and crowded condition could help model models by federal and local governments to simulate debris flows. “Say, if it rains so much and I have that kind of stuff, how fast and how fast does it flow,” says Gerolmac.
In a more general way, Jerulmak and colleagues hope that the work, which combines theoretical and experimental sciences, will lead to more of these interdisciplinary approaches. “We can take recent discoveries in physics and actually relate them directly to a meaningful environmental or geophysical problem.”
Robert Kostynick et al, The rheology of debris flow materials is controlled by distance from the jammer, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2209109119
Presented by the University of Pennsylvania
the quote: Physics of Disaster: How Mudslides Move (2022, November 1) Retrieved on November 1, 2022 from https://phys.org/news/2022-11-physics-disaster-mudslides.html
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