The Thomas Fire ravaged nearly 300,000 acres of Southern California in early December 2017. The intense heat of the flames vaporized the roots of the trees and vegetation on the hillsides above Montecito.
A month later, in the early morning hours of Jan. 9, a strong storm dumped more than half an inch of rain in five minutes on the barren slopes.
The rootless soil churned down a creek-carved canyon, picking up boulders in the rush before fanning out at the bottom and barreling into homes. The disaster claimed the lives of 23 people.
Melding the theoretical and the applied
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New research led by Douglas Jerolmack of Penn's School of Arts & Sciences and School of Engineering and Applied Science, in collaboration with Paulo Arratia of Penn Engineering and researchers from the University of California, Santa Barbara (UCSB), answers these questions using cutting-edge physics, as per ScienceDaily.
Their research, which was published in the Proceedings of the National Academy of Sciences, used laboratory experiments to determine how the failure and flow behavior of samples from the Montecito mudslides were related to soil material properties.
Jerolmack took a sabbatical in the winter of 2018 and visited the Kavli Institute for Theoretical Physics at UCSB - but not to study mudslides.
However, the debris flows occurred three days after Jerolmack arrived. Thomas Dunne, a geologist at UCSB and coauthor on the paper, invited him to collect samples from Montecito about a month later when it was safe to do so.
It was a dreadful job. Some samples came from the wreckage of homes, where mud flows from the hillside were powerful enough to push massive boulders down creek beds up to - and sometimes through - houses.
"By the time we got close to the canyon's mouth," Jerolmack says, "it was almost like a phalanx of boulders."
"Houses had been buried to their roof lines, and cars had been pulverized and rendered unrecognizable."
Taking the samples back to the lab, the researcher's goal was to model how the mud's composition and the stresses it is subjected to influence when it begins to flow, overcoming the forces that lend substances rigidity, condition scientists refer to as a "jammed state."
This was not the first-time engineers and scientists attempted modeling from field samples.
Some studies attempted to simulate field conditions by placing shovelfuls of dirt and mud in large rheometers, which rapidly spin samples to measure their viscosity, or how their flow responds to a defined force.
Typical rheometers, on the other hand, only produce accurate results when a substance is homogeneous and well-mixed, which was not the case with the Montecito samples, which contained varying amounts of ash, clay, and rocks.
This limitation can be overcome by more advanced and sensitive rheometers that measure the viscosity of minute quantities.
They do, however, have one disadvantage: samples containing larger particles, such as rocks in mud, may clog their delicate workings.
Each team member's expertise was essential to the investigation. Hadis Matinpour, a postdoc at UCSB, prepared, recorded, and plotted the first samples and analyzed the composition of natural particles.
Sarah Haber, a Penn research assistant at the time, determined the chemical composition of the materials, including critical quantities such as clay content.
Those contributions relied on a grasp of cutting-edge physics concerning the forces at work in dense suspensions.
Friction occurs when particles rub against one another; lubrication occurs when a thin film of water helps particles slide past one another, and cohesion occurs when sticky particles such as clay bind together.
Shravan Pradeep, a Penn postdoc with a strong background in rheology, or the study of how complex materials flow, has also joined the team.
He determined precisely how the soil's material properties - particle sizes and clay content - determined its failure and flow properties.
His analysis revealed that understanding particle stickiness, as measured by "yield stress," and how tightly particles can pack together in the "jammed state," could almost entirely account for the Montecito samples' results.
Yield stress can be visualized by picturing toothpaste or hair gel, according to Jerolmack. These materials do not flow in a tube. They only begin to flow when a force is applied to the tube, such as a firm squeeze.
Landslides are caused by disruptions in a slope's natural stability. They can occur in conjunction with heavy rains or as a result of droughts, earthquakes, or volcanic eruptions, as per CDC.
Mudslides form when water quickly accumulates in the ground, resulting in a surge of water-soaked rock, earth, and debris.
Mudslides are typically caused by natural disasters and begin on steep slopes.
Landslides are particularly dangerous during and after heavy rains in areas where wildfires or human modification of the land have destroyed vegetation on slopes.
Mudslides occur when a large amount of water rapidly erodes soil on a steep slope.
Rapid snowmelt at the top of a mountain or a period of heavy rain can cause a mudslide because the large volume of water mixes with the soil, causing it to liquefy and slide downhill.
A mudslide can range from very wet mud to thick mud with tons of debris, which can include large boulders, trees, and even cars or houses.
Every year, mudslides kill many people and cause millions of dollars in property damage.
