Each year in the United States, landslides and debris flows kill an estimated 25 to 50 people and cause roughly $1 billion in property damage. A deeper understanding of the specific mechanisms through which landslides occur could help guide architects and engineers in reducing this loss of life and property. In an October 2022 study published in the Proceedings of the National Academy of Sciences, a joint team of geophysicists and mechanical engineers from the University of Pennsylvania and University of California, Santa Barbara applied recent advances in the physics of dense suspensions (heterogeneous mixtures of solids and liquids) in an attempt to better model complex debris flows. The team determined that an existing model for simple, ideal flows can be extended to account for complex, natural flows. The results accordingly offer a way to improve hazard assessment of hillslopes and their constituent materials. See also: Geophysical fluid dynamics; Geophysics; Landslide; Mass wasting; Suspension

In 2018, debris flows in Montecito, California, completely covered portions of U.S. Route 101. (Credit: Cal OES/State of California)

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Researchers examined soil samples from debris flows that occurred in Montecito, California, in 2018, resulting in 23 fatalities and 408 damaged homes. The Montecito flows occurred after heavy rainfall struck an area where, one month prior, several large wildfires had destroyed many plant root systems. Without those root systems in place, soil on hillsides readily washed away in the intense downpour, forming a slurry strong enough to dislodge large boulders and send them downslope and, devastatingly, into local residences. See also: Root (botany); Soil ecology

In the laboratory, the researchers sought to reproduce the conditions of the Montecito landslides by subjecting the soil samples to stresses measured by rheometers—devices that measure flow properties of solids. Sieving out larger, coarser chunks of material ultimately allowed the researchers to obtain sufficiently precise measurements by using highly sensitive rheometers that only tend to work well with more homogenous and potentially unrepresentative mixtures. See also: Rheology

With that wealth of data in hand, the researchers then applied recent advances in modeling physical parameters such as friction, cohesion, and lubrication in dense suspensions to the studied debris flows. Very generally, a debris flow moves when the stress (force per unit area) of gravity on the suspension exceeds what is termed the suspension’s yield stress. A similar effect can be seen in common household “viscoplastic” fluids, such as mayonnaise or toothpaste, in which the substance does not flow unless a force is applied to it—for instance, by shaking off a dollop of mayonnaise from a spoon—at which point the yield stress of the fluid is exceeded and the fluid begins to flow. Debris flows stop when they reach a state known as jamming, in which granular soil particles in the fluid become too dense for the suspension to continue flowing. See also: Friction; Shear; Stress and strain; Viscosity

Through their analysis, the researchers found that yield stress varies with a parameter called “distance from jamming,” which can be thought of as the difference between the density of soil in a suspension at a given time and the density of soil in the suspension that would cause the suspension to become jammed. Overall, these results clarify some disparities between previous rheological models and observations and thus provide a clearer picture of the mechanics involved in previously stable hillsides degrading into dangerous mudslides. They also provide guidance on how future researchers can most effectively study these flows. See also: Environmental fluid mechanics; Fluid mechanics; Soil mechanics