Ingrid Tomac

2020 Hellman Fellow

Assistant Professor, Structural Engineering
UC San Diego

Project title: Post-Wildfire Mudflows Explained with Transport Phenomena of Hydrophobic Particle-Air-Water Mixtures

Project description: Post-wildfire mudflows are unpredictable with sudden occurrence and rapid travel downhill, turning into debris flows and mobilizing unusually large and heavy boulders while producing “tumbling and roaring sounds”. Mudflows have led to catastrophic infrastructure damage and loss of life. A well-established the trigger for mudflows on recently burned slopes is rain. For example, in January 2018, up to 0.75 cm of rain fell on the Santa Ynez mountains above Montecito, California in an intense 15-minute burst, resulting in a devastating debris flow which caused 21 deaths, led to $421 million in damages, and closed key transit corridors. We know that soil becomes water repellent (hydrophobic) due to condensing of gasses and wax induced by burning of vegetation, which prevents rain infiltration leading to water over-flow and erosion of hillslopes 1±6. However, post wild-fire mudflows behave differently than other soil-water slurries. Post-mudflow forensic research, including rheological investigations of collected samples, indicates the presence of shear-thinning mixtures with flow characteristics of dense networks of rills and viscous flow features indicating progressive failure, which is much different from the expected slurry behavior. By looking at the small-scale particle-fluid behavior, we may be able to explain these differences. For instance, fundamental studies reveal how hydrophobic particles attach to air bubbles when mixed in water, and particle-air-water mixes form interesting structures (bubbles, pipes and clusters) whose shapes are primarily governed by a balance between the gravity effects on different particle sizes associated with their weight and the attractive forces at the contacts between the non-wettable air bubbles and water repellent particle surfaces 8,9. What remains unknown is a link between mudflow composition and shear flow behavior, as well as, how the mixture particle arrangement changes during the flow process. There is an urgent need for a fundamental understanding of how hydrophobic-particle-air-bubble aggregation forms and evolves. There is a critical need to assess how such complex three-phase mixtures affect shear flow dynamics. If the knowledge about how mudflow composition is related to its devastating flow behavior remains unclear, it will be unattainable to model and predict mudflow damage and establish critical early warning systems.