UC San Diego

This work provides a novel understanding of the internal structure and the rheological behavior of post-wildfire debris flows. During the wildfire process, the topsoil layer becomes hydrophobic due to the condensation of chemicals that are released from burning vegetation and organic matter. Hydrophobic soil causes problems such as reducing water infiltration rate, prolonging the wetting process, increasing soil erosion, forming preferential flow paths, changing hydrological characteristics of the watershed, and inducing severe mudflow, debris flows, or mudslides. This work hypothesizes that air, intermixed into a flowing mudflow, plays a critical role in modifying post-wildfire mudflow behavior compared to regular mudflows. Liquid marbles form because mudflows can entrap hydrophobic soil particles. Liquid marbles have air bubbles in the center, covered by hydrophobic soil particles. Existing investigations and research include many different rheological models for post-wildfire mudflow slurries, ignoring the role of entrapped air bubbles. This research provides new insights to explore the air entrapment mechanism and to incorporate entrapped air into the rheological responses of post-wildfire mudflow. For example, preliminary findings reveal that fully covered liquid marbles (i.e., air bubbles with the entire surface covered with hydrophobic sand particles) will replace all partially covered liquid marbles (i.e., air bubbles with part of the surface vacant of sand particles) after 50 seconds of mixing. Micromechanical analysis of submerged hydrophobic particle to air bubble attaching and detaching forces in the particle-bubble interaction reveals conditions that lead to stable liquid marbles in post-wildfire mudflows. The amount of new liquid marbles depends on mixing speed, mixing time, and sand particle diameter. Larger sand particles and higher air entrapment lead to a collision-dominated flow regime. In addition, what has not yet been understood is the role of hydrophobic particles and air entrapment in the case where only a portion of material enrolled into a mudflow is hydrophobic. This work also investigates the flow of such complex mixture in an open channel using the GeoPIV method. Overall, this work provides a greater understanding of the internal structure and rheological behavior of post-wildfire mudflows and contributes to better analysis of future debris flow runoff and sediment transport events.