Abstract

Post-wildifre mudflows are devastating to watershed environment, life, and infrastructure. Burned scars tend to form catastrophic mudflows when rained upon shortly after fires, flow very fast, quickly blasting obstacles on the way and carrying large boulders and debris. Internal composition of post-wildfire mudflows has recently become of interest, with a goal to understand better mechanisms and differences between post-wildfire and natural mudflows flow and transport. This paper shows critical new insights into how air entrapment affects the properties of rain-induced post-wildfire mudflows as a mixture of air bubbles, water, and hydrophobic sand. The idea of mudflows' internal structure containing trapped air bubbles is novel. Such mixtures can flow down slopes at incredible speeds, quickly blasting obstacles on the way and carrying large stone boulders and objects. The surficial soil particles turn hydrophobic due to the deposition of combusted organic matter during wildfires. Afterward, raindrops, splash, and erosion form devastating mudflows. We propose and experimentally investigate a new paradigm in which a significant amount of air remains entrapped in post-wildfire mudflow via hydrophobic particle-air attraction. Specific findings quantify the amount of air trapped within sand-water volumetric concentrations, the effect of intermixing energy, gravity, and sand particle size on outcome mudflow internal structure. As a result, little agglomerates of sand particles covering air bubbles characterize the mudflow mixture's internal structure.