Post‐Wildfire Debris Flow Rheology of Mixed Hydrophobicity Sands

Abstract

Post‐wildfire debris flows can have devastating effects on structures and transportation lifelines. Surficial sandy soil layers become hydrophobic during wildfires, and post‐wildfire rainfall has been shown to lead to air entrapment and excessive erosion. This paper investigates the internal structure and rheology of the sediment portion of the debris flow matrix, excluding large boulders and large debris. The sediment portion of the debris flow is composed of sands with varying degrees of hydrophobicity, which affects the air‐entrapping process, water content, and mixture behavior. Rheometer measurements are performed using clay, hydrophobic sand, and hydrophilic sand at different air‐to‐water ratios. Fine sand particles have peak air entrapment at 75% hydrophobicity, while coarse sand particles and fine sand‐clay mixtures have peak air entrapment at 100% hydrophobicity. This study also discusses the applicability and limitations of the classical Herschel‐Bulkley fluid model for describing the rheological behavior of such a complex material and proposes new rheological relationships for different flow regimes. This research provides a fundamental understanding of how sand hydrophobicity affects air entrapment in postfire debris flows and alters the rheology of the sediment portion of debris flows.