Assessment of Physical Properties of Water-Repellent Soils

This note reports the physical and mechanical properties of two postwildfire sands collected in Southern California and chemically induced hydrophobic sand. Hydrophobicity decreases the attraction between water molecules and solid surfaces and manifests a modification of hydraulic soil properties that lead to postwildfire rain soil erosion and mudflows. Wildfires induce different hydrophobicity levels in shallow soil layers based on fire severity, vegetation, and soil chemistry. This note assesses the applicability of laboratory-made hydrophobic soils for studying mudflows by comparing them to natural hydrophobic soils collected from two burned sites that will help understand and predict postwildfire soil erosion and mudflow mechanisms. Results show that soil water retention curve (SWRC) can be unreliable for assessing the hydraulic properties of burned soils on slopes impacted by rain because suction develops with significant time delay or is absent. Flash floods on inclined hydrophobic soil surfaces occur rapidly during rainfall, and the overflow water does not pond on slopes. The water entry value is linearly related to the water drop contact angle and characterizes the immediate hydrophobic soil response, which is relevant to mudflow onset.