Effects of Bed Hydrophobicity on Post‐Fire Debris Flow Entrainment and Momentum Growth

Abstract

Forest wildfires leave vast areas with burned topsoil that is water repellent, susceptible to erosion by surface runoff and a potential source of debris flow. To mitigate post‐fire debris flow hazards, debris resisting barriers are usually constructed in the flow path. However, the fundamental interaction mechanisms of debris flow on a water repellent bed and the momentum exchange process that governs the destructive potential of debris flows have yet to be elucidated. This study investigates the influence of bed sediment hydrophobicity on entrainment by debris flows and the impact force on a downstream barrier using a series of physical flume tests. Bed sediment with increasing contact angles (50°–130°) were used to model bed hydrophobicity from wettable to hydrophobic conditions, and volumetric water content (0%–30%) to simulate the natural unsaturated condition after rainfall in fire‐burned hillslopes. Compared to a wettable bed, a hydrophobic bed exhibits a failure pattern with slab‐by‐slab entrainment and en masse failure with a sixfold increase in the average erosion depth. A new dimensionless number is proposed to quantify the effects of contact angle on soil erosion. The flow momentum increases up to 21% after entrainment of hydrophobic bed compared to non‐erodible bed. The peak impact force on the downstream barrier can increase up to 80% for hydrophobic bed as a consequence of momentum gains after entrainment. The hydrodynamic coefficient for estimating impact force reveals the inadequacy of current design criteria of debris resisting barriers for fire‐prone vegetated hillslopes, suggesting further investigations are needed.

Fire in the Earth System: Introduction to the Special Collection

“…Rengers et al (2021), also working in the San Gabriel Mountains, used airborne and terrestrial lidar to determine that only 7% of postfire sediment erosion originated from channels, whereas 93% was derived from hillslopes; postfire erosion rates were of the same magnitude as millennial-scale bedrock erosion rates, suggesting that fires account for a majority of longterm erosion in that region. Flume experiments by Ng et al (2022) investigated the effects of soil hydrophobicity (water repellency) on debris-flow entrainment and momentum growth. Their study demonstrated unique erosion patterns and showed that the erosion depth of a debris flow can be six times greater when occurring in a hydrophobic bed similar to one that would be present after a wildfire.…”

“…Flume experiments by Ng et al. (2022) investigated the effects of soil hydrophobicity (water repellency) on debris‐flow entrainment and momentum growth. Their study demonstrated unique erosion patterns and showed that the erosion depth of a debris flow can be six times greater when occurring in a hydrophobic bed similar to one that would be present after a wildfire.…”

Experimental Investigation on Scouring v.s. Mass Failure of Unsaturated Soil Bed: Implications for Debris Flow Initiation and Erosion

“…Existing literature (McCoy et al., 2012; McGuire et al., 2017; Ng et al., 2022; Wilcock & McArdell, 1993; Winterwerp et al., 2012) demonstrates that progressive scouring and mass failure are two main mechanisms by which a soil bed is eroded. Scouring occurs when a soil bed is eroded at its surface grain‐by‐grain by hydrodynamic shear stress (Fraccarollo & Capart, 2002; Shields, 1936).…”

“…Mass failure occurs when a soil bed fails en masse as a block along a failure plane (Ng et al., 2022; Takahashi, 1978). Takahashi (1978) proposed that mass failure of a soil bed is prone to occur in loose saturated soils, where soil is liquefiable under the influence of external loading imposed by a debris flow.…”

“…Infiltration of flow water results in an increase in the water content of soil near the surface and changes its unsaturated strength. Therefore, considering the effects of flow infiltration on soil bed erosion is a crucial feature to capture, even though it is not considered in existing erosion models (McCoy et al., 2012; McGuire et al., 2017; Ng et al., 2022; Winterwerp et al., 2012).…”