Experimental study of rheological behavior of debris mixtures subjected to burning: implications to postfire debris flow mobility

Abstract

Wildfire-induced changes to soil and vegetation decrease infiltration and increase runoff, making slopes susceptible to the occurrence of postfire debris flows. These flows, which occur in burned watersheds, are typically characterized by an abundance of fire-burned debris, resulting in expected alterations in the flow rheology. Burning tests were conducted to investigate the effects of the burn temperature and combustion of organic matter on the rheologic properties of postfire debris flows. Representative organic matter was combusted at temperatures typically observed during wildfires from 100 to 400 °C. Rheologic testing was subsequently employed to measure the rheologies of fire-burned debris by mixing the burned debris with water. The findings reveal that burned debris produces more diluted slurries, resulting in a decrease of up to 100 % in yield stress and 80 % in viscosity. In addition, the underlying physics of the rheology are elucidated. These systematic investigations on burned debris suggest that wildfires have the potential to increase the mobility of debris flows in burned watersheds. This insight provides theoretical underpinnings for hazard assessment models that depend on rheological characteristics as crucial inputs.