Changes in Soil Properties over Time after a Wildfire and Implications to Slope Stability

Postwildfire forests are dynamic environments. Wetting-induced shallow landslides are observed at varying times after wildfires, but the reasons are not fully known. This study investigates the time-dependent changes in soil properties and mechanical and hydrologic soil behavior of hillslopes after the 2019 Williams Flats Wildfire near Keller, WA and demonstrates the implications of these changes to slope stability. Soil water repellency, organic content, fine content, soil water retention curve, hydraulic conductivity, friction angle, and in-situ suction-saturation data provides initial evidence that soil properties fluctuate over a year after the wildfire. The results therefore suggest that soil properties that are measured immediately after the wildfire are misleading for long-term slope stability analysis. The stability of a steep (45°) slope is found to be most affected by the fluctuations in friction angle and soil water retention over the year. The comparison of suction-saturation response near a burned and unburned location demonstrates the effects of macropores that are formed after the wildfire and evapotranspiration on slope stability. The hillslope stability at the unburned location reduces rapidly in April upon snowmelt, whereas the stability at the burned location, which has macropores, shows the rapid decrease in late-January, with the onset of snowfall.