Wildfire disturbs hydrologic and geomorphic systems by incinerating vegetation, decreasing hydraulic roughness, and altering soil hydraulic properties that control rainfall-runoff partitioning. These changes promote increases in runoff and sediment transport that increase the likelihood of flash flooding and debris flows. Assessing debris-flow hazards throughout the post-wildfire recovery period is complicated, in part, by the myriad of wildfire-induced hydrologic changes that take place and their nonlinear relationships with sediment transport and runoff generation processes. To address this challenge, we have been monitoring rainfall, runoff, and debris flow responses in a number of burned areas throughout the southwestern U.S. Numerical modeling at the watershed scale suggests that the transition from water-dominated runoff to high-sediment-concentration debris flows is associated with the exceedance of a slope-dependent discharge threshold. By characterizing the initiation threshold for runoff-generated debris flows in terms of a discharge criteria instead of rainfall intensity, as is typically done, we can explore how temporal changes in soil hydraulic properties (e.g. sorptivity and saturated hydraulic conductivity) and hydraulic roughness influence debris flow activity as a landscape recovers from fire. Measurements of soil hydraulic properties at our study areas indicate that moderate and high severity wildfire often leads to decreases in sorptivity and/or saturated hydraulic conductivity. However, decreases in hydraulic roughness associated with the removal of litter and duff may sometimes be the most important factor controlling debris flow activity in burned areas. Results demonstrate the utility of debris flow thresholds based on discharge relative to those based on rainfall intensity and duration and highlight opportunities for additional collaboration among geomorphologists, hydrologists, and atmospheric scientists in the study of post-wildfire hazards.
Luke McGuire is an Assistant Professor in the Department of Geosciences at the University of Arizona. Prior to coming to UArizona in 2016, he worked as a postdoctoral researcher with the U.S. Geological Survey’s Landslides Hazards Group. He is a geomorphologist with broad interests in natural hazards and landscape evolution.