EXPLORING THE IMPACTS OF SUCCESSIVE HIGH-SEVERITY WILDFIRE ON DEBRIS-FLOW PROPERTIES

Understanding how successive wildfires influence debris-flow properties is becoming increasingly important as fire regimes across the western United States shift to larger, more frequent high severity wildfires. Ponderosa pine forests in Arizona, for example, have historically burned in frequent, low severity fires, but recent changes in climate and land use practices have led to more frequent high severity fires in this ecosystem. This is evident in the San Francisco Peaks, an isolated, high-elevation mountain range located north of Flagstaff, Arizona. In 2010, the Schultz Fire burned over 15,000 acres of ponderosa pine forest on the slopes of the San Francisco Peaks, mostly at high severity. Twelve years later, the 2022 Pipeline Fire reburned 75% of the area burned by the Schultz Fire, again often at moderate-to-high severity. While debris flows initiated following each fire, the magnitude and mobility of the events were substantially different. Following the Schultz Fire, 19 watersheds produced debris flows that ranged in volume from 200-14,000 m³ and ran out between 0-2.5 km downstream of watershed outlets, remaining confined to slopes upstream of residential communities. On the other hand, following the Pipeline Fire, several watersheds produced debris flows that were much larger than what was observed following the Schultz Fire. Here we study the flow properties, including volume and runout distance, of a post-Pipeline debris flow that was significantly larger than the post-Schultz debris flows, with a volume of 115,000 m³ and a runout distance of 7 km. This debris flow was 28 times larger and ran out six times longer than the debris flow that initiated in the same watershed following the Schultz Fire. Unlike the post-Schultz debris flows, it impacted downstream residential areas before transitioning to flood flow. This is despite the fact that the intensity of the debris flow-producing storm and the area burned at moderate and high severity, both factors that contribute to larger debris-flow volumes, were lower following the Pipeline Fire than following the Schultz Fire. This suggests that the impacts of successive high severity wildfires, including increased hydrologic connectivity and sediment availability, may influence the volume and runout of post-wildfire debris-flows.