Vulnerability of California roadways to post-wildfire debris flows

Post-wildfire debris flows represent a significant hazard for transportation infrastructure. The location and intensity of post-fire debris movements are difficult to predict, and threats can persist for several years until the watershed is restored to pre-fire conditions. This situation might worsen as climate change forecasts predict increasing numbers of wildfire burned areas and extreme precipitation intensity. New insights are needed to improve understanding of how roadways are vulnerable to post-fire flows and how to prioritize protective efforts. Using California as a case study, the vulnerability of transportation infrastructure to post-fire debris flow was assessed considering geologic conditions, vegetation conditions, precipitation, fire risk, and roadway importance under current and future climate scenarios. The results showed significant but uneven statewide increases in the number of vulnerable roadways from the present to future emission scenarios. Under current climate conditions, 0.97% of roadways are highly vulnerable. In the future, the ratio of vulnerable roadways is expected to increase 1.9-2.3 times in the representative concentration pathways (RCPs) 4.5 emission scenarios, and 3.5-4.2 times in the RCP 8.5 emission scenarios. The threat of post-fire debris flow varies across the state, as precipitation changes are uneven. The vulnerability assessment is positioned to (a) identify, reinforce, and fortify highly vulnerable roadways, (b) prioritize watershed fire mitigation, and (c) guide future infrastructure site selection.