Assessing roadway vulnerability to post-fire debris flows in Arizona in current and future climates

Abstract: Wildfires are a growing threat to infrastructure, and roadways in particular, in Arizona. In addition to causing direct damage, they can destabilize soil and lead to post-fire debris flows when rainfall occurs over recently burned areas. Post-fire debris flows can damage pavement, block drains and culverts, and result in disruptions to roadway services. Both wildfire frequency and intensity and rainfall intensity are expected to evolve in response to climate change, which may lead to increased debris flow threats. Public agencies who manage roadways will need tools to support decision-making around where to expend resources for mitigation of fire and debris flow risk. In this work, we provide a statewide assessment of roadway vulnerability to wildfire and post-fire debris flow in Arizona under current and future climate conditions. We use a state-of-the-art regression-based model to estimate debris flow likelihood on each roadway segment in Arizona. Debris flow threat is based on terrain ruggedness, burned area intensity, and soil characteristics. We then assess vulnerability of each roadway by overlaying debris flow threat, roadway criticality (measured using betweenness centrality), traffic data, and sociodemographic variables. We identify roadways that are most vulnerable in current conditions, and those that are expected to be most vulnerable in future climate conditions. We assess climate uncertainty by using a range of future scenarios covering multiple emissions pathways and using multiple global climate models. Our results indicate that the roadways facing the highest debris flow threat are concentrated in mountainous Northern Arizona in the Four Forest Restoration Initiative (4FRI) Region, and in the rugged Southeastern mountains. We find that changes in future debris flow threat vary geographically and across climate scenarios, with some roadways expected to see an increase in debris flow likelihood, and other roadways expected to see a decrease. These results will provide guidance for agencies and decision-makers on where to focus their resources to mitigate the evolving threats of post-fire debris flows.