Roadway vulnerability to post-wildfire debris flows in Arizona: a current and future assessment

Abstract Wildfires are a growing threat to roadway infrastructure. While causing minimal direct damage, wildfires cause substantial indirect damage, including through post-fire debris flows (PFDFs), which occur when moderate intensity rainfall occurs over recently burned areas where soil has been destabilized. PFDFs damage pavement systems, block drains and culverts, and disrupt roadway services. Wildfire frequency and intensity and rainfall intensity are evolving with climate change, which may lead to increased PFDF threats. Public agencies managing roadways need tools to support decision-making that accounts for limited resources and appropriately mitigates fire and debris flow risk. While there have been targeted assessments of fire and PFDF threat in specific areas of Arizona, there is no comprehensive assessment of statewide threat. In this work, we create a statewide assessment of roadway vulnerability to wildfire and PFDFs in Arizona in current conditions and future climate scenarios. We use a state-of-the-art regression-based model to estimate debris flow likelihood on each roadway segment in the state. Our model adapts novel geological methods to assess PFDF threats and engineered infrastructure data to characterize roadway threats. PFDF threat is affected by terrain ruggedness, burn intensity, soil characteristics, and rainfall intensity. Vulnerability of roadways is assessed by overlaying PFDF threat, roadway criticality (using betweenness centrality), and traffic data. We identify roadways most vulnerable in current conditions and estimate how threats change in future climate scenarios. Our results indicate roadways facing the highest PFDF threat are concentrated in the rugged mountains of Southeastern Arizona, the White Mountains of Eastern Arizona, and the Mogollon Rim in Central Arizona. We find that projected changes in precipitation patterns lead to an increase in PFDF threat in much of the state. The framework and results will provide guidance for agencies and decision-makers on where to focus their resources to mitigate the evolving threats of PFDFs.