Integrated debris flow hazard assessment in critical infrastructure zones: a case study of the 2022 Cubatão event, Serra do Mar, Brazil

Abstract Debris flows are among the most destructive natural processes in tropical mountainous regions, especially when they affect critical infrastructure. However, integrated methodologies that combine susceptibility analysis, numerical modeling, and structural vulnerability assessment remain scarce, particularly in humid tropical environments such as the Serra do Mar. This study proposes an integrated hazard assessment approach combining morphometric susceptibility metrics, RAMMS-DF numerical simulations, and infrastructure-oriented vulnerability diagnostics. The methodology was applied to a catchment in the Serra do Mar (São Paulo), where a debris-flow event in March 2022 damaged a roadway and threatened the Presidente Bernardes Refinery (RPBC). Susceptibility was estimated using morphometric parameters, while vulnerability was assessed through simulations calibrated for local tropical conditions. The results indicated flow velocities exceeding 30 m/s and impact pressures above 200 kPa in the most critical channel sections. The main outcome is a hazard index that integrates geomorphological controls and infrastructure exposure, enabling the classification of catchment sectors according to hazard level. Validation with the extreme event that occurred in São Sebastião in February 2023 confirms the replicability of the approach. The findings offer a practical tool to support mitigation planning, guide land-use decisions, and strengthen risk management strategies in tropical mountainous regions with critical infrastructure.