Hydrogeological risk assessment of existing bridge portfolios through an index-based approach

In recent years, several collapses of existing bridges have highlighted the need for rapid risk analysis methods to support infrastructure managers in the prioritisation of detailed assess- ments and, where necessary, the implementation of risk mitigation actions. Among other natu- ral perils, hydraulic and geological hazards, particularly floods, scour, and landslides are primary causes of bridge failure, compounding risks in multi-hazard scenarios. The frequency and intensity of extreme weather events has also increased due to climate change, overstraining existing infrastructure and exposing it to multiple hydrogeological risks. This study presents a quantitative, index-based methodology for risk assessment and prioritisation of existing bridge portfolios, focusing on flood, scour and landslide hazards. The proposed methodology inte- grates structural vulnerability, exposure, and the consequences of failure into a multi-hazard, hydrogeological risk indicator. Single-hazard indices are developed separately for flood/scour and landslide hazards, accounting for key factors such as bridge geometry, material properties, environmental conditions, and surrounding topography. These indices are then combined to facilitate a comparative analysis across bridge portfolios. The methodology is applied to a case-study portfolio of roadway bridges in northern Italy, generating a risk-based ranking that identifies critical structures requiring immediate attention or further detailed assessment. The results are analysed and discussed, highlighting the influence of the considered hazards and corresponding variables on the final risk prioritisation, providing infrastructure managers with a quantitative tool for large-scale bridge risk assessment and prioritisation, enabling risk-tar- geted maintenance and mitigation strategies.