Cascading flood-landslide scenarios reframe the hazard assessment of exposed infrastructure

Conventional multi-hazard assessments in catchment-infrastructure configurations based on spatial overlay do not always capture physically interacting hazard chains, because they lack systematic protocols for identifying possible cascading activation. This structural limitation means that secondary impact pathways, possibly determined by geomorphological and hydraulic conditions, can remain undetected until damage occurs. This study addresses this gap by proposing a hierarchical stepwise framework that links regional screening with targeted field inspections and two-dimensional rain-on-grid hydrodynamic modelling to identify cascading geo-hydrological impact pathways threatening wastewater treatment plants (WWTPs) in small steep ungauged coastal catchments. Applied to 26 catchments along the Tyrrhenian coast of Calabria Region (southern Italy), the selection procedure identifies and classifies sites according to their predisposition to cascading hazard activation. Among these cases, the Maddalena catchment, a site exhibiting multi-pathway cascading potential based on field inspections, is selected as demonstrator for a full process-based modelling. Results show that landslide-induced channel obstruction generates backwater effects that reduce considerably the flood return period required to inundate the most vulnerable WWTP components. The reduction increases the frequency of a 1-in-500 year event under clear-water conditions to approximately a 1-in-50 year event under cascading conditions. Sensitivity analyses confirm that this threshold shift persists across different sediment distribution configurations and for landslide volume reductions up to 40%, demonstrating the robustness of the cascading mechanism. These findings show that multi-hazard overlay approaches can substantially underestimate latent flood-related environmental hazards in data-scarce environments, and that systematic cascading hazard screening provides actionable information for prioritising monitoring and protection of critical infrastructure in flood-prone coastal regions.