Multi-hazard risk assessment in coastal urban settlements: application to the RETURNVILLE virtual testbed

ABSTRACT Over the past few decades, economic losses from natural hazards have continued to increase due to the combined effects of climate change, population growth, and rapid urbanization in hazard-prone areas. This effect is particularly relevant in coastal urban areas, where high population density, critical infrastructure concentration, and exposure to multiple hazards may generate complex, spatially heterogeneous risk conditions. In such contexts, traditional single-hazard approaches may be insufficient, requiring integrated multi-hazard frameworks to capture the interactions between processes, compound events, and cascading effects. This study addresses these challenges within the framework of the Italian RETURN project, which aims to advance for multi-risk methodologies and operational tools under changing climate conditions. Specifically, this research applies a suite of innovative tools developed in the RETURN project to the RETURNVILLE Coastal (RV1) Virtual Testbed, a digital environment designed to simulate a coastal urban settlement exposed to multiple hazards. The assessment considers two physically distinct, potentially concurring hazards: tsunami-induced coastal flooding and pyroclastic density currents. Adopting a suitably developed two-step top-down statistical framework, the available exposure data for RV1 is improved to obtain a high-resolution building exposure model, including the georeferenced distribution of the building stock. Hazard-specific vulnerability models are integrated within a scenario-based framework using risk storylines and impact chains to model damage and cascading effects on buildings and infrastructures. Moreover, a consequence-based multi-hazard harmonization based on functionality levels is proposed, adopting the mean functionality as a uniform, consistent indicator for operational consequences of different hazard impacts on the urban system. The findings highlight the potential of Virtual Testbeds to support multi-hazard risk analysis in urban contexts, providing a flexible platform for the testing of advanced methodologies and significantly improving the representation of exposure, vulnerability, and impact interactions in coastal cities.