Rail2FLood: A nationwide classification of the Italian Railway Network exposed to flood hazard

The Italian Railway Network is one of the major transport infrastructures that can be negatively impacted by flood events. Due to the considerable lengths and differences between the various routes, several structural damages and disruptions can occur. To understand and predict future impacts of floods on the railway infrastructure and to develop appropriate adaptation strategies, it is necessary to analyze the relationships between such infrastructure and potential hazardous scenarios. This study leverages a methodology [1] capable of identifying and classifying paths along the railway infrastructure that are vulnerable to fluvial flood hazard and potential debris-flows. Following the EU Flood Directive and Flood Risk Management Plans (FRMP, updated for Italy to 2020), three flood hazard scenarios are here considered, namely, H1 (with return period, Tr, up to 500 years), H2 (where Tr=100-200 years) and H3 (where Tr=20-50 years), which correspond to low, medium and high hazard, respectively. The analysis is performed nationwide at the river basin level, where 273 river basins are investigated. By overlapping the railway geographical location and the three flood hazard scenarios, we identify that 37%, 27% and 15% of the Italian Railway network is exposed to low, medium and high hazard scenarios (H1, H2 and H3, respectively, see Figure 1a). By adopting a DEM-based procedure, also suitable for large-scale applications, we outline contributing areas relevant to railway stretches considered at risk and compute key hydrological parameters, such as the time of concentration [2], the sub-basin contributing area, the river slope at the sub-basin outlet, and the presence of debris-flow, as influenced by topography-related triggering thresholds, including the slope angle of surrounding areas within a 1 km buffer from railways. These parameters are then used for classifying river sub-basins according to different flood hazard classes, which depend on the type of flooding phenomenon they are exposed to, the damage mechanism, and the severity of the expected flooding scenario [1]. More specifically, the flood hazard classification proceeds as follows: river basins with a river slope larger than 4% belong to Class 1-2 (mountainous-hilly flood areas), whereas river basins with a river slope <4% are distinguished as a function of the time of concentration, either smaller or larger than 12 hours, corresponding to Class 3-4 and Class 5-6, respectively. Class 1, 3, and 5 are identified if the triggering area is greater than 10 hectares and the slope angle is greater than 27%, otherwise the sub-basin belongs to Class 2, 4, or 6. Results for the low flood hazard scenario (H1) indicate that the majority of flood hazard classes in Italy (Figure 1b) belong to Class 4 (41%) and Class 3 (26%), while Class 1 and 2 collectively represent a lower percentage (13%). A nationwide dataset is compiled, processed and analyzed to provide a comprehensive overview of the Italian Railway Network affected by floods. 91 This analysis represents a significant step forward in enhancing our understanding of flood dynamics and exposure analysis of railway infrastructure, thereby contributing to more informed decision-making processes in flood risk management and disaster mitigation efforts.