Hydrological and CFD investigations for the flood risk assessment of roads, railways and existing bridges

Increasingly frequent and intense flood events, combined with the significant industrialization of cities, are placing transportation networks under stress. The loads that roads and railways must withstand are often greater than those originally considered during their design; furthermore, the ageing state of these infrastructures is such that any disturbance, such as a flood, could lead to total or partial traffic interruptions, resulting in socio-economic losses. To support planning and flood risk management, a streamlined methodology is proposed for estimating the risk and potential impacts of inundation on infrastructure. The method assigns a variable risk class (High, Medium, Low, and Null) based on road categories (e.g., highways, main roads, secondary roads, service roads, and others) or railway types (e.g., high-speed or conventional railways), considering both the structural and functional characteristics of the infrastructure (e.g., type of traction, traffic flows) and the magnitude of the expected event. By accounting for the topography of the terrain, the methodology distinguishes between elevated sections (bridges, viaducts) and underground sections (tunnels, underpasses), determining, in addition to the spatial extent of the impacted system and the possible economic damages, the number of flooded railway stations and bridges. During floods, bridges can be partially or completely submerged, having to endure higher hydrodynamic actions that can cause structural collapse. Using Computational Fluid Dynamics (CFD) modelling, the hydrodynamic forces acting on a beam bridge under various hydraulic conditions (under pressure, limited hydraulic clearance) were analysed. The numerical model, calibrated using existing experimental data on forces induced by debris accumulation around piers of different geometries (circular, ogival, semi-circular), and validated with laboratory measurements in the absence of solid material, generated reliable synthetic curves. This practical instrument, transferable to engineers, constitutes a more robust force estimation criterion, targeted at improving the resistance of existing bridges and the design of new ones.