Hydrodynamic modelling of historical flood events in the Iberian Peninsula: implications for flood risk assessment and mitigation

Historical flood events provide critical insights into extreme flood dynamics that are often underrepresented in instrumental records. This contribution presents the application of hydrodynamic modelling to analyse major historical flood events in the Iberian Peninsula and to assess their relevance for flood risk assessment and mitigation. Several catastrophic floods affecting Portugal and Spain between the late 19th and the 20th centuries are investigated, including riverine and flash-flood events with severe societal impacts. A modelling framework is implemented using the physically based Iber+ hydrodynamic model. It integrates precipitation reconstructed through multiple methodologies, including interpolation of gridded and measured data, and topography compiled using different sources, ranging from historical maps to field-based measurements. This framework enables the estimation of peak river flows, one of the main unknowns in historical events, and reproduces flood propagation, inundation extent, water depths, and flow velocities. Model performance is evaluated against historical watermarks, documentary evidence, and witness testimonies, showing good agreement despite the scarcity of direct measurements and the associated uncertainties. The simulations enable a detailed analysis of the key drivers controlling flood severity, including the exploration of plausible scenarios, providing insights into the main causes of these events where uncertainties in flood development persist. The results highlight the role of hydraulic bottlenecks, infrastructure blockage, and local topographic constraints in amplifying flood impacts. Scenario-based simulations further demonstrate the potential of hydrodynamic modelling to explore mitigation strategies, such as optimized dam operation and improved infrastructure maintenance, and to assess extreme but plausible flood scenarios under current terrain and infrastructure conditions, supporting flood mitigation. Overall, the results emphasize the value of integrating historical flood information with modern hydrodynamic modelling. This approach contributes to improving flood hazard mapping, reduce uncertainties in flood risk estimation, and support more robust flood risk management strategies under current and future climatic conditions, in which flood events of comparable or even greater intensity than historical floods are expected.