Physics-based Two-Dimensional Hydrodynamic Model of Riverine Flood Disasters Across Costal Cities

Flood disasters are increasing in frequency and intensity worldwide due to climate change and sea-level rise. Meanwhile, rapid urbanization is concentrating populations, assets, and critical infrastructure systems within cities, amplifying their vulnerability to flood impacts. In this context, it is crucial to develop a principled understanding of the real-time evolution of flood-induced disruption in urban environments, to enable a robust risk assessment and informed decision-making on the emergency response. However, most existing studies rely on statistical indicators - particularly maximum water depth derived from scenario-based return-period analyses - which cannot capture the time-varying hydrodynamics of flood events, including the spatiotemporal variations in flow depth and velocity. To address this challenge, this study develops a physics-based modelling framework capable of tracking the hydrodynamics of riverine flooding in complex urban settings. The framework employs a two-dimensional hydrodynamic model that integrates in-channel flow conveyance, overbank inundation, and floodplain propagation. Its applicability is demonstrated through a case study of Cardiff, a coastal city in South Wales, as one of the most flood-prone areas in the UK. The model is first validated against Storm Claudia in 2025, which caused moderate flood-related disruption across the city. The simulated hydrodynamic behaviour shows strong agreement with the recorded observations, revealing the framework's ability to reproduce the event's temporal progression. The framework is then applied to Storm Dennis in 2020 - that triggered the most severe flood-related disruption in Cardiff since 1979 - to examine its performance under extreme conditions. The results demonstrate that the proposed framework enables nuanced, time-resolved modelling of flood dynamics, thereby establishing a robust foundation for the risk governance of urban communities imperilled by riverine flood disasters.

Keywords: Riverine flooding, Two-dimensional hydrodynamic model, Urban community, Flood risk..