Threshold-controlled flood risk response to extreme dam releases in a regulated nile river reach

Abstract Flood risk in regulated rivers is strongly influenced by dam operations, particularly under extreme release scenarios. This study applies a two-dimensional hydrodynamic model to assess flood hazard and infrastructure vulnerability along a Nile River reach between the Aswan High Dam and the Esna Barrage under discharge conditions of 3500–11,500 m³/s. Results reveal a distinctly nonlinear hydraulic response governed by a critical threshold of approximately 5800 m³/s. Below this threshold; flow remains largely confined within the main channel with limited floodplain interaction. Once exceeded, the system transitions abruptly to a floodplain-dominated regime, characterized by rapid inundation expansion, enhanced lateral connectivity, and significant increases in flow depth and velocity. This behaviour is strongly influenced by floodplain topography, where low-gradient areas enable substantial horizontal expansion of inundation. Infrastructure analysis shows threshold-controlled vulnerability, with a sharp increase in pump station failures once water levels exceed critical elevations. These findings highlight the importance of threshold-based flood dynamics for improving flood risk assessment and dam operation in regulated river systems.