Morphodynamic Feedback on Flood Hydraulics in Alluvial River Systems: The Polish Middle Vistula River Case

ABSTRACT This study investigates the influence of bed mobility on flood hydraulics and inundation characteristics in the Middle Vistula River (Poland), downstream of the Dęblin hydrometric station, under three flood return periods (2, 5, and 10 years). Using hydrodynamic simulations assuming fixed‐bed and moveable‐bed conditions, the results demonstrate that bed mobility significantly alters the spatial distribution and intensity of flood‐related dynamics. Compared with fixed‐bed conditions, assuming a movable bed means that flow velocity increased by up to 22%, stream power rose by nearly 500% for the 2‐year flood, and the Froude number more than doubled for the 10‐year event, indicating a stronger and more dynamic flow behavior. Despite minor changes in the inundation area, flood intensity (depth × mean flow velocity 2 ) increased considerably by up to 85%, underscoring the significant impact of bed mobility on flood hazard characteristics. The observed patterns highlight the critical role of sediment transport in shaping flood behavior and challenge the adequacy of fixed‐bed assumptions for hazard mapping and flood risk management. These findings pinpoint the necessity of integrating river morphodynamics into future flood modeling approaches to enhance prediction accuracy and resilience planning, particularly in dynamic alluvial river systems like the Vistula.