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This study analyzes the extreme flood event that occurred on the Wien River in Vienna, Austria, in September 2024, classified as a 1000-year flood due to its peak discharge of 450 m³/s. The flood posed a major threat to Vienna’s critical urban infrastructure, especially the subway system running close beneath the river. Despite significant flood protection systems such as concrete riverbeds and retention basins, localized scouring and near-overflow conditions revealed weaknesses in current urban river flood management strategies. The study aims to understand the causes of these failures, particularly the formation of critical flows and hydraulic jumps that led to severe riverbed erosion.

Through calibrated one-dimensional hydrodynamic-numerical modeling, the study found that scouring occurred exactly at points where flow transitioned to critical or supercritical regimes, forming hydraulic jumps that produced intense erosional forces. These effects were intensified by structural constrictions in the river’s path, such as bridges and narrowed sections. When modeled with increased roughness, such as that caused by vegetation or macro-bedforms, the system shifted to subcritical flow conditions, reducing erosion risk. The findings support Grant’s theory on river flow dynamics and call for balancing roughness, discharge capacity, and erosion prevention in urban river design.