Critical flows at the Wien River during the 1000-years event in September 2024 – causes, consequences and possible management options for urban river flood management

In September 2024, a thousand-year flood event of the Wien River was recorded in the Austrian capital Vienna with 450 m3s− 1 peak discharge. The flood flows had to cross the city from West to East before discharging into the Danube side-channel. The water stages were close to flood the subway system in some critical parts of the city. However, major inundations in town could be prevented also by the technical control of the retention basins, located in Auhof at the entrance to the city. The Wien River, a typical urban river, contains vertical walls and large stones levelled out in a concreted bed with the aim of preventing morphodynamics during high flows. During the flood event in 2024, however, local major scouring occurred in different parts along the river. One of the scouring holes was up to 2.5 m in depth. Thus, the aim of this short-communication is to figure out what might be the causes for such local, but massive scouring, although heavy protection of the riverbed was implemented since the regulation of the river. The analysis was conducted based on calibrated one-dimensional hydrodynamic-numerical modelling to study the hydraulic forces, like flow velocity, bottom shear stress and possible critical flows in detail. The results showed that exactly in those parts of local scouring, critical flows, followed by hydraulic jump formations, occurred during the event. This indicates that this area experienced high energy dissipation with high erosional forces. The findings support the Grant´s theory that rivers are never over longer distances and over longer periods in critical flow conditions and that morphological adjustments during high flows would lead to sub-critical conditions. This was proven by implementing the large scouring into the model and re-calculating the flood flows of the event in September 2024. In conclusion, such sites with possible critical flows in urban rivers must be identified and should be monitored in detail that no weakening of the technical control might be possible. Similar implementations like those described is a technical standard for weirs and hydropower plants in hydraulic engineering practice.