Phase 1 – Large-scale physical model test to investigate two types of measures for the Wien River to achieve good ecological potential and identify the risk of possible riverbed erosion during extreme floods

The Vienna River currently fails to meet good ecological potential from its confluence with the Danube Canal to the retention basins in Auhof. This is due, among other things, to the lack of habitat caused by continuous, monotonous regulation and riverbed paving, as well as the obstruction of passability through regulation and transverse structures that are impassable for fish. In a study commissioned by MA45, ezb – TB Eberstaller GmbH developed types of measures for the inner-city section of the Wien River between Penzinger Steg (river km 8.5) and St. Veiter Brücke (river km 10.2) in 2023. In this area, the Wien River flows in a deep channel on the orographic right bank. The above-mentioned study proposes two specific measures: M1 - scour areas to create habitat and M2 - short groynes to create passability. Based on this concept, DonauConsult Ingenieurbüro GmbH (DonauConsult for short) was commissioned by MA45 to submit plans for the section between km 12.0 and km 8.9. A pilot section is planned between km 10.7 and km 10.2, where the renaturation measures are to be implemented first. For the pilot section, in which the Wien River currently flows in the middle of the discharge profile, low-water groynes alternating with scour areas are planned. The scours are to be covered with a layer of gravel. The design of the groynes is based on the results of the technical report “Development of innovative groynes for the Vienna River” (MA45 study, 2022 by BOKU/IWA). A groyne-scour sequence is to be replicated in a large-scale model experiment in the research flume of the BOKU River Lab. The aim is to optimize the scour geometry (length, width, depth, slope angle) in terms of erosion and sedimentation behavior. In a hybrid approach, the flow field of this groyne-scour sequence will be calculated in high-resolution, numerical 3D simulations. Furthermore, during the 2024 floods, the Wien River showed local erosion of the concrete bed and, in some cases, bank protection. The aim here is to gain a better understanding of the causes of these erosion phenomena. To this end, a numerical 1D model of the entire inner-city area of the Wien River is being created, which calculates and compares the flow with and without erosion phenomena.