Helmut Habersack

Rivers in Europe have been heavily modified over the last 200 years, with a significant impact on their ecology and environment. This also applies to rivers like the Wien River, Austria, which are designed as overwide concrete channels for the benefit of flood protection. To achieve a good ecological potential in such heavily modified water bodies, one key element is fish passability. This requires an increase in the water level at low flow conditions and a reduction of the flow velocity. The aim of this study is to assess whether groynes are suitable for this application. A design study was conducted to examine the effect of individual groyne parameters on water depths and velocities. Physical experiments were carried out in a laboratory flume at a scale of 1:8. In addition, a 2D numerical model was used. It was found that the groynes had to be submerged and the alignment had to be repelling to achieve both requirements. The configuration of the groyne height, distance and degree of obstruction parameters were crucial. The groyne angle and shape had a minor effect and can be used for fine‐tuning. The best groyne design created a passable section for fish. Thereby, and through sedimentation, the best design contributed to an ecological improvement. However, it did not create habitats and did not constitute a fullfledged restoration. In general, submerged groynes can fulfil the hydraulic requirements for fish passage in heavily modified water bodies with a fixed bed.