[Movement of a fluvial particle under the influence of coherent structures]. / Bewegung eines Einzelkorns unter dem Einfluss kohärenter Strukturen.

In a fundamental study the influence of coherent structures on the incipient motion of single fluvial grain was experimentally investigated. To fully characterize coherent structures, the three-dimensional velocity field must be known with high temporal resolution. Using a tr-3D PTV system (tr = time-resolved, PTV = particle tracking velocimetry) this could be achieved. The influence of hairpin vortices and counter-rotating longitudinal vortices (VLSM) on sediment transport has been postulated in several studies, but due to the missing 3D information, evidence has been lacking. In the present "Rolling Stones test series", the incipient motion of a single grain was investigated for smooth and rough beds. It could be shown for the first time that both hairpin vortices and VLSM trigger particle entrainment. Hairpin vortices could also be detected on rough beds, contrary to common hypotheses, and triggered particle movement. The long-term study should also find practical applications in the coming years and increase the accuracy of sediment transport calculations in rivers.

[Experimental and numerical investigation of fluid-particle-interactions in water]. / Experimentelle und numerische Untersuchung von Fluid-Partikel-Interaktionen in Wasser.

For the development of improved sediment transport models, the basic understanding of the interaction between the solid particle and the moving fluid (water) is important. In this article, current developments in the field of fluid-particle interaction are presented based on two research articles by Gold et al. (2023) and Worf et al. (2022). One presented in this article uses state of the art measurement methods to investigate the flow around spheres of different densities that oscillate in initially resting body of water. For the spherical pendulum a similar vortex shedding characteristic was observed for all investigated fluid density ratios (m*=ρS/ρF=1.14,14.95, density ratio between solid and fluid). A new object tracking method (DOT) is also presented, which enables temporally and spatially resolved analysis of flow structures in the fluid field. The experimental results of Gold et al. (2023) show, that vortex shedding occurs during the first period. This vortex propagates downward and eventually dissipates. Furthermore, a damping optimum of the spherical pendulum in the range of m*=2.50 was observed. Additionally, an experiment with a cylindrical pendulum with m∗=4.98 was investigated numerically utilizing an immersed boundary method. The process of creation and separation up to the dissipation of a vortex ring was described. Furthermore, this investigation by Worf et al. (2022) described the creation of tip vortices. These were connected with the development of the three-dimensional flow and added mass coefficient.

Evaluation of re-depositing hydropower trapped sediments at large rivers to improve flood protection and aquatic habitats - Case study Danube/Austria.

Sedimentation in impoundments of run-of-river hydropower plants is an ongoing and progressing management issue for hydropower operators because of its consequences for e.g., the maintenance of flood protection and waterway parameters. Current practices in sediment management are often unsustainable, associated with high costs, and can pose risks for downstream biota (e.g. during flushing). The present study elaborated a conceptual model of a novel sediment management strategy for impounded river sections consisting of a current-state and deficit analysis, and the application of a novel sediment management practice, which was practically implemented at the study site at the Austrian Danube River. This novel practice consists of (i) local dredging of gravel at locations in the impoundment, which are problematic in terms of flood protection and waterway maintenance, and (ii) the re-deposition of dredged sediments by artificial placement of gravel structures. The present study included morphological analyses of the impounded section at the study site by applying the channel profile budget technique. The knowledge of the long-term morphological development served as a basis for the evaluation of the sediment management measures and for the elaboration of the conceptual model. The combination of the morphodynamic characteristics in the impoundment with the implementation of the novel sediment management practice helped to derive generalized statements for the potential implementation of the presented conceptual model in large rivers with similar impoundment characteristics. We further defined several aspects, which are related to sediment dynamics in impounded river sections, flood protection, waterway demands, and ecological criteria that require consideration for an efficient realization of the proposed conceptual model.

Evaluation of hydropeaking impacts on the food web in alpine streams based on modelling of fish- and macroinvertebrate habitats.

Hydropeaking as a result of peak-load electricity production has been identified as one of the most significant pressures in alpine streams. Scouring of macroinvertebrates leads to downstream transport of aquatic organisms (catastrophic drift). Additionally, invertebrates are affected by periodic drying of wetted area during the dewatering of gravel bars and exposed areas along the banks. Even though fish are physiologically better adapted to switch to suitable habitats, artificial flow fluctuations may be followed by lethal stranding and quick alteration in habitat quantity and quality. Nevertheless, the interactions between pressures on fish and macroinvertebrates in terms of hydropeaking have not been investigated so far. The aim of this paper is to evaluate effects of flow fluctuations on potential epibenthic feeding grounds. Therefore, we evaluated changes in habitat distribution resulting from rapid flow fluctuations in river reaches with different river morphological characteristics, for five different macroinvertebrate taxa. Additionally, microhabitats for brown trout at two different life stages were calculated using representative peaking events (seasonal analysis) based on mid- to long term times series. Moreover, GIS-analysis allowed the evaluation of hydropeaking impacts (interaction) on both, macroinvertebrates and fish. In this study, it could be documented that feeding from the benthos for juvenile and subadult brown trout is inhibited during peak flow and is therefore reduced to times of base flow. Moreover, potential benthic feeding areas occurring at base flow have been found to increase with the level of morphological heterogeneity within analyzed river reaches. Likewise, hydrological sensitivity testing in terms of reducing ∆Q at different levels was performed and revealed that possible positive effects required heterogeneous river morphology as a precondition. However, this approach might be applied for estimating the impacts of hydrological mitigation measures in hydropeaked rivers concerning physical condition and/or growth rate of salmonids considering the river morphology of the investigated stream.