Anthropogenic structures often block or delay the downstream migration of fish in rivers, thereby affecting their populations. A potential solution at run-of-river hydropower plants (HPPs) is the construction of a fish guidance structure in combination with a bypass system located at its downstream end. Crucial to fish guidance efficiency and thus to fish behavior are the hydraulic flow conditions in front of the fish guidance structure and upstream of the bypass entrance, which have not thus far been investigated in depth. The present study aims to extend the knowledge about the flow conditions at these structures. Based on the results of 3D numerical simulations of two idealized block-type HPPs with horizontal bar rack bypass systems, the flow conditions were examined, and the fish guidance efficiency was predicted. Herein, a new method was used to represent the fish guidance structure in the numerical model. The results show that the approach flow to fish guidance structures at block-type HPPs varies significantly along their length, and areas with unfavorable flow conditions for downstream fish migration frequently occur according to common guidelines. Subsequently, eight variations were performed to investigate the effect of key components on the flow field, e.g., the bypass discharge. Finally, the results were compared with literature data and discussed.

Trash racks installed at hydropower plants cause head losses that reduce energy output. Previous research has thoroughly investigated head losses through both experimental and field studies. However, only a limited number of numerical studies have been performed, which have shown significant simplifications in terms of model complexity. In this study, the head loss coefficients ξ of circular bar trash racks (CBTRs) were analyzed using 3D Large Eddy Simulation (LES). Specifically, a single submerged bar oriented perpendicular to the flow direction was studied under homogeneous inflow conditions while (i) the blocking ratio P was varied between 0.043 and 0.444, and (ii) the flow velocity U was varied between 0.3 and 1.0 m/s. The model parameters were selected primarily based on the extensive literature on flow past circular cylinders, particularly at a Reynolds bar number Re_b of 3900. To ensure the validity of the parameters, systematic independence tests were performed, including simulations with three and five bars in the computational domain. The results confirmed the suitability of 3D LES as an appropriate tool to determine ξ of CBTRs. In general, ξ decreased continuously with decreasing P and increased with increasing U when Re_b ≥ 3981, which is consistent with comparable flow parameters observed in previous studies of flow past circular cylinders. Notably, the study found that the empirical formulas used for comparison tended to underestimate ξ when P was relatively low. Finally, the potential of the presented approach for future applications was discussed in detail.

In the 19th and 20th centuries, numerous alpine rivers were modified into straightened and monotonous channels by river regulations, which had significant negative effects on the ecology of the river systems and their floodplains. River revitalisations aim to restore river sections in order to create stepping stones in anthropogenic altered rivers. A large-scale example are the measures along the alpine Inn River between Stams and Rietz in Tyrol (Austria) over a length of about 3 river kilometres, which are implemented within the extension project of the hydropower plant Sellrain-Silz. During two low-water periods (October - April), starting in 2021, the existing bank protections were removed to a large extent, the river bed was widened up to 75 meters and a back water zone as well as a branch were created. As a result, the river stretch can develop by its own dynamics and ecologically valuable areas such as shallow water zones as well as gravel and sand banks are supposed to be formed.In order to assess the morphodynamics and ecological functionality of these measures, while also maintaining flood protection, a comprehensive monitoring program is being conducted. This includes nine photogrammetric surveys using an unmanned aerial vehicle (UAV) between March and October 2022. The UAV data were used to generate ortho-images and digital elevation models. The accuracy was assessed by comparison with airborne LiDAR data from one flight in March 2022. The objective is to quantify the erosion and deposition processes in the area of the measures and to determine the sedimentological processes that occurred during the survey period using the UAV results and hydro-morphological data (e.g., hydrograph, suspended sediment concentration) from nearby gauging stations on the Inn River. In addition, the suitability of high-resolution data from UAV surveys for monitoring sediment and bedload dynamics in alpine rivers will be evaluated. First results show that deposition processes dominated in the area of the measures, while relatively low discharge values were recorded throughout the study period. Based on the data analysis, we elaborate a suggestion for further monitoring in addition to the cross-section surveys already conducted since 1980, i.e., one UAV flight per year at low-flow conditions in order to establish a long term monitoring of morphodynamics.

Morphodynamic models are essential tools for estimating sediment dynamics along river reaches. A wide range of bedload transport equations is available, and applying these models to river restoration projects and river widenings requires simplifying assumptions about slope effects, secondary currents, and bank failure, introducing significant uncertainties. This study evaluates morphological changes during a flood event in the restored section between Stams and Rietz on the Inn River in Tyrol, Austria, using the 2D hydrodynamic model Telemac2D coupled with the morphodynamic model Gaia. As the flood occurred shortly after the completion of the restoration works, vegetation had not yet developed, allowing the influence of vegetation to be neglected in the numerical model. To enhance the applicability to similar restoration projects, the simulations were solely hydraulically calibrated and only relied on default morphodynamic calibration parameters. A comprehensive sensitivity analysis tested multiple bedload transport equations and key model parameters. The results suggest that the Meyer‐Peter & Müller and Hunziker equations effectively replicate erosional and depositional patterns, with volume estimates within a factor of two compared to the survey data. Transport rates varied significantly across all tested equations, underscoring the limitations of existing formulas for rivers characterized by coarse and broad gravel‐dominated particle size distributions and immobile coarse particle fractions. The sensitivity analysis also highlights the critical role of slope effects. While accounting for secondary currents and bank failure improved the model results, their impact was comparatively smaller. Overall, this study demonstrates that morphodynamic modeling can be a valuable tool for planning river restoration projects, particularly in estimating sediment budgets and predicting erosional and depositional patterns.

Zusammenfassung: Die fortschreitende globale Erwärmung erfordert eine Reduktion der Treibhausgasemissionen. In diesem Zusammenhang kommt der Wasserkraft eine zentrale Rolle in der regenerativen Energieerzeugung zu, insbesondere in Österreich. Wasserkraftanlagen zeichnen sich durch zahlreiche Vorteile wie hohe Energieeffizienz, niedrige CO₂-Äquivalentemissionen und hohe Erntefaktoren aus. Gleichzeitig sind jedoch große Teile der heimischen Fließgewässer durch anthropogene Einflüsse wie Wasserkraftanlagen sowohl in ihrer gewässerökologischen als auch morphologischen Beschaffenheit beeinträchtigt.Ein Beispiel hierfür ist die Untere Salzach an der Grenze zwischen Österreich und Deutschland. Durch Begradigungen und Befestigungen der Ufer weist der Fluss hier kaum Strukturvielfalt auf und befindet sich, auch bedingt durch Geschieberückhalt im Einzugsgebiet und Oberlauf, in einem Eintiefungsprozess. Der Fokus der flussbaulichen Sanierung liegt hier in der langfristigen Stabilisierung der Sohle und ökologischen Verbesserung nach dem Leitbild eines eigendynamischen voralpinen Kiesflusses. An einem derart stark anthropogen beeinflussten Gewässerabschnitt, an dem ohnehin flussbauliche Anpassungen erforderlich sind, erweist sich ein integratives Konzept zur Verbindung von Renaturierungsmaßnahmen mit klimaneutraler Energiegewinnung als besonders vielversprechend. Die Sanierungsvariante E1+ „Mehr Fluss“ an der Unteren Salzach stellt ein solches integratives Konzept dar, welches die Aspekte der notwendigen Sohlstabilisierung und Verbesserung der Gewässerökologie mit erneuerbarer Energienutzung kombiniert. Neben der Sohlstabilisierung durch eine aufgelöste asymmetrische Rampe sind Initialmaßnahmen zur eigendynamischen Aufweitung im Unterwasser geplant. Die durch die Rampe entstehende Fallhöhe soll durch ein Fließgewässerkraftwerk energetisch genutzt werden. Dieses ist so konzipiert, dass potenziell negative Auswirkungen herkömmlicher Flusskraftwerke, wie Störungen in der biologischen Durchgängigkeit, Sedimentrückhalt oder Verlust des Fließgewässercharakters, weitestgehend vermieden werden können.Das Konzept kann so einen Beitrag zum Klimaschutz leisten und gleichzeitig zu einer Verbesserung der Gewässer- und Gesamtökologie beitragen, während die Sohle der Unteren Salzach sofort nachhaltig stabilisiert und angehoben wird.