Turbulent flow around well-submerged, wall-mounted obstacles has been widely studied, but the effects of low relative submergence on the flow field remain less understood. While submergence effects have been examined in experiments, uncertainties persist in the three-dimensional flow characteristics around large obstacles, particularly regarding its role in shaping instream habitats and fish migration pathways. The complex flow generated by such obstacles plays a key ecological role in natural environments (e.g., rapids, tributaries), influencing sediment transport, refuge zones, and turbulence structures that affect aquatic species. However, volumetric time-resolved measurements covering the upstream, side, and wake regions remain scarce. To examine the impact of submergence on the flow field, this study conducts laboratory flume experiments employing 3D Lagrangian particle tracking to assess two submergence levels around a wall-mounted cube. The response of key mean flow parameters, including streamwise and vertical velocities, as well as the production of turbulent kinetic energy, was examined, and time-resolved flow fields were analyzed using power spectra and proper orthogonal decomposition. Major vortical structures around the cube, namely horseshoe and arch vortices, are identified and analyzed. The study reveals that variations in relative submergence and shallow flow conditions strongly influence the flow field, with lower relative submergence being associated with increased flow complexity. These variations affect flow structures that govern habitat formation and fish migration. The insights gained contribute to a better understanding of flow around large obstacles, supporting ecological engineering and habitat restoration efforts in shallow waterways.

Boulders in riverbeds significantly influence sediment transport and deposition by altering local flow patterns. This study employs coupled Computational Fluid Dynamics–Discrete Element Method (CFD–DEM) simulations to examine how boulder concentration affects sediment dynamics. Three boulder spacing scenarios, representing isolated, wake-interference, and skimming flow regimes, are evaluated for their impacts on flow structure, bed shear stress distribution, and sediment transport and deposition. The fluid phase is modelled using a large eddy simulation in a finite volume framework, while individual sediment grains are tracked with the discrete element method. The results reveal that increasing boulder concentration transforms the flow regime from isolated wakes behind individual boulders to a more coherent recirculation zone among neighboring boulders. This transition substantially reduces near-bed shear stresses between boulders and leads to a decline in total sediment transport rates. At high boulder concentration, sediment particles preferentially accumulate in sheltered inter-boulder corridors, forming stable depositional belts. In contrast, widely spaced boulders lead to isolated, localized deposition around individual boulders. To our knowledge, this is the first attempt to apply a coupled finite volume–DEM approach to simulate boulder–sediment interactions in open-channel flow, enabling analysis of sediment transport and deposition under varying boulder concentrations.

3D Lagrangian particle tracking dataset of flow around a surface-mounted cube in an open-channel flume at two relative submergence conditions. Particle tracks were reconstructed using Shake-The-Box and used to obtain Eulerian velocity fields. The dataset provides time- averaged velocity and acceleration fields for two relative submergence conditions: Low (RS = 1.25) and High (RS = 2.00). Velocity and acceleration data are stored in separate files. Each row in the files represents a single grid point in the reconstructed Eulerian field.

Increasing periods of zero-discharge and large fluctuations in discharge are expected in future hydropower operations due to changes in the electricity system, including greater reliance on solar and wind power, as well as increased variability in precipitation driven by climate change. In this study, several types of zero-flow periods were analyzed in a regulated northern river in Sweden. The results highlight different mitigation measures that may be suitable for reducing ecological impacts associated with hydropeaking. The study also evaluates potential improvements that could be achieved by implementing a mean annual low flow instead of zero flow. Overall, the findings demonstrate the value of conducting detailed river-specific analyses to identify effective ecological restoration measures in regulated river systems.

In this study, a three-dimensional hydrodynamic model is developed to investigate diurnal thermal dynamics induced by pumped hydropower storage operations. At this stage, the focus is on thermal mixing in a generic reservoir, with the aim of providing a methodology that can be adapted to various reservoir scenarios. Key issues include enhancing the understanding of how numerical grid resolution impacts modeling results and demonstrating a method for conducting mesh studies in standing water bodies influenced by flow fields, such as those generated by pumping. The model, being implemented in Delft3D FM, is designed to simulate the upper reservoir of a pumped hydropower plant under initial conditions of thermal stratification. A systematic mesh study was conducted by varying cell sizes in different directions to evaluate their influence on the modeling results. A Richardson analysis shows that the longitudinal resolution, along the main reservoir direction, has minimal impact, while the vertical and the lateral resolutions are critical to avoid thin layers in the mesh and prevent oscillations and numerical inaccuracies. The research demonstrates that pumped hydropower operations alter the thermal regime in the upper reservoir, leading to thinning and temperature fluctuations in the epilimnion, as well as weakening the thickness and strength of the thermocline. Additionally, these operations promote the formation of a large-scale recirculation zone. The adaptable model framework allows for changes in bathymetry, initial stratification conditions, and pumping scenarios, enabling new insights into general temperature dynamics and mixing patterns. 

Here presented is the development of digital twins for hydraulic modelling and sediment management of pumped hydropower storage within the Horizon Europe project Store2Hydro. Initially, themodelling of pumpedstorage hydropower in existing hydropower infrastructure wasperformed on generic casesin Work Package2 (WP2)of the projectin order to demonstrate the technology for a wide range of morphologic and hydraulic conditions representing different types of European rivers. The results show that morphologicparameters such asriver slope, cross-section shape and river size are important, not only for the storage capacity, but also for the local hydraulic conditionswhen virtually converting conventional hydropower plants to pumpedstorageplants.Simulations including the sediment dynamics also show thatcyclic pumping influences both sediment deposition/erosion patterns close to the power plant in the short term and the long-termbuild-upin the reach.These models willbe extended to real geometries and test-cases within WP5during the next two years, which will also includeenvironmental impact assessments.

This experimental work on discharge capacity investigates the flow distribution in a downscaled, multi-outlet experimental hydraulic rig. The water passing over each individual outlet is gathered and measured separately by collecting it into a large tank suspended on weight cells. This is repeated to ensure repeatability of the mass flow measurement method. To provide a large and robust set of data for CFD validation, six different inflows are tested, ranging from 60 l/s to 180 l/s. The findings show that for a larger inflow, and thus a higher water level, the discharge distribution over the spillway gets exacerbated by increasing the relative differences in the flow through each outlet. The differences in the measured water levels at different points in the channel leading up to the outlets also increase. ADV is used to measure velocities in the flow leading up to the channel for two of the six tested inflows. The ADV data shows persistent recirculation zones in different dimensions, as well as how the flow moving towards the outlets changes subtly as inflow is increased. Finally, comparisons are made to previous experiments on similar setups, indicating that a larger water column under the crest of a spillway reduces risk of uneven flow distribution for a spillway with multiple outlets. The data presented, along with dimensions of the model, can be used as a case study to validate how well different computational methods can predict flow distribution and spillway capacity.

Downstream migration of salmonid smolts through regulated rivers remains a major ecological and engineering challenge, with high mortality and delay rates despite mitigation measures like bypasses and guidance systems. This study integrates Computational Fluid Dynamics (CFD) with fish telemetry to analyze how salmon smolts respond to local hydraulic conditions in a real riverine environment. By coupling detailed CFD flow models with two-dimensional smolt track data from a hydropower facility in northern Sweden, we identified behavioral tendencies linked to specific flow velocities. The analysis of fish movement patterns indicates a general tendency to follow the main current during migration, with occasional variations influenced by initial velocity and local flow conditions. This behaviorally informed CFD–telemetry approach provides a method for identifying behavioral patterns based on velocities and demonstrates its potential to improve fish passage models, supporting more ecologically effective hydropower design. This study highlights the need for broader datasets to fully capture smolt behavior and to develop standardized, transferable modeling frameworks for fish–flow interactions.