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.

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.

Climate change is projected to significantly alter hydrological conditions across the Northern Hemisphere, with increased precipitation variability, more intense rainfall events, and earlier, rain-driven spring floods in regions like northern Sweden. These changes will affect both natural ecosystems and hydropower-regulated rivers, particularly during ecologically sensitive periods such as the grayling spawning season in late spring. This study examines the impact of extreme spring flow conditions on grayling spawning habitats by analyzing historical runoff data and simulating high-flow events using a 2D hydraulic model in Delft3D FM. Results show that previously suitable spawning areas became too deep or experienced flow velocities beyond ecological thresholds, rendering them unsuitable. These hydrodynamic shifts could have cascading effects on aquatic vegetation and food availability, ultimately threatening the survival and reproductive success of grayling populations. The findings underscore the importance of integrating ecological considerations into future water management and hydropower operation strategies in the face of climate-driven flow variability.

Renewable energy sources such as hydropower are important to reduce the global emissions. Hydropower, however, comes with other environmental challenges by altering the ecological conditions in the rivers. Hydraulic models connected with fish habitat models could be one tool to assess the environmental impacts and evaluate mitigation measures for fish habitats. This study examines the limitations of steady-state hydraulic simulations in a low-sloping river located between two hydropower plants, where downstream regulations significantly influence the river flow dynamics. A 2D hydrodynamic model in Delft3D FM was applied to compare steady-state and transient simulations, focusing on how hydraulic variables affect the spawning habitat. The results show that steady-state models fail to capture time-dependent damping and delayed water level responses, leading to systematic underestimation of hydraulic variability. Peak bed shear stress values were under-predicted by the steady-state interpolation, which may under-predict spawning ground stability. Additionally, the steady-state approach failed to capture daily habitat fluctuations, resulting in a mean absolute error of 2910 m2 in spawning habitat area per hour. This study demonstrates how errors in hydraulic calculations propagate into habitat assessments, potentially leading to misleading long-term evaluations of fish populations. This study highlights the importance of selecting appropriate hydraulic modelling approaches based on river-specific flow dynamics. Future studies should investigate the sensitivity of fish habitat models to hydraulic inputs from steady-state and transient simulations by integrating these approaches into advanced fish modelling tools, such as individual-based models. This will help determine the optimal balance between computational efficiency and accuracy in long-term habitat assessments.

Shallow waterways such as rapids, tributaries and smaller streams can have important ecological functions in both free-flowing and regulated rivers. As more intermittent renewable energy is introduced to the energy system to reduce CO2 emissions, the operational conditions of hydropower plants are changing. This implies various flow scenarios that can lead to more locations with shallow depths and larger variations in water levels and velocities, resulting in increased impact on the riverine ecosystem. Accurate predictions of these impacts require an understanding of the flow dynamics near large roughness elements such as boulders or trees in shallow river regions. This study uniquely investigates the effect of relative submergence, i.e., water depth relative to boulder size, on the flow field, turbulence, and potential fish habitats around idealized stone shapes (hemispheres) in shallow open channel flow using time-resolved 3D particle tracking velocimetry. The results indicate that varying relative submergence significantly affects recirculation zones, velocity and vorticity distribution, as well as turbulent kinetic energy. Notably, larger regions of lower velocity downstream of the roughness elements were generated at lower submergences, which might be favorable for fish energy conservation. Valuable insights into ecohydraulic engineering and habitat restoration in shallow waterways can be gained by understanding the fundamental flow mechanisms at low submergence for the flow around large roughness elements.

The increasing integration of fluctuating renewable energy into our energy systems has created a high demand for efficient electricity storage solutions, aiming to meet demand under intermittent supply conditions and maintain grid stability. Pumped hydropower storage (PHS) can be seen as a key solution, already accounting for the majority of the electricity storage today. However, the impact of PHS on reservoir ecology remains poorly explored, with open questions regarding how the physics within the reservoir is affected by pumping. Thermal stratification is a main factor influencing the ecological state of a lake ecosystem. To exemplify, stratification is a decisive factor for both nutrient recycling and deoxygenation. Therefore, this numerical study investigates the influence of PHS on thermal stratification in reservoirs. A 3D hydrodynamic model of a generic PHS reservoir is used to test how different types of stratification phenology are disrupted by a diel pumping cycle. The study demonstrates that thermocline phenology may influence the ecological effects of PHS operations by determining the extent of vertical mixing and thereby nutrient redistribution and oxygen availability. Scenarios with a well-established thermocline exhibit different responses to pumping compared to weakly stratified conditions, leading to variations in thermal structure and potential biological impacts. This adds to the comparability of existing case studies and improves understanding of the thermal dynamics in hydropower reservoirs, necessary to optimize PHS operations as to the status of the ecosystem and enhance water quality management.

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.

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.

Hydropower and dam structures worldwide are facing evolving requirements due to changes in climate, better methods for flood estimates, combined with the needs of surrounding interests. Improved understanding of the hydraulic behavior of spillways, and the approach flow leading up to them, is important for evaluation of existing spillways and considering potential redesigns. There is limited research on the distribution of flow across a multiple outlet spillway, therefore a purpose built experimental setup is utilized to examine the impact of various geometrical changes on the flow distribution across a spillway with three outlets. The maximum difference measured between the different outlets were as much as 10%. While small changes to abutment and pier corners were found to reduce total discharge capacity up to 8%, with increased discharge and overflow height causing greater reduction in the capacity of the spillway. To further investigate the flow behavior leading up to the spillway outlets, ADV measurements were conducted to capture flow velocities. The measured flow cross sections indicate a stable flow field leading away from the inlet, stagnation zones and recirculation zones leading up to the spillway, with minor variations occurring for increasing inlet flow rates.

This work examines the relationships between flow restoration, natural flow regimes and riparian vegetation utilizing a case study in northern Sweden. The riparian zone is one of the most species-rich ecosystems and forms an important link between aquatic and terrestrial systems. The integrity of the riparian zone and its vegetation is harmed by flow alteration in numerous rivers, which calls for enhancing riparian management and ecological mitigation measures. Here, we take an ecohydraulic approach, combining a hydraulic model and data on inundation tolerance of riparian vegetation to evaluate the effects of reintroducing seasonal flow variation in a bypassed reach with minimum discharge. The results show that implementing the seasonal flow variation is projected to benefit riparian vegetation by extending the riparian zone and to lead to the development of distinct vegetation belts similar to riparian vegetation along free-flowing rivers. Additional simulations demonstrated that a further increase in riparian area could be achieved by increasing the magnitude of the minimum flow release. While the method assumes the riparian vegetation to be in equilibrium with the flow regime, continued monitoring is needed to assess how fast the riparian vegetation adjusts to new flow conditions.

Measurements of mass flow through a three-outlet spillway modeled after a scaled-down spillway were conducted. The inlet and channel leading up to the outlets were placed to lead the water toward the outlet at an angle. With this, measurements of the water level at three locations were recorded by magnetostrictive sensors. The volumetric flow rates for each individual outlet were recorded separately to study the differences between them. Additionally, Acoustic Doppler Velocimetry was used to measure water velocities close to the outlets. The conditions changed were the inlet volume flow rate and the flow distribution was measured at 90, 100, 110, and 200 L per second. Differences between the outlets were mostly within the error margin of the instruments used in the experiments with larger differences shown for the 200 L test. The results produced together with a CAD model of the setup can be used for verification of CFD methods. A simulation with the k-epsilon turbulence model is included and compared to earlier experiments and the new experimental results. Larger differences are seen in the new experiments. Differing inlet conditions are assumed as the principal cause for the differences seen.

Hydropower is an important tool in the struggle for low‐emission power production. In the Nordic countries, hydropower operating conditions are expected to change and work more in conjunction with intermittent power production. This in turn might increase the amount of hydropeaking events in the reaches downstream of hydropower plants. The current work investigates the influence of highly flexible, high‐frequency hydropeaking on the hydrodynamics in the downstream reach. By quantifying four different dynamic stages in the study reach, the influence of the hydropeaking frequencies was investigated in the bypass reach of the Stornorrfors hydropower plant in the river Umeälven in northern Sweden. The hydrodynamics in the study reach were numerically modelled using the open source solver Delft3D. Eight different highly flexible future hydropeaking scenarios, varying from 12 to 60 flow changes per day, were considered. A method for identifying four hydropeaking stages— dewatering , dynamic , alternating and uniform —was introduced. The hydropeaking frequency directly decided the stage in most of the study reach. Furthermore, a Fourier analysis showed a significant difference between the stages and their corresponding power spectra. The classification of stages put forward in this work provides a novel, simple method to investigate the hydrodynamics due to hydropeaking in a river reach.

In the near future, the operating conditions of hydropower plants in the Nordics are expected to change, in order complement expansion of intermittent power production such as solar- and wind-power. This in turn might increase the amount and frequency of short-term flow regulations events, so called hydropeaking. In recent years, hydropower's effect on local ecosystems has drawn much attention. Especially hydropeaking has been shown to affect the ecosystems and induce morphological changes in the reach downstream hydropower plants. In order to assess the potential impact of a change in operating conditions, different numerical models can be used to examine the downstream reach when subject to potential future operating conditions. In-flow parameters, such as depth and velocity, can be predicted by using numerical approximations of the Navier-Stokes equations in either one-, two- or three-dimensions. These modeling approaches have different advantages and disadvantages. In this work the uncertainties that arise naturally from field measurements and numerical modelling, and how it might affect in-flow parameters, specifically variables frequently used in ecohydraulics, with special emphasis on 2D models was investigated. The flow in a regulated reach was here modelled with the open-source hydrodynamics solver Delft-3D. A stretch of the river Umealven in northern Sweden was modelled based on bathymetry measurements from a multibeam sonar. The model was calibrated with measured water levels and validated with ADCP velocity measurements. The results show that the model is significantly dependent on the calibrated roughness. Additionally, it is shown that 2D models might be insufficient for some ecohydraulical applications. Lastly, a discussion on the merits of one-, two and three-dimensional models is presented.

Hydraulic modelling can be an important tool to assess ecological status of rivers and to evaluate where and how measures should be implemented to maximize their impact. This is becoming increasingly important in regulated rivers since hydropower's ability to balance intermittent electricity sources such as wind- and solar power is resulting in more frequent starts and stops of the power plants, which in turn is affecting the local environmental conditions. The resulting flow fields from the modelling can, for instance, be used to classify biologically important areas in rivers. Several relevant flow parameters can be predicted and applied, e.g depth and water velocities can be used to estimate habitat for specific fish species or the variation in water levels can be used to evaluate the risk of stranding for fish in different life stages. This work specifically involves numerical modelling of a heavily regulated reach in the Lule River in northern Sweden. Models are created in 1D, 2D and 3D to show strengths and weaknesses in the different modelling techniques. To ensure that the models capture reality, measurements of water levels and temperatures in the reach are performed using pressure/temperature loggers for validation purposes. River velocities are also measured with an Acoustic Doppler Current Profiler which are mainly used to validate the 3D model. The results derived using the different modelling methods are all shown to be useful depending on relevant application.

Hydropower plant operating conditions are expected to change to be more in tandem with intermittent power production so as to meet the requirements of the Paris Agreement, which in turn may negatively impact ecological conditions downstream of the hydropower plants. The current study investigates how highly flexible hydropower operating conditions may impact several salmonid species (European grayling, Atlantic salmon and brown trout) in the River Umeälven, a major river in northern Sweden; specifically, how changes in hydropeaking frequency may affect the area of the downstream watercourse that is hydraulically suitable for spawning (potential spawning area) and how changes in spill gate closing time may affect the propensity to stranding. River hydrodynamics were modeled using the open-source solver Delft3D, with a range of hydropeaking frequencies (from 10 to 60 starts and stops per day) and a range of spill gate closing times from (1–30 min). Increasing the hydropeaking frequency caused a reduction in potential spawning area, but also a reduction in dewatering of potential spawning area at low flows. Increasing spill gate closing time caused a decrease in propensity to stranding. Effects were dependent on both species and life-stage, and declined longitudinally with distance downstream from the spillway outlet. The modelling approach used here provides an effective method for predicting likely outcomes of flexible hydropower operating conditions, taking into account fish species and life-stages present and watercourse characteristics.

The ability to sense changes in oxygen availability is fundamentally important for the survival of all aerobic organisms. However, cellular oxygen sensing mechanisms and pathologies remain incompletely understood and studies of acute oxygen sensing, in particular, have produced inconsistent results. Current methods cannot simultaneously measure the key cellular events in acute hypoxia (i.e., changes in redox state, electrophysiological properties, and mechanical responses) at controlled partial pressures of oxygen (pO2 ). The lack of such a comprehensive method essentially contributes to the discrepancies in the field. A sealed microfluidic system that combines i) Raman spectroscopy, ii) patch-clamp electrophysiology, and iii) live-cell imaging under precisely controlled pO2 have therefore been developed. Merging these modalities allows label-free and simultaneous observation of oxygen-dependent alterations in multiple cellular redox couples, membrane potential, and cellular contraction. This technique is adaptable to any cell type and allows in-depth insight into acute oxygen sensing processes underlying various physiologic and pathologic conditions.

The operating conditions of Nordic hydropower plants are expected to change in the coming years to work more in conjunction with intermittent power production, causing more frequent hydropeaking events. Hydropeaking has been shown to be detrimental to wildlife in the river reaches downstream of hydropower plants. In this work, we investigate how different possible future hydropeaking scenarios affect the water surface elevation dynamics in a bypass reach in the Ume River in northern Sweden. The river dynamics has been modeled using the open-source solver Delft3D. The numerical model was validated and calibrated with water-surface-elevation measurements. A hysteresis effect on the water surface elevation, varying with the downstream distance from the spillways, was seen in both the simulated and the measured data. Increasing the hydropeaking rate is shown to dampen the variation in water surface elevation and wetted area in the most downstream parts of the reach, which could have positive effects on habitat and bed stability compared to slower rates in that region.