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Desanding facilities (DFs) limit particle sizes and reduce the suspended sediment load in hydropower plants. Multiple successive tranquilizing racks (TRs) at settling chamber inlets are a cost-efficient structural measure to homogenize the flow and reduce local velocities and turbulence in the chambers. We summarized the state of the art of TRs and conducted detailed flow velocity measurements in the settling chambers of a DF with TRs. In the first campaign, a pronounced flow concentration beneath the vertical bars of the TRs was found, resulting in flow velocities along the invert of up to 3.5 times the average cross-sectional velocity. Because this impairs the trapping efficiency, the TR bars were extended downward to achieve a more even distribution of the flow resistance over the entire flow section. A subsequent measurement campaign confirmed a more homogeneous flow field and a considerable reduction of the turbulence intensities and turbulent kinetic energy, which were the lowest compared to six other DFs. The results highlight the potential to modify existing TRs at other DFs with inhomogeneous flow fields and the practical importance of TR design for new DFs. Practical Applications: In hydropower plants (HPPs) with water intakes located at sediment-laden rivers, DFs are crucial for reducing the sediment load in the turbine water, consequently minimizing wear on the turbines. A DF usually consists of one or several chambers through which the water flows slowly, allowing sand particles to settle. However, for geometrical, hydraulic, and economic reasons, the flow at the chamber inlet is often not favorable for settling. In such situations, TRs can be installed to slow down and distribute the inflow more evenly. Because no general design guidelines for TRs are available, we reviewed the state of the art based on literature and present the field investigation of flow fields in a DF with TRs. We found an unfavorable flow concentration and modified the TRs to cover the whole flow section. This resulted in more favorable conditions for settling, also in comparison with other DFs. In DFs with inhomogeneous inflow conditions, suitably designed TRs enhance the trapping efficiency and contribute hence to the cost- and energy-efficient use of the hydropower potential at sediment-laden rivers. [ABSTRACT FROM AUTHOR]

Introduction: Sediment bypass tunnels (SBTs) are operated during floods to reestablish sediment continuity past dams with the ultimate goal of decreasing accumulation in reservoirs. Depending on the location of the intake structure and operational conditions, SBTs may release either bed load-laden or bed load-free flows in the form of sudden floods (pulses). When evaluating the impacts of SBTs, the traditional approach has been to monitor channel changes in the years following project completion. However, by the time these impacts become evident, mitigation is challenging. Methods: This research adopts a forward-thinking methodology, emphasizing the prediction of potential impacts during the project's early planning stages. We use a one-dimensional morphodynamic model to forecast the potential morphological response of a gravel-bed river with an idealized geometry to a series of SBT operations characteristic of projects in the Swiss Alps. Results: The morphological response of the downstream reach over the medium to long term is influenced by the alternating pattern of bed load-laden and bed load-free flow releases. This pattern is instrumental in forming a transition region near the SBT outlet hydrograph boundary layer (HBL). Discussion: Within the (HBL), fluctuations in bed elevation and slope are localized, while variations in grain-size distribution persist throughout the downstream region. [ABSTRACT FROM AUTHOR]

During flood events, river topography and fluvial habitats can change drastically, potentially affecting the ecological status. In case of multiple floods, whether each single event modifies the habitat characteristics in the same direction or not, is still an open question. We gathered high quality topographical data of one braided Alpine reach before, between and after multiple floods. Considering the full dynamics of the hydrological regime affected by hydropower production, we calculated water depth and flow velocities distributions for relevant discharge conditions using hydrodynamic modelling. We then calculated four ecological indicators related to habitat diversity, habitat quantity, habitat connectivity and stranding risk. Despite the consistent depositional morphological trend, the habitat diversity and stranding metrics returned to pre-floods values after an initial deviation. The habitat quantity and connectivity metrics did not show a clear trend towards an alternative state. Habitat prevalence varied seasonally and with hydropower water release, and also changed markedly between floods, possibly affecting species composition. We show the possible intrinsic variability in several ecological indicators which can aid in the management and restoration of river floodplains. [ABSTRACT FROM AUTHOR]

The authors developed a novel design equation based on numerical model results by taking into account the effects of water velocity and residual tunnel cross-sectional area above the water surface. The discussers acknowledge the presented approach, but also express their concerns regarding the following points: (i) the validation of the numerical model is superficial as it is based on bulk parameters only and ignores known model limitations; (ii) effects of the air vent characteristics are not considered in the numerical model and consequently neither in the presented design approach; (iii) air entrainment is not considered in the authors' approach but may be important for Froude numbers above 6; and (iv) the general applicability of the design equation due to the small range of considered Froude numbers and the negligence of air vent characteristics which have been shown to significantly impact air demand in outlet tunnels. This discussion aims at clarifying some facts not clearly presented by the authors and identifying limitations of the authors' design equation. Overall, we highlight that the approach describes the unrestricted air discharge and should be limited to the parameter range and assumptions considered for its derivation, i.e. for large air vent parameters (A* > 0.1) and flows with Froude numbers below 10. [ABSTRACT FROM AUTHOR]

Numerical hydrodynamic models enable the simulation of hydraulic conditions under various scenarios and are thus suitable tools for hydropeaking related assessments. However, the choice of the necessary model complexity and the consequences of modelling choices are not trivial and only few guidelines exist. In this study, we systematically evaluate numerical one‐dimensional (1D) and two‐dimensional (2D) hydrodynamic models with varying spatial resolution regarding their suitability as input for hydropeaking‐sensitive, ecologically relevant hydraulic parameters (ERHPs), and their computational efficiency. The considered ERHPs include the vertical dewatering velocity, the wetted area variation between base and peak flow and the bed shear stress as a proxy for macroinvertebrate drift. Furthermore, we quantified the habitat suitability of brown trout for different life stages. The evaluation is conducted for three channel planforms with morphological characteristics representative for regulated Alpine Rivers, ranging from alternating bars to a braiding river morphology. For the prediction of habitat suitability and bed shear stress, a 1D model appears to be always insufficient, and a highly resolved 2D model is suggested. Reducing the spatial resolution of 2D models leads to computational efficiency similar to 1D, while providing more accurate results. Thus, our results suggest, that while a highly resolved 1D model is sufficient for accurate predictions of the dewatering velocity and wetted area in the less complex alternating bar morphology, a 2D model is recommended for more complex wandering or braiding morphologies. This study can serve as guideline for researchers and practitioners in the selection and setup of hydrodynamic models for hydropeaking. [ABSTRACT FROM AUTHOR]

Large wood (LW) in rivers increases the flow variability and provides habitats for various species. During flood events, transported logs can accumulate at river infrastructures and increase the flood hazard. LW accumulations result in an upstream backwater rise and may increase local scour, for instance at bridge piers. Consequently, estimates of the resulting backwater rise and local scour are necessary to improve the flood hazard assessment. This study presents the findings of flume experiments with a movable bed on local scour and backwater rise due to LW accumulations. The approach flow conditions and the bed material were varied systematically for a specific LW accumulation volume. For all experiments, the initial condition for the bed material was defined as weak transport, since the bed shear stress was slightly below the critical bed shear stress for incipient motion. The inflow Froude number was identified as the governing parameter for backwater rise due to LW accumulations. The present study confirms the hypothesis that the resulting local scour reduces backwater rise. For the local scour, the unit discharge and the grain size diameter are the decisive parameters.

River widening is a common restoration approach to mitigate the adverse effects of past stream alterations on infrastructure and the riparian ecosystem by stabilizing the river bed and enhancing habitat heterogeneity. In this study, two river widening approaches, excavated and dynamic, are described for the case of moderately steep gravel-bed rivers in the Alpine foothills, with a focus on dynamic river widening. As most channelized rivers exhibit ongoing degradation due to the lack of sediment supply and efforts to restore sediment transport are increasing, the consideration of the response of river widenings to variable sediment supply is important. For this purpose, insights from regime theory are applied to river widening and several experimental flume and field studies on channel response to variable sediment supply are reviewed. Dynamic river widenings are expected to be morphologically active in weakly degraded rivers with sufficient sediment supply, while they may not be an appropriate restoration approach for highly degraded rivers due to persistent impairment of morphological activity.

Sediment transport in glacier basins and rivers, and hence reservoir sedimentation tend to increase under the impact of climate change. The raise of bedload transport rates results in an increase of hydro-abrasive erosion leading to bedrock incision in rivers and wear at hydraulic structures. Mechanistic abrasion models are a helpful tool for both river and landscape evolution and hydroabrasion of hydraulic structures. Therefore, knowledge of the physical processes of turbulent flow characteristics, bedload particle motion, and hydroabrasion and their interrelations is needed. Ongoing research at VAW of ETH Zurich aims at improving the required knowledge to address hydroabrasion issues by means of laboratory and prototype experiments in supercritical flows over fixed smooth and transitionally rough beds. A physical model investigation revealed that particle trajectories on fixed planar beds are rather symmetric, flat and long compared to alluvial beds. By applying newly developed particle motion equations, an existing mechanistic abrasion model was adapted and calibrated for concrete and natural rock by means of prototype data. This advances abrasion prediction modelling and contributes to a better understanding of river bed and landscape evolution and to a sustainable design and operation of hydraulic structures facing severe hydro-abrasive erosion.