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Sedimentation is one of the main eco-morphological and technological challenges associated with reservoirs. Sedimentation not only reduces the functional capacity of a reservoir by filling it, but also changes downstream sediment dynamics and habitat availability for the aquatic biota. Additionally, dams hinder free bi-directional fish passage, emerging as a major threat to species of migratory fish. In the past decades, mitigation measures aimed at reducing such environmental and technological impacts have been developed. Sediment bypass tunnels (SBTs) have been shown to successfully help prevent reservoir sedimentation, whereas fish passages have been found to be potential solutions to facilitate bi-directional passage of fish. However, the construction of such structures, in particular of SBT, can be extremely costly. The development of design solutions that can function both for downstream sediment transport and up- and downstream fish passage should be considered as they can mitigate ecological deficiencies of reservoir operations while accounting for economic feasibility. Possibilities and challenges of combining SBT and fish passage were explored by bringing together a team of interdisciplinary specialists on hydraulics, sediment transport and continuity, bypassing, hydraulic structures, hydropower engineering, aquatic biology, and fish passage in a two-day workshop. Here, we present potential solutions identified during the workshop for integrating SBT and fish passage. [ABSTRACT FROM AUTHOR]

The current geometric designs of fish guidance structures with vertical bars for run-of-river hydropower plants result in high head losses and asymmetric turbine admission flow. To address these issues, we develop innovative curved bar designs and experimentally investigate different rack configurations with curved bars in a laboratory flume. The present paper (Part II) focuses on the hydraulic performance of the novel curved bar designs with regard to flow fields, while the companion paper (Part I) reports the results on the head losses. The detailed flow fields obtained by 3D velocity measurements show that the curved bars promote flow conditions favourable for both fish guidance and turbine operation. The flow straightening effect of the curved bars leads to quasi-symmetrical turbine admission flow and reduced head losses. The findings are discussed with regard to fish protection and guidance, and optimal engineering application. [ABSTRACT FROM AUTHOR]

The hydraulic performance of fish guidance structures such as louvres and angled bar racks is an important design criterion to achieve high fish guidance efficiencies with a minimum impact on hydropower production. The current geometrical designs of such structures result in high head losses and asymmetric turbine admission flow and hence need to be optimized. We therefore developed an innovative curved bar design for fish guidance structures and experimentally investigated different rack configurations in a laboratory flume. The present paper (Part I) focuses on the hydraulic performance of the novel curved-bar racks with regard to head losses, while in the companion paper (Part II) we report the results on the flow fields. The effects of the main rack angle, bar angle, bar spacing, bar depth, and top or/and bottom overlays were systematically studied. The curved bar shape leads to significantly lower head losses compared to straight bar shapes. An existing head loss prediction formula was extended to account for the new bar shape and the application of overlays. [ABSTRACT FROM AUTHOR]

Unstructured block ramps are fish-friendly structures to stabilize a riverbed, characterized by large macroroughness elements randomly placed on the riverbed. Physical model tests were conducted to investigate their stability and their behavior in case of overload. Different parameters describing the block and the base material were tested to find an optimal combination in terms of ramp stability with a maximum ramp slope typically in the range of 1-3%. A model for the determination of the ramp stability is presented, where the equilibrium slope of the ramp is related to a dimensionless specific discharge including information on the block size, the block placement density, and a characteristic grain size of the base material. [ABSTRACT FROM AUTHOR]

Unstructured block ramps (UBR) are fish-friendly structures to stabilize a river bed. They are increasingly used to replace drops and represent large macro-roughness elements randomly placed on the river bed. Laser Doppler Anemometry measurements were conducted to determine the mean flow characteristics in a laboratory channel covered by block ramps. The resulting local time-averaged flow quantities allow for considerations of the flow field heterogeneity and for the identification of migration corridors for a certain target fish. Double-averaging (in time and in space) was applied to characterize the flow conditions taking into account the strongly heterogeneous bed of an unstructured block ramp. By applying this method, the characteristic vertical profiles describing the general flow conditions are presented. Based on geometrical and physical considerations, a suggestion is made to sub-divide the interfacial sublayer of macro-rough beds into upper and lower parts. The aim of this research is to present not only the flow field, but also to highlight the ecological value of UBR. [ABSTRACT FROM AUTHOR]

The article reflects on downstream fish migration by experimentally studying both inclined and angled fish-friendly trashracks. Topics discussed include concepts of trashrack flow hydraulics, investigations of inclined trashracks on the head-loss and the velocity distributions and the effects of vertical (slats) and horizontal (cylinders) elements not being included in the overall trashrack blockage ratio.

During their life cycle, fish carry out distinct movements within rivers and migrate upstream and downstream to reproduce, to feed, and to shelter in refuge habitats. During downstream movements, they can incur severe injuries that may be lethal directly or indirectly over time when passing through hydropower plants or when being entrained at other water intakes. Horizontal bar rack bypass systems are a state-of-the-art technology to protect and guide downstream moving fish towards a reasonably safe corridor around water intakes. They have been in operation at multiple hydropower plants for more than a decade, but only little is known about the potential fish protection and guidance efficiencies and the fine scale reactions of different fish species when encountering such racks. To resolve this, systematic live fish laboratory tests were conducted under various hydraulic conditions involving a diverse assemblage of riverine fish species differing in their swimming behavior and morphology. Six riverine species, namely spirlin (Alburnoides bipunctatus), barbel (Barbus barbus), nase (Chondrostoma nasus), brown trout (Salmo trutta fario), Atlantic salmon (Salmo salar), and European eel (Anguilla anguilla) were tested with a rack consisting of foil-shaped bars, clear bar spacings of 15 and 20 mm, a horizontal rack angle of 30° to the flow direction, and a full depth open channel bypass. Variations in fish behavior were observed between different species and hydraulic conditions, but the results suggest that the guidance and protection efficiencies primarily depend on the ratio of the fish width to the clear bar spacing. Larger fish were well protected by the horizontal bar rack, while smaller fish frequently passed through the rack. New equations are proposed to estimate the protection and guidance efficiencies as a function of the clear bar spacing and the fish species' biometry, which is highly relevant to assess the effect of horizontal bar racks as fish protection measures prior to installation. [ABSTRACT FROM AUTHOR]