To achieve the targets of the of the European Water Framework Directive and national energy acts, amongst others, downstream fish migration connectivities and a sufficient protection of fish populations are needed at water bodies. The installations of trash racks with horizontal bars that are arranged at an angle to the direction of the flow – so-called inclined screens – are seen as a solution at existing and new hydropower plants. These inclined screens shall not only protect the fishes migrating downstream from a passage through the turbine chamber but also lead them into the direction of a laterally positioned bypass. Even though there are several publications and applications of limiting values for different frame conditions, until now, no scientifically sound researches and ethohydraulic evaluations with inclined screens are available. Furthermore, the hydraulic losses, generated by the inclined screens, so-called screen losses, are unknown so far. The intention of the present thesis is to close knowledge gaps in terms of inclined screens. Therefore, hydrometric measurements for the evaluation of screen losses were taken and compared with the calculated results of known screen-loss formulas. Stream signatures within the close range of the inclined screen were identified and are shown as 2D- and 3D-graphics. Finally, ethohydraulic studies with salmon smolts and eels were conducted at different inclined screens with various geometric and hydraulic conditions. In addition to these ethohydraulic studies, the impact of an electrification of inclined screens was investigated in the studies with eels. All measurements and studies were conducted at the hydraulic research laboratory of Technische Universität Darmstadt, as well as at an inclined screen of the hydropower plant Ottenau at river Murg and at a temporary electric deterrent system at the hydropower plant Rotenfels, also at river Murg.

The results of the comparison of hydrometrically measured and calculated hydraulic losses at inclined screens demonstrated that there is no possibility to apply the known formulas for inclined screens. Due to the orientation of the screen bars, there was a different inflow caused by the inclined screen as compared to screens with vertical bars, which are perpendicular flowed by the stream. These inflows could not be described by means of the known loss formulas. The results showed a rise of losses with increasing flow velocity and with decreasing bar clearance between the screen bars. The inclined orientation of the screen also had an impact on the losses: the more inclined the screen is installed, the lower are the losses. The graphics in this thesis can be used for the evaluation of inclined screen losses.

The measuring of the stream signatures showed non-homogeneous flows at the investigated inclined screen. Amongst others, these flows are generated due to the construction of the screen with several pillars and supporting structures, silting because of floating debris, as well as due to a lateral inflow of the bypass. The velocities increased in direction of the bypass, floating debris at the bottom, baffles or screen cleaners caused the development of slow-flow sections. Flow peaks were generated at the rims of screen cleaners. The stream signatures harmonized again towards upstream.

The ethohydraulic studies showed that at high velocities, fishes often remained in slow-flow sections. Even though this reaction did not avoid a downstream fish migration, it decelerated it. In the studies, salmon smolts showed a behavior that favored a downstream fish migration and that could be supported using a suitable construction of the inclined screen. The detection of the bypass by the eels was merely determined at random so that a guiding effect was observed only rarely or not at all. A screen passage of eels through the bar clearances must thus be avoided by means of a suitable screen construction. Compared to investigations on conventional screens with vertical bars without an inclined streaming eels that passed through the bar clearances of the investigated inclined screens were monitored only rarely. Electrification reduced the number of screen passages of eels at screens with large bar clearances. It also reduced the lingering of eels at the screen. Hence, the electrification can reduce the eel mortality at screens. Fishes which were pressed against the screen due to a high water pressure was only observed in an insignificant number. The results of the field studies showed a high protective function of the investigated screen, but an inadequate protective function of the electric deterrent system for eels. The results of the ethohydraulic studies demonstrated that inclined screens do not only act as a purely mechanical barrier, but in fact also as a behavioral barrier. New thresholds for planning and design were established in this thesis. A combination of inclined screens and electrification – a so-called hybrid screen – can be a solution for an effective fish protection structure at hydropower plant at big rivers.