Lakes are integral components of river networks, constraining hydrological, biological, and chemical processes at both the ecosystem and network scales (Gardner et al., 2019;Jones, 2010;Schmadel et al., 2018). However, research has often focused on rivers and lakes in isolation and not from an integrated perspective that reflects the intimate relationship between these systems (Gardner et al., 2019;Jones, 2010). In particular, there is a need to develop scaling relationships that describe the morphology of river-lake networks (Gardner et al., 2019). Such relationships are central to the up-scaling approaches that are widely used to generalize understanding aquatic patterns and processes at regional to global scales (Downing, 2009). Hydrological scaling relationships are primarily based on characteristics that are easy to quantify on maps, including lake surface area and river order (Downing, 2009;Strahler, 1957). For example, there are wellknown scaling relationships between river abundance, mean river segment length, and mean upstream contributing area relative to river order (Strahler, 1957). For lakes, abundance, perimeter, volume, and mean depth scale predictably with surface area (Cael et al., 2017;Kent & Wong, 1982;Seekell et al., 2013). Some characteristics of lakes on river networks have been assessed. For example, lake abundance decreases, lake size increases, and spacing between lakes increases as river order increases (Gardner et al., 2019). Globally, the abundance of river inlets varies among lakes by about three orders of magnitude, but this aspect of lake-river connectivity is not described by these existing scaling relationships (Mark, 1983).In this study, we describe a simple theoretical scaling relationship between lake surface area and the number of river inlets based on the principle of line intercepts of topographic features. We evaluate how lake and landscape characteristics effect river inlet abundance based on this theoretical scaling relationship. Finally,
The flow field inside and downstream of an open channel placed near the surface of a free flow (such as the tail water of a turbine) is characterized in detail. The channel cross-section is U-shaped and in the downstream end is placed a ramp on the bottom which accelerates the flow passing through the channel. This flow is intended to catch the attention of fish and improve their entrance to fishways, which has also been successfully demonstrated in field tests.The flow through the channel is subcritical and the ramp thus blocks some water from passing through. To find the optimum vertical position of the channel, a down-scaled model channel is placed in a water flume and flow fields in vertical planes directed along the flow are visualized using particle image velocimetry. Results show that increasing the depth over the ramp has little effect on the maximum velocity while it makes the accelerated water more perceptible downstream the channel. This will most likely improve the channel's ability to attract fish. It is also shown that a recirculation zone is formed in the channel for the small depths tested. Finally, it is shown that a modest tilt of the channel will not affect the flow field in any significant way.
Turbulent secondary flows are motions in the transverse plane, perpendicular to a main, axial flow. They are encountered in non-circular ducts and can, although the velocity is only of the order of 1–3% of the streamwise bulk velocity, affect the characteristics of the mean flow and the turbulent structure. In this work, the focus is on secondary flow in semi-circular ducts which has previously not been reported. Both numerical and experimental analyses are carried out with high accuracy. It is found that the secondary flow in semi-circular ducts consists of two pairs of counter rotating corner vortices, with a velocity in the range reported previously for related configurations. Agreement between simulation and experimental results are excellent when using a second moment closure turbulence model, and when taking the experimental and numerical uncertainty into account. New and unique results of the secondary flow in semi-circular ducts have been derived from verified simulations and validating laser-based experiments.
Simulation-driven design with computational fluid dynamics has been used to evaluate the flow downstream of a hydropower plant with regards to upstream migrating fish. Field measurements with an Acoustic Doppler Current Profiler were performed, and the measurements were used to validate the simulations. The measurements indicate a more unstable flow than the simulations, and the tailrace jet from the turbines is stronger in the simulations. A fishway entrance was included in the simulations, and the subsequent attraction water was evaluated for two positions and two angles of the entrance at different turbine discharges. Results show that both positions are viable and that a position where the flow from the fishway does not have to compete with the flow from the power plant will generate superior attraction water. Simulations were also performed for further downstream where the flow from the turbines meets the old river bed which is the current fish passage for upstream migrating fish. A modification of the old river bed was made in the model as one scenario to generate better attraction water. This considerably increases the attraction water although it cannot compete with the flow from the tailrace tunnel.
A flow device that accelerates turbine tail water (or any free stream) to act as an attraction for migrating fish is field tested. The device consists of an open (U-shaped) channel which accelerates the incoming flow by a local constriction of the cross-sectional area. The velocity increase has previously been investigated in a lab-scale model and an increase of 38% has been established. In the summers of 2004 and 2005, a full-scale prototype of the attraction channel was tested at the Sikfors hydropower plant in the Pite River in Sweden. The channel was equipped with underwater cameras to monitor and record the fish swimming through it. The tests show that the fish do swim through the attraction channel. During the same time period in 2004 and 2005, 57 and 471 fishes swam through the channel, respectively. The major change of the channel between the two years was that it was painted black for 2005.
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