5.2. Velocity distribution below different fishway types

Velocity distribution below fishways is three dimensional, but in order to simplify the results, here only the velocity distribution at the mid-axis below the opening has been showed. Flow decay was visualized by dye and in addition, velocity measurements were made below each fishway entrance. The flow conditions below the fishway types studied differed from each other in both flow direction and the velocity distribution, and in the magnitude of the highest dimensionless velocity.

At the entrance of the pool-and-weir fishway, two different flow patterns could be distinguished depending on the discharge rate. With a very low discharge, the jet plunges immediately to the bottom, and vanishes there (Fig. 22 a). With high discharges, the flow from the fishway is directed away from the dam, mildly sloping towards the bottom, and the jet reaches the bottom only after a certain distance from the dam (Fig. 22 b). Velocities in the jet from pool-and-weir fishway were mostly 10 to 20 times the mean velocity at the cross section, and only in a limited area did the velocities exceed that (Fig. 23 a, b).

For pool-and-weir fishway with a V-shaped sharp crested weir for low discharge, the jet was directed into the middle of the water mass (Fig. 22 c). With high discharge, the jet was directed towards the bottom (Fig. 22 d). Velocities in the jet were about 30 to 50 times the mean velocity (Fig. 23 c, d).

For a vertical slot fishway the jet was directed into the middle of the water mass (Fig. 22 e). In the studies, velocity distribution across the cross section was measured. The highest velocities located directly downwards from the slot and almost in a straight line. The velocities in the jet were about 40 to 50 times the mean velocity at the cross section, with the highest velocities being as much as 80 times the mean velocity at the cross section (Fig. 23 e). Experiments for vertical slot fishway were carried out with only one discharge, as flow patterns in vertical slot fishways are independent of the flow depth, excluding very high or extremely low flow depths.

For Denil fishway, it seems apparent that the flow from the fishway disappears on the surface (Fig. 22 f). As the highest velocities are not situated along the centerline but on the sides in Denil fishways, velocity distribution across the cross section was measured. The jet spread over the water surface, and the highest velocities were on the sides of the jet. Flow decay was fast, and the differences in the velocities inside the jet were only 10 to 20 times the mean velocity at the cross section, independent of the discharge (Fig. 23 f). Due to the experimental arrangements, with the highest discharge (Q = 16.20 l/s) the jet from the fishway was so wide that the flow was affected by the walls. Thus no velocity measurements were made. However, it was observed in flow visualization that even in this case the jet was directed up to the surface. Even with drawdown effect, the jet was directed upwards to the surface. As in Denil fishways the highest velocities are located in the sides, dye was injected into the sides of the free opening immediately in the water outlet under a drawdown situation, and still the jet was up to the surface. Because of the colored water and high velocities, no photograps could be taken of the dye experiments with the highest discharge.