| Flow over weirs with application to fish passage facilities | ||
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All the studied designs differed from each other both in the flow development and jet formation and location. Velocities in the jet from pool-and-weir fishway were mostly 10 to 20 times the mean velocity at the cross section, only in a few points did the velocities exceed that. The jet was wide and with high discharges, the flow was directed mildly sloping towards the bottom. With low discharge, the flow was directed strongly towards the bottom immediately below the weir.
For Denil fishway, 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. The jet from the Denil fishway was spread over the water surface, and the highest velocities were on the sides of the surface jet. Even the highest velocities below the entrance of a Denil fishway were not higher than 20 times the mean velocity at the cross section. Flow decay was very fast, and flow from a Denil fishway disappears on the surface.
For pool-and-weir fishway with a V-shaped sharp crested weir, velocities in the jet were higher in the jet than for pool-and-weir or Denil fishways, about 30 to 50 times the mean velocity. With higher discharge, the jet was directed towards the bottom, and for lower discharges the jet was directed into the middle of the water mass.
For a vertical slot fishway, the velocities in the jet were higher than for other designs, 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. The jet was directed into the middle of the water mass. In the studies, velocity distribution across the cross section was measured. The highest velocities were observed to be located directly downwards from the slot and almost in a straight line.
The differences in maximum velocities in different fishway types are due to the three dimensional character of the flow. Flow from pool-and-weir and Denil fishways is spread widely over the cross-section. The flow from pool-and-weir fishway with the V-shaped crest and from the vertical slot fishway is concentrated into a narrower range over the cross-section.
All of the models, except for the Denil fishway model, were about the same in scale. The scale of Denil fishway model was larger, which actually gives too good of results about how discernible the flow from the fishway is, both vertically and horizontally, as well as concerning the velocities.