Chapter 5. Hydraulic conditions at the water outlet of fish passage facilities
- Table of Contents
- 5.1. Considerations of the scaling factors
- 5.2. Velocity distribution below different fishway types
- 5.3. Flow conditions below fishway entrances
In fishways, one of the most crucial places is the entrance. First of all, it must be properly situated, and secondly, it must operate in a way that attracts fish. There is a great variety in the successful fishway designs that are used for species with similar swimming capabilities (Orsborn 1986). In addition, different salmonids have unique requirements for water flow and velocity for optimal entry and passage.
At the Isohaara fishway, Northern Finland, the number of large salmon entering the fishway increased after the entrance was changed from a Denil fishway into a pool-and-weir fishway. To understand the difference in hydraulic conditions below the fishway before and after the change, a study was carried out at the hydraulics laboratory at the University of Oulu, Finland. Flow into standing water with no backwater or drawdown effect was chosen for further studies. In these studies, flow decay and velocity distribution below the entrance was studied for pool-and-weir fishways with horizontal and V-shaped weir crest and for vertical slot and Denil fishways.
5.1. Considerations of the scaling factors
When studying the backwater or drawdown effect with different discharges, the previously used custom of scaling by the free opening can be used. The disadvantage with this procedure is that it can only be applied for one fishway type at a time; it is not applicable for comparing different fishway types. Therefore, when one needs to compare different fishway types, the scaling factor must be chosen in a different way. In studying the flow into standing water with no backwater or drawdown effect, the discharge from the fishway can be used as a scaling factor. The parameters that need to be scaled are velocity and length.
Let us now consider the flow from the fishway. The discharge from the fishway is Q, which also is equal to the discharge in the flume or tailrace. Let us now define that the mean velocity at any cross section in the tailrace is V. Now
where A is the cross section area equal to B*Y, where B is the total width of the flume and Y is the water depth in the tailrace (Fig. 21). In these experiments, the flume width is set and it is independent on the scale of the model. In scaling the measured velocities the mean velocity at the cross section was chosen for a scaling factor. Now
where v* is dimensionless velocity at any point of the cross section and, v is the horizontal water velocity at any point. As a length scale, tailwater depth Y is used. The equations for the dimensionless distance, head and water depth at the weir are accordingly (25)
A lower water depth Y was chosen for the scaling factor instead of the constant width B, because it describes, at the same time, the effect of discharge. It can be used because flow conditions in the tailrace are set by defining that no backwater or drawdown effect should appear.
Due to the scaling and the accuracy of the velocity measurements, the lowest dimensionless velocities that could be distinguished were about 10 times the mean cross sectional velocities.