Velocity distribution was studied at a single V-shaped sharp crested weir and at almost a similar weir located in the middle of the fishway. For analysis, velocities, lengths, and discharges were transformed into dimensionless values.
It was observed that at a single weir with a V-shaped crest, velocity distribution was almost independent of the flow rate and the submergence. The highest velocities were located near the bottom. With high submergence, velocity distribution was more even compared to free nappe conditions.
With high relative water depths, i.e. high flows, at a weir inside a pool-and-weir fishway with a V-shaped crest, the highest velocities were located in the upper parts of the cross section near the surface. The velocity distribution resembles that of a Denil fishway. When the pool length was large compared to the depth of flow at the weir, the velocity distribution was different, having the lowest velocities located near the surface.
It was concluded that the difference was to be due to the flow from the upper weir, which has a great effect on the hydraulic conditions both inside the pool and at the weir. The same effect was missing when the flow was over a single weir in a channel with zero slope. The velocity distribution of the fishway flows with low relative depth of flow, i.e. when the pool length was large compared to the depth of flow over the weir, was the same as the velocity distribution characteristic of a single weir. This indicates that the weirs act like single weirs.
Velocity distribution over the weir for a chain of weirs changes depending on the relative depth of flow (compared to the pool length). When the relative depth of flow is small, the highest velocities occur near the region closest to the weir crest. In pool-and-weir fishways this mode of flow is actually called as plunging flow. When the relative depth of flow increases, the velocity distribution changes at the weir gradually, and the highest velocities are found near the water surface. Depending on the structure, the difference between the velocities at the upper part of the water mass changes compared with those at the lower part of the water mass.