Chapter 6. Velocity distribution at v-shaped sharp crested weirs
The velocity distribution at V-shaped sharp crested weirs was studied at a single weir and at a weir in the middle part of a pool-and-weir fishway with a V-shaped crest. Studies on a single V-shaped sharp crested weir were carried out in a flume of zero slope. Zero slope was chosen because flow pattern was to be observed only very close to the weir. The results were compared with the flow patterns in fishways, in which flume slopes are quite mild, less than 15 %. At the highest slopes in an unobstructed open channel, the approach velocities will grow and the flow is supercritical, but in obstructed channels like fishways, due to considerable flow friction, the approach velocities will remain lower. The results were transformed into dimensionless values.
6.1. Scaling factor for velocities
As a scaling factor for velocities, basically the same procedure was applied as when scaling velocities below fishway entrances, and similar definitions were used (Fig. 24).
For scaling, the mean velocity at the cross section was used. From Fig. 24 we get
where v* is the dimensionless velocity, v is the measured velocity, vm is the mean velocity at the weir, ho is the mean depth of flow at the weir, Q is the discharge, and β is the opening angle.
Accordingly, as a scaling factor for depths, the mean depth of flow at the weir was taken as (27)
where y* is the dimensionless water depth, y is the water depth measured from the bottom of the V-notch, and yo is the mean depth of flow at the weir. The same procedure was applied for analyzing the results both of the studies on a single weir and on a weir in a fishway.