Effects of Chute Slope on Stilling Basin
A factor which occasionally affects stilling basin operation is the slope of the chute upstream from the basin. The foregoing experimentation was sufficiently extensive to shed some light on this factor. The tests showed that the slope of chute upstream from the stilling basin was unimportant, as far as jump performance was concerned, provided the velocity distribution in the jet entering the jump was reasonably uniform.
For steep chutes or short flat chutes, the velocity distribution can be considered normal. Difficulty is experienced, however, with long flat chutes where frictional resistance on the bottom and side walls is sufficient to produce a center velocity greatly exceeding that on the bottom or sides.
When this occurs, greater activity results in the center of the stilling basin than at the sides, producing an asymmetrical jump with strong side eddies. This same effect is also witnessed when the angle of divergence of a chute is too great for the water to follow properly.
In either case the surface of the jump is unusually rough and choppy and the position of the front of the jump is not always predictable. When long chutes precede a stilling basin the practice has been to make the upstream portion unusually flat, then increase the slope to 2:1, or that corresponding to the natural trajectory of the jet, immediately preceding the stilling basin.
The most adverse condition has been observed where long canal chutes terminate in stilling basins. A definite improvement can be accomplished in future designs where long flat chutes are involved by utilizing the Type III basin. The baffle piers on the floor tend to alter the asymmetrical jet, resulting in an overall improvement in operation.
Typical Sloping Chute with baffle blocks (Pakistan)
The following rules have been devised for the design of the sloping aprons developed from the foregoing discussion: