Three-dimensional turbulent flow characteristics downstream of submerged weir with an opening.

Abstract:

It has been pointed out that river-crossing structures such as dams and weirs tend to interrupt the inherent continuity of rivers, eliminate the diversity of physical and biological environments, and suppress natural disturbance. In the United States, more than 500 weirs and dams have already been removed to improve the deterioration of dams including dams and the deterioration of the river environment.

As a previous study, Zhang et al. installed various weirs with openings in channel laid with various subgrade materials with different particle sizes and specific gravities, the scouring characteristics of the river bed and the local flow around the weir were investigated in the upstream area of the weir.

In this study, the three-dimensional structure of the flow along the sand bar formed in the downstream region of the upward weir and downward weir with an opening was compared with that of the right-angle weir.

The experiment used a canal with a total length of 10 m, width B = 40 cm, and height of 20 cm. A weir having an opening with a width of 10 cm was installed in the center of the channel, and silica sand was laid from 4.5 m upstream to 4.5 m downstream of the weir. There are two types of weir directions: right angle (θ=0°), upward (θ=10°) and downward (θ=-10°) with respect to the normal to the side wall.

Cross-section average flow velocity was set to about 20 cm/s at the point where the local scouring of the weir did not affect, and the relatively overflow depth H/Dw was systematically changed. The relative overflow depth H/Dw is the relatively overflow depth H divided by the weir height Dw.

As a result, the scouring in the upward and downward weirs was concentrated around the weir opening, and the area and depth of the scouring became smaller as the relative overflow water depth increased. In addition, the comparison of the upward weir and the downward weir showed that the spread of scouring was larger for the upward weir.

The turbulent energy downstream of the weir with opening was highest in the upward weir, and decreased in the order of right angle and downward. The thickness of the sand bar deposit at the center of the weir opening was greatest in the upward weir, and then decreased in the order of right angle and down.