Performance of hydraulics jumps over rough beds

Abstract

A hydraulic jump phenomenon is one of the most common phenomena in open channels. It is a process of transition of a supercritical flow into a subcritical flow by dissipating a large amount of energy. The available knowledge on the behavior of hydraulic jump is only for smooth, horizontal channel beds and very limited studied have been reported in literature on the performance of hydraulic jumps on rough beds which is the real situation in the field. This research attempts to investigate the characteristics of hydraulic jumps formed on rough, horizontal channel beds under different flow conditions. A series of laboratory experiments were carried out in a rectangular flume which consists of artificially roughened beds formed by placing rectangular wooden strips in specific intervals. The hydraulic jump parameters such as, initial water depth, sequent water depth, and flow rate were measured for different bed roughness by changing the roughness density (width of roughness element / Spacing) 8% to 37.5%. The analysis of experimental data showed that the rough bed reduces the distance to the jump from the gate than in those smooth beds while creating a high energy loss. Existing equations to describe the behavior of hydraulic jump on rough beds were not in agreement with our experimental results as most of them were derived by keeping the roughness density constant therefore a new theoretical equation was developed using dimensional analysis, a better correlation was found among hydraulic jump parameters that include the roughness density to represent bed roughness. where, y1, y2, t, d, Fr are initial depth, sequent depth, height of roughness element, roughness density and Froude number, respectively. It was found that maximum effect of roughness elements occurred at a roughness density of 0.23. The results of this study confirm the effectiveness of rough beds for energy dissipation below hydraulic structures