Introduction Free and submerged jumps are important phenomena in open channel flow because of their applications in energy dissipation downstream of hydraulic structures, such as spillway, barrage and weir. Because of their various engineering applications, jumps have been the focus of numerous prior investigations. Critical review on the significant contributions to the subject of jumps was put forward by Rajaratnam (1967a), Rajaratnam (1976), Launder and Rodi (1981, 1983), McCorquodale (1986) and Hager (1992). While the measurements of flow to date have greatly advanced our understanding of the characteristics of free and submerged jumps, the vast majority of these measurements were obtained on smooth beds and so our knowledge on turbulent characteristics of flow in jumps (especially submerged jumps) on
rough beds is deficient. Importantly, Long et al. (1990) and Wu and Rajaratnam (1995) recognized that the submerged jumps could be viewed as transitional phenomena between wall jets and free jumps. Therefore, a brief summary of the studies on wall jets and free jumps is given in addition to those on submerged jumps.
Glauert (1956) analyzed the wall jet on a horizontal bed in the context of boundary layer. He obtained the similarity solution of the velocity distribution for laminar flow, but in turbulent flow, he did not get a complete similarity. Schwarz and Cosart (1961) experimentally measured the velocity distributions of plane turbulent wall jet by a hot-wire anemometer. They theoretically computed the Reynolds and bed shear stresses from the solution of the equations of motion for a steady turbulent flow. Rajaratnam (1967b) experimentally studied the plane turbulent wall jet on artificial rough beds and measured the velocity and the bed shear stress using a Pitot and Preston tubes, respectively. He also calculated the bed shear stress neglecting the momentum of the backward flow. Wygnanski et al. (1992) explored the applicability of different scaling laws of the turbulent wall jet. Ead and Rajaratnam (2002a) presented a theoretical and experimental study of a plane turbulent wall jet with finite tailwater depth. They found that for low tailwater depths, the momentum flux of the forward flow in the wall jets decays appreciably with the distance from the nozzle due to the entrainment of reverse flow. The momentum decay was correlated with the tailwater depth ratio and relative streamwise distance. Tachie et al. (2004) reported the experimental findings of the effects of smooth and transitionally rough beds on the flow characteristics of a plane turbulent wall jet. The results showed that the bed roughness increases the inner-layer thickness, but the jet half-width is nearly independent of bed roughness.
Rajaratnam (1965) treated the free jump as a two-dimensional wall jet. The mean velocity distributions in the region external to the boundary layer were found to be self-similar when velocity and length scales were used as issuing jet velocity and jet thickness, respectively. Rajaratnam (1968) found that the free jumps on rough beds were significantly shorter than those on smooth bed. Narayanan (1975) represented the free jump as a plane turbulent jet spreading in presence of the reverse flow. He applied the momentum integral and continuity equations to calculate the kinematics of jumps. McCorquodale and Khalifa (1983), and Madsen and Svendsen (1983) simulated the free jumps numerically using the momentum integral approach. The physical characteristics of the free jump over rough beds were investigated by Hughes and Flack (1984). They reported that the bed roughness reduces the tailwater depth and the length of the jump, which was also found by Hager (1992). Long et al. (1990) measured the velocity, turbulence intensity and Reynolds stress by a laser Doppler anemometer (LDA) to study the flow characteristics of submerged jump on smooth bed. They found some degree of similarity in the flow of fully developed zone. Long et al. (1991) and Qingchao and Drewes (1994) used k-e turbulence model to simulate the submerged and free jumps, respectively. On the other hand, Gharangik and Chaudhry (1991) applied finite-difference scheme to simulate the free jumps. Wu and Rajaratnam (1995) explored the similarities and dissimilarities amongst free jumps, submerged jumps and wall jets. They found the submerged jumps as the transition between wall jets and free jumps. Effect of the corrugated beds on the free jumps was investigated by Ead and Rajaratnam (2002b). They observed a considerable reduction in the tailwater depth required to form the submerged jumps on corrugated beds than that for the corresponding jumps on flat smooth beds. There was also a manifold increase in bed shear stress on the corrugated beds. Recently, Dey and Sarkar (2006) analyzed the degree of similarity in flow and turbulence characteristics of submerged jets due to abrupt changes from smooth to rough beds.
Most of the aforementioned investigations focused either on the physical characteristics of the free and submerged jumps or the hydraulics of plane turbulent wall jets on smooth bed. There is a considerable dearth of understanding of the effect of bed roughness on the flow characteristics in submerged jumps. The present study aims to investigate the effect of bed roughness on the flow characteristics in submerged jumps, providing a comprehensive dataset and addressing some of the important scaling issues related to the flow and turbulence.
