The topic of scour downstream of an apron due to a submerged jet issuing from a sluice opening has been well explored due to its practical applicability in the context of the stability of hydraulic structures (Chatterjee and Ghosh 1980; Hassan and Narayanan 1985; Chatterjee et al. 1994; Balachanda et al. 2000; Dey and Westrich 2003; Kurniawan et al. 2004; Sarkar and Dey 2005; Dey and Sarkar 2006a,b). Sarkar and Dey (2004) compiled a comprehensive survey of the research on scour due to jets. Chatterjee and Ghosh (1980) conducted experiments to measure the horizontal velocity distributions in submerged jets issuing from a sluice opening as it developed over the apron followed by an equilibrium scour hole. They gave a series of empirical equations of velocity and length scales of submerged wall jets over the apron and at the location of maximum scour depth. They used the equation of boundary layer over a flat plat to calculate the time-variation of bed shear stress at the location of maximum scour depth. Hassan and Narayanan (1985) studied the flow characteristics and the similarity of scour profiles downstream of an apron due to a submerged jet. They concluded that the velocity and the length scales are capable to collapse all the data of apron and scour hole regions on a single band. They also gave a semi-empirical equation based on the characteristics of mean velocity in the scour hole to estimate the scour rate. Chatterjee et al. (1994) used their experimental data to derive the empirical equations of the time-variation of scour depth and the various dimensions of the equilibrium scour hole downstream of an apron. They observed that these scour hole parameters are asymptotic. Balachanda et al. (2000) measured the dynamic change of the velocity distributions during the development of a scour hole by the LDA to analyze the effect of the tailwater depth on the scour downstream of a very short apron. Dey and Westrich (2003) investigated the similarity of the velocity distributions within a scour hole in cohesive bed sediment downstream of an apron due to submerged horizontal jets. They studied the boundary layer characteristics within the scour holes and determined the bed shear stress by solving the von Karman momentum integral equation. Kurniawan et al. (2004) explored the trajectory of the issuing jet within the scour hole with and without an apron. They observed that the issuing jet inclines and impinges on the downstream portion of the scour hole if there is no apron, while the issuing jet flowing over an apron remains closer to the apron surface when it leaves apron. Recently, Sarkar and Dey (2005) studied the variations of the characteristic lengths of a scour hole downstream of an apron with the densimetric Froude number for uniformly and nonuniformly graded sediments. They found that all the characteristic lengths (except dune height) increase with an increase in densimetric Froude number, whereas these lengths decrease with an increase in nonuniformity of sediments. On the other hand, Dey and Sarkar (2006a) investigated the similarity of scour profiles that exist in both evolving and equilibrium scour holes, and the dependency of the equilibrium scour depth on apron length, sediment size, sluice opening, densimetric Froude number, tailwater depth and sediment gradation. They reported that the equilibrium scour depth decreases with an increase in apron length, sediment size and sluice opening; while it increases with an increase in densimetric Froude number. There exists a critical tailwater depth corresponding to a minimum equilibrium scour depth. The nonuniformly graded sediments reduce scour depth significantly forming an armor-layer. Based on the solution of the two-dimensional Navier-Stokes equations, Dey and Sarkar (2006b) developed a mathematical model for the computation of the equilibrium scour profiles and the time-variation of scour depth downstream of an apron due to submerged jets.
The flow in submerged jet over noncohesive bed sediment at a sluice gate structure is often coupled with the upward seepage from the permeable bed sediment. In reality, the bed sediment downstream of an apron is subjected to an upward seepage as a result of afflux (difference) of the flow level across the upstream and the downstream reaches of a sluice gate. Though the effects of upward seepage on unidirectional open channel flow and incipient motion of sediment have been studied (Cheng and Chiew 1998a,b; Cheng and Chiew 1999; Dey and Zanke 2004; Dey and Cheng 2005), little is known about the influence of upward seepage on the scour and the flow field downstream of an apron due to submerged jets. Cheng and Chiew (1998a) reported that the upward seepage from the bed is the cause of increasing the streamwise velocity in the upper-layer of the velocity distribution. On the other hand, Cheng and Chiew (1998b) gave the modified logarithmic law for the velocity distribution subjected to upward seepages. Also, Cheng and Chiew (1999) conducted experiments for the incipient motion of sediment with upward seepage and showed that the threshold shear velocity decreases with an increase in the seepage velocity. Dey and Zanke (2004) developed a mathematical model for the incipient motion of sediment under upward seepage. They validated their model by the experimental date of Cheng and Chiew (1998b). In recent times, Dey and Cheng (2005) derived the expression for Reynolds stress distribution in nonuniform-unsteady flow through open channels under upward seepage and concluded that the Reynolds stress distribution is considerably influenced by the upward seepage velocity.
In this study, the influence of the upward seepage on the characteristics of the scour hole and the flow field downstream of an apron due to submerged jets is investigated. Attempts are made to describe the variations of the characteristic lengths of the scour hole with upward seepage velocity, and to derive the time scale for the scour. To analyze the characteristics of the flow, the flow field in both the submerged jets over an apron and within a scour hole under upward seepage was measured by an acoustic Doppler velocimeter (ADV).
