Effect of Baffle Blocks on the Geometry of Scour Hole Downstream of Stilling Basins

Scouring around water structures, like resting basin, is one of the major issues which designers are always facing. Figure 1 shows the local scouring below the resting basin with a positive step in the Cybercrime deviation dam on the Agri River in Southern Italy (Huger, 1992). Different people have studied the factors affecting the scouring phenomenon in the downstream of resting basin. A study on the effect of floor blocks on hydraulic jump characteristics is done by Habibzadeh et al.(2011 and 2012). In his research, he investigated the effects of various parameters such as block dimensions, block distance from the gate and Froude number. The results showed that as the floor blocks slip away, the jet outlet valve changes; from the formation of the reverse wall jet stream (RWJ), only the diverted surface jet (DSJ) is formed. In the present study, the effect of floor blocks and block spacing from the drainage duct under different flow conditions on scouring the downstream of the resting basin was investigated. The experiments were done at the Hydraulic Laboratory of Hydraulic Structural Engineering Department, University of Tarbiat Modares. The flume used was 0.95 m wide, 1 m high and 12 m long. In order to observe the flow, the right wall of the flume was Plug-Glass and the left and bottom walls of the flume were made of concrete. To pump the water into the flume, an electric pump (type 125-200) was used, which was made by Iranian Pump Company. To regulate the flow rate, a coupling system was used to create a variable speed on the electromotor of the pump. To measure the flow rate, a leftover overflow was used at the flume end. The tailpipe depth was adjusted using a recessed overflow. The depth of water on the resting basin, downstream of the drainage door, was measured using a 0.1 mm precision point height. The moving bed topography was also harvested using a laser meter with a precision of 1 mm in a 5 ×5 cm network. In order to create a hydraulic jump, the sliding door made of Plexiglas sheet at a height of 0.75 m and a width equal to the flume width, at 4 m far from the beginning of the flume, is designed. The flume width decreased by 0.62 m using the ionolyte blocks and the outer surface was covered with vitrified glass. Due to the complexity of the scouring phenomenon, the factors affecting the scouring phenomenon are too many. Due to the experimental limitations in the study, it was impossible to investigate all effective factors. For example, with the provision of full turbulent flow conditions, the effect of the Reynolds number is ignored. Whereas, the gravity acceleration parameter (g) and the Froude number are considered. Research on all effective factors is impossible. Therefore, the effect of some parameters can be evaluated using the dimensional analysis technique and the recognition of effective parameters. It should be noted that the variable parameters in the present research are the Froude number, relative depths and block distance from the gate. Regarding three replications for each variable, 27 of the main tests and eight control tests were performed in the unblocked state; generally, 35 experiments were carried out in the present study.An experimental study was conducted to investigate the effects of baffle blocks on the geometry of scour hole downstream of sluice gate- stilling basins. The dimensional analysis was used in order to determine effective variables and their quantitative impacts. Results show that the inclusion of floor blocks has significant effect on the scour hole geometry. By increasing the blocks distance from the beginning of the gate, the scour hole dimension decreases; and after some distance, the scour hole shape is also affected. The change of the blocks distance from the gate, from 0.18 to 0.54 m, results in a 85% decrease in scour hole volume. Furthermore, the scour hole volume is definitely related to the Froude number, but it does not have a determined relationship with the relative tail water depth. The minimum volume of scour hole occurs for the blocks distance of 0.36 and 0.54 m, and with the relative tail water depth of 6.3.