Effect of V-shaped Blocks on Local Scour Downstream of Ogee Spillway

Flow over spillways has a great amount of potential energy, which is converted into kinetic energy downstream control structures. This energy should be dissipated to prevent the possibility of excessive scouring of the downstream waterway bed, minimize erosion and undermining of structures, which endanger the structure safety. Local scour downstream is usually one of the most important issues in designing process of hydraulic structures, which is mainly some applications have been proposed to reduce this occurrence. Present study has been carried out on the effects of V-shaped baffle piers in order to reduce the local scour downstream of the ogee spillway.

Experiments were carried out in a flume which is 6 m long, 0.5 m wide and 0.6 m deep with horizontal bed slope. A pump with a maximum discharge capacity of 14 lit/s circulated water from sump. A movable weir located at downstream of flume controlled water level. The hydraulic and geometric characteristics of these baffle piers were investigated with three different discharges (20, 24 and 28 Lit/s.m). The positions and height of these blokes were presented to decrease the amount of scour downstream. The position and height parameters were defined based on the length of stilling basins as follows respectively, Lb/Lf=(0.22, 0.44, 0.66, 0.88) and Hb/D=(0.66, 1, 1.33, 2). The test area in the channel was 2 m long and 12 cm in height, 2.5 m from the beginning of the channel. The experiments were carried out using non cohesive sediments with median diameter of 0.72 mm, specified gravity of 2.65 and geometric standard deviation of 1.12.

The presence of the block decreases the final depth of scour and the length of the scour which shows the role of the blocks and their role in this case. According to laboratory observations, the existence of the block distinguishes three types of flow around it. The first is a flow that jumps above the block. The second is the convergence flow between the blocks and the third the eddy flow that forms in front and inside the blocks. The presence of the block and the three types of flows resulted in greater turbulence and reduced jump energy, resulting in a shorter jump length and an earlier secondary depth. Taken together, these factors reduce the flow strength at the bottom of the plate and also cause less sediment transport downstream. The more the Froude number increases, the more stable the jump is due to the category of jump types. (Depending on the group of jumps created in this study) The amount of scour depth decreases due to the presence of blocks. Also, the result indicates that by decreasing the distance of baffle from weir toe, the length and the depth of scour have been remarkably diminished. Meanwhile the raise of baffle height decreases the scour depth across a longitudinal section. In addition, in different block positioning, as the height of the blocks increases, the length of the scour decreases. So that block with height (Hb/D=2) has a significant decrease during scouring compared to block with height (Hb/D=0.66).

Overall, the suitable geometrical condition proposed in order to reduce this sediment deposition which is propose based on length of stilling basins and Froude number as, Hb/D=2, Lb/Lf=0.22 It assumes that the scour depth is decreased between 30 to 76 percent and the scour length is decreased between 57 to 71 percent. The present study demonstrates the importance of positioning the blocks and their height at different Froude numbers. Therefore, pre-execution modeling is suggested for better conclusions to achieve the best position and height of the block.