Introduction
Spillways are constructed in a variety of types to discharge excess water and major flood streams to avoid damage to dams. Flowing over spillways dissipates energy; however, the non-dissipated portion of energy leads to bed erosion and holes downstream. Ogee spillways are hydraulic structures that have been of interest to researchers. They were developed first in concrete dams and then in embankment dams. These spillways are actually simple examples of a high roughness coefficient channel, considering the low energy dissipation along its length, in the toe of the dam there is no need to use energy dissipating structures. During the flow passing over the crest of the spillway structure, potential energy is converted to kinetic energy at the toe of the structure.Scouring occurs downstream of the stilling basin, and it is important to evaluate techniques to protect the downstream bed. Scholars have studied parameters influencing scouring downstream of stilling basins in recent decades. They analyzed the scouring mechanism and the effects of the structure shape and sediment particle parameters on the scour size. This study aims to obtain deeper insights into the effects of a gate used over the ogee spillway and scouring in the tailrace wall. Thus, an ogee spillway model was developed, and experiments were designed to test the system.
Materials and Methods
Experiments were designed to evaluate the effects of the gate size in the ogee spillway crest and tailrace scour. A plexiglass-walled flume with a metal bottom from Khuzestan Water and Power Organization was used. It had a length of 7 m, a width of 60 cm, and a height of 50 cm, and a total of 45 tests were performed to evaluate the effects of the gate shape and discharge rate on scouring downstream of the ogee spillway. The flow rate was also measured using a calibrated triangular weir and a manometer installed at the end of the flume. A depth gauge was used to read the water level in the required sections, mounted on the flume. A butterfly valve was used to adjust the input flow rate. Additionally, a triangular weir and a manometer were used to measure the flow rate. The flow rate was also measured using a calibrated triangular weir and a manometer installed at the end of the flume. A depth gauge was used to read the water level in the required sections, mounted on the flume. The model was prepared, and the gate was opened until the flow became steady in 40 min. Then, the scour width and length were measured. Furthermore, in order to achieve the goals of this research, the governing parameters of the hydraulic flow of the weir were first introduced, and then the effective governing parameters in this study were presented using the π (Buckingham) method.
Results and Discussion
For a given gate width, an increase in the gate opening reduced the Ds/Ls ratio; the Ds/Ls ratio reduced by 55% when the gate opening decreased by 500%. In fact, it can be said that with a 1% increase in valve opening, sedimentation decreases by 11.0%. As well, it can be concluded that increasing the valve opening leads to further sedimentation along the length and has less effect at depth. For a given gate opening, an increase in the gate width reduced the Ds/Ls ratio; a 400% increase in the gate width reduced the Ds/Ls ratio by 60%. Furthermore, the effect of H/b on the scour depth (Ds) was studied, and the scour depth increased as the gate width increased; a 400% increase in the gate width led to a 24% increase in the scour depth. The effect of H/b on the scour length (L) was evaluated, and a 400% increase in the gate width increased the scour length by 82%. It can be inferred that a further increase in the gate opening would increase the scour length, while an increase in the discharge reduced the Ds/Ls ratio; the Ds/Ls ratio increased by 15% when the discharge decreased by 50%.
Conclusion
One of the structures that is of great importance in water conveyance is the ogee spillways, which are used in many water conveyance networks. Scouring at the downstream of these structures has led researchers in this study to investigate the effect of dimensions and gate opening levels on the ogee spillway and the amount of sedimentation downstream of this structure. In other words, scouring is one of the problems in water structures that can lead to their destruction and, under the best conditions, can cause an increase in suspended sediments in river flow.This study evaluated the contributions of ogee spillways to scour reduction downstream. By comprehensively reviewing previous studies and conducting dimensional analysis, the dimensionless parameters affecting scouring were identified, and experiments were planned based on them. The results indicated that the scour depth could increase by 55% as the gate opening increased. Moreover, an increase in the gate width decreased the scour depth; a 6.5% increase in the gate width would lead to a 1% in the scour depth. A 400% increase in the gate width increased the scour depth by 82%. Moreover, A 1% rise in the discharge reduced the Ds/Ls ratio by 0.3%. Also, Increasing the width of the valve causes more sedimentation along the length and has less of an effect in depth. |
