Numerical simulation of the Chute Convergence effects on Forming the Transverse Wave in Flood Evacuation Systems

Introduction
Flow contact with the spillway piers causes rooster tail waves which could continue along the flood evacuation system. Therefore, the information about the height and location of these waves would be useful for the design of the spillway chute walls. On the other hand, the engineers try to design the converging chutes to reduce the excavation costs, and this convergence in many cases would increase the transverse flow depth. According to the literature, three kinds of waves could be formed in the spillway and chute system. The first wave is formed just after the spillway piers and called as rooster tail waves. The Flow passing through the two sides of a pier collide each other at a near distance after the pier and forming the first wave. The second wave is formed at the middle axis of the chute and could be considered as the result of the interaction of the first waves and the channel convergence. The third wave is formed due to the collision of the mentioned waves with the channel walls. Investigating the effect of the chute convergence on the depth and location of transverse waves is critical and necessary and could present suitable information about the three waves. In this paper, the numerical simulation of the Khair Abad dam flood evacuation system is performed for investigating the effects of chute convergence on the formation of the transverse waves along the chute.
Methodology
In this Research, the FLOW-3D is used for simulation of the effects chute convergence on transverse wave formation. The Khair Abad dam flood evacuation system laboratory tests results were used for verification of the numerical simulation. The present simulations considered four convergence angles of the chute, including 0, 3, 5 and 7 °. Also, the simulation was performed using three discharge rates consist of 3000, 7000 and 9000 m3 / s. The convergence angle of the Khairabad Dam spillway is approximately 5 degrees and the convergence angles were changed during the chute until the chute width reached from 66 m to 40 m. The chute width is kept constant after the convergence.
Results and discussion
The model validation was performed using the laboratory data of the Kheirabad Dam spillway Model developed by the Iran Water Research Institute. Since the size and number of the cells in the model affect the accuracy of the results and computational cost, this study concentrates in finding the optimum value of dimensions and number of cells for the simulation domain. This procedure could result in acceptable accuracy and suitable computational cost. The results of the verification showed that the smaller grid size and the greater number of cells, result in the higher correlation of the numerical results and the laboratory data. Hence, based on the above-mentioned method and available computational device, the best size and number of the cells were selected for simulating the effect of chute convergence on the three waves height.
Simulation results show that the height of the first wave increases with increases in the flow discharge, and the spillway convergence has not a significant effect on the first wave height. Also, the second wave height is increased by the increase in flow discharge. It should be mentioned, the second wave doesn't appear when there is no convergence in the chute, but with increasing the convergence angle the wave height increases and the maximum wave height will move to upstream stations of the chute. In other words, with the increase of the angle of convergence of the chute, the location of the second wave formation will be closer to the spillway crest and its height will increase. The simulation results demonstrated that the increase in the flow discharge could lead to an increase of third waves heights. Also, the results show that the third waves could not be formed when the channel convergence angle is equal to zero, and increase of convergence angle could result in an increase of the third wave height. Also, the increase in the angle of convergence of the chute results in transport of the location of the third wave formation into upstream stations. In other words, the third waves will form closer to the spillway axis by an increase in convergence angle. Numerical simulation results show that the height of the third wave which formed beside the wall can be more than twice the average flow depth at the same point. This fact should be considerate in the design of the chute with convergence. The results also showed that increasing the flow discharge will cause to increase of the wave height, but it led to the decrease of the ratio of the third wave height to the average flow depth. This means that increasing the discharge has less effect on increasing the transverse wave height.
Conclusion
Simulation results show that the height of transverse waves increases by increasing the discharge. But the ratio of the maximum height of the waves to the average depth of the same station will reduce by the increase of the discharge. Also, Results show that, by increasing the chute’s convergence angle, the height of the transverse waves will increase and the location of the primary transverse waves will transmit to upstream locations of the chute.