Experimental Studies on Sediment Flushing in the Upstream Approaching Channel of TGP

1. Experimental Studies on Sediment Flushing in the Upstream Approaching Channel of TGP Wang Ruiyu

2. Main Contents • Introduction • Model Experiment • Analysis of the Result • Conclusion

3. Protecting Dike Dam This is the satellite picture of the TGP. We are here Five-steps Ship Lock Ship Lift Flushing Sluice

4. The protecting dike is built to minimize the sedimentation in the navigation channel, and protect it from the fluctuation in the main channel, thus formed the approaching channel. The upstream approaching channel is 2674 m long, and 220 m wide at the entrance. The bottom elevation is 130m. The sediment deposition should not exceed 139m at the ship lock channel, and 140m at the ship lift channel. Downstream approaching channel Flushing tunnel Upstream approaching channel Upstream protecting dike Flushing Sluice Downstream protecting dike

5. This study focus on three points: 1. How high is the maximum flushing discharge under the conditions that the difference of the water levels on the two sides of the protecting dike does not exceed 2m. 2. What is the efficiency of sediment flushing. 3. How to minimize the waves in the navigation ffchannel induced by the shutdown of the flushing ffsluice.

6. Main Contents • Introduction • Model Experiment • Analysis of the Result • Conclusion

7. Water level, water velocity and deposition volume are to be measured during the experiment. Because the process is unsteady, the water level varies much with the time. An automatic measuring device was applied to precisely trace the change of water level. 7 sensors measure the water pressure, and then convert the pressure into water level.

9. The framework of the automatic measuring device Modulus-number converter Pipe 2 Column Experimental channel Pipe 1 Sensor

10. Sensor 0 Sensor 1 Sensor 2 Sensor 3 Sensor 4 Sensor 5 Entrance Sensor 6 Flushing tunnel Flushing Sluice

11. Main Contents • Introduction • Model Experiment • Analysis of the Result • Conclusion

12. Analysis of the Result of the Experiment • 1.Flushing Discharge We have mentioned above that the maximum flushing discharge under the precondition that the difference of the water levels on the two sides of the protecting dike does not exceed 2 meters has to be studied. The water level of the Three Gorges Reservoir during the flood season is 145 meter. This relative low water depth could increase the flushing velocity and thus improve the flushing efficiency. Therefore we will first study the condition of 145m reservoir water level, and the sediment deposition reaches 139m-140m.

13. Q=5000m3/s

14. We conducted experiments under different conditions and generalized the following diagram. We can see that the higher the reservoir water level is, the less the water level at the entrance of the flushing sluice drops.

15. Analysis of the Result of the Experiment • 2. Flushing efficiency The result show that: Compared with experiment 8, the reservoir water level of experiment 12 is 2m higher, and the flushed sediment volume has nearly doubled. The flushing discharge of experiment 15 is 1000 m3/s more than that of experiment 12, and the flushed sediment volume has again nearly doubled. The result show that increasing flushing discharge and raising reservoir water level are effective methods to improve flushing efficiency.

16. It can be seen that the flushing intensity at 500-600m from the entrance of the approaching channel is the highest. In the downstream part the flushing intensity is relatively low.

17. Analysis of the Result of the Experiment 3. Fluctuation in the upstream approaching channel • To safeguard the navigation, the water flow condition in the upstream approaching channel should satisfy the following requirements: the longitudinal flow velocity<2m/s, transverse flow velocity<0.5m/s, wave height<0.5m, converse flow velocity<0.4m／s．Otherwise the safety of the navigation will be compromised. Therefore it is necessary to study the fluctuation in the approaching channel induced by the shutdown of the flushing sluice.

18. These two diagrams are based on 4 experiments, all conducted with a discharge of 5000m3/s, but the water depths of the approaching channel of each experiment are different. We can see that both the duration of the fluctuation and the initial fluctuation amplitude has a good linear relationship with the water depth of the approaching channel. Although higher reservoir water level and thus greater water depth could improve the flushing efficiency, it may cause more turbulent fluctuation, and thus prolong the intermittence of navigation.

19. Main Contents • Introduction • Model Experiment • Analysis of the Result • Conclusion

20. Conclusion • Sediment flushing plays a vital part in the navigation facilities management in the Three Gorges Project. Aperiodic flushing is an efficient way to lower the upstream approaching channel bed. The result of the experiments shows that: • for the reservoir water level 145m, the flushing discharge should not exceed 3700m3/s for the safety of the dike; to increase the flushing discharge to 5000m3/s, the reservoir water level must be enhanced. • The higher is the flushing discharge the larger is the volume of scoured sediment. • The amplitude and duration of the waves induced by shutdown of the flushing gate in the approaching channel increase along with the water depth.