Morphodynamics Simulation on Zhitan Dam Removal to Improve Flooding Issue in Quchi and Guangsing Area

In Taiwan, due to the special terrain with high gradient slope and large flow velocity, 5 dams were built on Xindian River for water conservation. However, the existence of dams would cause sediment accumulation in the reservoir. Sediment deposition occurs the area of cross-section of river channel in upstream decreases, which leads to the residents beside the river banks would be easily impacted by the flood. In response to the Saola Typhoon in 2012, Jade Reservoir took releasing operation, which caused the Guangsing and Quchi area between Zhitan and Tskeng Dam were seriously impacted by flood disaster. Many residents in Guangsing area doubted that the gates of Zhitan dam had not be operated correctly during this typhoon event, so they proposed to remove the Zhitan Dam to resolve the flooding issues. Therefore, the main purpose of this research is to discuss the sediment distribution after removing the Zhitan Dam by using numerical simulation. This research was conducted in two phases. The first phase was referring to scientific and technical literatures relevant to dam removal. The second phase was using numerical simulation software SRH-2D to simulate fixed-bed and mobile-bed conditions. The range of numerical simulation was between Tsukeng Dam and Si-Yuan Bridge. In the conclusion, 60 hours simulation under 2 and 5-year return period with steady inflow cases show that the water elevation in Guangsing and Quchi area could only descend tens of centimeter, but the descent value would be higher than 1 meter under the steady inflow case of 10-year return period. However, the flooding issues still exist in those two areas. The result indicated that the flooding issue in Guangsing area could be completely improved and the water elevation in Quchi area would be lowers for two meters under 300 hours of 5-year return period steady flow simulation. Finally, we used Soudelor Typhoon inflow hydrograph as boundary condition to simulate the situation under unsteady inflow. We found that the water elevation of those two areas did not change after experience single Soudelor Typhoon, and after 7 soudelor Typhoon, the result was similar to 60 hours simulation under 5-year return period steady inflow case.