Hydraulic Model Tests for the Reconstruction and Improvement Works of the Shihmen Reservoir
Background
In 2004, Typhoon Aere brought immense quantities of sediment into the Shihmen Reservoir, causing severe siltation and giving rise to the challenge of "storing turbid water and discharging clear water," which compromised water supply capabilities and reservoir lifespan. To improve the reservoir's sedimentation mitigation capacity, a series of hydraulic model tests have been conducted since 2005 under the Shihmen Reservoir Reconstruction and Improvement Plan to guide engineering design and planning.
Test Features
Configured according to Froude scaling laws, fixed-bed hydraulic models of various scales were constructed to analyze and validate five major improvement works for the Shihmen Reservoir:
- Power Plant Reconstruction Project: Utilized 1:40 and 1:25 undistorted models to verify the hydraulic flow patterns and impacts on discharge capacity caused by the modified penstocks.
- Additional Stratified Intake Structures: Simulated intake hydraulics across varying water levels. The tests identified tendencies for vortex-induced air entrainment and air entrapment in the delivery tunnel in the original design, leading to recommended improvements such as installing 90-degree elbows and air release valves.
- Spillway Tunnel Reconstruction Project: Simulated sediment-laden flow trials under Typhoon Aere conditions to evaluate the desilting benefits of extending the downstream (No. 1) spillway tunnel intake to an elevation of EL. 190 m.
- Preliminary and Feasibility Planning for Flood Diversion and Desilting Tunnels: Compared the sediment prevention performance of various options and evaluated the engineering efficiency of Option C, which features an "Elephant Trunk Desilting Steel Pipe" extending the intake to EL. 190 m.
- Amuping Desilting Tunnel Project: Executed tests on multiple layouts for the flushing basin to analyze siltation/flushing efficiencies under different designs, optimizing desilting facility performance.
Application of Results
The test results provided essential design validation for the Shihmen Reservoir renewal projects, successfully supporting scheme optimizations:
- Spillway Tunnel Reconstruction: Achieved a total sediment discharge of 6.07 million tons in a single Typhoon Aere-scale event, representing a sediment sluicing ratio of 20.8%.
- Diversion and Desilting Tunnel (Option C with Elephant Trunk Pipe): A sediment reduction of up to 9.2 million tons during a single Typhoon Aere event, serving as the decision-making basis for expanding desilting functions.
These outcomes have been broadly applied to power plant reconstruction, stratified intakes, spillway tunnels, desilting tunnels, and the Amuping desilting project. They have enhanced flood discharge, sediment flushing, silt prevention, and water supply operations—mitigating siltation risks during extreme storm events, prolonging reservoir service life, and securing stable water supplies for northern Taiwan.