Background:
"In 2010, Typhoon Megi, accompanied by the northeast monsoon, brought intense and prolonged rainfall to the Suao area. Insufficient urban drainage and tidal surges led to severe flooding once again. In order to further improve flood control in the Suao area, the Water Resources Planning Branch of the Water Resources Agency, under the Ministry of Economic Affairs (MOEA), conducted the Regulation Planning of the Suao River in 2011. On December 31, 2013, the "Regulation Project of Flood-prone Areas the Third Stage Practicing Plan: A Diversion Plan of Yilan County-Governed Su-Ao River Basin" was approved by the Ministry of Economic Affairs. The environmental impact assessment for this project was also approved by the Yilan County Government on April 21, 2020."
Objectives:
The main goal of the project is to divert the main stream of the Suao River in the event of conditions similar to Typhoon Megi (approximately 1,084 mm of rainfall in 24 hours and 1,333.5 mm in 48 hours at the Suao Rainfall Station). The flood diversion plan aims to prevent overflow of the main stream embankment and reduce the water level while diverting water to the southeast Neipi coast, ultimately discharging it into the Pacific Ocean. The flood diversion ratio is set at 80%, with a diversion capacity of approximately 690 cubic meters per second (CMS). This strategy aims to reduce downstream flow to less than the 5-year recurrence period flow of 331 CMS, excluding upstream sediment and alleviating sedimentation issues in the middle, lower reaches, and Suao Harbor.
Implementation Strategies and Methods:
| Flood Diversion Facility |
Description |
| Flood Diversion Tunnel |
A flood diversion tunnel approximately 2.4 kilometers long will be constructed at cross-section No. 39 of the main stream of the Suao River. The tunnel will have a slope of 5/1000, about 10.2 meters in diameter, and will divert floodwaters to the southeast Neipi coast, discharging into the Pacific Ocean. |
| Check Dam |
Check dams will be installed at each of the confluence of Zuentoukeng River and Baimi River to intercept large rocks, soil, or drifting wood from upstream, reducing wear and tear on the flood diversion tunnel. |
| Barrage and Diversion Weir |
An unmanned, automatic diversion mode will be adopted. Diversion will commence when the water level in front of the diversion weir reaches 18.7 meters, with a diversion rate of 102 CMS for the water intercept weir. |
| Outlet Section |
The outlet section comprises three segments: a gradually transitioning section, a steep channel section, and an energy dissipation pool. The energy dissipation pool is designed to minimize environmental impact and maintain the original coastal landscape. |
| Fish Passage |
A fish passage will be constructed on the left bank of the dam site to mitigate the ecological impact of the check dam and barrage construction. This passage will provide opportunities for migratory fish and promote public access and ecological education |
Budget and Schedule:
The project will start from 2024 to 2027, with approximately one year for basic design and three years for detailed design and turnkey construction. The central government will compile the unit budget to subsidize the implementation by the Yilan County Government. The total budget for the project is approximately NT$5.413 billion.
Expected Results and Impacts:
1.Improved Flood Situation: After the Suao River diversion is completed, the risk of flooding on both banks will significantly decrease. The overall flooded area will be reduced by 57.3 hectares, with a maximum improvement of 32.39% in the flooded area. The average flooding depth will decrease from 2 to 2.5 meters to 1.25 to 2 meters. Significant improvements will be observed in the downstream areas along Suao River, Suao Elementary School, Suao Sports Park, and other regions.
2.Enhanced Harbor and Infrastructure Benefits: The flood diversion design, based on a 200-year recurrence rainfall event, can reduce approximately 100,000 cubic meters of sediment entering Suao Harbor with each diversion, reducing harbor sedimentation and saving government dredging expenses (approximately NT$33.18 million per year). This is conducive to the safety and maintenance of critical infrastructure in the entire harbor area, contributing to local economic development and national security.
3.Effective Control of River Water Levels: Flood diversion can effectively reduce downstream river flooding, extending the normal operation time of downstream pumping stations.
4.Maintenance of Existing Flood Control and Cross-River Structures: After flood diversion, the existing flood control structures in Suao Town can effectively protect the urban area from external water threats and no need for bridge reconstruction.
5.Promotion of Environmental and Tourism Development: Flood diversion lowers the main stream water level of the Suao River, eliminating the need for higher embankments and reducing visual conflicts with the landscape, thereby promoting tourism and recreation development.