圖書典藏及影音數位平台

秀巒防砂壩興建以來，已達到攔蓄泥砂、減緩土砂進入水庫之階段性使命，為恢復河道功能與生態連通性，實施分階段調降工程，採T型開口設計，分兩階段來降低壩體與開挖切口，形成最寬處約20公尺、最深處達9公尺的T字構造，希望可達到改善下游淘刷、加強護岸基礎安全、減少維護管理資源與改善生態廊道及棲地環境等效益。惟降壩後泥砂對上下游河道衝擊與生態系統影響，均需進行審慎評估與監測，以減少不可預期的負面因素，輔以後續完善配套措施以及詳實紀錄。秀巒降壩工程於民國 113 年 7 月完工，隨即遭遇幾次颱風考驗，主要為7月24日的凱米颱風及10月31日的康芮颱風。113年凱米颱風後壩上70公尺(斷面10)處深槽下切約2公尺並逐步穩定，壩上347公尺至548公尺處(斷面12-13)持續溯源侵蝕但趨緩，壩下68公尺(斷面6)處深槽亦於6至8月間淤積約0.5至1公尺。數值模擬（SRH-2D）預測結果與現地觀測趨勢一致，顯示秀巒壩上游持續發生溯源侵蝕，長期下會形成深槽連接，壩下以淤積為主，河道逐步朝平衡狀態演化。水質監測結果顯示氨氮及COD(除樣點1、2外)濃度呈下降趨勢；總磷與懸浮固體於降壩初期一度上升，推測可能受降壩影響外同時受到降雨沖刷有關，後續濃度逐步下降。RPI指數評估顯示降壩後各樣點均為「未（稍）受污染」等級，顯示並無水質惡化情況。整體而言，兩次颱風未對降壩後的壩體及護岸造成負面影響，河道樣貌則經洪流淘洗，形成上下游連續深槽，與降壩前上游為平坦河床、壩下游急遽落差有顯著不同，降壩工程有效促進河道水砂動態恢復，改善護岸安全、減少維護成本，並提升魚類遷徙能力與生態廊道連通性，對環境與生態具正向貢獻。

Xiuluan Check Dam, after its construction decades ago, have successfully fulfilled its mission of retaining and reducing sediment discharge into the downstream reservoir. It has entered a new phase focused on comprehensive river restoration. To revitalize natural river functions and reestablish ecologi-cal connectivity, a planned staged dam-lowering project was undertaken. This project featured an innovative T-shaped opening design which has implement-ed in two phase modification. This involved progressively reducing the dams height and excavating a central cut, ultimately creating a T-shaped structure approximately 20 meters wide and up to 9 meters deep. The primary objec-tives of this initiative were to enhance natural downstream scouring processes, thereby reinforcing the stability of bank protection foundations, to decrease long-term maintenance and management demands, and critically, to restore and expand ecological corridors and vital habitat environments.The Xiuluan dam-lowering project, completed in July 2024, was soon ex-amined by significant hydrological events, notably Typhoon Gaemi on July 24 and Typhoon Kong-rey on October 31. These events provided an early as-sessment of the projects impact and resilience. Post-Typhoon Gaemi, notable geomorphological evolution were observed. Approximately 70 meters up-stream of the dam, XS 10, a trench about 2 meters deep formed and subse-quently stabilized. Further upstream, between 347 and 548 meters, between XS 12–13, headcuting erosion persisted, but slowed down. Downstream, about 68 meters from the dam, XS 6, sediment deposition of approximately 0.5 to 1 meter was recorded between June and August. These field observa-tions aligned with SRH-2D hydraulic sedimentation transportation simulation results, which indicated ongoing upstream headcuting erosion, expected to eventually form a continuous trench, and predominantly sediment accumula-tion downstream, progressing gradually towards a state of equilibrium.Water quality monitoring following the dam lowering revealed positive trends. Concentrations of ammonia nitrogen and Chemical Oxygen Demand (COD) generally decreased, with the exception of sampling points 1 and 2. While total phosphorus and suspended solids initially rose, likely due to the dam intervention activities and rainfall-induced runoff, they subsequently showed a gradual decline. Importantly, RPI (River Pollution Index) evalua-tions categorized all sampling points as unpolluted or slightly polluted after the dam lowering, suggesting no deterioration in overall water quality.In summary, the lowered dam structure and associated bank protection successfully withstood the impacts of the two typhoons. The channel evolu-tion induced by these events actively reshaped the creek, creating a more con-tinuous trench both upstream and downstream. This presents a significant improvement from the pre-lowering conditions, which were characterized by a flat upstream riverbed and a steep at the dam site. The Xiuluan dam-lowering project has thus far be considered as effectively restored natural sed-iment dynamics, improved bank stability, and promoted a healthier, more natural channel morphology.