圖書典藏及影音數位平台

北港溪支流部分河段歷年河床淤積現象屢現，雖未嚴重影響防洪安全，但地方民眾對河床淤積現象可能影響河防迭有反映。為顧及淤積河段之河床泥砂平衡，並適度回應地方開發需求，兼利用河川中有限之砂石資源，故有必要評估河床淤積成因，並評估部分河段之河床利用疏濬方式清淤後所可能造成之影響。惟天然河川泥砂演變特性複雜，疏濬後雖局部河段有效通水斷面增加，水流挾砂能力驟增，但上游輸送至下游之泥砂待水流挾砂能力回復正常後，泥砂亦可能逐漸落淤於更下游段。故疏濬後之河道演變及疏濬作業對防洪安全、自然環境所可能造成之影響，亟有必要重新審視現有環境條件，進行詳細之專業評估。

Beigang River is administered by the central government. Repeated sedimentations can be seen in its branches over the years, which impact river safety and the surrounding environment. Therefore, considering river safety and the use of the river’s limited sand and gravel resources, dredging operations are planned for the river branches with serious levels of sedimentation. However, the dredging operations will inevitably influence the river’s sediment transport balance, so this study plans to evaluate the factors contributing to river sedimentation, and discuss the characteristics and changes in sediment transport to obtain a better understanding of the long-term stability of the river in the first year. In the second year, the dredging project will be further developed and analyzed to propose a solution that comprehensively stabilizes the riverbed and corrects the river, while preserving safety and the natural environment. The study adopted the HEC-RAS model to assess the impact of the dredging operations on cross-river structures and flood control structures. The NETSTARS model was used to evaluate the influence of tributary dredging operations on the satiability of the main stream’s lower reaches. Afterwards, the sediment reduction ratio and sediment reduction efficiency were evaluated to determine the efficiency of the dredging operations, and were compared to the yearly sediment transport before and after dredging operations to select the most appropriate dredging option. In addition, river reaches at high flood-risk were further discussed through the CCHE-2D model, during which contingency suggestions were brought up and used as a reference for future river correction projects by the 5th River Management Office. Riverbed stability analysis suggests that the Lunzih River reaches are stable, narrow, and deep channels, therefore we do not recommend dredging; yet, certain reaches should still be corrected. Shihniou River and Dahukou River are divided into 5 dredging and correction analysis programs within the project, which includes the Shihniou River Shueiduei South Bridge reaches, the Shihniou River Shangong Bridge reaches, the Shiniou and Lunzih River confluence, the Dahukou River Xing-geng Bridge reaches, and the Dahukou River Zizhi Bridge reaches. Aside from the above dredging operations and contingency plans, suggestions are also being made to tear down or reconstruct the Sanjie Bridge due to its excess pillars and insufficient flood capacity. In addition, the above operations must be incorporated with periodic dredging and the relevant flood mitigation measures. The bottom of the aqueduct in the upper reaches of the Dahukou River at the Shing-geng Bridge appears to be too deep with a small water flow cross-section, which contributes to serious sedimentation in the upper reaches. Aqueduct reconstruction is recommended along with periodic dredging to ensure river safety. Because the coastal erosion of cross-section 100 to 105.1 of the lower reaches at the Dahukou Guanghua Bridge is relatively severe, and is accompanied with the loss of the armoring layer and the exposure of soft rocks in the riverbed, we recommend utilizing many series of energy dissipators—such as consolidation work and aprons—to prevent longitudinal and lateral corrosion.

這份「北港溪支流淤積原因及河道穩定研究 (2/2)」旨在解決北港溪支流（石牛溪、崙子溪、大湖口溪）長期淤積對河防安全的影響。研究運用HEC-RAS、NETSTARS及CCHE-2D等模式，分析淤積成因、輸砂特性及疏濬效益。 結果顯示崙子溪河段較穩定，不建議疏濬但需整理；石牛溪與大湖口溪則針對五個易致災河段提出疏濬及河道整理方案。並針對三結橋、行耕橋上游渡槽及光華橋下游灘岸淘刷等問題，提出橋樑改建、渡槽改建及設置固床工等具體對策。 研究評估支流疏濬對北港溪主流河道及跨渠構造物影響極小，但部分防洪構造物通洪能力不足，需進一步治理。最終提出結合工程與非工程措施的整體河床穩定與河道治理對策。