本研究針對潮流往復流條件下單樁基礎周圍局部沖刷之發展行為進行水工模型試驗，模擬臺灣西部離岸風場常見之潮流環境，探討流速變化與流向交替對坑形演化、沖刷深度發展與床面穩定趨勢之影響。彰化沿海地區潮流流速約介於 0.5–1.5 m/s，具顯著往復流特性，對基礎周圍之沖刷與沉積行為具有重要影響；然而現有研究多以單向流條件為主，對潮流交替作用下之沖刷演化機制仍缺乏系統性實驗驗證。
本研究採用幾何相似比 1/36 之水工模型試驗，以模型流速 0.125–0.250 m/s（對應原型流速 0.75–1.50 m/s）模擬潮流往復過程。試驗以 120 分鐘為一完整潮流循環，包含正向與逆向共八階段流速配置，並延伸進行 240、360、480 與 600 分鐘之長時程觀測，以分析沖刷坑形與最大沖刷深度隨時間之演變特性。
試驗結果顯示，潮流往復條件下之沖刷發展呈現「沖刷—回填—再沖刷」之週期性行為特徵。於初期階段，迎流側坑深隨流速增強而快速增加；當流向反轉時，原迎流區域因流場重新分布而出現局部回填，使沖刷深度增長趨緩。隨潮流循環次數增加，床面形態變化幅度逐漸降低，迎流坡與背流坡之坡度差異隨之減小，坑形逐步趨於相對對稱之配置，顯示系統進入動態平衡狀態，但尚未達最終平衡。
與文獻中單向流條件下之試驗結果（Chen et al., 2019）相比，潮流往復條件下之最大沖刷深度相對較小，顯示流向反轉所引致之回填作用與能量分散機制能有效抑制局部侵蝕之累積。即使於高流速條件（V=0.250 m/s）下，沖刷影響範圍擴大，整體床面仍呈現趨於穩定之演化趨勢。
本研究成果顯示，若僅依單向流經驗公式評估潮流主導環境下之沖刷行為，可能導致沖刷深度之保守高估。研究結果可作為離岸風場單樁基礎設計與長期維運評估之參考基礎，並可進一步發展適用於潮流條件下之沖刷修正模型，以提升離岸結構物設計之安全性與合理性。

With the rapid development of offshore wind power, the stability of monopile foundations under tidal current environments has become an important engineering concern. Local scour induced by tidal currents can significantly affect the structural stability of offshore monopile foundations. This study aims to investigate the evolution characteristics of local scour around monopile foundations subjected to reversing tidal currents with different flow velocities and directions through large-scale physical model experiments.
The experiments were conducted at the Tainan Hydraulics Laboratory, National Cheng Kung University, using a large flume measuring 60 m × 7 m × 1.2 m. A Froude similarity scale of 1/36 was adopted, corresponding to a prototype monopile diameter of 6 m. The model monopile, with a diameter of 0.167 m, was vertically installed in uniform sand with a median grain size (d₅₀) of 0.15 mm. Four flow velocities (0.125 m/s, 0.167 m/s, 0.208 m/s, and 0.250 m/s) were tested, representing prototype tidal current velocities ranging from 0.75 m/s to 1.5 m/s. Each experiment was carried out for a total duration of 600 minutes, including alternating forward and reverse flow conditions to simulate reversing tidal currents.
The results show that local scour develops rapidly during the initial stage of the forward flow and gradually approaches a stable development trend over time. After flow reversal, partial backfilling occurs within the existing scour hole, while new scour develops on the opposite side of the monopile, resulting in asymmetric scour morphology. Increasing flow velocity leads to deeper maximum scour depths and faster scour development. The experimental results provide useful reference data for understanding local scour behavior under reversing tidal currents and for the design and safety assessment of offshore monopile foundations in tidal environments.

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