七股沙洲位於臺灣西南沿海，屬於典型障壁島 (barrier island) 與潟湖系統，具有阻隔外海波浪能量、保護內側潟湖與濕地環境之重要功能。然而，受波浪、海流與極端氣候事件作用下，岸線以每年 10 至 28 公尺之速率持續向東南退縮，同時也面臨沙洲破裂與潮口擴張等危機。為因應此危機並尋求有效之防護對策，本研究以 XBeach 數值模式為主要工具，建立七股沙洲海域之高解析巢狀網格模式，模擬 2022 年七股海域之水動力與地形演變過程，接著利用將軍潮位站、七股浮標站及歷史地形變遷資料進行水動力及地形變遷驗證。本研究研究驗證良好並進一步以人工養灘作為海岸防護對策。設計不同養灘情境，分別探討養灘放置位置、養灘量體與沿岸配置長度對七股沙洲侵淤變化與防護效益之影響。透過岸線變化量、斷面高程變化、侵蝕堆積分布、平均淨變化及養灘砂體流失情形等指標進行綜合評估。研究結果顯示，網仔寮沙洲北端之潮口在潮汐驅動下形成水位差與水道束縮效應，導致潮口流速明顯增加並產生劇烈淘刷，造成破口大幅擴張。在長期波浪與海流交互作用下，網仔寮沙洲呈現向東南遷移趨勢，全年平均岸線退縮達 26.27 公尺，反映出沉積物持續流失且沙洲向內陸推進之自然演變趨勢。在人工養灘模擬結果顯示，養灘放置位置為影響防護效益之主要因素，將養灘體以配置於灘線前之方案防護效益最佳。增加養灘量體或改變沿岸配置長度雖可影響局部地形變化，但防護效益並非與養灘量成正比，其中，養灘體沿岸長度為變因中的寬廣配置，為本研究各養灘情境中防護效益最佳之方案；其平均岸線退縮量由無養灘情境之 26.27 公尺降至 18.11 公尺，且平均變化量由-48.363m³/m 減至 -26.119 m³/m，顯示適當延伸養灘配置範圍可有效提升網仔寮沙洲之整體防護能力。

Qigu Sandbar forms the seaward boundary of the Qigu barrier-lagoon system in southwestern Taiwan and protects Qigu Lagoon, wetlands, aquaculture areas, and the low-lying hinterland. The barrier has experienced persistent shoreline retreat, inlet enlargement, sediment loss, and southeastward migration under the combined effects of monsoon waves, typhoons, tidal currents, reduced sediment supply, and coastal development. This study established a high-resolution XBeach model to identify the principal erosion mechanisms of Wangzailiao Sandbar and to evaluate alternative beach-nourishment configurations.
The simulations were driven by full-year conditions for 2022. Wave and wind data were obtained from the Qigu buoy, while water levels were reconstructed from eight major constituents in the TPXO10-atlas database. A two-level nested-grid system was applied. The outer grid covered approximately 39 km by 32 km at a 500 m resolution, and the inner grid covered approximately 9 km by 10 km with 148,830 grid points, a minimum cross-shore spacing of 10 m, and an alongshore spacing of 20 m. XBeach was operated in Surfbeat mode using the Van Thiel-Van Rijn sediment-transport formulation, a median grain size of 0.25 mm, a 5 m active sediment layer, and the avalanching mechanism.
The model reproduced observed water levels, waves, and tide-dominated reversing currents with acceptable agreement. The simulated southeastward migration, northern-inlet enlargement, and mean annual shoreline retreat of 26.27 m were also consistent with the historical retreat range of 10-28 m per year.
Eight scenarios were examined: one no-nourishment baseline and seven nourishment cases addressing placement depth, nourishment volume, and alongshore length. The no-nourishment case produced a mean net sediment-volume change of -48.363 m³/m. Shallow shorefront placement provided stronger shoreline feeding than placement near the 4.74 m inner depth of closure or in deeper water. Nourishment volume showed a nonlinear relationship with average shoreline protection. The 1,000 m broad configuration, Case 7, provided the best system-wide performance, reducing mean shoreline retreat to 18.11 m, an improvement of 8.16 m or 31.1%, and improving the mean net sediment-volume change to -26.119 m³/m. The results demonstrate that nourishment performance is controlled primarily by placement geometry and exposure to active nearshore transport rather than by sediment volume alone.

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