| 摘要: | 媚公三角洲長期受到複雜機制交互作用，導致嚴重的土地流失、設施毀損、影響生態以及沿海養殖產業。因此掌握淤積及侵蝕機制對於制定適當海岸保護策略為至關重要之議題。本研究旨在於調查媚公河口海岸線變化。研究使用陸地衛星影像推估海岸線變化速率，以及利用Delft3D模式建立水力模型評估海岸線變化行為。結果顯示自1988年起至2021年止，區段3處之最大侵蝕率為33.9 m/year而最大淤積率為86.3 m/year。其侵蝕行為主要發生於凸岸，淤積行為則頻繁發生於凹岸處。水力模型對於預測水位之表現良好。An Thuan觀測站及Ben Trai站之觀測與模擬水位之間有相當良好之吻合，其綜合表現相關係數皆超過0.8。Binh Dai 站以及Tran De站之綜合表現相關係數皆超過0.69，唯獨Binh Dai站之NSE僅顯示合理結果，而結果介於0.54 ~0.64。Vung Tau站因鄰近邊界，其綜合分析成果良好，有較高相關係數，但其NSE較差。模式結果顯示，河流流量和海洋動態隨季節而變化。乾季河川低水位之流量小於7000 m3/s，東北季風造成東北-西南方風向，其風速主要為5 至10 m/s 以及10 - 15 m/s。對該地區的植被和波浪、潮汐造成影響，導致它們直接侵蝕海岸線凸岸。海洋動力將海淡水交界面由東北推向西南方靠近海岸線處。雨季河水入海量達22000 m3/s，西南季風使風從西南向東北吹，風速為5至10 m/s。因此，海動力因素與豐水期河流流量和豐富的泥沙來源相結合，凹形岸線泥沙沉積提供適宜的紅樹林沼澤生長條件。根據岸線變化和水動力模型結果，該區海岸線變化機制可能受到岸線位置、水動力季節變化和海岸線沉積物特徵的影響。最終，研究結果可作為擬訂保護沿海地區適當策略的參考。;The Mekong Delta shoreline has suffered complex variations that induced severe land loss and infrastructure destruction, impacting eco-tourism and coastal aquaculture areas. Therefore, it is necessary to understand the accretion and erosion mechanisms to introduce appropriate strategies to protect the coasts. This research aims to investigate the shoreline variations in the Mekong estuary zones. The study utilized Landsat satellite images and Delft3D software to estimate shoreline change rates and develop a hydrodynamic model to evaluate shoreline variations behavior. The results show that from 1988 until 2021, the maximum erosion rate was 33.9 m/year, and the maximum accretion rate was 86.3 m/year in segment 3. The erosion mainly occurred in the convex shoreline, while accretion frequently occurred in the concave shoreline. The hydrodynamic model performance generally is good in predicting the water levels. An Thuan and Ben Trai stations gave excellent results over 0.8, indicating high agreement between simulated and observed water levels. Binh Dai and Tran De stations, which produce good results, all metrics are over 0.69, except for the Binh Dai station′s NSE metric, which gives reasonable results (0.54 – 0.64). Vung Tau station is adjacent to the boundary, and the metrics are good, except the NSE is poor. The model results revealed that the river flow and sea dynamics vary by season. In the dry season, when the river flow is low, under 7,000 m3/s in a day, the northeast monsoon makes the wind blow in the northeast-southwest direction with the speed of 5-10 m/s and 10 - 15 m/s dominantly, causing impacts on the vegetation and waves, tides, in this area, leading to them directly attacking the convex shoreline causing erosion. Sea dynamics push the fresh-saltwater interface from the northeast to the southwest and are located near the shoreline. In the wet season, river water discharges into the sea up to 22,000 m3/s in a day, and the southwest monsoon makes the wind blow from southwest to northeast with the speed of 5-10 m/s. Thus, sea dynamic factors combined with abundant river flow and sediment during the wet season generated suitable conditions for sediment deposition in the concave shoreline and developing mangrove marsh and tidal flat. Based on the shoreline variations and hydrodynamic model result, the shoreline change mechanism in this area might be impacted by the shoreline position, seasonal variations of hydrodynamics, and the shoreline sediment characteristics. Eventually, the study findings can be used as a reference for developing adequate strategies for protecting coastal areas. |
