中臺灣海峽近岸區沙波的遷移及演化

Abstract

沙波為常見於淺水或近岸海域之海床形貌，因沉積物與海流相互作用下所形成，並會隨潮流遷移，進而可能對海事工程的安全與營運造成影響，故成為重要的研究議題。過去研究發現臺灣海峽因其地質環境與潮流作用，海域內有大範圍的沙波發育，但由於缺乏全面、重複性及高解析的調查資料，過去對於沙波的遷移與演化尚未能詳細瞭解。本研究透過高解析多音束水深及反射震測資料，結合區域海水面變化與近岸區的陸域岩心資料，分析與探討中臺灣海峽近岸區沙波之形貌、遷移的時空特性及發育過程。地形分析顯示區域內沙波之波長為35至295公尺，波高為0.6至8.2公尺，於兩年半間最大移動量達95公尺，最大高程變化達6.7公尺，且沙波變遷方式有垂直於波向遷移及沿著波向匯聚增高兩種方式。而根據5次地形資料的分析結果，在兩年半間沙波移動平均速率為每月0.76公尺，但於兩個月間卻為每月10.31公尺，且在小於400公尺之距離內可觀察到移動方向相反之沙波遷移模式，證明沙波遷移不僅具有往覆性，時空變化也具高度複雜性，呈現非線性變化模式。震測及層序地層分析結果顯示，研究區域之地層演化可分為四期。第一期(14ka前)：於末次冰盛期陸棚出露於地表形成侵蝕面；第二期(14ka-8ka)：海進時期隨海水面快速上升，研究區域轉變為河口及濱海環境，沉積物向上加積；第三期(8ka-4.5ka)：海水面上升減緩，進入高水位時期，由濱海轉變為淺海環境，海峽的物理海洋環境也趨於穩定，開始接收自陸源向海進積之沉積物；第四期(4.5ka至今)：穩定高水位時期，沉積物在潮流作用下於近岸地區形成許多沙波及沙脊，且在海床下2至15公尺深處，形成沙波基底面。本研究透過多重地球物理方法，分析臺灣海峽近岸區的沙波地形特徵、變遷方式並提出其地層環境之演化模式，希望對於後續海事工程發展能有所幫助，但結果也顯示沙波變遷呈現複雜地非線性變化，若能再配合更多次的觀測及收集小尺度的流場資料，相信將能對臺灣海峽的沙波特徵與變化有更進一步的瞭解。

Submarine sand waves are common bedforms observed in shallow or nearshore seas, they are formed under the interaction of tidal flow and non-cohesive deposits, and can migrate with the tidal current. As the migration of sand waves may cause hazard to offshore installations, it has been an important study issue. Previous studies show that a large number of sand waves have been developed in the Taiwan Strait, induced by the high sediment supply and tidal effects. However, due to lacking of high-resolution and repeated geophysical surveys, detailed characteristics and migrating features of the sand waves in Taiwan Strait were poorly understood. In this study, we use high-resolution bathymetry data and seismic reflection profile data, combined with other geophysical and geological information to investigate the characteristics, migration and evolution of sand waves in the nearshore area of Central Taiwan Strait. The results of bedform analyses show that in the study area, the wavelengths of sand waves range from 35 to 295 m, their heights range from 0.6 to 8.2 m, the migration distance can be up to 95 m in 2.5 years, and the seafloor elevation difference can be up to 6.7 m. In terms of the bedform changes, there are migrations orthogonal to the sand wave ridge crests, and also growth and stretching of the sand wave ridges. From repeated mapping data at various time-scales, the migration rate was 0.76 m/month from a 2.5-year period observation, but from the 2-month period observation data, the migration rate was 10.31 m/month. Based on the analyses of sequence stratigraphy and seismic stratigraphy, we propose that there are four stages in the depositional environment evolution of the study area. In the first stage (before 14 ka), the continental shelf was initially terrestrial under fluvial erosion in the Last Glacial Maximum. In the second stage (14 ka-8 ka), land became inundated as the sea level rise, and transitioned from shoreface to offshore. In the third stage (8ka-4.5ka), the rising of sea level came to a pause, tidal environment of the strait became stabilized, and sediment started to prograde seaward to the shallow marine environment. In the fourth stage (4.5ka to present), accumulated sediment that aggraded at the river mouth formed the river delta and then prograded seaward, which induced the formation of sand ridges and sand waves in nearshore area, and the modern sand waves are proposed to develop and migrate with the sand ridges in this stage. By combining the analyses of seafloor topography and sediment stratigraphy, we characterize the morphology, migration patterns and the complexity of sand wave bedform changes in the Taiwan Strait, and suggest more intensive and smaller scales of physical oceanographic observations are needed to assist marine engineering in the future.