台灣北部及西部外海近岸海床地形分析之研究

Abstract

臺灣周圍的淺水海域，因水下地形資料不如陸地上的地形資料豐富，且未有人作系統化的整合與資料判識工作，以致於不能十分了解臺灣周圍海域的地形或地貌變化。本研究將搜集到的單音束水深資料經編輯校正後，透過相交測點偏差量之檢核法，分析資料之品質與誤差量，再配合相關之規範規定(如國際水文組織)進行編整，將資料做最完善的運用，以提高整體資料的可用度。 依據結果，繪圖討論的地形區分為：一、北部近岸段，以淡水河出海口處為分界點，向北海岸延續經鼻頭角至麟山鼻外海的地形特徵，屬噴發形成的大屯火山區的外圍地貌，由陸向海延伸的地形表現顯著；向南海岸則屬林口台地外海，地殼抬升後受河流切割侵蝕後影響，水下地形相對於北岸，較為平緩。在水深15∼50公尺處，北岸坡度約為1/110(0.5度)；南岸坡度約1/192(0.2度)，呈現北陡南緩的地形特徵；二，海峽北部、中部近岸段，主要是受河流輸出物在海床堆積搬運作用及潮汐影響，在地形地貌上，有著其顯著的海床沉積物搬運的特殊形貌，除了在台中外海一帶近岸區存在一個寬約10公里、南北延伸30公里的構造台地外，在水深40公尺以外的海床上，可以看到很多明顯的大型海底沙丘，使得等深線在此區顯得較為紊亂，但由等深線的分布可推論沉積物在此由南向北搬運；三、海峽南部近岸段，沿岸有廣大的潮浦地，一個位在東石外海的外傘頂沙洲，另一為位在南邊曾文溪外發育較早的海埔新生地；在這兩個沙洲的尖嘴處其離岸坡度最陡，坡度在5度以內，水深值由20公尺下降到120公尺。 另外，在彙整出的地形圖上，近岸地形受陸域構造延伸至海外的有龍洞斷層、崁腳斷層與金山斷層等。在西部外海，只有在高雄港北方外海出現一個嵌向陸地的線型低區，其形成可能和由陸地向海延伸的旗山斷層的左移運動有關。

There had been no sufficient bathymetrical data as abundant as those collected on land in nearshore area off Taiwan coast, such that systematical gathering of bathymetrical data was undone and there was merely a few studies related with the morphology or underwater topography research off the coast of Taiwan. In this study, single beam sounding data had been gathered and checked using the cross over check method. After that, sets of data had been edited by ruled out those inaccurate measurements. The data thus was used to make the plots according to density distribution and precision of data sets. Area off the coast was divided into the following regions: 1、Northern region, northern part of the coast extended north from DanShuei River, it passes Pitoujiao to Linsanpi, and surrounds the Datun volcanic mountain. The ocean bottom behaves same feature as shown on land, and the descending slope near the coast is at 1/110, which is steeper than the slope shown in coastal area south to DanShuei River, gradient at 1/192, in which is the area extended from Linkou Terrace.；2、Middle strait region, this portion of seabed has been covered by recent sediments, where the sediments have been transported under the influence of tidal flow. Except in the offshore region outside Taichung coast, where a structural coastal terrace extended offshore from coastal line for 10 kilometers, and along shore for 30 kilometers long. Seafloor in this region is covered by sand waves, and together with the pattern shown by underwater topography, it is indicated that sediments have been moved from south to north in this area..；3、South strait region, it is occupied by a vast area of tidal flat; there are two tidal flats in the region, northern one is called Waisandingjou,which is located offshore of Tonshi, southern one is located off Tseng-wen River estuary. Two tidal flats have formed in triangle shape, with their headlands being the place with greatest descending slope off the coast line. They all show with about 5 degree descending angle from 20 meters to 120 meters water depths. We can observe possible topographical lineation of Lungdung Fault、Kanchiao Fault and Chinshan Fault on land , extended into the ocean floor in the bathymetrical maps produced in this study. In addition, a triangular shape linear depression is shown in the northern offshore seafloor of Kaohsiung Harbor area. This depression is located just outside of Chishan Fault on land, and was formed by left lateral transcurrent motions.