The Topography and Morphology of the Taoyuan Offshore Seabed

利用多年蒐集之近岸單音束和多音束測深，以及側掃聲納資料，針對桃園市海岸向外海延伸約15公里以內的區域做分析研究。這個區域位於桃園台地坡腳向外海延伸的地帶，其形貌和北方新北市海岸不同，也和其南方新竹縣的海岸景觀不同。根據水深資料畫出的地形圖顯示桃園外海地形離岸坡度屬於較陡的地區，越往南坡度越陡；因此，位在最南側的新屋區，離岸7公里水深會降到80公尺深(坡度~0.85度)，接近台灣海峽陸棚區的深部深度，此坡度值為林口台地海岸離岸坡度的兩倍。資料也顯示此區域離岸在近岸區看不到比較明顯的近岸台地，海岸坡度由岸向外表現得很一致，不像桃園和新北市交界的北部海岸，例如林口台地外側，水深40公尺內會出現一個連接海岸帶的平坦台地地形。側掃聲納海床影像資料指出，桃園外海域往北海床上會有大型礫石分布的現象，直徑達30公分以上的石頭，會往外海海床上散落，越接近林口台地外海區域這現象愈明顯。往南延伸的桃園海岸帶由北方的蘆竹區到至新屋區，全長約40公里，沿岸的地形與地貌有明顯變化，由北而南由卵礫石海床、變成沙質海床、到藻礁質海床，且外海會有中型的沙丘出現；資料指出，在南方的觀音區，大型沙丘會出現在最靠近岸30幾公尺深的海床上，但出現的區域侷限，似乎和海床上的底質分布特性有關。本研究除了介紹近岸海床地形地貌的形貌外，也會探討分析它們是否和相鄰陸地的地形地質存在有特殊的相關性。This study is the analyzes the coastal zone 6 km away from shoreline of Taoyuan, with years of surveys from Single-beam Echo Sounding System, Multi-beam Echo Sounding System, and Side Scan Sonar. The coastal zone is located at the region extending from the edge of Taoyuan Plateau. Its topography differs from those of in New Taipei City and Hsinchu. According to bathymetric map, the slope of the Taoyuan’s offshore topography is steeper than that of western coast. The more southern of the western offshore topography, the steeper the slope is. Therefore, the water depth of 7 meters from the shore of the most southern part of Taoyuan, Xinwu District, goes to 80 meters. The drop height is near that of the continental shelf of Taiwan Strait. Besides, its slope is one time higher than that of offshore from Linkou Plateau. The image database from Side Scan Sonar also indicates that there are large gravels scatter on the seabed along Taoyuan offshore to the north. The gravels are more than 30 cm in diameter. The coastal zone from the north to south of Taoyuan, from Luzhu District to Xinwu District, has a total length of approximately 40 km. The topography of its seabed change from the north to the south, including gravels, sands, and algal reefs. In addition, medium sand dunes also appear here. In accordance with the bathymetric map, large sand dunes show up on the seabed to a depth of over 30 meters, situated nearest the shore of Guanyin District. This study not only provides an introduction of the characteristics of the seabed topography in nearshore coastal zone of Taoyuan, but analyzes the relation between the seabed topography and adjacent continental physiographic and geologic