臺灣西南部臺南海岸平原地形變遷之研究

Abstract

The Tainan coastal plain in southern Taiwan had been formed since late Pleistocene. The coast was marked by the offshore bars and lagoons, and have prograded to the west about 13 km since last three hundred years. The large lagoon was named as "paleo Taichiang Inland Sea" in Ching dynasty historical document had been filled up and reclaimed as fish ponds and salt pans due to the rapid sedimentation. However, the west coast of offshore bars have been eroded with a maxium rate about 14 m/yr in Tingtouerh bar in this century. The coastal dunes are distributed mainly in Salun, Tainan and Kunshen. Among them, Salun and Tainan dunes were deposited in Tahu Transgression stage (3500-4000 yr B.P.) where as the Kunshen dunes developed on offshore bars in last three centuries. Besides, a series of dunes is still exsited along the former Tsengwen River. Due to the expansions of the cultivated and built areas, all the dunes have decreased in magnitude. The river channel changed significantly both in hills and coastal plains. In the hills, 13 meander neck cutoffs can be observed in Erhjen River and Tsengwen River. On the coastal plain, river channel shifted by meandering and avulted suddenly in typhoon season because of epesodic sedimentation in the river bed. There were four big channel avulsions recorded since 19 century in Tsengwen River and the range of channel avulsion from north to south is about 25 km. However, the channel was confined after the dyke had been constructed in 1934. Overall, the geomorphological changes of Tainan coastal plain were caused by long term physical processes but modified strongly by human agency in the recent time. The geomorphological changes have induced some impacts on the human beings such as the decline of port, moving of village to prevent flooding, confusion of administrative boundary, etc.