臺灣東部河口地區之地形學研究

Abstract

The vincinity of the estuaries is one of the most populated area in the world. In order to ultilize the resources of this area more reasonably, the understanding of its geomorphological, hydrological and sedimentary characteristics becomes a main and basic task. Since estuary is located on the interface of river and sea, the morphology of estuary is influenced by the factors coming from both environments. The drainage basin area, shoreline length, shape factor, channel gradient, runoff, sediments and monthly discharge, etc. are factors coming from land, while the wind, current, wave and tide etc. coming from the sea. This study focuses on eleven estuaries of the eastern coast of Taiwan. Based on the stream slope, number of channels, discharge, etc., these estuaries can be divided into straight and braided pattern. The former has sandbars or spurs, such as Hualien Hsi, Hsiukuluan Hsi and Kangkou Hsi, while the latter have braided channels, sandy-gravel sediments and changeable sandbars, such as Hoping Hsi, Liwu Hsi and the estuaries in Taitung coastal area. The eastern coast is classified as wave-dominated type according to average wave height and tidal range. The wave impacting upon the area between Hoping Hsi and Hsiukuluan Hsi is severer. The forms of the estuaries in eastern Taiwan have significant seasonal change due to the seasonal differences of discharge, monsoon and typhoon. In the summer, the channels usually become wider and the sandbars are gentler and extended seaward for most estuaries. In the winter, however, these sandbars become narrower, steeper and extend toward right bank. The mouth of channels will be narrowed or even closed when typhoon coming. The forms of estuaries are stable in the long-term scale. Eleven estuaries have only slight change during the period from 1904 to 1989, moving back and forth within the distance of 250m. The channel migration of most estuaries, however, is obvious.