1999年集集地震後濁水溪河道變遷

Abstract

短時距的河流地形演育是很複雜的，了解十年尺度的河流變化是地形學一直以來想要關注的目標。然而十年尺度的河流快速變化的性質，使得保留這些變化較長時間尺度來得更困難。西元1999年發生在台灣中部的集集地震（Mw = 7.6），沿著南北走向的車籠埔斷層產生了約100公里長的地表破裂，切穿了台灣中部許多河流，其中包含台灣中部地區流量最大的河流——濁水溪，根據野外調查，通過濁水溪的地表破裂約為3公尺。濁水溪有非常強的河川侵蝕功率（stream power），且在地震後上游興築集集攔河堰，這些因素都會加速河道地形的改變。因此在如此大的地表錯移與人造建物的相互影響下，濁水溪自名竹大橋到集集攔河堰河段提供了了解河流地形如何快速演變的獨特機會。 本研究利用航空影像帶入SfM-MVS的方法產製DSM與正射影像，並展示自1999年地震以來歷年的地形變化。根據數化正射的結果，2009年之前的河道是以沖積型河道為主，而2009年後轉為底岩型河道；另外，河道型態從1998年到2016年間可分為三個階段，原始的辮狀河道轉為曲流最後形成單一且直的河道。利用河道縱剖面結果分析，在2009年之前河道遷急點（knickpoint）在車籠埔斷層投影範圍內來回移動，直到2011年開始後退，直到2016年的後退量約為1000公尺。另外2009年到2016年間河道底岩下切為5-9公尺，這樣的下切量已經超過了同震抬升。本研究結果顯示，這樣的變化是同時受到構造運動與人類活動的交互作用所致。

Short-term fluvial process, which could be influenced by stream power, channel morphology, lithology, or anthropogenic activities, is very complicated. Understanding decadal river changes is a long-standing target in geomorphology, but the channel rapidly changes rarely allows for detailed measurement of landscape processes and evolution. The 1999 Chi-Chi earthquake (Mw =7.6) in central Taiwan created an approximately 100-km-long surface rupture along the north-south trending Chelungpu fault cutting through lots of east-west trend river. Especially in the Choushui River, which is the largest river in Taiwan with very large stream power, along with the fact that the surface rupture across the Choushui River is around 3 meters based on field observations, it is an opportunity to examine how the fluvial landscape evolves under rejuvenation of large coseismic uplift and following processes. In order to evaluate the history of channel morphology and processes of knickpoint after the earthquake, we construct DSMs and orthoimages derived from aerial photos by using the SfM-MVS method and evaluate annual river topography changes since 1999. According to the longitudinal profiles, transverse sections and channel mapping of Choushui River derived from the DSMs and orthoimages, the channel changes are not spatially and temporally steady. The first decade after earthquake, the channel pattern changes from braided channels to one-channel river by sweeping surficial sediments. The knickpoint retreated ~1000 meters from the fault scarp between 2011 to 2016. There are 5-9 meters of incision between 2009 to 2016, which is more than coseismic uplift consistent with the observation in Ta-an River. As a result, the dynamic channel response is an interaction from tectonic movements and human actives.