九二一地震對於大甲溪及大安溪河床演變之影響

Abstract

The northern segment of Chelunpu fault passed through Dajia and Daan River. It induced 11.2 m liftings near Shinkang Dam, 15 m at Meizih Bridge and 5~6 m in Zhuolan after 921 earthquake. Furthermore, the stratum lifting has become the main factor of river morphology. The earthquake has already occurred for over ten years since 1999, and the behavior of sediment transport, which caused the change of river morphology in Dajia and Daan basin has been changed. Therefore, the purpose of the study was to analyze the changing process of river morphology, river planform, longitudinal, lateral changing and quantification of souring of the channel bed. The study chosed six channel characteristic factors, which are channel profile, river width, cross-section area, swinging of main channel and unit stream power, to analyze the behavior of Daan and Dajia River during the past ten years. The study discusses the change of the river morphology which was due to the 921 earthquake. by analyzing the orthophoto base maps, aerial photos and the river cross section data over pass 10 years. Finally, we found the river morphology changing process is quite different between Dajia and Daan River. The sediment deposition in Shinkang Dam result in baselevel uplifting of upstream reach; therefore, the incision rate of the uplifting block reach in Dajia River are less than the uplifting reach in Daan River. Furthermore, the earthquake lead to rapidly block uplifting formed the knick point and headcutting in this reach. Because the different channel conditions such as positions, geology and the knick point in the watershed. It caused the different responses of river morphology.

車籠埔斷層北段通過大甲溪及大安溪，在九二一地震時造成石岡壩附近垂直位移達11.2公尺，梅子鐵橋附近垂直抬升量更達15公尺；大安溪卓蘭一帶垂直落差約也在5~6公尺，地表大幅抬升，成為地震後河相演變之主導因子。地震後至今已十餘年，上游來砂與河川輸砂行為仍有演變，促使大甲溪及大安溪流域河道型態改變。本研究選取六個因子，針對河道之平面型態、縱橫向及量化其河道斷面之沖刷潛勢，以分析探討河相演變之不同。其中河床演變特性因子分別為：河道平面型態、主深槽高程、槽變量、河寬、兩期斷面積變化及單位河川功率。 本研究利用地震後至今各期影像及大斷面測量資料，進行河道特性分析，探討河相演變之成因。結果得知大甲溪與大安溪河相演變過程並不相同，石岡壩水庫之淤砂如同侵蝕基準面上升，使大甲溪抬升段河道的下切幅度，無須達到與大安溪相同即可取得平衡。此外，地震造成河道快速抬升，使斷層出露處形成遷急點與溯源侵蝕的現象。河道因其在集水區中之位置、地質條件不同，及遷急點是否突破抬升段之不同，造成河相演變上會有不同之反應。