以空照應用PIV探討大安溪蘭勢大橋河段變遷

Abstract

河川除受水流侵蝕垂直下切外，水平地貌亦受搬運與堆積作用影響甚劇。本研究使用具時序性之正射影像計算其水平位移，觀察921地震後大安溪蘭勢大橋上游河川行水區的變遷情況。傳統地滑監測藉由歷年航空照片進行人工立體判釋，確立欲調查之範圍後，實際進入研究區域設立GPS控制點施以多時期測量，求得地滑區位移情況。此方法耗費許多時間和金錢，且不同操作者判釋之成果也不一致。利用PIV技術計算影像位移場，可大幅降低進行現地調查所花費之時間、人力與金錢成本。 本研究使用90年~97年共8個年度的大安溪大峽谷河段正射影像，分別以SIFT以及互相關函數法進行影像位移場計算，在90~91年觀察到研究區內行水區靠上游段有往兩岸擴張情形，靠下游段則有縮減情況；91~92年間行水區無明顯移動；92~93年行水區靠上游段仍有往兩岸擴張情形；93~94年行水區有整體往往東北方遷移的趨勢；94~95年靠下游段有整體往西南方移動的趨勢，靠上游段則有往東北東遷移的趨勢；95~96年行水區沒有明顯遷移情況，僅在靠下游段有沙洲堆積情形；96~97年除了靠下游段沙洲往西北西方向移動，行水區有整體往東北方移動的趨勢。

The horizontal terra of rivers is also greatly influenced by transmission and accumulation besides vertical erosion. This study use a time serise of orthophotos to detect horizantal displacement of Daan Shih Lan-Shih Bridge Section after Chi-Chi earthquake.In tradition monitoring landslide, after establishing the scope of investigation by conducting annual aerial photograph, researchers visit the region and set up the GPS control points for imposing multi-period measurement to obtain the displacement of slide area.However, the results are inconsistently acquired by different operators, and such procedure costs a considerable amount of time and money. The use of PIV could solve the problem and substantially reduce the cost of time labor, and money in the on-the-spot investigation. In this study, we calculate the displacements by PIVview and SIFT with a series of orthophotos of Daan Shih Lan-Shih Canyon section from 2001 to 2008. From 2001 to 2002, the river boundaries expands in the upstream section while scales down in the downstream section;2002 to 2003, there is no significant movement of the river boundaries;2003 to 2004, the river boundaries continuously expands in the upstream section; 2004 to 2005, the river boundaries has a trend to entirely move towards northeast; 2005 to 2006, there is an overall trend to southwest by the downstream section, while a trend to east-northeast by the upstream segment;2006 to 2007, there is no significant migrations of the river boundaries, only accumulation of the shoal by the downstream; 2007 to 2008, in addition to the shoal moving to the west-northwest by the downstream section, the Daan river has an entire movement towards northeast.