應用DEM區塊法探討八掌溪河道變遷

Abstract

本研究針對八掌溪之行圓橋至國道3號八掌溪橋河段，使用1999、2002、2006及2009年之歷史航照經由空中三角測量、影像匹配等程序建置河道地形數據。研究目的為使用歷年數值高程模型與正射影像了解河道變遷狀態，包含分析河岸與河床之變遷情形，以及探討其變遷因素。河岸變遷主要觀察歷年河岸線平面變化與河岸地形高程變化；河床變遷則是觀察河道邊界內地形高程變化，藉此評估河床整體泥沙沖淤情形。在時間序列中，河道的變化具有立體性質，若將多期數值高程模型相減，獲得高度與體積之變化速率後，亦可提供另一維度的河岸線資訊，因此另一研究目的為比較套疊河道邊界比較法與區塊法所獲得之河岸變遷分析成果，並從中探討其成果之同異處。套疊河道邊界比較法是由人工於正射影像上判釋並描繪河道邊界，再將多時期的河道邊界套疊，藉此分析歷年河道平面變化情形；區塊法則是結合區塊與DoD (Difference of DEMs)，計算河岸變遷速度與河岸體積變化量。 若以區塊法計算河岸整體區塊的變遷速度平均值，可發現相對於1999年的三個時期之左右岸皆以後退現象為主，且多個區域皆有持續性的後退，左岸雖僅有後退7.20至9.62 cm/yr，右岸也只有後退1.34至7.07 cm/yr，但若將左右岸切成多個區塊後，即可發現河岸後退主要集中在某些區域。套疊河道邊界比較法與區塊法所發現的河岸變遷位置幾乎一致，但也有少數區域有明顯河岸拓寬情形，而區塊法分析成果卻顯示該區域只有些微變遷，或有些位置雖河岸拓寬不明顯，但區塊法卻認定該位置河岸變遷顯著，主要原因是區塊法分析公式考量了河岸地形高程變化量，故若能結合兩方法共同探討河岸變遷應會更為客觀。

This study employs DEMs and ortho-photos generated via aerial triangulation and image matching technique from the 1999, 2002, 2006 and 2009 historical aerial photos to analyze the geomorphologic and change factors of Pa-Chang River (Xingyuan Bridge to Pa-Chang River Bridge). The major work includes the analysis of river bank and river bed. The observation of river bank includes the changes of river channel and bank geomorphology. River bed analysis focuses on the topographic changes in the river boundary. Due to the changes of river banks are in 3D, taking the multi-temporal DEMs differences to calculate the elevation and volume changes provides another aspect of bank analysis. Compartment analysis method and Difference of DEMs (DoD) combined together can analyze the river channel changes. The result is validated with the changes identified from manual interpretation with multi-temporal ortho-photo. With the integrated method, both right and left bank were identified as retreating. The overall retreat rate in left bank and right bank ranges from 7.20 to 9.62 cm/yr and 1.34 to 7.07 cm/yr, respectively with concentration in several compartments. Moreover, the experiments indicated that the results are quite similar with manual interpretation. Discrepancies were observed in compartment with large areas where erosion and accumulation balanced out in the same compartment. Compartment analysis and DoD schemes are complimentary to each other.