以多期數值地形資料評估山崩區及河道地形之變遷

Abstract

近年來遙測技術在硬體和資料處理上不斷地發展，數值地形資料的精度與解析度也隨著不斷提升，高解析度數值地形資料是一項可以廣泛應用於國土管理與防災減災的基礎工具，也在許多領域可以發揮其效益，尤其是在地質科學領域中如山崩、河道地形的變遷等重要課題。經濟部中央地質調查所自2005年開始以空載光達技術進行數值地形資料測製，至2016年將完成臺灣全島1m x 1m的高解析度數值地形資料。本論文則以地調所產製之數值地形資料為出發點，選擇發生大規模崩塌及受颱風季節影響的河道範圍，在這些範圍進行重複空載光達測製作業或以歷史資料產製不同時期的數值地形資料，藉由多期數值地形資料的比對，以了解這些造成地形變遷的地質作用。 本論文以多期數值地形資料分別進行3個不同主題之地形變遷的研究。第一主題為利用航空攝影測量、空載光達、無人載具等不同遙測技術所獲取之6個不同時期的數值地形模型資料，探討資料誤差分析及山崩後地形變遷之應用。多期數值地形資料的應用裡，誤差分析結果為重要的關鍵訊息。本研究發現在植生茂密、地形陡峭之高山地區，可應用至少1期之空載光達產製之高程資料為評估基準，來解決高山林區高程檢核點實測困難而量不足的問題，結果顯示精度也可符合分析需求。由各期資料產製及比對過程誤差分析之結果驗證，這些多期資料能有效應用至山崩之地形變遷比對。經由多期資料比對，南投眉原山崩塌後之土方量搬運，主要受豪大雨事件影響其土砂遞移率而逐年遞減，此崩塌在發生後數年仍有約半數土方量仍留存於集水區內。研究成果也說明多期航空攝影測量(含無人機)和空載光達數值地形資料，可以在坡地災害後觀測於下游河道地形變遷及其潛在影響。 第二主題為藉由多時序的數值地形資料比對可以直接觀測與判釋地形變遷之過程，本研究選擇在1999年集集地震所誘發之草嶺山崩地區，使用3期空載光達數值地形資料，探討歷經約10年後之蝕溝侵蝕的發展。採用固定間隔斷面取樣以及條帶式剖面(Swath profile)連續取樣等2種蝕溝基岩下切速率計算方法，其中連續斷面方法獲取較完整的蝕溝變化資料，可為較合理也便利之方法。藉由多期數值地形資料的比較加上誤差評估，可以直接呈現不同時期地形演化，而能減少費時且大量的野外工作。以3期空載光達數值地形資料所求的坡面蝕溝基岩侵蝕下切速率，結果界於0.23至3.98 m/yr，呈現出基岩抗侵蝕能力與所組成岩性有關，且颱風豪雨事件確實為引發地形變化主要作用，本區下切速率也明顯快於其他相關研究，反映本區域快速之侵蝕作用也是影響山崩多次發生、循環的因素之一。 第三主題為在2008年蘭陽溪地區經歷卡玫基（Kalmaegi）、鳳凰（Fung-Wong）、辛樂克（Sinlaku）、薔蜜（Jangmi）等四次重大颱風事件，造成河床地形變化甚劇，本研究利用兩次空載光達測製之數值地形資料，分析比較颱風事件前後河床地形以及河道輸砂之變化。研究成果呈現空載光達技術，在大流域面積測製河道地形上的極佳應用，以颱風季節前後飛航測製所產製之不同時期之數值地形資料，應用高程差值之地形計量方法，計算在颱風季節前後河道土石量之體積變化量，也可推估蘭陽溪在此河段颱風季節前後的輸砂量。從河道地形高程值變化，觀察蘭陽溪不同河段所程現的沉積物搬運特性，主要仍受河道坡度所影響。 本論文分別利用空載光達、立體航測、無人飛行載具等技術所獲取的多期數值地形資料，以多期資料比對方法能直接觀測山崩與河道地形變遷之過程，另經由誤差評估和地形計測方法，也明確表示出各種數值地形資料應用的範圍和其誤差值。針對這些地形變化所引發的坡地災害，未來可持續利用這些技術來獲得有效的基礎資訊，而提供後續了解地質災害致災潛勢、機制和影響範圍等相關需求。

In recent years, the precision and resolution of digital terrain model (DTM) data are constantly increasing with the advancement of the hardware and data processing of remote sensing technology. High resolution DTM is the basic data that could be widely applied in land management and disaster prevention, and also shows its analytical benefits in many fields, especially in geological science such as landforms, landslides, and fluvial changes. Central Geological Survey, MOEA has carried out LiDAR-DTM survey since 2005, and complete high-resolution (1m x 1m) LiDAR-DTM data of the whole island of Taiwan in 2016. In this dissertation, we chose the areas affected by large-scale landslides and the river channels affected before and after typhoon season, and used repeated airborne-LiDAR survey or historical data processing to acquire multi-period DTM data. We analyzed the terrain changes from the multi-period DTM data to better understand the effects of landslides and fluvial evolution. The dissertation mainly consists of three subjects. In the first subject, six different periods of DTM data obtained from various flight vehicles by using the techniques of aerial photogrammetry, airborne LiDAR (ALS), and unmanned aerial vehicles (UAV) were adopted to discuss the errors and applications of these techniques. Error estimation provides critical information for DTM data users. This study conducted error estimation from the perspective of general users for mountainus and forest areas with poor traffic accessibility using limited information, including error reports obtained from the data generation process and comparison errors of terrain elevations. Our results suggested that the precision of the DTM data generated in this work using different aircrafts and generation techniques is suitable for landslide analysis. Especially in mountainous and densely vegetated areas, data generated by ALS can be used as a benchmark to solve the problem of insufficient control points. Based on digital elevation model (DEM) differencing of multiple periods, this study suggests that sediment delivery rate decreased each year and was affected by heavy rainfall during each period for the Meiyuan Shan landslide area. Multi-period aerial photogrammetry and ALS can be effectively applied after the landslide disaster for monitoring the terrain changes of the downstream river channel and their potential impacts. In the second subject, we used three periods of ALS DEM data to analyze the short-term erosional features of the Tsaoling landslide triggered by the 1999 Chi-Chi earthquake in Taiwan. Two methods for calculating the bedrock incision rate, the equal-interval cross section selection method and the continuous swath profiles selection method, were used in the study after nearly ten years of gully incision following the earthquake-triggered dip-slope landslide. Multi-temporal gully incision rates were obtained using the continuous swath profiles selection method, which is considered a practical and convenient approach in terrain change studies. After error estimation and comparison of the multi-period ALS DEMs, the terrain change in different periods can be directly calculated, reducing time-consuming fieldwork such as installation of erosion pins and measurement of topographic cross sections on site. The gully bedrock incision rate calculated by the three periods of ALS DEMs on the surface of the Tsaoling landslide ranged from 0.23 m/yr to 3.98 m/yr. The local gully incision rate in the lower part of the landslide surface was found to be remarkably faster than that of the other regions, suggesting that the fast incision of the toe area possibly contributes to the occurrence of repeated landslides in the Tsaoling area. In the third subject, the Lanyang River region experienced four major typhoon events in 2008: Kalmaegi, Fung-Wong, Sinlaku, and Jangmi. These typhoons changed the riverbed terrain dramatically. Therefore, we used two occasions of digital terrain data measured using the airborne LiDAR system to conduct analytical comparisons of the riverbed terrain and river sediment transportation changes before and after typhoon events. Our results shows the excellent applications of the airborne LiDAR system for measuring river terrains in a large watershed area; digital terrain data were produced for periods before and after a typhoon season. We used the elevation difference topography measurement method to calculate river terrain changes before and after a typhoon season. This study’s results showed that the airborne LiDAR high resolution digital terrain model data produced excellent results when used to calculate river terrain and sediment volume changes or change amounts. In this dissertation, multi-period DEM differencing was applied to study the processes of landslides and fluvial morphological changes by means of airborne LiDAR, photogrammetry and unmanned aerial vehicle. Through error estimation and geomorphometric analysis, the errors of DEMs and their applicability in topographic analysis were stated and systematically discussed. For disasters caused by the terrain changes on slopes and channels, the proposed methods in the dissertation can be used to analyze and understand the disaster potential, mechanisms and affected areas, thereby, offering useful information for the assessment of geological hazards.