台灣西南部地區泥岩邊坡發育之研究

Abstract

本研究利用地面光達(Leica Scanstation II)，以小尺度來觀察泥岩邊坡地形變動的特徵，並以此歸納泥岩邊坡的發育特徵與模式。研究樣區位於台南市龍崎區牛埔戶外水土保持教室，以及高雄市田寮區月世界，兩者都位於泥岩岩層(古亭坑層)。觀察的事件日期從2011年2月至12月。 研究資料顯示，降雨是啟動鑰匙，未超過啟動閾值時，泥岩邊坡不會發生變動；而隨著降雨量遞增，邊坡侵蝕會從紋溝擴散到整個邊坡，表面侵蝕量也從3-5公分擴大到7-10公分。降雨強度與侵蝕量有較好的對應關係，而邊坡坡度則控制了泥岩邊坡的對降雨的回應方式。高坡度區(>75°)不會受到降雨強度的影響，均呈現出高侵蝕量；中坡度區(30-75°)是變動幅度較小的區域；低坡度區(< 30°以下)則會受到降雨強度控制反應，低降雨強度時以堆積為主，高降雨強度時則呈現侵蝕。這是因為紋溝的侵蝕作用因為高降雨強度而被加強，甚至可以將邊坡上的物質帶出邊坡系統。 本研究認為從型態變化上，泥岩邊坡的變化型態介於Young理論的第5型與第6型之間，原因在於泥岩風化層厚度與低坡度區(坡腳)的移除速率，顯著影響到泥岩邊坡發育。泥岩風化層的厚度在不同的坡度，不單由泥岩本身的風化速率決定，還有來自高坡度區的干擾。坡腳移除的速率被降雨所影響，決定坡腳會延伸還是縮短。 本研究使用高空間解析度的資料，觀察事件型的邊坡演育作用，記錄了在泥岩區邊坡的快速變化。但觀察結果無法全然套用或印證前人研究的成果。如此高空間解析度的邊坡資料，相對於過往的模式研究，是一個全新領域，看到了更多的邊坡細節。後繼者能把所有地形計測參數統計完成，並以此檢驗目前的各種模式與參數進行修訂，除了完善模式之外，尺度效應帶來的因子影響力改變會成為未來修正邊界條件的重點。本研究利用地面光達蒐集資料，跨入小尺度的研究領域，也為未來跨地形學、邊坡水文學、土壤沖蝕、甚至土木工程提供一個橋樑，相互結合各領域的研究成果，提出能通用這些領域的物理變化模型，這是本研究在修正邊坡型態變遷之外的貢獻。

This research aims to identify the slope evolution processes by detecting the morphological characteristics of the mudstone slope in Tainan and Kaosiung Cities, southwest Taiwan, 2011. The study methods were obtaining the high spatial resolution DTM by terrestrial LiDAR at two study areas, and analyzing the changes for slope erosion pattern and seeking the main factors which gave the most effect. There were seven scans obtained in 2011, and converted to two and three observation periods. The permanent target points (TP) method and DEMs of differences were used to check the accuracy of point cloud. The results showed that the average erosion rate was 5 cm during the dry period in 2011. Following the typhoons, the erosion rate increased 1.4 times to 7 cm and was better correlated with the increase in the rainfall intensity than with general precipitation amounts. The hillslope gradient combined with rainfall intensity played a significant role in the geomorphic process. This research found that at high gradient area ( > 75°) with larger rainfall intensity showed more erosion that at other gradients. The lower gradient area (< 30°) would shift their reaction of morphological change by the rainfall intensity. The gradient also influenced the rill development, which occurred at middle and low gradients but not at high gradients. The rills also created a transition zone for erosion and deposition at the middle gradient where a minimal change occurred. This research also modified the slope evolution forms from Young’s theory. The realistic morphological change closed between forms no.5 and 6. The thick of wearing layer and removal rate of slope feet defined the shape type.