Erosion of Soft Bedrock by River and Jet Flows: Small - Scale Experiment, and Field Exploration

台灣近年來，除了多數人研究的沖積河川外，受到921大地震地層抬升,以及攔砂壩等水工結構物的影響，河道中岩盤的露出、侵蝕、下切也逐漸受人重視。本篇論文介紹台灣多處岩盤侵蝕的情況及可能成因，並以此做為出發，設計縮尺物理實驗模型來模擬岩盤侵蝕。在材料的選擇方面，我們以石膏和砂混水做為河床流域基礎，經由調配適當的比例，使得我們的模型足以表現岩盤的強度，但又能快速的表現沖蝕。沖蝕環境設定在攔砂壩下游，磨料控制在不完全淤積但足以沖蝕的量；模型渠道的地表設計則包含地形突變、不同強度材料相互影響以及河川蜿蜒影響。 我們記錄了縮尺物理模型實驗中的沖蝕過程，結合階段性的地形雷射掃描、表面流速分析，並設計一種新的岩石強度測試方式，以射流水柱沖蝕的深度來判斷實驗及現地的材料強度。我們也設計了一種以極座標原理出發的網格來展現DTM，克服了岩盤沖蝕常會遇見的複雜峽谷狀地形，比一般平面網格更能立體地呈現地形。最後我們將實驗成果與現地（集集攔河堰）做比較，可發現我們的模型沖蝕結果，與大部分現地現象都相似。而射流水柱測試強度的方法，需要更多的沖蝕測試來提高準確度並建立材料強度及射流沖蝕深度的關係。In Taiwan, the issue of soft bedrock erosion has become of increasing concern following the uplifting of the 921 Earthquake. Following this event, bedrock has become exposed downstream of many hydraulic structures like dams and weirs. This thesis is going to introduce lots of bedrock erosion examples at different places in Taiwan. We use these examples as motivation to design small scale laboratory experiments simulating soft bedrock erosion. For the bedrock material of our experiments, we mix plaster, fine sand and water. By carefully choosing the mixing ratio, the physical model can approximate the characteristic of bedrock, and can be eroded in a short time. We set the experiment environment as the reach downstream of a dam, and add abrasive to accelerate erosion. Other surface design for experimental channel including: stairs, interaction of different material and river curvature. We recorded the evolution of river topography using laser scanning, measured surface flow velocity using particle tracking, and developed a water-jet strength test to determine the material strength of soft bedrock from the experiment and from the field. We also developed a mesh generator using polar coordinate system to address the difficulties of complex topography. In the end, we compare our experimental results with field observations, especially of Chichi Weir, and found a lot of similarities. As for the water-jet strength test, we need more data and more testing to improve the accuracy and construct a relation between bedrock strength and water-jet eroded depth