軟弱岩床河道沖蝕率模式建立 - 以大安溪與八掌溪為例

Abstract

由於自然環境變遷或人為因素影響，台灣西部河川中、下游大多流經軟弱岩床河道，並出現嚴重河道下切現象。會造成如此劇烈之沖蝕行為，在大安溪是因921地層抬昇隆起，造成河川快速下切，在八掌溪，是由於上游興建攔河堰、固床工等建設，影響河道砂石向下游的運輸，造成嚴重河道下切。國內對於河道沖蝕問題的探討及處理，多侷限於沖積層河道，缺乏對於岩石河床、岩石河岸的研究。 基於國內多缺乏於軟弱岩床河道的研究，所以本論文特對台灣西半部軟弱岩床河道進行分析研究。如何正確掌握軟岩之沖刷機制及評估方法，是現在比較急迫的問題。雖然現在國內學者近年已對軟弱岩石力學性質與行為、基礎工程行為、邊坡工程行為、隧道工程開挖行為有了略為了解，但對於水流與岩床河道相互行為尚未有深入的探討。 本研究藉由大安溪及八掌溪的地表地質調查、沖蝕現象調查、歷年地文及水文條件整理分析等結果，透過回歸分析方式提出沖蝕率的經驗式，經驗式採用之重要因子包括洪水事件之最大流量、累積流量等水文因子，以及河道坡降等地形因子，沖蝕量則是採用河道主深槽的深度變化量。

Because of environmental changes or human factors, middle and downstream of rivers of western of Taiwan are flowing through the soft rock bed channel which appears drastic river-bed incision problems. The erosion of soft rock bed channel at two rivers, the Da-An river and the Ba-Chang river is studied in this thesis. The reasons to cause the strong erosion between two rivers are different. The Da-An river is caused by the river bed uplift induced by the 1999 Chi-Chi earthquake. The Ba-Chang river is caused by the the weir building and gravel mining. In Taiwan, Study and mitigation of riverbed erosion is limited to alluvium riverbed. This thesis aims to propose an empirical erosion rate model based on the study on the two rivers. To development the model, the erosion mechanisms for soft rock bed channel are investigated. Based on the field results, the factors affected the erosion rate are explored including the stream hydraulics, geology, geomorphology, etc. The erosion depth of main channel is calculated from DEM preparing from areal photos and LiDAR with different periods. Also, the corresponding hydraulics data are collected. Then, regression analysis is adopted for developing the models. An empirical erosion rate model in terms of channel slope, peak discharge, and accumulated discharge is presented in this thesis.