軟岩河床沖蝕行為及沖蝕能力

Abstract

台灣西部地區甚多河川(八掌溪、頭前溪、大漢溪等)的部分河 段，常於豪雨後產生劇烈沖蝕，長時間累積了數公尺至數十公尺的沖 蝕量。本研究團隊曾針對八掌溪軟岩河床劇烈沖蝕段進行勘查，勘查 結果顯示磨蝕現象明顯，亦存在抽離現象，河床或河岸因乾濕風化現 象形成龜裂，為抽離或磨損做準備的現象處處可見，亦有因差異侵蝕 形成的沖蝕現象。綜合目前針對岩石河床沖刷機制行為的相關研究文 獻，既有的沖蝕模式大多未考慮沖蝕機制、行為、岩性等等，應用於 軟弱岩床河道沖蝕量計算的結果均與河道斷面地形量測結果有甚大 之差距，而且國內至今未有詳細且有系統的探討岩石河床的沖蝕現 象、機制、及模式。因此，本整合性研究計畫擬針對國內外學術領域 於岩石河床的沖蝕及其對工程行為的影響尚有不足處加以深入探 討，本子計畫主要目的將完成下列工作： 1. 由現場沖蝕現象調查結果，探討並建立軟岩河床的主控沖蝕機 制。 2. 室內沖蝕試驗儀器設計、建立及測試，進行試驗室沖蝕試驗（含 磨蝕及抽離機制），並由沖蝕試驗結果修正或建立適合軟岩河 床的沖蝕率模式。 3. 由地質觀點（或岩體分類法）修正或建立岩石河床沖蝕速率與 能量消耗及抗沖蝕指數的關係。

Some intensively eroded reaches after flooding were found in rivers of west Taiwan, such as the Ba-Chang River, the Da-Han River, and the Tou-Chien River. The accumulated erosion quantity is from several to tens of meters. The river bed of the intensively eroded reaches is composed of soft rock. Various erosion phenomena, including abrasion, plucking, pothole, weathering crack, unequal erosion, etc., have been observed in our investigation at the Ba-Chang River. Since civil/hydraulic engineers do not pay much attention to rock-bed channel erosion/scouring, there are no suitable theoretical, analytical, or empirical methods for modeling these types of problems. In order to improve the understanding on this category of problems, a three-year project aims to study the soft rock-bed river erosion mechanisms in engineering scale. This project plans to determine the dominant erosion mechanism and the rock erodibility for various conditions of rock properties, loading types, flow conditions, and so on. The major scopes of this project include (1) development of a conceptual model for individual erosion mechanisms based on investigation results at intensively eroded reaches; (2) Design and assembling a multi-function experimental set for laboratory erosion test, developing erosion models suitable for a soft rock-bed river based on results of various erosion tests; (3) From the viewpoints of engineering geology, to present the relationship between the erosion rate, the energy dissipation rate of flowing water; and the erodibility index for a soft rock-bed river.