地形劇烈變化河道之沖蝕特性及其對河道穩定性的影響及治理策略-總計畫暨子計畫:地形劇烈變化軟弱岩質河道之沖蝕行為探討(I)

Abstract

台灣西部河川之中下游多流經軟弱年輕地質區，該河川區域在受到自然環境變化的 影響(如地震抬升、氣候條件變遷等)或人為開發利用的影響(水利或交通設施的興建、砂 石之開採等)下，或因地震抬升造成的遷急點上溯沖蝕、或因跨河構造物（攔河堰、橋 樑、固床工）形成的人為遷急點的下切沖蝕，已有多處軟弱岩質河床發生護甲層流失或 劇烈沖蝕等現象。由於軟岩抗侵蝕能力低，洪水作用下之沖蝕速率可能與硬岩相差達到 一個數量級以上，快速的下切侵蝕作用可造成河川型態持續發生變化，造成河岸或人工 構造物的不穩定進而發生破壞。因此，為預防或治理軟弱岩質河床的下切侵蝕，必須充 分掌握其沖蝕機制、沖蝕速率、及沖蝕率模式等，方能進行適當的河川規劃治理。本研 究團隊自2008 年以來陸續執行軟岩相關研究計畫，並已提出適用於河道坡度變化較少 的沖蝕率模式等相關的研究成果。本期將針對遷急點的沖蝕行為進一步加以探討。 本計畫屬整合性計畫『地形劇烈變化河道之沖蝕特性及其對河道穩定性的影響及治 理策略』之子計畫一『地形劇烈變化軟弱岩質河道之沖蝕行為探討』，本子計畫主要目 的在於: (1)由現場遷急點（地形劇烈變化河道）上溯現象調查結果，建立軟岩河床遷急點上 溯機制及溯（倒退）率模式。 (2)由室內沖蝕試驗探討濁水下水平磨蝕及沖刷坑形成的行為。 (3)由現地固床工調查、觀測之結果探討固床工破壞原因及建立固床工破壞機制。

The middle/down stream of most rivers in west Taiwan flows through young geological areas, which are composed of weak sedimentary rocks. Due to the uplift of the 1999 seismic fault, the scour of weirs, or the abandoned hole of sand mining, knick points are easily found in the river channels of the rivers. The knick point migration is drastic, which may cause the river channel deeply and narrowly and induce the instability of bridges or weirs across the channel. The erosion behavior of weak rock river-bed has been investigated by this research group for four years. Study on the erosion behavior of weak rocks river-bed in the channel with drastic variation of longitudinal profile will be conducted in the following three years. This project belongs to the integrated project of “Erosion behavior of rock river-bed in the channel with drastic variation of longitudinal profile and its effects on the stability of river channel, and suggestion of counter measures for mitigating the erosion” The major purposes of this project include (1) propose the processes of knick point migration and the migration rate model (2) study the erosion behavior of weak rocks with laminar turbidity flow and the processes of scour hole formation under turbidity jet in laboratory (3) investigate the failure mechanism of counter measures for weal rock river-bed..