軟弱岩床河道的沖蝕機制與抗沖蝕力評估─以八掌溪為例

Abstract

我國西部麓山帶之地質年代極為年輕，出露之岩層主要為上新世及更新世的地層，此類岩體在強度性質上為介於岩石與過壓密土壤之間的「軟岩」材料。當河道護甲層沖失使此類岩體出露於河床後，在水流的作用下會產生嚴重的沖刷侵蝕，且其沖蝕行為與顆粒性材料有相當大之差異，所以瞭解軟岩河床的沖蝕為研究我國西部河川沖蝕的重要課題。 目前對於河床沖蝕的分析，大都採用計算分布於河床上的剪應力或流功的方式來評估沖蝕的量與速率，很少考慮到岩床本身地質條件的不同影響到沖蝕機制，進而改變沖蝕的結果。本研究藉由八掌溪發生嚴重侵蝕的河段，透過歷史資料比對，加上詳細的現場調查獲得地質條件作為基礎，用以追溯河道侵蝕的演進，提出該河段可能的軟岩河床沖蝕機制，進而對不同沖蝕機制下作沖蝕速率的比較。本研究中亦使用Annandale (2006)提出的沖蝕指數(erodibility)評分方法，評估河床材料受沖蝕的可能性，探討該方法在軟岩河床的適用性。

In the Western Foothills of Taiwan, sedimentation and diagenesis for the Pliocene and Pleistocene formations are still short in geological timescale. Their engineering behaviors are tended to be like soft-rock, e.g. poor cementation, high deformability, and low strength. Severe erosion in river channel caused by water flow is often observed once the armor layer has vanished. The processes of river erosion on soft-rock beds are not the same with for granular materials beds. So river incision models used commonly considering only on shear stress or stream power are not suitable for soft-rock. Geological condition, e.g. discontinuity properties, strength, etc. do have significant effects on the river erosion rate. The Pa-Chang River is studied in this research. Based on the historical surveying data and the results of intensively field investigation, the erosion mechanisms are proposed and used to compare different erosion rates. The erodibility index proposed by Annandale (2006) is also adopted for assessing the erosion possibility of the river.