河道軟岩質遷急點模型之磨蝕行為

Abstract

台灣西部河川常因自然原因或人為因素，致使河道坡度與落差急遽變化而形成遷急點，於此處易產生快速沖蝕與河道變遷，而國內、外尚甚少以室內模型有系統的探討軟岩質遷急點磨蝕的行為。因此，本研究利用郭炳宏(2010)所建置之多功能沖蝕試驗儀為基礎，建立一小型渠道試驗區，參考軟體COMSOL水理模擬進行渠道主體設計與建構，渠道試驗區製作完成後進行測試與遷急點磨蝕倒退試驗，並探討試體消散耐久性、水流含砂粒徑及水流濃度條件對沖蝕量之影響，同時觀察試體遷急點谿線剖面隨時間之變化。 本研究試體岩塊取自湖山水庫築壩材料借土區之砂岩，在水流流量及遷急點角度條件固定下，改變水流含砂粒徑與水流含砂濃度來進行遷急點磨蝕試驗，試驗結果顯示沖蝕量與岩石試體消散耐久第一循環指數Id1呈負相關；沖蝕量與含泥砂濃度呈高度正相關；水流含砂粒徑與沖蝕量為一正相關；而由遷急點谿線依時間順序觀察其變化，本試驗應屬均質硬材料的沖刷，破壞模式為削頭型式(Replacement)。

Natural disasters or human activities in major rivers located in western Taiwan may cause the steep drop and rapidly slope change of channels. The location of a river channel with draft slope change is called as a knickpoint. The presence of a knickpoint tends to accelerate the bedrock progressive abrasion Due to the lack of experimental study on the abrasion behavior of weak rock knickpoints, this study aims to build up a small model channel in laboratory for conducting a series of knickpoint abrasion tests. The model channel is established on the Multi-functional Erosion Test Apparatus (META) developed by Kuo (2010). Based on the results of the hydraulics calculation using COMSOL (a multi-physics numerical model), the configuration of the model channel is determined. Then , twelve model knickpoints with same geometry were adopted for performing the tests under constant discharge with three types of turbidity water (0, 5000ppm, 10000ppm). The knickpoint models were prepared from sandstone blocks taken from the slopes in the area of a new reservoir, the Hushan reservoir, Yunling, Taiwan. The sandstone is poorly cemented and very weak. The topographical changes of the model surface were measured using a laboratory laser scanner after each test. Then, the erosion rate and erosion behavior can be investigated. The physical properties of the rocks, including the slake durability index (Id1), were also measured. Based on the test results, we can find: 1. The erosion rate is negative-correlated with the slake durability index (Id1). 2. The erosion rate is highly positive-correlated with the suspended sediment concentration. 3. The erosion rate is positive-correlated with the particle size of turbidity flow. 4. From the chronologically evolution of knickpoint profile with time, the experimental specimen is a homogeneous material and the mechanism of the knickpoint retreat is “replacement”.