GEOMORPHIC CHANGE INDUCED BY THE FLOW OF MUD AND DEBRIS SURGES INTO RIVER CHANNELS

• Main Authors: Peng, Szu-Hsien, 彭思顯
• Other Authors: Chen, Su-Chin
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/74490403214396894158

博士 === 國立中興大學 === 水土保持學系 === 92 === This study examines various ways in which flow slides and river currents interact, including blocking of rivers due to either accumulated debris or to the formation of landslide dams across the channel width. In other situations, local narrowing of a channel can occur due to partial invasion by a debris fan. At the outlets of debris flow gullies, thick deposits may be left for water currents to slowly transport downstream. The study is composed of three parts: (1) systematic field observations; (2) laboratory experiments; (3) computational simulations. The purpose of this work is to study this confluence behavior of a hyperconcentrated tributary inflowing the main river. In the first part, it collects numerous previous studies and field investigations in order to study confluence modes of debris slides and river flows in Taiwan. A preliminary classification is established according to the basic concepts of sediment transport. We have analyzed several examples further and applied them to our catalogue to illustrate the classification method in detail. The confluence mechanism can be realized through this classification but it still needs more study, including of experimental research, as well as field investigation to better establish the classification framework. In the second part, it uses digital image processing to survey the confluence behavior of a tributary with hyperconcentrated flow entering the main river. The experiment has two parts: in the first, digital terrain data were computed via coordinate transformation from profiles extracted by laserlines; in the second, velocity fields were obtained by using Voronoï imaging method to analyze the images through low-pass and high-pass filters when trace-particles were added. The measurement approach is described and preliminary results are presented. Through the discussion of alluvial fan morphology, we can better understand the interaction of this mechanism. Finally, we proposed a numerical scheme to simulate the confluence phenomenon by using two-layer shallow water equations. The rheology behavior of mudflow or debris flow was regarded as the Herschel and Bulkley or Bingham fluid. Thus the shear stress on rigid bed can be derived from the constitutive equation. The computational approach uses the HLL scheme as a basic building block, treats the bottom slope by lateralizing the momentum flux, then refines the scheme using the Strang splitting to deal with the frictional source term. This study successfully set up 1D and 2D two-layer shallow water computations on rigid bed. The numerical model can describe the variety of depths and velocities, including water and mud, when the hyperconcentrated tributary flows into the main river. The results in this study will be helpful for advanced research sequentially and design or plan of hydraulic engineering structures.