Development and application of soft bedrock mobile-bed model

• Main Authors: Liao, Chung-Ta, 廖仲達
• Other Authors: Yeh, Keh-Chia
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/29367236035167876505

博士 === 國立交通大學 === 土木工程系所 === 101 === Steep slope and severe bed change are the general characteristics of Taiwan’s rivers. These characteristics often cause the bed armour layer flushed away, make the bedrock exposed, and then increase the channel incision rate. Due to the bedrock exposed and the channel characteristics, it makes the incision problem more seriously. Most mobile-bed models of past few decades aimed at the sediment transport of alluvial channel. However, the soft bedrock channel incision processes are different with the alluvial channel. Some empirical formula and mechanisms are needed to build in the model to simulate the erosion processes. In this study, a bedrock river evolution mechanism is included in a 2-D mobile-bed model, called the explicit finite analytic model. The model can consider both incision and deposition over the bedrock, by combining a stream power type of bedrock erosion rate formula with the flow and sediment transport modules. The multiple bed layers distributed in bedrock river are also considered. In the theorem of non-equilibrium suspended sediment concentration, the velocity scale is set to adjust the overloaded or under-loaded situation over the bedrock riverbed. The downstream reach of Chi-Chi Weir of Choshui River and the Taan River are chosen as the study sites for the model calibration, validation and comparison. In the field cases, the proposed model is stable in the simulation of complex river morphology with the main channel and floodplains. For the stream power threshold bedrock erosion rate formula and knickpoint evolution mechanism, the comparison of the calculated bed changes with field data show the model is capable of predicting bedrock incision, with reasonable accuracy and reliability. The simulated bed changes, thalwegs and cross-sectional profiles agree well with the measured data. It also indicates that the velocity, shear stress and critical stream power of flow could form a proper relation of erosion threshold in bedrock rivers. This model could be served as the prediction tool for the bedrock rivers.