| Summary: | 碩士 === 逢甲大學 === 水利工程與資源保育學系 === 101 === Due to the traffic demand extension built to make the river straightened or reduced river width in urban river. It’s using concrete or anti-erosion of the material on both sides of embankments, cause the flow fixed on the instream, in order to achieve effects such as flood control and anti-erosion. Thus, it affected the river of sediment and migration mechanisms, make the riverbed morphology appear to change. In 2003, the high-speed railway construction, it cause the downstream of Fa-Zi river straightened. After several years of sand and water interaction, alternate bars formed on the channel, and mature and stable development. According to previous observed the alternate bars have mostly formed in the river channels, seldom has a similar local case from a straight condition to alternating bars, such as Fa-zi river. But it’s only three photos record by satellite images, the process of change can only speculate.
In the study, fluvial process simulation the downstream of bridge of Fa-Zi river by using the two-dimensional depth-averaged sediment transport model CCHE-2D. Discussed the process and characteristics of alternate bars by generated model. Numerical channel settings to field conditions basis, slope 0.0076, median diameter of 120mm gravel, river width is 150m, the upstream set an inflow angle of 20 degrees, three kinds of steady flow and two kinds of particle size distribution as simulated conditions, Explore the evolution and characteristics of alternating bars.
The simulation results and field aerial photographs approximation, and complementary the possibility of process between photos. Other results, such as the greater flow of cases, alternating bars migrate faster and longer wavelengths formation, formation of a stable time required quickly. In addition, the calculation results of numerical models, Explain the evolution of transport situation of alternating bars. According to the condition of the main channel flood defined two bed form stages, floodplain of drop water and meandering of dry bed form respectively. The former is a flow diffuse floodplain conditions with alternating bars growth. The latter is the dry bed generation, make the main flow changed meandering flow and stable state. Meanwhile, in the migration process, alternate bars hourly rate of growth is about 1.2 to 1.3%. In addition, in order to the force system for alternate bars, the adaption length for bed load of the river width ratio of about 0.04 to 0.1 of the simulation results better.
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