Migration processes of alternating bars in Fa-Zi river by CCHE-2D modeling

Due to the high-speed railway construction, the downstream reach of the bridge in Fa-Zi river has been straightened. After several years of sand and water interaction, alternate bars formed on the channel. The change from a straight condition to alternating bars was recorded by three satellite images, but the process of change can only be speculated. In the study, fluvial process in the downstream reach of the bridge in Fa-Zi river was simulated by using the two-dimensional depth-averaged sediment transport model CCHE-2D; and the process and characteristics of alternate bars generated by the model were discussed. The simulation results showed high similarity to the field aerial photographs, and thus may illustrate the potential changing process between the photos. The evolution of alternate bars can be set as two bed form stages according to whether the flow overtopping floodplain or only within the main channel, floodplain-overflow and meandering form, respectively; the key point of morphological changes is the formation of dry node. Same characteristics shown in difference recurrence interval cases, such as the cases of higher flow discharge, alternate bars formed a faster migration rate and longer wavelengths formation, and the time required to stable faster, too. Meanwhile, in the migration process, the hourly rate of growth of the alternate bars is about 1.2 to 1.3% of the original wave length. In addition, the ratio of bed load adaptation length to the river width is about 0.04 to 0.1 for better simulation results for the example of alternate bars in Fa-Zi River.