Modeling migration of alternating bars by both flume experiments and 2-D simulation

Urban rivers in Taiwan were usually straightened for saving lands and confined by dikes on both sides. By natural processes, however, alternate bars may or may not occur afterword. In this study, a simplified moveable-bed flume, which generated features of alternating bars, was used as a simulation tool to investigate a focused river reach. Considering the site conditions, theoretical analysis was applied, based on existing data of alternating bars in the literature, on the experimental design to overcome the limitation of flume facility. Experimental results were compared with that of a numerical two-dimensional depth-averaged moveable-bed river model. An inflow angle was given as perturbation, sediment transport formula by Wu et al. (2000) was employed. Simulation results showed high similarity to the field aerial photographs, and thus may illustrate the potential changing processes. The evolution of alternate bars can be set as two bed form stages which are (A) floodplain-overflowing stage and (B) meandering form stage, respectively, according to whether the flow overtopping floodplain or only within the main channel (or thalweg). The key factor on morphological changes is the formation of dry nodes. Same characteristics shown in cases with difference recurrence discharges, such as the cases of higher flow discharge, alternate bars formed with a faster migration rate and longer wavelengths formation, and the time required to reach stable was also faster.