Sedimentary numerical simulation of the development mechanism in sandy braided-meandeing transitional river

Braided river and meandering river are possible to form a braided-meandering transitional river, which has been concerned by scholars from sedimentology, geomorphology and hydrology. Traditional studies based on satellite data, field outcrop and drilling data while lacking understanding of its formation process and mechanism. In this study, sedimentary numerical simulation based on hydrodynamics is adopted to reproduce the development process of braided-meandering transitional river and compare it with modern river sedimentary data for verification. Through analyzing different sedimentary elements such as bar, channel and bank, The development process of sandy braided-meandering transitional river is revealed as follows: 1) Due to the extension and lateral accretion of the tail of the unite bar, a large number of secondary braided channels are filled, forming a compound bar, and the main channel gradually emerges, controlling the morphology development and evolution of the river bed; 2) The sediment and discharge of the channel on both sides of the compound bar are asymmetrical, and the sediment of the main channel is carried by denudation to the secondary channel near the bank and then unloaded and filled. Finally, the compound bar is connected with the embankment, and the main channel gradually presents a curved shape. 3) The main channel constantly adjusts and transforms the banks and bars. The concave bank is eroded and the sediments are transferred to the convex bank due to tranverse circulation. At this time, the compound bars develop multi-stage lateral deposits and the main channel migrated laterally. This understanding is of significance to the characterization of palaeochannel sediments.