Abstract:
Abstract: [Objective] Discharge regulates the sedimentary evolution and morphological scale of braided rivers; however, the degree of its influence on braided channels, bars, and internal architecture has not been systematically analyzed.[Methods] Five groups of three-dimensional sedimentary numerical simulation experiments with different flow rates were carried out using Delft3D software to explore the evolution, morphological patterns, and scale differences of braided channels and bars under various flow rates, reproduce the internal architecture patterns of multi-type bars in braided rivers with different flow rates, and summarize the sedimentary characteristics and evolution of sandy braided rivers.[Results and Discussions] (1) The evolution rate of sandy braided rivers positively correlates with discharge, which also influences the co-evolutionary process of braided channels and bars. In low-discharge braided rivers, bars primarily accumulate downstream and overlap as they migrate downward. This pattern makes it difficult for the bars to be divided or re-merged by the channel. In contrast, high-discharge braided rivers typically exhibit lateral accumulation, leading to complex stacking. This results in a more frequent dynamic equilibrium process of bar merging and fragmentation. (2)Discharge control braided channel plane style and channel depth. As discharge increases, the braided channel becomes wider and deeper, which results in a reduced overall number of channels. However, the chute of the channel bars becomes more developed. In contrast, lower-discharge braided channels are shallower and primarily consist of narrow, numerous, and relatively fixed interwoven river networks. (3) The discharge affects the shape, architecture, and scale of channel bars. As discharge increases, a larger portion of the channel area becomes active, resulting in a greater hydrodynamic force on the bars due to the transformation of water flow. Consequently, there is a smaller proportion of preserved bars. With higher flow rates, the degree of transformation also increases, leading to collisions among multiple unit bars and compound bars, which creates more complex overlap relationships. Large-scale compound complex bars and sidebars can develop extensively, especially at the contact points where chutes form between the bars.(4) By comparing the effects of high and low hydrodynamic forces on the internal growth of braided rivers, we find similarities and differences in how channel bars form. Both types of bars have a lobate shape and consist of multiple layers of sediment accretion. These layers include downstream accretions at the core, aggradational accretions that are layered above and extend over a long-range, which are thin and have a small dip angle, as well as lateral accretions deposited on the sides or chutes. The lateral accretions are characterized by high inclination, greater thickness, and smaller scale.The key difference is that as discharge increases, the proportion of downstream accretions within the bar decreases, while the proportion of lateral accretions increases.(5) The flow rate positively correlates with the inclination angle, length, width, and thickness of the accretions, but negatively correlates with the length-to-width ratio and width-to-thickness ratio. [Conclusions] It is proposed that the flow rate is directly proportional to the size of the accretions within the channel bars of sandy braided rivers. This relationship clarifies the sedimentary characteristics and scale differences of sandy braided rivers influenced by flow rate, providing an essential basis for understanding the architecture of complex underground reservoirs.
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