Flooding in a sediment-laden floodplain is affected by riverine beach vegetation and the shape of a meandering compound channel. The laws of water and sediment exchange and the deposition distribution in beach troughs are very complex. These factors play a significant role in the formation and development of secondary suspended rivers, in the adjustment of the beach horizontal gradient, and even in the evolution of the flood control situation. In this study, we used a combination of experimental simulation and theoretical research to carry out a generalized model test of floodplain flooding evolution, analyzed the lateral distribution characteristics of sediment-laden flow and sediment factors in a meandering compound channel under the conditions of beach vegetation, and revealed the pros and cons of beach vegetation on the adjustment of the beach and channel siltation. The model test results of the flooding in the floodplain in the compound channel with meandering vegetation showed that the main stream was not only concentrated in the main channel but also appeared near the foot of the left and right bank levees and formed flood discharges along the embankment. As the riverine beach siltation was mainly concentrated at the riverine beach lip, the vegetation on the riverine beach had a significant effect on slowing down the flow velocity. Whether it was a row or full vegetation on both sides of the bank, this played an important role in the stability of the main channel. When there was no vegetation on the riverine beach, the main channel was easy to move. The arrangement of full vegetation on the riverine beach had a uniform effect on the velocity distribution of the riverine beach, which reduced the phenomenon of excessive velocity at the foot of the riverine beach and increased the velocity effect in the main channel. These results will provide a theoretical basis for the utilization of riverine beach areas and river management in the lower Yellow River and have a great significance for enriching the basic theory of water and sediment movement and promoting the integration of hydraulics, river dynamics, and ecology.

The construction of a river regulation project has changed the cross-sectional shape of a river and significantly impacted the evolution of the river regime. In this paper, an asymmetry index was proposed to characterize the changes in the shape of the river cross-section and the distribution of water and sediment factors. According to the transverse distribution formula of the river section and water and sediment factors, the asymmetry of the cross-sectional shape as well as water and sediment factors along with the transverse distribution in the wandering reach of the lower Yellow River before and after the construction project was calculated, respectively. The results showed that the cross-sectional shape of the river channel before and after the building was asymmetric, and the cross-sectional shape of the river channel after the construction was more asymmetric than that of the free development channel without engineering constraints; at the same time, under the action of a limited control boundary, the asymmetry of cross-section flow velocity and sediment concentration and other water and sediment factors along the transverse distribution were more prominent, the flow velocity and sediment concentration along the transverse distribution increased, the river flow was more concentrated, and the sediment transport capacity of the channel improved significantly under a large flow (5000 m³/s). This study revealed the positive effect of river regulation projects on the river regime evolution of the wandering river and provided new ideas for the study of river regime evolution.

The change of water and sediment conditions in wandering channels has a great impact on the stability of river regime. The quantitative relationship between them is still unclear. The qualitative influence of water and sediment conditions on the river regime stability was analyzed by a model test. The response relationship between the upward or downward moving distance of the main stream zone and water and sediment conditions was quantitatively studied by using the measured water and sediment data and large-section data over the years. The results showed that when the upstream water and sediment inflow conditions change, the stability of a wandering channel with relatively stable river regime under the control of finite boundary will still change. When the river channel is at 1000 m³/s under the action of long-term small water, or silting thickness is about 0.53 m, the main stream next to the project moves upwards about 1170 m along the way. In the case of a large flood, such as 8000 m³/s, or scouring depth is about 0.39 m, the main stream next to the project moves downwards about 870 m along the way. The study provides a certain scientific basis for river regime stability and river flood control early warning, and provides a certain method reference for quantitative study of river regime evolution of other rivers at home and abroad.