Xiang Zhang

The relationship between water and sediment in the lower reaches of the Yellow River is uncoordinated, leading to frequent floods. In this area, the floodplain is situated below the main channel and embankment foundations, increasing the likelihood of overbank flooding. Ecological protective forests serve as a nature-based mitigation measure by reducing flow velocities along embankments and lowering the risk of structural failure during near-bank flood events. To assess the role of ecological protective forests, laboratory experiments were conducted, and field data informed parameterization and geometry selection. A total of 24 scenarios were designed, combining four forest arrangements (A1, A2, A3, and A4), two submergence degrees (H₀/H = 0.5 and 1.0), and three water and sediment conditions. Results show that sediment deposition increases with vegetation density. Under constant vegetation density and embankment-aligned flow, a larger along-flow to cross-flow spacing ratio promoted deposition upstream, whereas a smaller ratio extended deposition further downstream. Deposition thickness was greater under fully submerged conditions than under semi-submerged conditions. Among the arrangements, sediment deposition effectiveness followed the order A1 > A2 > A4 > A3, with arrangement A1 providing the strongest promotion of deposition. Under varying flow–sediment conditions, the A1 arrangement enhanced sediment deposition by 6.8% to 20.6%. Flow structure was also modified: under semi-submerged conditions, the vertical profile of longitudinal velocity approximated a logarithmic distribution, whereas full submergence produced a different profile due to combined drag from tree trunks and canopy. Vertical sediment concentration profiles were similar under both submerged states, with minimum values near the water surface and maximum concentrations near the bottom. These changes confirm that ecological protective forests contributed to reducing flow velocity and diminishing sediment transport capacity.

Neck cutoff is a sudden phenomenon that occurs during the evolution of meandering rivers, but its process and mechanism remain unclear. In this investigation, the neck-narrowing process before cutoff, the formation of a new river, and the appropriate conditions for cutoff under constant and variable discharges were studied using experimental methods in a laboratory flume. The results indicated that bank erosion upstream and downstream of the neck was the main factor that triggered cutoff, regardless of whether the discharge changed. The sinuosity of the model river was approximately 7, the ratio of the neck width to the average river width reached approximately 0.43, and the experimental material was nonuniform natural sand. Under the conditions that the unit width discharge ranged from 0.0039 to 0.0069 m²/s and the longitudinal gradient was 1‰, neck cutoff could occur after sufficient bank erosion. When neck cutoff was imminent, the sinuosity of the channel increased and the channel rotated downstream. After neck cutoff, the formation of a new river experienced three stages, namely, the short-term longitudinal scouring stage, the long-term horizontal widening stage, and the long-term transverse widening and normal slow development stage. The discharge resulting in neck scouring was the effective discharge contributing to the cutoff, and this discharge directly affected the cutoff time and position. We attempted to explain the factors leading to neck cutoff from an energy consumption perspective. Neck cutoff was a sudden phenomenon that occurred when the energy consumption of a meandering river increased to its maximum value upon entry to the recession stage from the relative equilibrium stage. To a certain extent, this study is significant in understanding river evolution and neck cutoff.

Although it is a consensus in academic circles that regulation works will change a channel’s shape, the quantitative relationship between the finite control boundary and the channel’s shape parameters is still unclear. In this study, a flume model test was carried out, and the evolution of channel’s shape under different engineering densities was qualitatively analyzed. Based on the construction of river regulation works and measured hydrological data of the wandering channel in the lower Yellow River from 1960 to 2014, the channel shape parameters (bending coefficient, swing range of the main streamline and river facies coefficient) of the wandering channel in different periods were calculated, and the variation in the channel shape parameters with the density of regulation works in different periods was analyzed. The results showed that the river regime cannot be effectively controlled with less dense regulation works; in this case, the regulation works have little impact on the channel’s morphology. With an increase in the density of regulation works, the evolution of channel morphology was obviously different. Specifically, the river bending coefficient increased as a whole from 1.14 in 1949–1960 to 1.32 in 2011–2014, indicating that the regulation works have had an obvious restraining and improving effect on the river’s morphology. The swing range of the main stream obviously decreased, with an average value of about 500 m; the river facies coefficient decreased dramatically from 20~88 before 1960 to 15 in 2014, and the channel morphology has developed with a tendency to be narrow and deep. The research showed that a finite control boundary can significantly improve the channel’s shape, limit the wandering characteristics of the river reach to a certain extent, stabilize the main stream and control the river’s regime.