Studying the effects of different riparian vegetation densities on river channel evolution has practical significance for predicting the river channel evolution process during flood periods and ecological river engineering via the artificial planting of vegetation. In this study, we simulated the formation and evolution processes of river channels under different riparian vegetation coverage rates in laboratory conditions. The riparian vegetation coverage rates were set as 0, 20, 40 and 80%, on unilateral and bilateral sides of a river channel bank. Given the same flow and sediment boundary conditions, experiments were carried out in a 4 × 1.5 m tank. This paper focuses on the comparative analysis of main stream stability characteristics, bend migration characteristics, river bank erosion characteristics and sediment transport intensity. The results showed that different amounts of riparian vegetation cover created different characteristics of river channel evolution and strongly impacted the stability of the banks and bed. River channel evolution under unilateral vegetation cover is often accompanied by alternate development of the main stream and branch, and the bend stability under unilateral riverbank vegetation cover is worse than under bilateral cover. For a bilateral vegetation-covered river channel, a narrow and deep regime channel more easily forms with a higher vegetation coverage rate; the curvature of the stable river bend is smaller, but the adaptation period of the flow to the river channel bed increases. Planting of riparian vegetation played a positive role in the erosion resistibility, which effectively reduced the lateral migration rate of the riverbank. The higher the vegetation coverage rate, the greater the flow shear stress needed for the same river channel migration rate. While effectively reducing lateral migration, riparian vegetation coverage increased the vertical migration and led to a trend in overall scour depth along the riverbank.

Energy dissipation is one of the most important factors in choosing stepped spillways. However, very few studies have investigated energy dissipation with different horizontal face angles. In this paper, the realizable k-ε turbulent model was used to study the flow field, energy dissipation rates and turbulent kinetic energy and its dissipation rate for different stepped spillways with five horizontal face angles in the skimming flow regions. Results showed that the field and direction of the flow were changed by the horizontal face angles of the stepped spillway, which produced some unique characteristics and thus caused better energy dissipation. The fluctuation of free water surface will be larger with increasing horizontal face angles and the energy dissipation rate decreases with an increasing unit discharge and increases for the enlargement of the horizontal face angles. This conclusion could provide a reference for the relevant research of V shaped stepped spillways.

Water wave dynamics and its engineering application have always been a key issue in the field of hydraulics, and effective and efficient numerical methods need to be proposed to perform three-dimensional (3-D) simulation of large-scale water fluctuation in engineering practice. A single-phase free-surface lattice Boltzmann method (SPFS-LB method) is coupled with a large-eddy simulation approach for simulating large-scale free water surface flows, and the simulation is accelerated on a GPU (graphic processing unit). The coupling model is used to simulate the evolution process of dam-break wave after complete and partial dam-break. The formation mechanism of horizontal and vertical vortices in water after partial dam-break and the advance and evolution process of dam-break flow on non-contour riverbed are analyzed. The method has been verified to be reasonable and can obtain a more accurate time curve of water level fluctuation. Applying this method to practical arch dams, discharge coefficients consistent with empirical formulas can be obtained by comparison and analysis, and the surface flow phenomena (such as tongue diffusion, surface fragmentation, and surface fusion) can be well simulated by this method. In addition, based on the key technology of parallel computing on a GPU, the implementation of the SPFS-LB model on a GPU unit achieves tens of millions of lattice updates per second, which is over fifty times higher than that on a single CPU chip. It is proved that the proposed method for large-scale water fluctuations can be used to study practical engineering problems. The mathematical model method realizes the efficient and accurate simulation of practical physical problems.

In the Loess Plateau of China, loess is widely distributed and forms a unique geomorphic feature of the world. Meanwhile, the Yellow River water and sediment regulation system is under construction. Nonetheless, the morphological characteristics of constrained meandering rivers in the Loess Plateau are still unknown due to the difficulty of extracting the sediment-laden water body. An improved method is proposed based on Landsat 8 imagery, which automatically extracts the multi-band spectral relationship of high-sediment-concentration rivers in valleys. This study analyzes the morphological characteristics of constrained meandering rivers in the middle reaches of the Yellow River Basin, including their sinuosity, periodicity, curvature, and skewness based on the deflection points bend segmentation and continuous wavelet transform methods. These characteristics are then compared with those of other constrained meandering rivers and alluvial meandering rivers. The results show that the sinuosity of the constrained river bends is generally low (with an average of 1.55) due to limitations imposed by the riverbanks, which prevent full development. The average dimensionless curvature radius of the constrained rivers is 18.72, lower than that of alluvial rivers. The skewing angle of the constrained river bends typically inclines upstream, with a proportion reaching 59.44%. In constrained river bends, as the sinuosity increases, the proportion of bends skewing upstream initially increases and then gradually decreases. This indicates that constrained river bends can develop similarly to alluvial bends at lower sinuosity but are limited by the mountains on both sides at medium sinuosity. The analysis of river characteristics in regions with different geological structures reveals the effect of geological structures on the formation of constrained rivers in the Loess Plateau. These findings can provide a reference for selecting reservoir dam sites and are important for the dredging engineering layout in the middle reaches of the Loess Plateau. They also offer quantitative explanations for the meandering characteristics of these rivers.