Recent studies have indicated that low-slope bedrock reaches are more common in nature (the bedrock surface slope is slightly lower than 0.005). This paper focuses on the hydrodynamic structure and turbulent characteristics of low-slope and critically sharp bedrock bends under flood conditions. Three experiments under different flood frequencies were conducted to investigate the influence of the ratio of centerline curvature radius to depth ($R_c/H$ ) on the distribution and characteristics of the secondary flow, turbulence, and momentum transport measured by acoustic Doppler velocimeter (ADV). The results were then compared with the flow structure of a sharp bend. Our results indicated that the distribution area and the intensity of the secondary flow decreased with $R_c/H$ , especially in the outer-bank cell and the center-region cell. The lower $R_c/H$ was, the more obvious the flow separation at the inner bank was. The core area of the turbulent kinetic energy moved upstream gradually, and its intensity increased with $R_c/H$ . The friction factor increased in the upstream bend and did not move further upstream when $R_c/H$ was below 13.6. The intensity of the transverse bed shear stress decreased with $R_c/H$ in a critically sharp bend, and the maximum value near the upstream centerline implied the potential erosion in the bedrock bend. These experimental results are conducive to understanding the flow process of erosion and deposition in the bedrock bend reach.

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.