The calculation of sediment transport capacity is of great importance in modelling of morphodynamic processes in alluvial rivers. Recently, worldwide fluvial sediment load is reducing especially in the Lower Yellow River (LYR), while the commonly used formulas of sediment transport capacity are specially developed for relatively high sediment-laden flow, which needs some improvement for the varying flow and sediment regime. The measurements during a long period (1956–2020) covering 4974 datasets in a quasi-equilibrium state were collected at seven hydrometric stations of the LYR, and the hydrodynamic variables related to suspended bed-material load were classified, in order to determine the critical parameters (K and M) in the widely used formula proposed by H.W. Zhang. The calculation relations were firstly developed between K and M and the comprehensive flow-sediment factor (STP), under various combinations of flow and sediment, which reduce the uncertainty related to modifying model input parameters. The improved formula was further applied to simulate the morphodynamic processes in the LYR. Results indicated that the improved formula presented the best accuracy in the simulation of morphodynamic processes induced by low or high incoming sediment amount, compared with the other two formulas. This improved formula can be directly applicable to the varying flow-sediment regime and achieves the accurate simulation of the hydrograph of low sediment concentration in the LYR.
The key factor influencing flood evolution and the progression of riverbed erosion and silting in the wide floodplain of the Lower Yellow River (LYR) is channel roughness. The distribution law of channel roughness has changed since the implementation of water storage and sand control in Xiaolangdi Reservoir in 2002, and the change in roughness has had a number of effects on the design of flood control projects and ecological environment protection planning of the lower reaches. Manning's formula was used to calculate the roughness based on cross-section data and hydrological data collected by typical hydrological stations in the wide floodplain of the LYR from 2002 to 2020. The evolution law and influencing factors of channel roughness of typical sections before and after floods were also examined. The findings indicate that: (1) The post-flood roughness was slightly higher than the pre-flood roughness in the same area. The post-flood roughness in the Huayuankou and Sunkou sections significantly increased compared to the pre-flood roughness, with an increase rate of 14.64% and 19.37%, respectively. (2) The time-to-space variation of roughness in the wandering section exhibits a decreasing trend, whereas the time-to-space variation of roughness in the transition section exhibits a slightly increasing trend. (3) The channel roughness decreases with an increase in Froude number, but there is no clear relationship between the roughness and sediment content. (4) Using Huayuankou and Gaocun stations as examples, the channel roughness is inversely proportional to the flow, the average sectional velocity, and the cross-section area of the flow. The channel roughness of the wandering section is inversely proportional to the ratio of width to depth, and the channel roughness of the transition section is proportional to the ratio of width to depth. Roughness and average velocity were determined to have the best-fitting connection, with R2 values of 0.62 and 0.78, respectively. (5) The main elements impacting channel roughness in the wide floodplain of the LYR are section shape and flow condition.
Sediment is one of the main factors affecting the ecological environment of rivers, and its eco-environmental effect plays an important role in maintaining the balance of water environment and aquatic biodiversity. Sediment in mountain rivers has obvious characteristics such as wide gradation, which has unique impacts on the ecological environment. In addition, the increasingly intense human activities in mountain rivers, such as the construction and operation of large-scale cascade reservoirs, lead to further complicated changes in the ecological and environmental effects of sediment. In this paper, the environmental effects of mountain river sediment in adsorption, desorption and transport and the ecological effects on aquatic microorganisms, animals, plants and the entire food web were systematically reviewed. The problems existing in relevant researches were discussed, and the research prospects were presented, in order to provide guidance for the protection of mountain rivers.
Under the action of earthquake, concrete-filled steel tubular（CFST） columns are often subjected to torsion action or combined action of compression-bending-shear-torsion.The existence of torque will aggravate the damage of CFST columns.In order to study the pure torsional failure and size effect behavior of CFST columns, a numerical model of CFST columns on pure torsional failure was established by using the three-dimensional meso-scale numerical simulation method, considering the meso-heterogeneity of concrete and the contact between steel tube and concrete.The influence of structural size, confinement coefficient and cross-sectional shape on torsional failure of CFST columns with different sizes were analyzed.Finally, the Ba6ant size effect law was compared and revised, and the formula for the nominal torsion strength in the full size range was proposed.The results show that:（1）Pure torsion failures of plain concrete columns show significant brittle failure characteristics, while pure torsion failures of CFST columns show certain ductile failure characteristics;（2）The pure torsion failure of the CFST column has the size effect, and the size effect of the square columns is stronger than that of the circular columns;（3）Under low constraints（use confinement coefficient to mark）,the size effect of the pure torsion failure of the CFST column is more obvious, with the increase of the confinement coefficient, the size effect of the nominal torsion strength is weakened.
The Yellow River is the “mother river” of the Chinese nation.The unbalanced relationship between water and sediment and frequent flood disasters have always been the main challenges in the governance of the Yellow River Basin or YRB.In recent years, the Yellow River channel have undergone major changes caused by the fluctuated water-sediment relationship, meanwhile, along with frequent extreme weather events, the social economy in the basin has developed rapidly.Under this changing environment, a river health diagnosis for the YRB becomes essential.In this study, methods such as information entropy and entropy weight method are adopted to study the characteristics of the healthy development and evolution of the Yellow River Basin through a new index “River Health Index”,shortly RHI.The study results indicate:（1） A health analysis of the river system shows that the YRB was in a healthier state from 1980 to 1986 and was generally unhealthy from 1987 to 1999;and from 2000 to 2019,the river system developed towards a healthy and well-aligned direction.（2） An analysis of the trend of the RHI finds that the river system developed better from 1980 to 1984,showed an increasingly unhealthy trend from 1984 to 1999,and a positive trend from 2000 to 2019;（3） The sensitive factor of the river system was the flood capacity of the main channel of the Ningxia-Inner Mongolia river section before 1986;from 1987 to 1999,the sensitive factors were the flood capacity of the main channel of the Ningxia-Inner Mongolia river section and the total water volume of Huayuankou; since the 21st century, the sensitive factors are the total scouring and silting as well as the incoming sediment of the lower reaches of the Yellow River.For the entire period, the sensitive factors are the total scouring and silting of the lower Yellow River and the flood capacity of the main channel of the Ningxia-Inner Mongolia river section.These results can provide support for the ecological conservation and high-quality development of the YRB.
