Influenced by climate change and human activities, especially the completion and operation of cascade reservoirs in the middle and lower reaches of Jinsha River since 2012, new changes have taken place in the water and sediment characteristics of the Three Gorges Reservoir (TGR) in recent years. In this paper, a one-dimensional unsteady water and sediment mathematical model of the main and tributary rivers of the TGR is established, and the main calculation parameters of the model are calibrated with the measured water and sediment data from January 1, 2008 to December 31, 2017. In view of the different combinations of inflow water and sediment that may occur in the TGR under the condition of new water and sediment, the long-term changes of sediment erosion and deposition and the balance of reservoir deposition in the TGR are studied using the model. The results show that: (1) Under the new conditions of water and sediment, the amount of sediment in the TGR accounts for only 14.8% and 35.8% of that in 1956–1990 and 2003–2012, respectively; (2) The variation process of water level, discharge and sediment concentration of each station along the route calculated by the model is basically consistent with the measured results, and the calculated values of total deposition amount and deposition distribution are also basically consistent with the measured results. The verification results of the model are in accordance with the measured values; (3) Under the water-sediment conditions during 1961–1970 and 1991–2000, the model predicted the estimates of 320 and 430 years for the TGR to reach a sedimentation balance, respectively. Under the new water-sediment conditions, it takes 560 years at most and 450 years at least to reach the sedimentation balance for the TGR, and the corresponding condition is the typical year with less water-less sediment and more water-more sediment, respectively. The research results of this paper can provide a new reference for the long-term safe operation and operation optimization of the TGR.

Dominant discharge is an important parameter reflecting the water inflow process and river channel shape. Since the impoundment and implementation of the Three Gorges Reservoir (TGR), the bed-forming effect of the river channel in the reservoir area has changed significantly. Based on the observed water and sediment data from four hydrological stations in the upper Yangtze River from 1980 to 2017, the dominant discharge of the Zhutuo, Cuntan, Qingxichang, and Wanxian stations before and after the TGR became operational is calculated using the Makayev method, Han Qiwei empirical formula, and flow guarantee rate method. The results show the following: (1) The Makayev method takes both the sediment transport capacity and bed formation duration into account. The physical meaning is clear and the calculation results are relatively reasonable. (2) After the impoundment of the Three Gorges Project (TGP), the dominant discharge of the river decreased in the variable backwater area, while increasing in the perennial backwater area. With the water level rise in front of the dam, the dominant discharge in the variable backwater area remained basically unchanged, while increasing in the perennial backwater area. (3) The dominant discharge in the reservoir area is affected by incoming water processes and the reservoir operation mode, and shows a strong correlation with annual average discharge, flood season average discharge, and flood peak discharge. Before impoundment, the correlation between the dominant discharge and these three parameters was high. After impoundment, the correlation between the dominant discharge and the average flow during flood season was more apparent, further confirming that the bed-forming period of the reservoir is the flood season.

This article introduces a new method for mutation detection, the approximate entropy method, which is based on the complexity of time series. The mutation of average annual sediment discharge in Wu Jiang River Basin from 1960 to 2016 is detected by the introduced approximate entropy method, and compared with the results of double cumulative curve method and B-G segmentation algorithm. The mutation physical mechanism of the sediment discharge is discussed from the aspects of sediment source, annual distribution and interannual variation, climate change, impact of water conservancy and water conservation projects on sediment transport. The results show that mutation points occurred at 1984 and 2008 at Wu Long station, and the sediment discharge has a significant change after 2008. The mutation of average annual sediment discharge in Wu Jiang River Basin is caused by both climate change and human activity. Sediment reduction effect of the hydraulic engineerings built since 1990s climate is main and direct, and the climate change have secondary effect on sediment discharge change.

This paper analyzes the incipient motion mechanism of consolidated cohesive sediment. An experimental device based on previous studies was designed to investigate the influencing factors of the incipient shear stress, including the consolidation time, the density of dry bulk, cohesive particles content, and the composition of sediment mixtures. The experimental results indicated that the incipient shear stress of cohesive sediment increased with the increase of consolidation time, dry bulk density, and content of cohesive particles. The incipient motion mechanism of cohesive particles was further investigated using experimental data and theoretical analysis. A formula of the incipient shear stress for cohesive sediment was proposed herein, which is related to both the content of cohesive particles and the relative dry bulk density. The proposed formula was validated by the experimental data, and the calculated values of incipient shear stress using the formula were in good agreement with the experimental results.

The impacts of backwater due to large dam construction on flow may lead to navigation or flood control problems in curved rivers. This study conducted flume experiments to investigate the effects of backwater on the velocity distribution characteristics of a 90-degree bend. The experimental results show that the backwater degree (η, defined as the ratio of flow depth under backwater to that under non-backwater conditions) has significant impacts on the three-dimensional velocity distribution in the bend. The depth-averaged velocities decrease with increasing backwater degree, and the deflection degrees of depth-averaged velocities are found to be highly related to the backwater degree and cross-sectional position. In this experimental setup, the mean cross-sectional velocity decreases by 67.2% as η increases from 1.00 to 3.64 for Q = 35 L/s; 63.7% as η increases from 1.00 to 3.26 for Q = 52 L/s; and 60.1% as η increases from 1.00 to 2.80 for Q = 52 L/s. The maximum values of transversal and vertical velocities near the riverbed gradually shift to the inner bank as the backwater degree increases at the 45° cross section. The center of the high transversal velocity area shifts about 0.1 m toward the inner bank as the backwater degree increases from 1.00 to 3.26 for Q = 52 L/s, which can reduce the erosion of the riverbed near the outer bank. In the current study, we also demonstrate that the growth and decay processes of secondary flow cells under backwater conditions are similar to those under non-backwater conditions. However, the scales and positions of the secondary flow cells change continuously with different backwater degrees. From the entrance to the exit of the bend, the secondary flow intensity first increases, and then decreases, with its maximum values occurring at the 45° cross section. The findings detailed in this manuscript provide insights for navigation channel design in reservoir backwater zones.

Based on the hydrological and sediment observation data of the main stream and main stream of the upper Yangtze River from 1956 to 2018, using M-K test, Wavelet analysis, Approximate entropy and Lyapunov exponent, this paper analyzes the spatiotemporal evolution characteristics from the perspectives of catastrophe, periodicity, complexity and chaos of incoming water and sediment in The TGR (TGR), defines the connotation of “new water and sediment conditions” in the TGR, and discusses the main influencing factors of spatiotemporal variation of incoming water and sediment in the TGR. The results show that: (1) Except Pingshan station of Jinsha River, Fushun station of Tuojiang River and Wulong station of Wujiang River, the runoff of other main and tributaries and TGR in the upper Yangtze River has no significant change, but the sediment discharge has obvious mutation, and the runoff and sediment have periodic changes in different degrees; (2) The complexity of annual runoff and sediment transport in the TGR increases gradually, and the complexity of sediment transport is greater than that of runoff. Both annual runoff and sediment transport are chaotic, and the complexity of runoff and sediment transport sequence increases gradually from the upper reaches to the lower reaches; (3) The connotation of “new water and sediment condition” shows that there is no obvious mutation in runoff and sediment transport in time scale, the main cycle of water and sediment change is basically the same, and the complexity and chaos of sediment transport in space scale are significantly greater than runoff, and the change degree of sediment transport is more significant than runoff; (4) From 1991 to 2002, climate change (rainfall) was an important factor affecting the sediment discharge in the upper Yangtze River. From 2003 to 2018, the impact of climate change was relatively insignificant, and human activities became the most important factor.