Structure function analyses are increasingly used to investigate spatial correlation patterns of river bed roughness. However, traditional structure function analyses remain mostly qualitative as the statistical significance of correlation patterns has not been explicitly addressed. This paper proposes a methodology for the analysis of 2D second-order structure functions based on statistical significance testing. Using data from flume experiments with gravel beds, a method to create surrogate rough beds is proposed. The surrogate beds are subsequently used to carry out Monte-Carlo simulations which in turn are used to evaluate statistical significance of the structure functions values. The method is applied to two datasets of gravel surfaces and the results are used to investigate the physical meaning of the obtained significance patterns. The results confirm previous studies that the orientation of large grains can be described by 2D second-order structure functions through a central ellipse reflecting areas of high spatial correlation. The main axes of the ellipses can hence be interpreted to define characteristic roughness length scales. The novelty of the present paper is that the proposed significance testing allows for a more rigorous definition of the spatial extent of the central ellipse.

The ecological condition of waterways has attracted increasing public attention in recent years, and the evaluation of the ecological status of waterways is of practical and scientific significance. To carry out an objective and credential evaluation of the ecological condition of waterways, this study compares the performance of two artificial neural networks (ANN) models, including the traditional Back Propagation (BP)-ANN model and the Particle Swarm Optimization (PSO)-BP-ANN model. The traditional BP-ANN model is characterized by a local search strategy and usually converges to local minima. This study combined the PSO algorithm with the BP-ANN model to improve the performance of the latter. An evaluation indicator system was established first based on the main features of waterway ecology, which includes 13 indicators in four aspects including the transportation function, the ecological function, the landscape function, and the economic function. A waterway ecology dataset was then established based on major waterways in China. The results show that the prediction accuracy of the BP-ANN is around 0.6 and unstable due to its local searching strategy. In contrast, the coefficient of determination of the PSO-BP-ANN model reaches 0.98, indicating the high prediction accuracy of the model. On this basis, this study further analyzed the relative importance of the four functions using the PSO-BP-ANN model. Results showed that the evaluation grades can be significantly improved by advancing the landscape function and the transportation function, and the improvement of these two scores can greatly reduce the probability of the lowest rating. The highest rating requires all four functions to be greater than certain threshold values.

Flood control, power generation and agricultural irrigation are the primary functions of reservoirs. The main reason for the serious deposition in the Yellow River reservoir is the high concentration of sediment in the water flow. Due to the establishment of the Yellow River cascade reservoir and the joint scheduling among reservoirs, the flow rate of the downstream reservoir decreases in flood season, which reduces the hydrodynamic flushing efficiency in the reservoir area. Wanjiazhai Reservoir is selected as the research sample in the present study, which is a first-level cascade reservoir in the connecting area of the upper and middle reaches of the Yellow River. In this study, a numerical model was built to analyze factors that influenced the operation and flushing of the upstream reservoir. And a formula was recommended to apply to the Wanjiazhai Reservoir on the basis of the sediment deposition pattern discrimination formula. Besides, various flushing methods and methods reducing the sediment deposition were carried out and analyzed. Results indicated that when the inflow rate increased, the maximum particle diameter of the incipient sediment was larger. When the inflow rate reached 1200 m³/s, the diameter of the initial the incipient sediment varied from 0.015 mm to 0.094 mm. And the diameter range of the initial the incipient sediment was 0.014 mm–0.094 mm, when the inflow rate was 1800 m³/s. When the inflow rate reached 2500 m³/s, the diameter of the initial the incipient sediment varied from 0.010 mm to 0.094 mm. Notably, all sediment were discharged from the reservoir. Moreover, the sediment of the reservoir was flushed at a flow rate of 1200 m³/s–1800 m³/s under the premise of meeting the upstream water storage.

The tidal reaches are characterized by unsteady and non-uniform flow (UNF), which is significantly different from the commonly assumed steady and uniform flow (SUF) in hydraulics. The SUF shows invariant temporal and spatial flow characteristics, and thus flow acceleration is absent in a prismatic channel. However, for the UNF, the variation of flow velocity and depth in both temporal and spatial scales causes the loss of flow energy, and thus increases the flow resistance. In order to clarify the variation of flow resistance and its influencing factors in tidal reaches, this study investigates the flow resistance characteristics under UNF conditions. In this study, a typical tidal section of the Lower Yangtze River (LYR) – Kouanzhi Waterway (KW) – was selected as the study area, where the temporal variation of water surface along the river course at different tide levels, the bathymetry of multiple cross-sections, the distribution of cross-sectional flow velocity and its temporal variation were measured in detail. Based on these field measurement data, the contribution terms to the energy slope were calculated and evaluated, by decomposing the momentum equation. The calculated contributing terms include water surface gradient, local acceleration, and convective acceleration. The results showed that the local acceleration and convective acceleration have a substantial impact on the energy slope during specific time periods, which was found to be more significant than the findings in previous studies. The results show that the local acceleration term is more significant than the convective acceleration term except when the water surface slope is close to zero, and its contribution is significant throughout the flood tide and the initial ebb tide periods. The above research results are of great significance for the investigation of flow resistance mechanisms and numerical simulations in tidal rivers.

River flow in the Lower Yangtze River (LYR) is influenced by the combined effect of runoff and tides, and the complex flow conditions tend to cause sediment deposition in waterways, which in turn affects navigation conditions. In order to improve the understanding of the river mechanism of tidal reaches, this study selected a typical reach in the LYR – the Kouanzhi Waterway (KW) – as an example to investigate the flow characteristics that affect sediment transport processes. Sedimentation annually occurs at the entrance of the KW and a large sidebar constantly increases in size, causing the reduction of navigation depth and width, but the causes of the sedimentation are still unclear. In this study, a three-dimensional (3D) model of the KW was established based on the Delft3D, and the k-epsilon model was chosen to simulate the turbulent flow. The model simulates the flow processes during flood tides in flood and dry seasons. The results of the numerical simulation show that a significant difference exists in the large-scale flow structures between the flood and the dry seasons. In the flood season, flow at the entrance of the KW is extremely turbulent and a large-scale vortex shedding phenomenon is formed downstream of the channel entrance, which causes a great sediment transport rate at the entrance. In addition, because the river width widens in the middle part of the KW, a large-scale circulation flow structure is developed near the Sanyiqiao sidebar. The circulation is supposed to trap and deposit sediment on the sidebar. In the dry season, the intensity of flow turbulence is greatly reduced, which causes the sediment transport rate at the entrance decreases accordingly. In addition, the circulation flow structure at the Sanyiqiao disappeared, and thus the sidebar is supposed to be under erosion during this period. The results of this study provide a vital reference for the engineering works of waterway regulations in tidal river reaches.