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Hydrodynamic processes in compound channels are closely related to the cross-sectional geometry of the river. The cross-sectional shape of natural meandering rivers is always irregular, with a curved main channel (MC) and one or two floodplains (FPs), and most are confined within levees. The geometric shape of the cross-section of such rivers changes greatly in a flood, which influences the exchange of momentum and energy between the MC and FPs. Many previous studies have focused on flow mechanisms in compound meandering channels of relatively regular and simple cross-section, which is not representative of natural rivers. An experimental study of a sine-shaped meandering compound channel of irregular shape and with levees was thus conducted. Using detailed experimental data, the development of streamwise flow, secondary flow and turbulent shear stresses over the cross-sections were analysed. Special emphasis was placed on the momentum exchange between the MC and FPs. The results showed that flow transport in the MC plays a leading role and the momentum exchange between the MC and FPs is more intensive for a high water level. The formation, development and decay of secondary flow in compound meandering rivers are closely related to the momentum exchange between the FPs and the MC. Finally, transverse flow plays a dominant role in the process of momentum exchange and the effects of turbulence are negligibly small.

Construction of large dams is attractive because of their great benefits in flood control, hydropower generation, water resources utilization, navigation improvement, etc. However, dam construction may bring some negative impacts on sediment transport and channel dynamics adjustments. Due to the effects of recent water and soil conservation projects, sediment retention in the newly constructed large upstream reservoirs, and other factors, the sedimentation in the Three Gorges Reservoir (TGR) is quite different from the amount previously predicted in the demonstration stage. Consequently, based on the measured data, characteristics of sedimentation and the related channel deformation in the TGR were analyzed. The results imply that sediment transport tended to be reduced after the Three Gorges Project (TGP). Sedimentation slowed dramatically after 2013 and indicated obvious seasonal characteristics. Due to the rising water level in the TGR in the flood season, the yearly sediment export ratio ( E r a t i o ) was prone to decrease. The water level near the dam site should be reasonably regulated according to the flow discharge to improve the sediment delivery capacity and reduce sedimentation in the TGR, and to try to avoid situations where the flood retention time is close to 444 h. The depositional belt was discontinuous in the TGR and was mainly distributed in the broad reaches, and only slight erosion or deposition occurred in the gorge reaches. Sedimentation in the broad and gorge reaches accounted for 93.8% and 6.2% of the total sedimentation, respectively. The estuarine reach located in the fluctuating backwater area experienced alternate erosion-deposition, with a slight accumulative deposition in the curved reach. Sedimentation mainly occurred in the perennial backwater area. The insight gained in this study can be conducive to directly understanding of large reservoir sedimentation and mechanism of channel adjustment in the reservoir region in the main channel of large river.

A online social interaction-based norm learning study, we use manipulated social feedback to testify our hypotheses.

In order to understand the root morphology distribution and mechanical properties of typical herbaceous plants, and to evaluate the ability of soil reinforcement by the plant roots, root morphology investigation, single root tensile test in laboratory and root cohesion evaluation by the Wu-Waldron model were carried out on two local representative herbaceous plants, Kochia scoparia (L.) Schrad and Artemisia sacrorum Ledeb. in the Loess Plateau of China. The results showed that the root morphological indexes (root number, single root diameter, root cross-sectional area, root surface area, root volume and root area ratio) of the two herbaceous plants decreased with the increase in soil depth, and the ratio of root to shallow soil layer was the highest in the 0–10 cm soil layer. The efficiency of root reinforcement could be higher in the shallow soil layer less than 10 cm. A positive correlation was observed between the root tensile force and root diameter in power function or exponential function, and a negative correlation was observed between the root tensile strength and root diameter in power function. The root cohesion of Kochia scoparia (2.73 kPa, or 0.92 kPa–1.37 kPa) was greater than that of Artemisia sacrorum (1.60 kPa, or 0.54 kPa–0.8 kPa), which could be used as the preferred herbaceous plant species for soil erosion control. The results could provide a scientific basis for selecting dominant species in the fields of ecological slope protection and soil and water conservation plant engineering in the loess area.

This study is devoted to advancing hydroenvironmental modeling of large-scale shallow water systems to the times of three-dimensional (3D) simulations, where the solution of advective trans...

This paper presents a model to simulate the growth of cohesive sediment flocs in 3-D space, which takes into account: Brownian motion contribution, gravity contribution, flow contribution, flow shear stress, floc breakage, and non-uniform initial sediment size distribution. Results of validation tests indicate the model is feasible to simulate the growth of cohesive sediment floc under different contributions. Floc structure in different water depths and the influence of initial sediment size distribution were tested using the new model, taking fractal dimension as an index. The simulated results show that, the sediment floc at the lower region possesses a larger fractal dimension than that of the upper region. It is mainly due to the reproduction of new stronger, denser flocs from the breakage and reform under the higher flow shear stress in the lower region. Additionally, on the assumption condition that the initial sediment size distribution follows normal distribution, an increase in the standard deviation results in the fractal dimension presents a trend from increasing to decreasing.

A prediction&ndash, correction solver is presented here for rapid simulation of free-surface flows in dendritic and looped river networks. Rather than solving a large global algebraic system over the entire domain of river networks, the model solves subsystems for subdomains of branches in two steps: prediction and correction. With the help of the prediction&ndash, correction method (PCM), the partial linearization technique, the semi-implicit method, and the Eulerian&ndash, Lagrangian method (ELM), the model only needs to solve tridiagonal linear systems for branches and is free of any iteration. The new model was tested using a hypothetical looped river network with regular cross sections, the Three Gorges Reservoir (TGR) dendritic river network, and the Jing-south looped river system (with seasonally flooding branches). In the first test, a time-step sensitivity study was conducted and the model was revealed to produce accurate simulations at large time steps when the condition for application of the PCM to river networks was satisfied. In the TGR test, the PCM model provided almost the same histories of water levels and discharges as those simulated by the HEC-RAS model. In the Jing-south test, the mean absolute error in simulated water levels was 0.07&ndash, 0.24 m, and the relative error in simulated cross-section water flux was 0.5&ndash, 4.9% compared with field data, the conservation error was generally 2 &times, 10&minus, 4 to 3 &times, 4. The PCM model was revealed to be 2&ndash, 4 times as fast as a reported model, which solves local nonlinear subsystems using two-layer iterations, and 1.2&ndash, 1.4 times as fast as the HEC-RAS. Using a time step of 1200 s, it took the sequential code 26.8 and 23.1 s to complete a simulation of a one-year unsteady flow process, respectively, in the TGR river networks (with 588 cells) and the Jing-south river system (with 662 cells).

A two-dimensional (2D) flux-form Eulerian-Lagrangian method (FFELM) on unstructured grid is proposed for solving the advection equation in free-surface scalar transport models. The scalar c...

River deltas are actively propagating systems as redundant fluvial sediment accumulated hereon after part of the amount being taken away by marine currents. Anthropogenic activities in drainage basins strongly modified such propagation processes by increasing sediment productions over the past millennia and decreasing sediment loads in the past century. The most significant feature is the delta erosion or flooding as a result of sediment trapping in upstream reservoirs in combination with relative sea-level rise. In addition, human interventions within deltaic regions may complicate the morphological patterns with strong spatial variations. Understanding the controlling factors and future trends of delta erosion is of high importance for sustainable development and comprehensive protection of these dense-populated areas.

Real-time coupling of a hydro-environmental model and an optimization model is difficult because hydrodynamic model (HDM) and scalar transport model (STM) are both time-consuming, especially when the computational domain is large (e.g., river-lake system). This study presents a new 1D hydro-environmental model for free-surface flows and scalar transport in large river-lake systems. Using a prediction–correction method, hydrodynamic is simulated by solving local linear systems based on domain decomposition. A flux-form Eulerian-Lagrangian method (ELM) is constructed to solve advective transport of scalars in 1D grid systems, and a nested technique is proposed to reduce its startup cost. The resulting STM and HDM both allow large time steps for which the Courant–Friedrichs–Lewy number (CFL) is greater than 1, and they are parallelized using the open multiprocessing technique (OpenMP). Moreover, the STM is good at solving multiscalar transport and has low startup cost. The new model is tested using the Jing-Dongting (JDT) system which is covered by a grid of 2382 cells (with 113,600 sub-grids). Stable and accurate simulations are achieved at large time steps for which the CFL can be larger than 5. A sequential run of the new model runs tens of times faster than that of a conventional 1D model such as the Mike11. Moreover, the efficiency of the new model can be further improved by the OpenMP parallel technique. In the test of scale property of the HDM + STM model (using 32 kinds of scalars and 16 cores), a parallel run is 11.8 times faster than a sequential run, and it only takes the new model 33.3 s to complete a simulation of a 1-year process of unsteady flow and scalar transport in the JDT system.

A semi–implicit numerical model with a linear solver is proposed for the free-surface and pressurized mixed flows in hydraulic systems. It solves the two flow regimes within a unified formulation, and is much simpler than existing similar models for mixed flows. Using a local linearization and an Eulerian–Lagrangian method, the new model only needs to solve a tridiagonal linear system (arising from velocity-pressure coupling) and is free of iterations. The model is tested using various types of mixed flows, where the simulation results agree with analytical solutions, experiment data and the results reported by former researchers. Sensitivity studies of grid scales and time steps are both performed, where a common grid scale provides grid-independent results and a common time step provides time-step-independent results. Moreover, the model is revealed to achieve stable and accurate simulations at large time steps for which the CFL is greater than 1. In simulations of a challenging case (mixed flows characterized by frequent flow-regime conversions and a closed pipe with wide-top cross-sections), an artificial slot (A-slot) technique is proposed to cope with possible instabilities related to the discontinuous main-diagonal coefficients of the linear system. In this test, a slot-width sensitivity study is also performed, and the suitable slot-width ratio (ε) for the linear solver is suggested to be 0.05–0.1.

Morphological evolution of estuaries and deltas at the decadal timescale is becoming a global issue in recent decades due to their economic and environmental significances. Present study explores the decadal morphological evolution under high river discharge and decreasing river sediment. Quantitative analysis of bathymetric data indicates that frequent river floods in the 1990s enhanced erosion of the inner estuary superimposed with river sediment decline. A process-based modeling approach (Delft3D) is applied to investigate the physical mechanisms of river flooding on morphological change. Hydrodynamic simulations indicate that the water level gradient and residual transport in the inner estuary increase with river discharge. High water level gradient occurs simultaneously with peak ebb flow, and this status can last for about 5-6 hours. This hydrodynamic condition with sufficient long period facilitates channel erosion and sandbar incision. Morphological simulations indicate that erosion along the main channels is enhanced under higher river discharge, especially in the upstream part. The enhanced erosion can be offset by the increase in sediment load. River flooding superimposed with further decreased sediment supply in the future may induce more significant modifications of channel-shoal systems within the inner Yangtze Estuary than the present.

River-communicating lakes in the middle and lower reaches of the Yangtze River connect with river through culverts and sluices except Poyang Lake and Dongting Lake. Its water quality is poorer due to the lack of natural exchange of water, especially in dry season. Taking Huayang Lake group as an example, the feasibility of ecological operation and the specific plan during non-flood period were studied by data analysis, field investigation and numerical simulation. Data analysis result presents the concentration of TN and TP decreases with the increase of water level, but the tendency slows down when the water level exceeds a certain value. In addition, based on the current sluice gate dispatching mode, relationship between river and lakes, flood control security, improvement of water environment and the simulated results, the ecological operation plan was obtained, that is, using Yangwan and Huayang sluices to control the water level in the lake from October to December at 13 m, January to February at 12.5 m and March to April at 11.8 m.

Responses of irregular rotational tidal flows to an outlet regulation (the Guyuan Sand (GYS) regulation) in the three-level branching Yangtze Estuary are studied by a high-resolution numerical model and theoretical analysis. The project is launched around GYS at the outlet of the North Branch of the Yangtze Estuary. The tidal flows around GYS are rotational and become irregular under the influences of the runoff-tide interactions, rapidly varying topographies and complex solid boundaries in coastal areas. Three designs of GYS regulation were studied, including various diversion dikes and new outlets of different widths. The regulation disturbs the irregular rotational flows around GYS, and further changes the estuarine tidal processes and the water exchange between different branches of the branching Yangtze Estuary. It was interesting to find that additional current and additional storage are formed along the North Branch when a southward outlet and the clockwise rotational flow met around GYS. This special phenomenon is named “guide effect” in this study. The guide effect, together with common resist effect (arising from the narrowed outlet channel), reshapes the estuarine tidal processes. Based on the simulation result and a theoretical analysis, response mechanics of irregular rotational tidal flows to the outlet regulation in complex branching estuaries are quantitatively studied.

The 1998 big floods in the Yangtze River caused basin-wide severe damage. So far numerous studies on the general characteristics of the floods are found in the literature. However, surprisingly enough, little can be found on the propagation features of the floods, especially in the Jingjiang Reach, the most flooding-vulnerable reach of the Yangtze River. Using the prototype observational data, the propagation features of the 1998 big floods in the Jingjiang Reach has been analyzed and summarized. The results are of significance for a better understanding of the law of the floods, for reasonable flood regulation and for the flood protection decision-making.