Hongwei Fang

Three-dimensional calculations of flow and suspended sediment transport in the neighborhood of the dam for the Three Gorges Project (TGP) reservoir in the Yangtze River

The calculation of flow and sediment transport is one of the most important tasks in river engineering. The task is particularly difficult because of the many complex and interacting physical phenomena that need to be accounted for realistically in a model that has predictive power. The threedimensional mathematical model of Wu et al. (2000) is validated without any further calibration by calculating the flow and suspended sediment transport for the reservoir generated by the dam of the Three Gorges Project (TGP) in the Yangtze River. The sedimentation and morphological processes are simulated over a period of 76 years after the dam starts operation, and the results are compared with laboratory measurements obtained by Tsinghua University. Overall, the computational simulations agree well with the experiments and show that the code is a promising tool for natural river predictions. In general, however, some calibration of model parameters, especially for bed roughness, will be necessary.

One-dimensional numerical simulation of non-uniform sediment transport under unsteady flows

One-dimensional numerical models are popularly used in sediment transport research because they can be easily programmed and cost less time compared with two- and three-dimensional numerical models. In particular, they possess greater capacity to be applied in large river basins with many tributaries. This paper presents a one-dimensional numerical model capable of calculating total-load sediment transport. The cross-section-averaged sediment transport capacity and recovery coefficient are addressed in the suspended load model. This one-dimensional model, therefore, can be applied to fine suspended loads and to hyperconcentrated flows in the Yellow River. Moreover, a new discretization scheme for the equation of unsteady non-uniform suspended sediment transport is proposed. The model is calibrated using data measured from the Yantan Reservoir on the Hongshui River and the Sanmenxia Reservoir on the Yellow River. A comparison of the calculated water level and river bed deformation with field measurements shows that the improved numerical model is capable of predicting flow, sediment transport, bed changes, and bed-material sorting in various situations, with reasonable accuracy and reliability.

Effects of sediment particle morphology on adsorption of phosphorus elements

Abstract Sediment particle surface morphology affects phosphorus adsorption. This paper studied the phosphorus adsorption on sediment particle by using a Scanning Electron Microscope (SEM) and an Energy Dispersive X-ray Spectroscopy (EDS). Sediment samples from the Yangtze, Yellow and Yongding rivers in China were chosen to measure the particle surface morphology, surface gray scale and element distribution maps. These samples were firstly cleaned and put into phosphate solution for adsorption. Both the Langmuir equation and Freundlich equation were used for descriptions of adsorption-desorption isotherms for sediments. Particles were then dried and scanned. The results show that the adsorption of phosphorus depends on the surface morphology of particles. Phosphorus exists mostly in the ridges and channels, while a few exist in the slopes, and casually in the peaks, passes and pits.

Environmental assessment of heavy metal transport and transformation in the Hangzhou Bay, China

The environmental impact of heavy metal (Cu, Cd, Zn, Pb, Ni, (90)Sr and (137)Cs) transport and transformation in the Hangzhou Bay (China) was assessed through a comprehensive model that integrates hydrodynamics, sediment and heavy metal transport. A mechanistic surface complexation model was used to estimate the adsorption and desorption of heavy metal by suspended sediment under different aqueous chemistry conditions. The dynamics of metal exchange to and from the seabed was also assessed. The primary processes regulating heavy metal distribution, i.e., convection-diffusion, adsorption-desorption, sedimentation-resuspension, as well as other physical and chemical processes related to mass exchange between adjacent sediment layers, were considered in detail. The accidental discharge of (137)Cs was simulated as an example and results showed that (137)Cs transported along the coast driven by tidal flow. Most (137)Cs distributed near the outfall and accumulated in the seabed sediment. The proposed model can be a useful tool for predicting heavy metal transport and fate and provide a theoretical basis to guide field sampling, assessment of risks and the design of remediation strategies.

Biofilm effects on size gradation, drag coefficient and settling velocity of sediment particles

Sediment particles are often colonized by biofilm in a natural aquatic ecological system, especially in eutrophic water body. A series of laboratory experiments on particle size gradation, drag coefficient and settling velocity were conducted after natural sediment was colonized by biofilm for 5, 10, 15 and 20 days. Particle image acquisition, particle tracking techniques of Particle Image Velocimetry and Particle Tracking Velocimetry were utilized to analyze the changes of these properties. The experimental results indicate that the size gradation, the drag force exerted on bio-particles, and the settling velocity of bio-particles underwent significant change due to the growth of biofilm onto the sediment surface. The study proposes a characteristic particle size formula and a bio-particle settling velocity formula based on the regression of experiment results, that the settling velocity is only 50% to 60% as the single particle which has the same diameter and density. However, biofilm growth causes large particle which the settling velocities are approximately 10 times larger than that of primary particles. These results may be specifically used in the low energy reservoir or lake environment.

Calculations of Nonsubmerged Groin Flow in a Shallow Open Channel by Large-Eddy Simulation

AbstractRigid structures, such as groins or spur dikes, are constructed along riverbanks for various purposes, which pose computational challenges for unsteady flow in engineering mechanics. This paper presents a study of turbulent flow past a series of groins in a shallow, open channel by large-eddy simulation (LES). A direct-forcing immersed boundary method (IBM) was implemented to approximate complex boundaries around groins with round heads. The time-averaged velocities and turbulence intensities at the water surface obtained by an experiment using particle image velocimetry (PIV) were employed to validate the LES model, finding a satisfactory agreement between laboratory data and model results. Subsequently, the numerical model was employed to investigate the impact of groin parameters (i.e., head shape, aspect ratio L/D, and length L) on the flow properties. Model results showed that a rectangular-headed groin generates higher turbulence intensities and larger vortices than a round-headed groin. On ...

Mobility of phosphorus induced by sediment resuspension in the Three Gorges Reservoir by flume experiment

The mobility of phosphorus (P) induced by sediment resuspension have been examined in a circulated flume. During the flume run, the water level and velocity were monitored, and water samples were taken for measurement of sediment and P concentrations. Peak values of both the P and sediment concentrations existed at x=4m, and then decreased slightly along the flume due to deposition. A faster P release was observed for coarser sediment, while a more sustained P release for finer sediment. Combining with the measured data from Yangtze River and sorption experiment, the relation between the load of total P (LTP) and sediment load (Qs) was estimated, and the expressions of distribution coefficient Kd and the concentration of particulate P (PP) were obtained. This study established a bridge between the small-scale sorption experiment and the field observation of natural scale, providing references for the management of contaminated sediment in natural rivers.

Biostabilization and Transport of Cohesive Sediment Deposits in the Three Gorges Reservoir.

Cohesive sediment deposits in the Three Gorges Reservoir, China, were used to investigate physical and geochemical properties, biofilm mass, and erosion and deposition characteristics. Biofilm cultivation was performed in a recirculating flume for three different periods (5, 10 and 15 days) under ambient temperature and with sufficient nutrients supply. Three groups of size-fractionated sediment were sequentially used, including 0–0.02 mm, 0.02–0.05 mm and 0.05–0.10 mm. Desired conditions for erosion and deposition were designed by managing high bed shear stress at the narrow part of upstream flume and low shear stress at the wide part of downstream flume. Biostabilization and transport characteristics of the biofilm coated sediment (bio-sediment) were strongly influenced by the cultivation period, and the results were compared with clean sediment. The bio-sediment was more resistant to erosion, and the mean shear stress was increased by factors of 2.65, 2.73 and 5.01 for sediment with 5, 10 and 15 days of biofilm growth compared with clean sediment, resulting in less sediment being eroded from the bed. Simultaneously, the settling velocity was smaller for bio-sediment due to higher organic content and porosity (i.e., lower density). Additionally, there was a smaller probability of deposition for sediment with a longer cultivation period after erosion, resulting in more retention time in aquatic systems. These results will benefit water management in natural rivers.

Effect of biofilm on the rheological properties of cohesive sediment

Biofilm, a product of metabolic activity, has an important effect on the physico-chemical properties of cohesive sediment. However, little effort has been made to determine the substantial effects of biofilm growth on specific sediment properties, for example rheological properties. Understanding the changes associated with biofilm growth and quantifying the time scales over which these changes occur are important for understanding how biofilms mediate sediment properties and processes and the development of sediment transport mechanics. The effect of biofilm on the rheological properties of cohesive sediment was investigated experimentally. The rheological properties of sediment slurries with and without biofilm at different growth phases were measured and compared. Measurement showed biofilm growth has a significant effect on the rheological properties of cohesive sediment. Rheological equations for biofilm sediment and expressions for rheological properties which change over time are proposed. These equations, and information on biofilm sediment, are important for inclusion of biosedimentological processes in models of sediment dynamics.

A COUPLED 1-D AND 2-D CHANNEL NETWORK MATHEMATICAL MODEL USED FOR FLOW CALCULATIONS IN THE MIDDLE REACHES OF THE YANGTZE RIVER

A coupled one-dimensional (1-D) and two-dimensional (2-D) channel network mathematical model is proposed for flow calculations at nodes in a channel network system in this article. For the 1-D model, the finite difference method is used to discretize the Saint-Venant equations in all channels of a looped network. The Alternating Direction Implicit (ADI) method is adopted for the 2-D model at the nodes. In the coupled model, the 1-D model provides a good approximation with small computational effort, while the 2-D model is applied for complex topography to achieve a high accuracy. An Artificial Neural Network (ANN) method is used for the data exchange and the connectivity between the 1-D and 2-D models. The coupled model is applied to the Jingjiang-Dongting Lake region, to simulate the tremendous looped channel network system, and the results are compared with field data. The good agreement shows that the coupled hydraulic model is more effective than the conventional 1-D model.