Rheological Properties and Incipient Motion of Cohesive Sediment in the Haine Estuary of China

The Haihe cohesive sediment, which is typical in China, is studied systematically for its basic physical and incipient motion properties. Following the requirements of dredging works in the Haihe Estuary, cohesive sediment samples were taken from three locations. Laboratory experiments were conducted to determine the rheological properties of these samples and to examine the incipient motion of the cohesive sediment. It is found that the cohesive sediment has an obvious yield stress tau(b), which increases with the mud density in a manner of an exponential function, and so does the viscosity parameter eta. The cohesive sediment behaves like a Bingham fluid when its density is below 1.38 similar to 1.40 g/cm(3), and when denser than these values, it may become a power-law fluid. The incipient motion experiment also revealed that the incipient velocity of the cohesive sediment increases with die density in an exponential manner. Therefore, the incipient motion is primarily related to the density, which is different from the case for non-cohesive sediment in which the incipient motion is con-elated with the diameter of sand particles instead. The incipient motion occurs in two different ways depending on the concentration of mud in the bottom. For sufficiently fine particles and a concentration lower than 1.20 g/cm(3), the cohesive sediment appears as fluidized mud, and the incipient motion is in the form of instability of an internal wave. For a higher concentration, the cohesive sediment appears as general quasi-solid-mud, and the incipient motion can be described by a series of extended Shields curves each with a different porosity for newly deposited alluvial mud.

Experimental Study on the Viscosity of Soft Cohesive Sediments Around A Vibrating Pillar

Offshore structures are subject to environmental loads such as waves,currents,or wind,which may induce cyclic lateral vibration at the foundations.These cyclic vibration loadings may affect the rheological property of the sediments adjacent to the foundation and the stability of the structures.This is especially true when the structures are founded on cohesive sedimentary bed.In this study,the viscosity of soft cohesive sediments adjacent to a vibrating pillar was considered,and as a primary index of the rheological characteristics of the sediments.The investigation was performed using the sinking ball method.The experimental findings indicated that the viscosity of cohesive sediments decrease with increase of the liquidity index and vibration intensity.A simple semi-empirical formula was proposed.The structures of the cohesive sediments were destroyed due to the mechanical vibration,and the sediments were fluidized during vibration loads.The shear strength of the cohesive sediments decreased with increased vibration intensity,not only because of the increased pore water pressure but also the decreased viscosity of cohesive sediments following sediment fluidization.

EXPERIMENTAL STUDY OF SCOUR RATE IN CONSOLIDATED COHESIVE SEDIMENT

The erosion process of a cohesive sediment after consolidation was studied experimentally in a closed conduit system. The test samples investigated in this study are mixtures of sand and clay with variable compositions and different consolidation times. The main concern of this study is the effects of the dry density of the consolidated sediment on scour rate. A scour rate formula is derived and further interpreted based on the experimental results.

EFFECTS OF CONSOLIDATION TIME AND PARTICLE SIZE ON SCOUR RATES OF COHESIVE SEDIMENT

By means of flumes, experiments have been done in order to determine the effects of consolidation time and particle size on scour rates of cohesive sediment. Experimental results shown dry unit weight increased and scour rates decreased during the course of consolidation, the resistance to scour was related to consolidation time, but it presented different consolidation properties that depended on particle size. For the finer particle, the consolidation process was longer, the range of dry unit weight altered was greater, the consolidated sediments moved in chunks. On the other hand, based on analysis of the relationship of dry unit weight change with consolidation time, the simplified analytical expressions of dry unit weight and scour rates were derived, the results were checked with experimental data in the same flow situation and good agreements were achieved. Since the effects of different particle size were considered during the process of consolidation, the variations of consolidation properties were well reflected in these formulas.

Flocculation properties of cohesive fine-grained sediment in the Three Gorges Reservoir under variable turbulent shear

Sediment flocculation is a key process for the deposition of fine-grained sediments in the Three Gorges Reservoir(TGR)of China.Sediment flocculation influences the evolution of the river regime,but also hampers the smooth navigation in the long term.However,the flocculation process and its controlling factors are poorly understood.We experimentally determined the flocculation properties of cohesive sediment of samples from the TGR(predominantly a mixture of clay and silt)over a range of turbulent shear rates and sediment concentrations.The experiments were conducted in an almost isotropic turbulence field,which was simulated by an array of horizontal oscillating grids in a water tank.Sediment flocculation was recorded by a camera and investigated by image analysis.Our new data indicate that flocculation is generally a response in equilibrium median floc size(d_(f,50))to the increase of the shear rate G.The peak value is attained at G=16.5 s^(-1),where d_(f,50) is 81.3μm(for the suspended sediment concentration(ssc)=0.4 g/L)and 107μm(for ssc=0.7 g/L),respectively.At low shear rates(G<16.5 s^(-1)),the equilibrium floc sizes d_(f,50) increase with rising shear rate G and isinversely related to the Kolmogorov micro length scale η.We attribute this variability to an insufficient deposition time of the sediment flocs in the water tank.Settling velocities,calculated from our experimental data of the floc sizes,are almost consistent with in-situ measured settling velocities,and are ten times larger than the terminal settling velocity of primary particles as calculated from Stokes'law.

Sediment transport following water transfer from Yangtze River to Taihu Basin

To meet the increasing ：need of fresh water and to improve the water quality of Taihu Lake, water transfer from the Yangtze River was initiated in 2002. This study was performed to investigate the sediment distribution along the river course following water transfer. A rainfall-runoff model was first built to calculate the runoff of the Taihu Basin in 2003. Then, the flow patterns of river networks were simulated using a one-dimensional river network hydrodynamic model. Based on the boundary conditions of the flow in tributaries of the Wangyu River and the water level in Taihu Lake, a one-dimensional hydrodynamic and sediment transport numerical model of the Wangyu River was built to analyze the influences of the inflow rate of the water transfer and the suspended sediment concentration （SSC） of inflow on the sediment transport. The results show that the water transfer inflow rate and SSC of inflow have significant effects on the sediment distribution. The higher the inflow rate or SSC of inflow is, the higher the SSC value is at certain cross-sections along the ：river course of water transfer. Higher inflow rate and SSC of inflow contribute to higher sediment deposition per kilometer and sediment thickness. It is also concluded that a sharp decrease of the inflow velocity at the entrance of the Wangyu River on the river course of water transfer induces intense sedimentation at the cross-section near the Changshu hydro-junction. With an increasing distance from the Changshu hydro-junction, the sediment deposition and sedimentation thickness decrease gradually along the river course.

Settlement prediction model of slurry suspension based on sedimentation rate attenuation

This paper introduces a slurry suspension settlement prediction model for cohesive sediment in a still water environment. With no sediment input and a still water environment condition, control forces between settling particles are significantly different in the process of sedimentation rate attenuation, and the settlement process includes the free sedimentation stage, the log-linear attenuation stage, and the stable consolidation stage according to sedimentation rate attenuation. Settlement equations for sedimentation height and time were established based on sedimentation rate attenuation properties of different sedimentation stages. Finally, a slurry suspension settlement prediction model based on slurry parameters was set up with a foundation being that the model parameters were determined by the basic parameters of slurry. The results of the settlement prediction model show good agreement with those of the settlement column experiment and reflect the main characteristics of cohesive sediment. The model can be applied to the prediction of cohesive soil settlement in still water environments.

Experimental study on mechanical properties of methane-hydrate-bearing sediments

Mechanical properties of methane hydrate- bearing-sediments （MHBS） are basic parameters for safety analysis of hydrate exploration and exploitation. Young＇s modulus, cohesion, and internal friction angle of hydrate- bearing sediments synthesized in laboratory, are investigated using tri-axial tests. Stress-strain curves and strength parameters are obtained and discussed for different compositions and different hydrate saturation, followed by empirical expressions related to the cohesion, internal friction angle, and modulus of MHBS. Almost all tested MHBS samples exhibit plastic failure. With the increase of total saturation of ice and methane hydrate （MH）, the specimens＇ internal friction angle decreases while the cohesion increases.

Study on transport of powdered activated carbon using a rotating circular flume

This study employed a rotating flume to examine the Powdered Activated Carbon （PAC） transport with water flow. The initial PAC concentration was 10 mg/L-30 mg/L, and PAC concentration versus time under a specified cross-sectional averaging fluid shear was observed. Results show that compared with PAC deposition in still water, PAC is depleted to zero faster under a fluid shear of 0.02 Pa, due to PAC agglomeration with the fluid shear. However, since PAC floc size only ranges from a single particle （2 μm） to approximate 6 μm, an increasing of instantaneous turbulent fluctuations could counteract the force of PAC floc settling downward, and as a result the steady PAC concentration increases with the increase of shear stress. It is found that the critical shear stress for PAC deposition is about 0.60 Pa, and further the PAC deposition probability is presented according to the experimental scenarios between 0.02 Pa and 0.60 Pa. Combining the PAC transport and deposition formula with PAC-pollutant removal model provides an insight into PAC deployment in raw water aqueduct for sudden open water source pollution.