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.

Vertical Distribution of Momentum Exchange Coefficient and Sediment Concentration in Estuarine and Coastal Waters

Vertical structures of momentum exchange coefficient and sediment concentration are the keys in the research on es- tuarine and coastal suspended sediment transport. Based on the parabolic mixing length distribution pattern, the distribu- tion pattern of vertical momentum exchange coefficient which is suitable for estuarine and coastal waters is constructed. A compari^n with steady flow and measured momentum exchange coefficient during one tidal cycle in the Menai Strait of England shows that the result of this pattern is closer to the measured values than those of commonly used Rouse＇ s and van Rijn＇s patterns, and the pattern is also suitable for estuarine and coastal waters. Successively, based on the precon- dition that momentum exchange coefficient is equivalent to sediment turbulent diffusion coefficient, and combining with the Diffusion Theory, we obtain the exponential vertical distribution pattern of sediment concentration, which is also suitable for estuafine and coastal waters, Thereby, using measured vertical stratified sediment concentration data of the South and the North Passages of the Yangtze Estuary and Zhoushan archipelago waters for fitting calculation, and comparing the results with those from Rouse＇s （1938） and Zhang et al. ＇s （1989） formulas, the results show that the exponential vertical distribution pattern of sediment concentration obtained in the present study not only overcomes the defect of Rouse＇s formula that the surface sediment concentration will be 0, but also has a holistic higher precision along the vertical lines than those of Rouse＇s and Zhang et al. ＇s formulas.