Effects of Suspension of Cohesive Sediments on Shear Stress and Transport

摘要

The variability of rheological properties in fluid-mud suspensions is studied asa function of salinity and sediment concentration. It was found that the steady-state shear stress increases exponentially with increasing sediment concentration and increases logarithmically with increasing salinity of the suspension. An analytic model predicting shear stress as a function of electroviscous properties is developed. The model shows that the effect of salts in the suspension is to decrease the zeta potential by compressing the electric double layer, and thereby, elevating the shear stresses acting on the shearing planes at each particle-fluid interface in the suspension. These stresses increase with increasing salt content. The model incorporates the classical double-layer theories of Gouy-Chapman and the Helmholtz-Smoluchowski theory for electrokinetics of charged particles. The model shows good correlation with experimental data at low sediment concentrations where the basic assumptions of the Gouy-Chapman formulation are satisfied. Mixing length arguments show that the elevated shear stresses transport suspended sediment vertically upward across the lutocline, thereby reducing the abundance of suspended sediment directly adjacent to the consolidated bed. This action reduces the deposition rates that were found to obey a power law over several orders of magnitude. This power law was invoked in a vertical advection-diffusion model to calculate the variation in deposition flux with increasing distance along a channel. These calculations were compared with field measurements conducted at two different sites. The calculations based on the power law were found to correctly predict deposition flux behavior in the far field. (MM).