Chapter 5 - Rheological Properties of Cohesive Sediment Suspensions

Abstract

This chapter relates the viscosities and Bingham yield strengths of cohesive sediment suspensions with the density and strength of aggregates of various orders. The rotating cylinder viscometer is the appropriate tool for the development of higher order aggregates. The capillary viscometer can only generate zero-order aggregates, i.e., flocs. These studies led to various conclusions that include salinity higher than 1 g/L or 100 ppm seems to have negligible effect on both the viscosity and the yield strength of the suspensions, both viscosity and yield strength increase with increasing sediment concentration. The Bingham strength was found to be proportional to the 5/2 power of the sediment concentration; there is also a strong increase of the Bingham strength with CEC. This is to be expected, since higher CEC values are associated with higher physicochemical bonds, which are responsible for the yield strength and the differential viscosity of the suspensions, and the shear strength of the flocs is much higher than the erosive shear stresses on a deposited bed. This observation led initially to the conclusion, confirmed by subsequent detailed studies, that erosion takes place by the rupture of inter-aggregate bonds. Moreover, according to the same studies on erosion and deposition of cohesive sediments, the shear stresses for the initiation, degree, and rates of erosion are independent of the mass density and the mass strength of the bed, whereas the Bingham shear strength increases rapidly with sediment concentration. This means that neither the differential viscosity nor the Bingham strength can be used as a criterion for erodibility of deposited cohesive sediment beds. They can be helpful, however, for comparative studies of aggregate properties and for an estimate of aggregate densities, settling velocities, and strengths to be used in mathematical models for cohesive sediment transport studies.