The Significance of Aggregate Properties to Transport Processes
Aggregates of cohesive suspended particles in estuarial waters are formed by Brownian motion, differential settling, and velocity gradients. Aggregates formed by Brownian motion and differential settling are weak and have low densities, compared to those formed in moderate velocity gradients. The densities and shear strengths of aggregates formed in a range of velocity gradients were measured using a capillary viscometer and a concentric cylinder viscometer. It was found that different aggregate structures could exist over short ranges of velocity gradients, and that the structures stable at lower ranges of velocity gradients had lower shear strengths and lower densities. The structures can be described as aggregates of aggregates, with successively higher orders at successively lower ranges of velocity gradients.
Sediment beds formed from such aggregates have a structure that is one order higher than that of the depositing aggregates. The rate of deposition of suspended aggregates under a given bed shear stress depends on the strength of the bond between the depositing aggregate and the bed. Further, as material accumulates on the bed, the mounting overburden crushes the lower aggregates, depending on their strength, causing increasing bed density and shear strength below its surface. The properties of the depositing aggregates determine the rate of sediment deposition and the rate of increase of resistance to erosion for a short distance below the bed surface.
KeywordsShear Strength Velocity Gradient Settling Velocity Cohesive Sediment Capillary Viscometer
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