Floc properties and settling velocity of San Jacinto estuary mud under variable shear and salinity conditions

Abstract

The flocculation properties of a natural silt–clay mud taken from the San Jacinto estuary near Houston, TX were investigated over a range of suspended sediment concentrations, salinities, and turbulent shear rates. The study was conducted in a laboratory using a paddle mixer to create a turbulent shear field for driving the flocculation process; floc settling velocity and size attributes were measured in a settling column with a camera system and image analysis. In general, maximum floc sizes were observed at turbulent shear rates less than 30 s −1. For turbulent shear rates less than 50 s −1, flocs originating in saline water at 10 and 15 ppt were larger and more loosely packed than those created in freshwater at identical turbulent shear rates and suspended sediment concentrations. At turbulent shear rates of 50 s −1, floc properties of size, submerged specific gravity and fractal dimension were approximately equal for all conditions. Two ranges of behavior were observed in regards to the fractal dimension of flocs at a constant turbulent shear rate. In the first region, for floc sizes less than 200 μ m , a variable fractal dimension was needed to describe the submerged specific gravity as a function of floc size. It is suggested that this variability in fractal dimension may physically be the result of ploysized primary particles. In the second region, for floc sizes greater than 200 μ m , a constant fractal dimension was found to suffice in describing the submerged specific gravity. The constant fractal dimension for this second region was n f =2.3 for freshwater flocs and n f =1.95 for saltwater flocs. Based on the observation of the two-region behavior, a new variable fractal dimension model is presented using a simple exponential decay which asymptotically approaches a constant fractal dimension at a specified floc size. The model itself is based on the uniform-size primary particle formulation of the relationship between floc size and fractal dimension, but the equation was able to model the average shift from variable to constant fractal dimension, which was thought to primarily be due to polysized primary particles.